Introduction
============

Drought is one of the most severe abiotic factors limiting agricultural production in arid and semi-arid regions ([@b11-69_18104]). Drought can be prolonged or seasonal, since rainfall in such areas is very seasonal, and periodic drought occurs regularly. About 230 million ha of farm land is used for wheat cultivation world-wide, and half of this area is regularly affected by drought ([@b42-69_18104]). Xinjiang has a temperate continental climate, with low rainfall and low humidity, and is typical of an arid or semi-arid desert region. The data on wheat drought tolerance research in Xinjiang is valuable for wheat breeders, especially for those working in drought-prone regions of the world.

Drought-stress results in average wheat yield losses of 17%--70% ([@b28-69_18104]). Breeding crop cultivars for the ability to withstand drought is an important strategy, and is necessary to withstand both mild and severe stress conditions. However, better characterization of the ranges of drought-stress responses among available germplasm, and a comprehensive understanding of the physiological mechanisms associated with drought-stress response is crucial to ensure acceptable yield when drought occurs ([@b38-69_18104]). Three physiological and morphological mechanisms, namely drought escape, drought avoidance and drought tolerance, are known to be exhibited in response to drought ([@b20-69_18104]), and crop plants may use combinations of different stress-withstanding mechanisms to cope with drought-stress at any one time ([@b7-69_18104], [@b49-69_18104]).

Breeding crop cultivars with the ability to withstand drought should use a combination of different relevant traits as selection criteria, rather than a single trait. The traits for selection (or screening) for drought escape, avoidance or tolerance and the drought-stress breeding framework depend on the level and timing of stresses in the targeted area ([@b1-69_18104]). Some physiological traits are related to drought coping, such as rapid ground cover or early vigor ([@b37-69_18104]), "stay-green" ([@b27-69_18104]), canopy temperature and canopy temperature depression ([@b29-69_18104], [@b32-69_18104]), stable carbon isotope discrimination ([@b5-69_18104], [@b31-69_18104]), the normalized difference vegetation index (NDVI) ([@b9-69_18104], [@b24-69_18104]) and chlorophyll content (measured as SPAD) ([@b2-69_18104], [@b15-69_18104]). Physiological and morphological traits under drought-stress conditions, such as grain yield per plant ([@b6-69_18104], [@b26-69_18104]), leaf rolling ([@b4-69_18104], [@b17-69_18104]), and plant height ([@b16-69_18104]), are also associated with drought coping, and can be used to screen for high-yield germplasm accessions under drought-stress conditions. Grain plumpness is significantly positively correlated with grain yield under stress conditions ([@b43-69_18104]). The number of grains per spike and grain weight per spike have a positive association with grain yield. However, grain number per spike, thousand-grain weight and especially grain yield are more sensitive to drought-stress than are plant height and the number of spikelets per spike ([@b12-69_18104], [@b33-69_18104]). Therefore, breeders usually place greater emphasis on yield performance under drought-stress conditions.

Drought indices (DIs), measures of drought based on the reduction of grain yield under drought-stress conditions, compared with that under normal irrigated conditions, have been used for drought tolerance evaluation. The stress susceptibility index (SSI) has also been proposed for measurement of yield stability, based on the differences between potential and actual yields under stress environments ([@b45-69_18104]). Meanwhile, the drought resistance coefficient ([@b8-69_18104]) and the drought resistance index (DI) were used to identify genotypes achieving high yield under both stress and no-stress conditions ([@b19-69_18104], [@b18-69_18104]). Drought yield index (DYI), water yield index (WYI) and yield high water-use efficiency index (YH-WUEI) were suggested to be used together for evaluating drought tolerant genotypes, and also for simultaneous screening of genotypes for high water-use efficiency ([@b46-69_18104]). Genotypes identified on the basis of high values of DYI, WYI and YH-WUEI have greater adaptability and stability, and achieve higher yields under both drought-stress and no-stress conditions. Higher DYI, WYI and YH-WUEI values (\>1.05) were reported for accessions with high water-use efficiency and lower values (\<0.95) were observed for accessions with low water-use efficiency, whereas values between 0.95 to 1.05 were shown to be associated with accessions exhibiting moderate water-use efficiency ([@b46-69_18104]).

Wheat is one of the most important food crops in China and is the principal food crop in Xinjiang, China, a typical arid or semi-arid region. Drought-stress is the major yield-limiting abiotic factor for wheat production in Xinjiang, which has an average annual rainfall of approximately 200 mm, with less than 100 mm rainfall occurring during the wheat-growing season ([@b22-69_18104]). Without irrigation, there is no yield from crops grown in most regions of Xinjiang, as evaporation losses in the plains area are very high. However, irrigation water in Xinjiang is routinely in short supply during the wheat grain filling stage because of limited water availability. The main breeding objectives for wheat in Xinjiang are high and stable yield, with good adaptability to uncertain drought-stress conditions. In order to screen for the selection index of high yield and high grain quality under drought-stress in wheat, a set of 145 CIMMYT WPHYSGP nursery lines and seven local spring wheat varieties were phenotyped and evaluated for physiological and yield traits under two irrigation regimes during the 2011 and 2012 growing seasons in Xinjiang, China. In this paper, we report the evaluation and phenotyping of these wheat genotypes in response to drought, and the identification of the best-adapted genotypes, exhibiting high yields under both stress and no-stress conditions, as potential parents in wheat breeding programs in Xinjiang and areas with similar drought-prone climates.

Materials and Methods
=====================

Wheat genotypes
---------------

A total of 152 wheat genotypes were used in the trials, including seven Xinjiang locally bred spring wheat varieties, well adapted to conditions in the north of Xinjiang, and 145 lines from the CIMMYT Wheat Physiological Germplasm Screening (WPHYSGP) nursery in Mexico, where screening had been carried out for improved drought response. Of the seven local varieties, Xinchun 6, Xinchun 11, Xinchun 17 and Xinchun 29 are leading varieties in different regions of Xinjiang, while 'Xinchun 6' is also the local check variety in regional trials in Xinjiang.

Experimental conditions
-----------------------

Trials were conducted in two growing seasons (2011 and 2012) at the Junhu wheat experimental station of Changji, Xinjiang Academy of Agricultural Sciences, China: 43°96′N, 87°01′E, altitude 717.2 m, gray desert soil, soil depth 70 cm, soil pH\~7.8, and 200 mm average annual precipitation.

Sowing was done by hand in plots with four rows, 2 m length, and 0.2 m inter-row spacing and 624 seeds m^−2^ sowing density. Seeds were planted on April 11, 2011 and March 31, 2012. Before planting, 15 g and 6.8 g of N and P, respectively, were applied per plot in the seedbed. At the seedling (two to three leaves) and jointing stages, 13.8 and 14.5 g N per plot, respectively, were applied. The herbicide 2-methy1-4-chlorophenoxy acetic acid was applied once at the rate of 0.15 g m^−2^ during the jointing stage.

In each of the two growing seasons, the experimental layout was randomized complete blocks design with three replicates under each of the irrigation regimes, namely no-stress (non-limited irrigation) and drought-stress (limited irrigation) conditions. Both of the no-stress and drought-stress treatments used drip irrigation which was independently controlled according to the irrigation regime, and the areas under the different irrigation regimes were separated by 4 m isolation zone. No-stress plots (T1) were watered seven times between April 28, 2011 and June 28, 2011, and eight times between April 18, 2012 and June 28, 2012, with irrigation intervals of 10 d. Drought-stress plots (T2) were irrigated twice in 2011 at the jointing and heading stages, respectively, and three times in 2012 at the jointing, heading and early grain filling stages. During the 2011 growing season, all plots of T1 and T2 received 105.3 mm rainfall, and plots T1 and T2 received an additional 420 mm and 120 mm irrigation water, respectively. During the 2012 growing season, all plots of T1 and T2 received 95 mm rainfall, and plots T1 and T2 received an additional 480 mm and 180 mm irrigation water, respectively.

Grain yield and DYI, WYI, and YH-WUEI
-------------------------------------

In both the 2011 and 2012 growing seasons, all the plants (except for those harvested earlier for individual agronomic traits) in the T1 and T2 plots were harvested by hand, threshed by machine, cleaned by hand, and the yield per plot weighed by hand. Weights were expressed at a moisture content of 13%. Performance of the genotypes in each year was evaluated by calculation of the drought yield index (DYI), the water yield index (WYI) and the yield high water use efficiency index (YH-WUEI):

DYI

=

Y

S

/

Y

¯

S

WYI

=

Y

P

/

Y

¯

P

YH-WUEI

=

\[

(

DYI

\+

WYI

)

/

2

\]

=

\[

(

Y

S

/

Y

¯

S

\+

Y

P

/

Y

¯

P

)

/

2

\]

where Y~S~ and Y~P~ are the grain yields of each genotype under drought-stress (T2) and no-stress conditions (T1), respectively, and Ȳ~S~ and Ȳ~P~ are the mean grain yields of all the genotypes under drought-stress and no-stress conditions, respectively.

Agronomic traits
----------------

In both the 2011 and 2012 growing seasons, days to heading, plant height, grain plumpness, thousand-grain weight, spike number per plant, spikelet number per spike, spike length, grain number per spike, grain weight per spike, and grain weight per plant were determined. Days to heading was measured as the number of days from planting until 50% of the main culm spikes emerged from the boot in each plot. Plant height was measured as the distance from the ground to the top of spike (excluding the awns) at maturity. Physiological maturity was recorded when the green color was about to disappear from the upper portion of the main culm peduncle and there was complete loss of green color from the flag leaves. Days to physiological maturity was counted as the number of days from planting to physiological maturity. Grain plumpness was classified on a nine-class scale, from 1 (very shriveled) to 9 (well rounded). Ten whole-plant samples from each T1 and T2 plot were taken before harvesting and used for collecting spike number per plant, spikelet number per spike, spike length, grain number per spike, grain weight per spike, and grain weight per plant. Weights were expressed at a moisture content of 13%.

Physiological measurements
--------------------------

Previous studies had shown that reduction in canopy temperature at solar noon correlated better with yield than it did in the morning or late afternoon ([@b36-69_18104]). CT was measured in both 2011 and 2012 at the mid-grain filling stage (18 d after flowering), using a hand-held infrared thermometer Optris LS LT (Optris Infrared Sensing, Portsmouth, NH, USA) between 13:00 and 15:00 h during the day, selecting days with clear skies and low wind, keeping the sun behind the operator and measuring the temperature of the canopy exposed to the sun ([@b36-69_18104]).

Normalized difference vegetation index (NDVI) was determined with the Green Seeker 505 (NTech Industries Inc., Sunnyvale, CA, USA) on the central rows of all plots. In both the 2011 and 2012 growing seasons, NDVI was measured at the jointing stage, flowering stage and mid-grain filling stage. NDVI reflected the stay-green traits of plants. It mainly reflected the growth potential of plants during the vegetative growth period, and reflected the transformation ability of photosynthesis products of plants after flowering. Some studies have shown that stay-green traits have great potential in selecting for water-stress adaptation ([@b9-69_18104], [@b10-69_18104]).

The SPAD Minolta 502 Plus (Konica Minolta, Tokyo, Japan) was used to non-destructively determine chlorophyll content, measuring 10 main culm flag leaves from each replicate plot, and recording the mean data. In both the 2011 and 2012 growing seasons, SPAD was measured at the flowering stage and the mid-grain filling stage.

Statistical analysis
--------------------

Restricted maximum likelihood (REML) variance components analysis was conducted on the data in 2011 and 2012 (genotype, irrigation and year were the fixed model and row and column were the random model). Pearson's correlation coefficients were determined between grain yield and other evaluated traits within the drought-stress and no-stress conditions. Parametric linear regression analyses and multiple linear regression analysis were conducted among evaluated traits within each of the two irrigation regimes. Significant differences among genotypes and irrigation regimes were determined using Fisher's protected Least-Significant Difference at α0.05. Data analysis was performed using Genstat v.17.1 (VSN International Ltd., Hemel Hempstead, UK) and SPSS Statistics v.17.0.1 (IBM Corporation, Armonk, NY, USA).

Results
=======

Differences in agronomic traits between different genotypes and irrigation regimes
----------------------------------------------------------------------------------

REML variance components analysis of the 152 genotypes showed significant differences in grain yield (p \< 0.001), thousand-grain weight (p \< 0.001), grain plumpness (p \< 0.001), CT (p \< 0.01), NDVI (p \< 0.001), SPAD (p \< 0.001), grain number per spike (p \< 0.001), grain weight per spike (p \< 0.001), days to heading (p \< 0.001) and plant height (p \< 0.001) within and between the two irrigation treatments in both the 2011 and 2012 growing seasons ([Table 1](#t1-69_18104){ref-type="table"}). The seasonal effect was not significant for grain yield, grain plumpness, CT, NDVI at flowering and grain filling stages, grain number per spike, grain weight per spike and days to heading. Genotype × Irrigation treatment interaction was also significant (p \< 0.05) with regard to grain yield, thousand-grain weight, grain plumpness, CT, NDVI at the flowering and grain filling stages, days to heading and plant height in both the 2011 and 2012 growing seasons.

Assessment of a set of CIMMYT WPHYSGP nursery lines and local varieties
-----------------------------------------------------------------------

Comparison of the mean data of the 152 genotypes under trial showed that drought-stress significantly reduced grain yield, grain weight per spike, NDVI at the flowering and grain filling stages, SPAD at the grain filling stage, grain plumpness, grain number per spike, thousand-grain weight, plant height and CT under each of the two irrigation regimes ([Fig. 1](#f1-69_18104){ref-type="fig"}). The mean grain yields of all the genotypes under no-stress and stress conditions averaged over the two seasons were 6440.8 kg ha^−1^ and 3360.8 kg ha^−1^, respectively ([Table 2](#t2-69_18104){ref-type="table"}), representing a significant decrease of 47.8% under drought-stress conditions. Grain weight per spike, NDVI at the flowering and grain filling stages, SPAD at the grain filling stage, grain plumpness, grain number per spike, thousand-grain weight, and plant height decreased by 50%, 45.2%, 30.7%, 22.9%, 22.7%, 21.8%, 20.5%, and 19.5%, respectively, under drought-stress, while CT increased by 12.2% under drought-stress conditions ([Fig. 1B--1D](#f1-69_18104){ref-type="fig"}). Spike length, spikelet number per spike and heading date values were very similar under both irrigation regimes ([Fig. 1B](#f1-69_18104){ref-type="fig"}). NDVI continued to rise from the seedling stage onwards, reaching its peak at the flowering stage then decreasing at the grain filling stage ([Fig. 1D](#f1-69_18104){ref-type="fig"}). NDVI at the jointing stage under the no-stress and stress conditions were similar because drought-stress occurred mainly after the flowering stage. NDVI under drought-stress was lower than that under no-stress conditions at the flowering and grain filling stages. Chlorophyll content at the flowering stage was higher under drought-stress conditions, compared to the no-stress conditions, but decreased sharply at the grain filling stage under the stress conditions ([Fig. 1C](#f1-69_18104){ref-type="fig"}).

Evaluation of grain yield of test genotypes under two irrigation regimes
------------------------------------------------------------------------

Under no-stress conditions, 22 genotypes showed at least 10% higher grain yield and water yield index (WYI) than the mean value of the 152 genotypes. These genotypes were 9645, 'Xinchun 29', 9611, 9650, 9637, 9653, 9657, 'Xinchun 6', 9692, 9638, 9655, 9606, 'Xinchun 23', 9639, 9744, 9608, 9727, 9630, 9649, 'Xinchun 11' and 9729. Among these genotypes, 'Xinchun 29', 'Xinchun 6', 'Xinchun 23' and 'Xinchun 11' were local varieties, while the others were CIMMYT lines ([Table 3](#t3-69_18104){ref-type="table"}). Of the 22 genotypes, five genotypes (9645, 'Xinchun 29', 9611, 9650 and 9637) achieved 3% higher yield than the local check variety 'Xinchun 6'. CIMMYT genotypes 9672, 9703, 9678, 9701, 9711, 9659, 9661, 9706, 9677, 9702, 9663, 9605, 9662, 9732, 9704, 9684, 9675, 9665, 9670, 9660, 9679, 9740 and 9664 showed 10% lower grain yield than the mean grain yield of the 152 genotypes and 21% lower grain yield than the check variety 'Xinchun 6', and also had lower water yield index values (WYI) ([Table 3](#t3-69_18104){ref-type="table"}).

Under stress conditions, 26 genotypes achieved grain yields 10% higher than that of 'Xinchun 6' as well as high DYI. These genotypes were 9643, 9646, 9612, 9689, 9642, 9640, 9692, 9729, 9698, 9638 9645, 9606, 9650, 9720, 9627, 9699, 9735, 9611, 9730, 9637, 'Xinchun 29', 9644, 9725, 9655, 'Xinchun 33' and 9726. Of these superior genotypes, only 'Xinchun 29' and 'Xinchun 33' were local varieties, while the others were from CIMMYT. CIMMYT genotypes 9660, 9679, 9662, 9716, 9691, 9661, 9676, 9634, 9674, 9620, 9678, 9668, 9733, 9731, 9670, 9673, 9677, 9732, 9602, 9664, 9617, 9740, 9619, and 9705 had lower yields, with values 10% and 9.5% less than the mean grain yield of the 152 genotypes and the check variety, 'Xinchun 6', respectively.

Higher DYI, WYI and YH-WUEI values \>1.05 were observed in 24 genotypes, consisting of twenty-one CIMMYTY nursery lines and three local varieties ([Table 4](#t4-69_18104){ref-type="table"}). These genotypes showed high grain yield potential and good yield stability under both no-stress and drought-stress conditions. Acceptable plant height was considered to be between 70 and 90 cm, which was adapted to and acceptable for local irrigated wheat production. Ten genotypes, 'Xinchun 29', 9692, 9638, 9606, 'Xinchun 23', 9639, 'Xinchun 11', 9729, 9746 and 9730 exhibited acceptable plant heights and had high and stable yield under both no-stress and stress conditions. Twenty-four CIMMYT genotypes were classified as exhibiting low water-use efficiency (DYI \< 0.95, WYI \< 0.95, YH-WUEI \< 0.95) and they were 9675, 9663, 9659, 9711, 9712, 9713, 9672, 9665, 9660, 9679, 9662, 9716, 9661, 9674, 9620, 9678, 9670, 9673, 9677, 9732, 9664, 9617, 9740 and 9705. The other 106 genotypes were classified as exhibiting moderate water-use efficiency. Among them, CIMMYT genotypes 9630, 9741, 9671, 9625 and 9616 showed high grain yield under no-stress conditions (WYI \> 1.05), but low grain yield under drought-stress conditions (DYI \< 0.95), indicating that they were very sensitive to drought-stress. CIMMYT genotypes 9702 and 9669 showed high grain yield and drought yield index (DYI \> 1.05) under drought-stress conditions. However, low grain yield and WYI (\<0.95) were observed for these two genotypes under no-stress conditions.

Relationships between grain yield and agronomic traits
------------------------------------------------------

Significant negative correlations were obtained between grain yield and CT at the mid-grain filling stage, with coefficients of −0.440 and −0.494 (p \< 0.001) for no-stress and drought-stress conditions, respectively ([Table 5](#t5-69_18104){ref-type="table"}). At the mid-grain filling stage, CT also correlated significantly negatively with plant height, days to heading, spikelets per spike, spike length, grain number per spike and grain weight per spike under either irrigation regimes, and with thousand-grain weight but only under drought-stress conditions.

Grain yield was positively correlated with NDVI at the jointing (correlation coefficients of 0.551 \[p \< 0.001\]), flowering (r = 0.543; p \< 0.001) and mid-grain filling stages (r = 0.249; p \< 0.001) in each of the two irrigation regimes. The correlation coefficients were more significant at the jointing and flowering stages than at the mid-grain filling stage under no-stress conditions. However, under drought-stress conditions, the correlation between NDVI and grain yield at the flowering stage was more significant than that at the jointing and mid-grain filling stages. At the jointing stage, NDVI was significantly correlated with plant height, days to heading, thousand-grain weight, spike length, spikelet number per spike, grain number per spike and grain weight per spike under either irrigation regimes, and with grain plumpness under only drought-stress conditions. At the flowering stage, NDVI was significantly correlated with plant height, days to heading, spike length, spikelet number per spike and grain weight per spike under either irrigation regimes, and with thousand-grain weight and grain plumpness under drought-stress conditions. At the mid-grain filling stage, NDVI was significantly correlated with thousand-grain weight, grain plumpness, grain weight per spike and days to heading under drought-stress conditions. However, at the mid-grain filling stage, NDVI was correlated with days to heading only under no-stress conditions.

At the mid-grain filling stage, chlorophyll content (SPAD) was positively correlated with grain yield under drought-stress conditions. However, no correlation was observed between SPAD and grain yield at the flowering stage under drought-stress nor at the flowering or mid-grain filling stages under no-stress conditions. At the mid-grain filling stage, SPAD was also positively correlated with thousand-grain weight, grain weight per spike and days to heading. Significant positive correlations were obtained between grain yield and the traits plant height, days to heading, spike length, spikelet number per spike, grain number per spike, grain weight per spike, grain plumpness and thousand-grain weight.

Multiple linear regression analysis indicated that grain yield was significantly (p \< 0.001) related to grain weight per spike, plant height and grain plumpness under the no-stress conditions ([Fig. 2A](#f2-69_18104){ref-type="fig"}). The three principal factors, grain weight per spike (r = 0.318; p \< 0.001), plant height (r = 0.549; p \< 0.001) and grain plumpness (r = 0.172; p \< 0.001) explained 61.8% of the total phenotypic variation in grain yield under no-stress conditions in 2011 and 2012. Multiple linear regression analysis also found significantly (p \< 0.001) relationships between grain yield and CT, plant height and grain plumpness under drought-stress conditions ([Fig. 2B](#f2-69_18104){ref-type="fig"}). The three principal factors, CT at the grain filling stage (r = 0.244; p \< 0.001), plant height (r = 0.244; p \< 0.001) and grain plumpness (r = 0.239; p \< 0.001) together explained 44.8% of the total phenotypic variation in grain yield under drought-stress conditions in 2011 and 2012.

Discussion
==========

CIMMYT hexaploid spring wheat germplasm has played a global role in assisting wheat breeding with respect to high yield potential and quality improvement ([@b3-69_18104], [@b44-69_18104], [@b48-69_18104]). We evaluated the drought-stress response of a set of WPHYSGP lines, introduced from CIMMYT, under the two different irrigation regimes during the 2011 and 2012 growing seasons. Based on yield, DYI, WYI, and YH-WUEI, 24 elite genotypes with high and stable yield under drought-stress and no-stress conditions were identified. In view of the demand for varieties of appropriate suitable plant height, 10 of these genotypes can be recommended as core parents for breeding for improved drought response in Xinjiang, China. The DYI, WYI and YH-WUEI were shown to be very useful for evaluating drought tolerance and to be powerful in identifying genotypes with high yield potential and high water-use efficiency as well ([@b21-69_18104], [@b46-69_18104]). Interestingly, three out of seven (43%) locally bred varieties were included in the 10 selected superior lines, compared with seven out of 145 (4.8%) CIMMYT lines.

In the present study, drought-stress was associated with a noticeable decrease in grain yield, plant height, grain number per spike, grain weight per spike, thousand-grain weight, grain plumpness, NDVI at the flowering and grain filling stages and chlorophyll content at the grain filling stage, and to an increase in CT and chlorophyll content at the flowering stage. Plant height and grain weight per plant generally exhibit above-average heritability and have been shown to be very sensitive to drought-stress, so could be recommended as selection criteria for drought-response improvement ([@b6-69_18104], [@b10-69_18104]). However, another study also showed that the yield components number of kernels per spike and thousand-grain weight, and especially grain yield were even more sensitive to drought-stress than was plant height ([@b12-69_18104], [@b25-69_18104]). Improvements in grain yield in recent years were often accompanied by increased thousand-grain weight ([@b13-69_18104], [@b40-69_18104], [@b41-69_18104]). Our study showed that grain weight per spike, grain yield, NDVI at the flowering and grain filling stages, grain plumpness, grain number per spike, thousand-grain weight, plant height and CT were more sensitive to drought-stress than were NDVI at the jointing stage, spikelet number per spike, spike length and days to heading. Plants were watered at both jointing and heading stages of drought-stress, and spike development was completed before the heading stage, so drought-stress had little effect on NDVI at the jointing stage, spikelet number per spike, spike length and days to heading. The drought stress affected the growth of wheat after flowering stage, especially grain filling, thereby resulting in more change in grain weight per spike, grain yield, NDVI at the flowering and grain filling stages, grain plumpness, grain number per spike, thousand-grain weight, plant height and CT. Grain yield was negatively correlated with CT and positively correlated with plant height, grain number per spike, grain weight per spike, thousand-grain weight, grain plumpness, and NDVI. Low CT and high NDVI reflected better growth potential. The grain number per spike, grain weight per spike, thousand-grain weight and grain plumpness all reflected results of grain filling, so they were positively correlated with grain yield. We also found that grain weight per spike, plant height and grain plumpness were the three principal factors related to yield and could explain 61.8% of the total phenotypic variation of grain yield under no-stress conditions, so that grain weight per spike and grain plumpness could be used as selection indicator for high yield under stress conditions wheat breeding. Plant height was a key parameter affecting lodging and thus grain yield and grain quality, so the appropriate plant height was an important indicator for wheat breeding.

Many studies have reported that chlorophyll content (measured here as SPAD) was positively correlated with yield under drought-stress conditions ([@b15-69_18104], [@b47-69_18104]). In our study, chlorophyll content at the grain filling stage was positively associated with yield under drought-stress conditions but not under no-stress conditions. Furthermore, chlorophyll content at the flowering stage was not associated with grain yield under either irrigation regime. The reason for this may be the result of the SPAD measurement affected by leaf glaucousness.

Cooler canopies has been associated with increased stomatal conductance and increased grain yield under irrigated condition ([@b14-69_18104]) and with increased rooting depth and greater ability to extract moisture from deeper soil profiles under drought-stress conditions ([@b23-69_18104], [@b39-69_18104]). CT, dry weight stem^−1^, grains spike^−1^ and water-soluble carbohydrate concentration had high mean heritability and were recommended for use in selection for stress tolerance in plant breeding programs ([@b30-69_18104], [@b41-69_18104]). Lower CT was a trait that was proposed for breeding and early-stage selection, aimed at increasing genetic gain for grain yield in water-limited environments ([@b35-69_18104], [@b39-69_18104]). Our study confirmed that CT was negatively associated with grain yield under well-watered conditions as well as under drought-stress conditions. CT, especially at the grain filling stage, plant height and grain plumpness were the three principal factors associated with grain yield under drought-stress conditions.

NDVI has been used as a criterion to estimate relative biomass before heading ([@b34-69_18104]) and the relative character of "stay-green" after flowering ([@b9-69_18104]). "Stay-green" traits may provide cumulative effects, together with other traits, which improve adaptation under stress conditions ([@b10-69_18104], [@b24-69_18104]). NDVI showed a positive relationship with grain yield under well-irrigated condition, with an even stronger association with grain yield under drought conditions ([@b10-69_18104]). In the present study, we found that NDVI in the jointing stage, the flowering stage and the mid-grain filling stage were all significantly positively correlated with yield under both no-stress and stress conditions.

Wheat drought tolerance is a complex quantitative trait and it is difficult to identify a single trait index which reflects both wheat yield potential and drought tolerance ([@b6-69_18104]). Based on our study, plant height, grain number per spike, grain weight per spike, thousand-grain weight, grain plumpness, NDVI and CT could be recommended as indicators of drought tolerance improvement in spring wheat. Especially when breeding for improved varieties for cultivation in irrigated regions, more attention should be paid to screening for low values of CT and high values of grain weight per spike and grain plumpness, in combination with appropriate plant height in spring wheat breeding programs.

This work was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFD0300110), the GCP project (Grant No. G7010.02.01-5), the basic research funding of non-profit scientific research institutes in the Xinjiang Uygur Autonomous Region (Grant No. KY2017066), and a project of the Key Laboratory of Crop Ecophysiology and Farming System in Desert Oasis Region, Ministry of Agriculture (Grant No. 25107020-201605). We thank International Science Editing (<http://www.internationalscienceediting.com>) for editing this manuscript.

CT

:   canopy temperature

DI

:   drought resistance index

DYI

:   drought yield index

GNPS

:   grain number per spike

GP

:   grain plumpness

GWPS

:   grain weight per spike

GY

:   grain yield

HD

:   days to heading

NDVI

:   normalized difference vegetation index

PH

:   plant height

SL

:   spike length

SPAD

:   soil and plant analyzer development

SPS

:   spikelet number per spike

SSI

:   stress susceptibility index

TGW

:   thousand-grain weight

T1

:   no-stress conditions

T2

:   drought-stress conditions

WPHYSGP

:   Wheat Physiological Germplasm Screening Nursery

WYI

:   water yield index

YH-WUEI

:   yield high water-use efficiency index

![(A) The mean ± SD grain yield of 152 spring wheat genotypes under two irrigation regimes, T1 (no drought stress) and T2 (drought stress conditons) in 2011 and 2012. (B) The mean ± SD grain weight per spike, grain plumpness, grains per spike, TKW, plant height, spike length, spikelet per spike, days to heading, of 152 spring wheat genotypes under two irrigation regimes, T1 (no drought stress) and T2 (drought stress conditons) in 2011 and 2012. (C) The mean ± SD CT at grain filling stage, SPAD at flowering stage, SPAD at grain filling stage of 152 spring wheat genotypes under two irrigation regimes, T1 (no drought stress) and T2 (drought stress conditons) in 2011 and 2012. (D) The mean ± SD grain NDVI of 152 spring wheat genotypes under two irrigation regimes, T1 (no drought stress) and T2 (drought stress conditons) in 2011 and 2012. The different letters showed significant at the 0.05 probability level according to the ANOVA.](69_18104_1){#f1-69_18104}

![Regression analysis between grain yield and other traits under no-stress (A) and drought-stress (B) in 2011 and 2012.](69_18104_2){#f2-69_18104}

###### 

REML variance components analysis for two treatments in 2011 and 2012 growing seasons

  Trait                      Source of variation   df              Wald statistic                                        F value
  -------------------------- --------------------- --------------- ----------------------------------------------------- ---------------------------------------------------
  GY, kg/ha                  Genotype (G)          151             1810142.85                                            2.0[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     3085829921.37   3431.8[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}   
  Year (Y)                   1                     899176.21       1.0NS                                                 
  G × I                      151                   41996942.06     46.7[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  G × Y                      151                   1020276.18      1.1NS                                                 
  I × Y                      1                     216462509\.56   240.7[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I × Y                  151                   894905.09       1.0NS                                                 
                                                                                                                         
  TGW, g                     Genotype (G)          151             101.10                                                10.4[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     15938.45        1644.9[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}   
  Year (Y)                   1                     12.37           1.3[\*](#tfn1-69_18104){ref-type="table-fn"}          
  G × I                      151                   22.19           2.3[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  G × Y                      151                   26.70           2.8[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  I × Y                      1                     1203.53         124.2[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I × Y                  151                   12.24           1.3[\*](#tfn1-69_18104){ref-type="table-fn"}          
                                                                                                                         
  Grain plumpness            Genotype (G)          151             2.65                                                  4.4[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     324.14          532.6[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  Year (Y)                   1                     0.83            1.2NS                                                 
  G × I                      151                   48.25           79.3[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  G × Y                      151                   1.11            1.8[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  I × Y                      1                     67.14           110.3[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I × Y                  151                   0.69            1.1NS                                                 
                                                                                                                         
  CT mid-grain filling       Genotype (G)          151             60.88                                                 21.0[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     2433.76         840.1[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  Year (Y)                   1                     2.72            0.9NS                                                 
  G × I                      151                   85.75           29.6[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  G × Y                      151                   2.88            1.0NS                                                 
  I × Y                      1                     2.13            0.7NS                                                 
  G × I × Y                  151                   2.11            0.7NS                                                 
                                                                                                                         
  NDVI jointing              Genotype (G)          151             0.03                                                  6.5[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     0.32            71.7[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  Year (Y)                   1                     3.61            801.4[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I                      151                   0.00            1.1NS                                                 
  G × Y                      151                   0.01            2.6[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  I × Y                      1                     0.50            110.1[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I × Y                  151                   0.00            0.8NS                                                 
                                                                                                                         
  NDVI flowering             Genotype (G)          151             0.02                                                  2.5[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     20.54           2548.9[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}   
  Year (Y)                   1                     0.01            0.9NS                                                 
  G × I                      151                   8.86            1099.7[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}   
  G × Y                      151                   0.01            1.3[\*](#tfn1-69_18104){ref-type="table-fn"}          
  I × Y                      1                     1.47            182.5[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I × Y                  151                   0.01            0.9NS                                                 
                                                                                                                         
  NDVI mid-grain filling     Genotype (G)          151             0.02                                                  2.0[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     28.12           3034.3[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}   
  Year (Y)                   1                     0.01            0.9NS                                                 
  G × I                      151                   0.27            29.1[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  G × Y                      151                   0.01            1.4[\*](#tfn1-69_18104){ref-type="table-fn"}          
  I × Y                      1                     0.54            58.4[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  G × I × Y                  151                   0.01            1.2NS                                                 
                                                                                                                         
  SPAD flowering             Genotype (G)          151             37.79                                                 3.8[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     593.15          59.9[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  Year (Y)                   1                     444.73          44.9[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  G × I                      151                   8.60            0.9NS                                                 
  G × Y                      151                   10.61           1.1NS                                                 
  I × Y                      1                     2.04            0.2NS                                                 
  G × I × Y                  151                   7.54            0.8NS                                                 
                                                                                                                         
  SPAD mid-grain filling     Genotype (G)          151             139.75                                                2.0[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     41963.19        596.2[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  Year (Y)                   1                     53581.70        761.3[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I                      151                   116.49          1.7[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  G × Y                      151                   128.74          1.8[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  I × Y                      1                     47971.88        681.6[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I × Y                  151                   106.55          1.5[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
                                                                                                                         
  Spike length               Genotype (G)          151             24.81                                                 1.5[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     52.10           3.3NS                                                 
  Year (Y)                   1                     1215.27         75.9[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  G × I                      151                   14.68           0.9NS                                                 
  G × Y                      151                   16.24           1.0NS                                                 
  I × Y                      1                     34.90           2.2NS                                                 
  G × I × Y                  151                   15.47           1.0NS                                                 
                                                                                                                         
  SPS                        Genotype (G)          151             16.74                                                 16.2[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     1.3             1.1NS                                                 
  Year (Y)                   1                     7644.53         7386.7[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}   
  G × I                      151                   0.86            0.8NS                                                 
  G × Y                      151                   2.40            2.3[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  I × Y                      1                     14.42           13.9[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  G × I × Y                  151                   0.77            0.7NS                                                 
                                                                                                                         
  GNPS                       Genotype (G)          151             129.42                                                4.3[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     17242.87        575.5[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  Year (Y)                   1                     35.47           1.2NS                                                 
  G × I                      151                   11016.32        367.7[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × Y                      151                   39.59           1.3[\*](#tfn1-69_18104){ref-type="table-fn"}          
  I × Y                      1                     4026.35         134.4[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I × Y                  151                   26.80           0.9NS                                                 
                                                                                                                         
  GWPS                       Genotype (G)          151             0.20                                                  3.0[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     107.78          1581.8[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}   
  Year (Y)                   1                     0.10            1.1NS                                                 
  G × I                      151                   0.07            1.0NS                                                 
  G × Y                      151                   0.07            1.0NS                                                 
  I × Y                      1                     12.10           177.6[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I × Y                  151                   0.06            0.9NS                                                 
                                                                                                                         
  Days to heading            Genotype (G)          151             46.07                                                 33.0[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     1.35            1.0NS                                                 
  Year (Y)                   1                     5.02            3.6NS                                                 
  G × I                      151                   2.26            1.6[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  G × Y                      151                   5.27            3.8[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  I × Y                      1                     107.09          76.8[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}     
  G × I × Y                  151                   1.57            1.1NS                                                 
                                                                                                                         
  Plant height               Genotype (G)          151             387.63                                                11.9[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}
  Irrigation treatment (I)   1                     80000.46        2457.1[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}   
  Year (Y)                   1                     25697.52        789.3[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I                      151                   77.61           2.4[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}      
  G × Y                      151                   40.74           1.3[\*](#tfn1-69_18104){ref-type="table-fn"}          
  I × Y                      1                     18939.96        581.7[\*\*\*](#tfn3-69_18104){ref-type="table-fn"}    
  G × I × Y                  151                   37.04           1.1NS                                                 

Significant at the 0.05 probability level,

Significant at the 0.01 probability level,

Significant at the 0.001 probability level,

NS, non-significant at the 0.05 probability level.

###### 

Characteristics of a set of CIMMYT WPHYSGP nursery and local varieties in two regimes in 2011 and 2012 growing season at Changji, Xinjiang

  Trait                      No stress condition   Stress condition                              
  -------------------------- --------------------- ------------------ -------- -------- -------- --------
  GY, kg/ha                  6440.8                3870.0             7995.0   3360.6   2066.0   4492.0
  TGW, g                     35.6                  27.0               48.0     28.3     21.0     37.7
  Grain plumpness            4.4                   2.5                6.2      3.4      1.8      5.0
  CT mid-grain filling, °C   22.2                  21.3               23.4     24.9     23.0     27.2
  NDVI jointing              0.5153                0.3726             0.6439   0.4806   0.3259   0.6327
  NDVI flowering             0.8027                0.6433             0.9064   0.5565   0.3668   0.7068
  NDVI mid-grain filling     0.6619                0.5515             0.7808   0.3625   0.2646   0.4948
  SPAD flowering             52.5                  44.7               61.3     54.0     49.0     61.8
  SPAD mid-grain filling     52.3                  45.0               60.6     40.3     25.0     63.0
  Spike length, cm           7.8                   4.8                9.8      7.7      5.1      9.8
  Spikelet per spike         14.4                  9.4                17.6     14.1     10.3     17.4
  Grains per spike           33.9                  15.4               48.3     26.5     16.1     37.2
  Grain weight per spike     1.2                   0.5                1.7      0.6      0.3      1.1
  Days to heading            47.5                  41.8               52.7     47.4     42.3     52.2
  Plant height, cm           80.7                  57.0               101.3    65.0     45.5     82.0

###### 

Mean grain yield of 152 genotypes under no-stress conditions (T1) and drought-stress conditions (T2) averaged over the two growing seasons

  Genotype    Original   GY in T1, kg/ha   GY in T2, kg/ha   Genotype    Original   GY in T1, kg/ha   GY in T2, kg/ha   Genotype   Original   GY in T1, kg/ha   GY in T2, kg/ha
  ----------- ---------- ----------------- ----------------- ----------- ---------- ----------------- ----------------- ---------- ---------- ----------------- -----------------
  9645        CIMMYT     7995 (1)          3881 (11)         9696        CIMMYT     6707 (51)         3077 (116)        9699       CIMMYT     6234 (99)         3841 (16)
  Xinchun29   Local      7635 (2)          3793 (21)         9629        CIMMYT     6703 (52)         3386 (71)         9615       CIMMYT     6218 (100)        3297 (86)
  9611        CIMMYT     7622 (3)          3815 (18)         9632        CIMMYT     6698 (53)         3091 (115)        9603       CIMMYT     6200 (101)        3413 (65)
  9650        CIMMYT     7574 (4)          3873 (13)         9621        CIMMYT     6697 (54)         3288 (87)         9728       CIMMYT     6196 (102)        3035 (122)
  9637        CIMMYT     7573 (5)          3798 (20)         9633        CIMMYT     6688 (55)         3249 (94)         9676       CIMMYT     6189 (103)        2946 (130)
  9653        CIMMYT     7474 (6)          3474 (56)         9720        CIMMYT     6678 (56)         3868 (14)         9695       CIMMYT     6166 (104)        3331 (79)
  9657        CIMMYT     7362 (7)          3418 (64)         9656        CIMMYT     6677 (57)         3539 (43)         9686       CIMMYT     6160 (105)        3556 (40)
  Xinchun6    Local      7356 (8)          3341 (77)         9736        CIMMYT     6662 (58)         3608 (33)         9723       CIMMYT     6141 (106)        3032 (123)
  9692        CIMMYT     7322 (9)          4124 (7)          9658        CIMMYT     6662 (58)         3526 (47)         9714       CIMMYT     6136 (107)        3333 (78)
  9638        CIMMYT     7315 (10)         3894 (10)         9626        CIMMYT     6660 (59)         3531 (45)         9721       CIMMYT     6096 (108)        3466 (58)
  9655        CIMMYT     7300 (11)         3704 (24)         9745        CIMMYT     6656 (60)         3069 (117)        9609       CIMMYT     6069 (109)        3271 (93)
  9635        CIMMYT     7291 (12)         3203 (99)         9654        CIMMYT     6652 (61)         3599 (35)         9718       CIMMYT     6050 (110)        3299 (85)
  9606        CIMMYT     7267 (13)         3875 (12)         9680        CIMMYT     6651 (62)         3534 (44)         9613       CIMMYT     6032 (111)        3329 (80)
  Xinchun23   Local      7264 (14)         3621 (32)         9624        CIMMYT     6609 (63)         3412 (66)         9712       CIMMYT     6032 (111)        3092 (114)
  9639        CIMMYT     7259 (15)         3605 (34)         9726        CIMMYT     6599 (64)         3698 (26)         9617       CIMMYT     6018 (112)        2513 (144)
  9744        CIMMYT     7241 (16)         3507 (50)         9628        CIMMYT     6599 (64)         3480 (55)         9708       CIMMYT     6012 (113)        3309 (83)
  9608        CIMMYT     7163 (17)         3199 (100)        9634        CIMMYT     6595 (65)         2934 (131)        9669       CIMMYT     6002 (114)        3544 (42)
  9727        CIMMYT     7161 (18)         3356 (74)         9691        CIMMYT     6588 (66)         2973 (128)        9674       CIMMYT     5978 (115)        2933 (132)
  9630        CIMMYT     7156 (19)         3132 (110)        9652        CIMMYT     6573 (67)         3048 (120)        9667       CIMMYT     5965 (116)        3320 (81)
  9649        CIMMYT     7151 (20)         3468 (57)         Xinchun33   Local      6558 (68)         3701 (25)         9673       CIMMYT     5938 (117)        2755 (139)
  Xinchun11   Local      7149 (21)         3591 (36)         9685        CIMMYT     6557 (69)         3525 (48)         9724       CIMMYT     5891 (118)        3508 (49)
  9729        CIMMYT     7103 (22)         4052 (8)          9709        CIMMYT     6544 (70)         3458 (60)         9707       CIMMYT     5888 (119)        3404 (68)
  Xinchun17   Local      7093 (23)         3460 (59)         9610        CIMMYT     6523 (71)         3486 (54)         9713       CIMMYT     5871 (120)        3059 (119)
  9746        CIMMYT     7085 (24)         3577 (37)         9648        CIMMYT     6521 (72)         3365 (73)         9666       CIMMYT     5848 (121)        3280 (90)
  9622        CIMMYT     7071 (25)         3629 (31)         9717        CIMMYT     6503 (73)         3319 (82)         9620       CIMMYT     5798 (122)        2910 (133)
  9730        CIMMYT     7054 (26)         3808 (19)         9737        CIMMYT     6499 (74)         3219 (98)         9716       CIMMYT     5791 (123)        2987 (127)
  9741        CIMMYT     7051 (27)         3066 (118)        9668        CIMMYT     6486 (75)         2856 (135)        9672       CIMMYT     5784 (124)        3037 (121)
  9642        CIMMYT     7050 (28)         4169 (5)          9742        CIMMYT     6485 (76)         3487 (53)         9703       CIMMYT     5687 (125)        3351 (75)
  9688        CIMMYT     7038 (29)         3553 (41)         9647        CIMMYT     6449 (77)         3561 (39)         9678       CIMMYT     5686 (126)        2902 (134)
  9738        CIMMYT     7037 (30)         3400 (69)         9683        CIMMYT     6429 (78)         3500 (52)         9701       CIMMYT     5683 (127)        3281 (89)
  9646        CIMMYT     7000 (31)         4262 (2)          9690        CIMMYT     6427 (79)         3348 (76)         9711       CIMMYT     5657 (128)        3095 (113)
  9734        CIMMYT     6996 (32)         3237 (96)         9682        CIMMYT     6426 (80)         3393 (70)         9659       CIMMYT     5611 (129)        3133 (109)
  Xinchun10   Local      6990 (33)         3234 (97)         9643        CIMMYT     6425 (81)         4492 (1)          9661       CIMMYT     5596 (130)        2952 (129)
  9604        CIMMYT     6978 (34)         3368 (72)         9612        CIMMYT     6405 (82)         4251 (3)          9706       CIMMYT     5571 (131)        3275 (92)
  9671        CIMMYT     6958 (35)         3127 (111)        9743        CIMMYT     6394 (83)         3561 (39)         9677       CIMMYT     5550 (132)        2714 (140)
  9698        CIMMYT     6942 (36)         3901 (9)          9687        CIMMYT     6391 (84)         3113 (112)        9702       CIMMYT     5540 (133)        3672 (29)
  9631        CIMMYT     6931 (37)         3246 (95)         9614        CIMMYT     6383 (85)         3189 (102)        9663       CIMMYT     5522 (134)        3135 (108)
  9627        CIMMYT     6926 (38)         3853 (15)         9693        CIMMYT     6376 (86)         3154 (105)        9605       CIMMYT     5520 (135)        3503 (51)
  9625        CIMMYT     6925 (39)         3147 (107)        9733        CIMMYT     6369 (87)         2828 (136)        9705       CIMMYT     5506 (136)        2066 (147)
  9618        CIMMYT     6902 (40)         3234 (97)         9735        CIMMYT     6367 (88)         3833 (17)         9662       CIMMYT     5504 (137)        2998 (126)
  9651        CIMMYT     6874 (41)         3404 (68)         9700        CIMMYT     6353 (89)         3682 (28)         9732       CIMMYT     5452 (138)        2705 (141)
  9636        CIMMYT     6822 (42)         3574 (38)         9694        CIMMYT     6329 (90)         3198 (101)        9704       CIMMYT     5391 (139)        3420 (63)
  9641        CIMMYT     6822 (42)         3528 (46)         9697        CIMMYT     6310 (91)         3161 (104)        9684       CIMMYT     5383 (140)        3302 (84)
  9616        CIMMYT     6819 (43)         3183 (103)        9602        CIMMYT     6301 (92)         2668 (142)        9675       CIMMYT     5304 (141)        3150 (106)
  9644        CIMMYT     6816 (44)         3791 (22)         9710        CIMMYT     6296 (93)         3691 (27)         9665       CIMMYT     5165 (142)        3032 (123)
  9725        CIMMYT     6767 (45)         3758 (23)         9715        CIMMYT     6296 (93)         3278 (91)         9670       CIMMYT     5146 (143)        2796 (138)
  9719        CIMMYT     6764 (46)         3411 (67)         9689        CIMMYT     6292 (94)         4191 (4)          9660       CIMMYT     5140 (144)        3022 (124)
  9623        CIMMYT     6748 (47)         3351 (75)         9619        CIMMYT     6278 (95)         2287 (146)        9679       CIMMYT     5051 (145)        3012 (125)
  9722        CIMMYT     6729 (48)         3422 (62)         9731        CIMMYT     6274 (96)         2819 (137)        9740       CIMMYT     4303 (146)        2480 (145)
  9607        CIMMYT     6719 (49)         3451 (61)         9739        CIMMYT     6269 (97)         3665 (30)         9664       CIMMYT     3870 (147)        2525 (143)
  9681        CIMMYT     6711 (50)         3285 (88)         9640        CIMMYT     6235 (98)         4143 (6)                                                  

Ranking order of the mean grain yield of genotypes in no-stress (T1) conditions and drought-stress conditions (T2) is shown in brackets.

###### 

Screening of 24 genotypes with the highest drought yield index (DYI), water yield index (WYI) and yield high water use efficiency index (YH-WUEI) over 1.05 averaged over the two growing seasons

  Genotype    Origin   Plant height, cm in T1   Plant height, cm in T2   GY in T1, kg/ha   GY in T2, kg/ha   DYI    WYI    YHWUEI
  ----------- -------- ------------------------ ------------------------ ----------------- ----------------- ------ ------ --------
  9645        CIMMYT   91.74                    71.64                    7995              3881              1.15   1.24   1.20
  Xinchun29   Local    84.67                    68.62                    7635              3793              1.13   1.19   1.16
  9611        CIMMYT   95.98                    67.76                    7622              3815              1.14   1.18   1.16
  9650        CIMMYT   95.15                    71.07                    7574              3873              1.15   1.18   1.16
  9637        CIMMYT   91.25                    72.82                    7573              3798              1.13   1.18   1.15
  9692        CIMMYT   85.18                    64.79                    7322              4124              1.23   1.14   1.18
  9638        CIMMYT   87.09                    64.86                    7315              3894              1.16   1.14   1.15
  9655        CIMMYT   91.69                    68.89                    7300              3704              1.10   1.13   1.12
  9606        CIMMYT   85.38                    71.68                    7267              3875              1.15   1.13   1.14
  Xinchun23   Local    89.61                    69.56                    7264              3621              1.08   1.13   1.10
  9639        CIMMYT   85.82                    67.04                    7259              3605              1.07   1.13   1.10
  Xinchun11   Local    74.61                    58.11                    7149              3591              1.07   1.11   1.09
  9729        CIMMYT   90.01                    75.29                    7103              4052              1.21   1.10   1.15
  9746        CIMMYT   83.90                    68.52                    7085              3577              1.06   1.10   1.08
  9622        CIMMYT   100.75                   81.98                    7071              3629              1.08   1.10   1.09
  9730        CIMMYT   88.87                    70.08                    7054              3808              1.13   1.10   1.11
  9642        CIMMYT   77.20                    64.32                    7050              4169              1.24   1.09   1.17
  9688        CIMMYT   88.26                    69.58                    7038              3553              1.06   1.09   1.07
  9646        CIMMYT   85.87                    73.90                    7000              4262              1.27   1.09   1.18
  9698        CIMMYT   88.63                    67.16                    6942              3901              1.16   1.08   1.12
  9627        CIMMYT   90.66                    65.97                    6926              3853              1.15   1.08   1.11
  9636        CIMMYT   86.39                    68.24                    6822              3574              1.06   1.06   1.06
  9644        CIMMYT   90.90                    74.93                    6816              3791              1.13   1.06   1.09
  9725        CIMMYT   80.22                    69.65                    6767              3758              1.12   1.05   1.08

Genotypes are listed according to their ranking for mean grain yield under no-stress conditions.

###### 

Pearson' correlation coefficients under no-stress conditions (A) and drought-stress conditions (B) averaged over 2011 and 2012

  \(A\)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
  --------- ----------------------------------------------------- ---------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ---------------------------------------------------- ------------------------------------------------ ---------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ---------------------------------------------------
  TGW       0.450[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
  SPS       0.512[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
  SL        0.602[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.229[\*\*](#tfn8-69_18104){ref-type="table-fn"}     0.824[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
  SPAD-GF   NS                                                    0.284[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   NS                                                    NS                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
  SPAD-F    NS                                                    0.301[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   NS                                                    NS                                                    0.469[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                                                                                                                                                                                                                                             
  GP        0.413[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.546[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   NS                                                    NS                                                    NS                                                   NS                                                                                                                                                                                                                                                                                                                                                                                                                                        
  PH        0.740[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.389[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   0.651[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.703[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                   NS                                               0.354[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                                                                                                                                       
  NDVI-GF   0.249[\*\*](#tfn8-69_18104){ref-type="table-fn"}      NS                                                   NS                                                    NS                                                    NS                                                   NS                                               NS                                                   NS                                                                                                                                                                                                                                                                                                                                  
  NDVI-F    0.543[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                   0.656[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.692[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                   NS                                               NS                                                   0.645[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.449[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                            
  NDVI-E    0.551[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.282[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   0.551[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.576[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                   NS                                               NS                                                   0.604[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                    0.637[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                      
  HD        0.541[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                   0.709[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.734[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                   NS                                               NS                                                   0.728[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.339[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.770[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.639[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                
  GWPS      0.567[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.397[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   0.642[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.734[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.162[\*](#tfn7-69_18104){ref-type="table-fn"}       0.171[\*](#tfn7-69_18104){ref-type="table-fn"}   NS                                                   0.512[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                    0.471[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.382[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.491[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                          
  GNPS      0.316[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    −0.198[\*](#tfn7-69_18104){ref-type="table-fn"}      0.693[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.683[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                   NS                                               −0.1791[\*](#tfn7-69_18104){ref-type="table-fn"}     0.342[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                    0.409[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.238[\*\*](#tfn8-69_18104){ref-type="table-fn"}      0.509[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.716[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    
  CT-GF     −0.440[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   NS                                                   −0.424[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.477[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.209[\*\*](#tfn8-69_18104){ref-type="table-fn"}    NS                                               NS                                                   −0.532[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.275[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.535[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.439[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.512[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.291[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.222[\*\*](#tfn8-69_18104){ref-type="table-fn"}

  \(B\)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  --------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ---------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- ----------------------------------------------------- -----------------------------------------------------
  TGW       0.379[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
  SPS       0.301[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.369[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
  SL        0.300[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.462[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.863[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
  SPAD-GF   0.173[\*](#tfn7-69_18104){ref-type="table-fn"}        0.519[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.396[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.270[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
  SPAD-F    NS                                                    NS                                                    −0.242[\*\*](#tfn8-69_18104){ref-type="table-fn"}     −0.251[\*\*](#tfn8-69_18104){ref-type="table-fn"}     NS                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
  GP        0.489[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.698[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.219[\*\*](#tfn8-69_18104){ref-type="table-fn"}      0.2448[\*\*](#tfn8-69_18104){ref-type="table-fn"}     0.388[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                                                                                                                                                                                                                                                                                                                                                                                                             
  PH        0.494[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.334[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.440[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.447[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                    −0.241[\*\*](#tfn8-69_18104){ref-type="table-fn"}     0.386[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                                                                                                                                       
  NDVI-GF   0.222[\*\*](#tfn8-69_18104){ref-type="table-fn"}      0.348[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                    NS                                                    0.468[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                    0.387[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   NS                                                                                                                                                                                                                                                                                                                                  
  NDVI-F    0.442[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.492[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.545[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.539[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.369[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    −0.287[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   0.417[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   0.426[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.474[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                                                                            
  NDVI-E    0.255[\*\*](#tfn8-69_18104){ref-type="table-fn"}      0.484[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.524[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.583[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.264[\*\*](#tfn8-69_18104){ref-type="table-fn"}      NS                                                    0.300[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   0.436[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                    0.506[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                                                                      
  HD        0.321[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.562[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.736[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.687[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.503[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    −0.275[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   0.436[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   0.335[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.378[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.631[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.560[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                                                                                
  GWPS      0.482[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.463[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.590[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.634[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.385[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                    0.411[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   0.252[\*\*](#tfn8-69_18104){ref-type="table-fn"}      0.249[\*\*](#tfn8-69_18104){ref-type="table-fn"}      0.424[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.252[\*\*](#tfn8-69_18104){ref-type="table-fn"}      0.435[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}                                                          
  GNPS      0.268[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    −0.189[\*](#tfn7-69_18104){ref-type="table-fn"}       0.422[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    0.377[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    NS                                                    NS                                                    NS                                                   NS                                                    NS                                                    NS                                                    NS                                                    NS                                                    0.650[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}    
  CT-GF     −0.494[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.290[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.492[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.393[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.265[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   NS                                                    −0.228[\*\*](#tfn8-69_18104){ref-type="table-fn"}    −0.274[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.276[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.476[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.309[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.471[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.393[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}   −0.294[\*\*\*](#tfn9-69_18104){ref-type="table-fn"}

Significant at the 0.05 probability level.

Significant at the 0.01 probability level.

Significant at the 0.001 probability level.

NS, non-significant at the 0.05 probability level.

[^1]: Communicated by Takao Komatsuda

[^2]: These authors contributed equally to this work
