Photosynthesis. We assume that a 10% increase in light conversion efficiency could be achieved in the future. Using a model of canopy photosynthesis, () showed that the value of parameter (Rubisco specificity factor that represents the discrimination λ between CO2 and O2) found in current C3 crops exceeds the level that would be optimal for the present CO2 concentration ([CO2]), but would be optimal for [CO2] of about 220 ppm, the average over the last 400,000 years. The simulation results showed that up to 10% more carbon could be assimilated, if was optimal for the current [CO2] level. λ In Sirius, radiation use efficiency (RUE) is proportional to [CO2] with an increase of 30% for doubling in [CO2] compared with the baseline of 338 ppm, which is in agreement with the recent meta-analysis of field-scale experiments on the effects of [CO2] on crops (). A similar response was used by other wheat simulation models, e.g. CERES () and EPIC (). Phenology. Three cultivar parameters are directly related to phenological development of wheat, i.e. phylochron Ph, daylength response Pp and duration of grain filling Gf (Table 2). Modifying the duration and timing of crop growth cycle in relation to seasonal variations of solar radiation and water availability may have significant effects on yield (; ). An optimal flowering time has been the single most important factor to maximise yield in dry environments (). The phyllochron Ph is the thermal time required for the appearance of successive leaves, and is a major driver of phenological development (; ; ). Details of the response of final leaf number to daylength could be Pp found in (; ). By modifying phyllochron Ph and daylength response Pp we alter the rate of crop development and, therefore, the date of flowering and maturity. Increasing the duration of the grain filling period Gf has been suggested as a possible trait for increasing grain yield in wheat (). In Sirius, Gf is defined as a cultivar-specific amount of
Photosynthesis. We assume that a 10% increase in light conversion efficiency could be achieved in the future. Using a model of canopy photosynthesis, () showed that the value of parameter λ (Rubisco specificity factor that represents the discrimination between CO2 and O2) found in current C3 crops exceeds the level that would be optimal for the present CO2 concentration ([CO2]), but would be optimal for [CO2] of about 220 ppm, the average over the last 400,000 years. The simulation results showed that up to 10% more carbon could be assimilated, if λ was optimal for the current [CO2] level. In Sirius, radiation use efficiency (RUE) is proportional to [CO2] with an increase of 30% for doubling in [CO2] compared with the baseline of 338 ppm, which is in agreement with the recent meta-analysis of field-scale experiments on the effects of [CO2] on crops (). A similar response was used by other wheat simulation models, e.g. CERES () and EPIC (). Phenology. Three cultivar parameters are directly related to phenological development of wheat, i.e. phylochron Ph, daylength response Pp and duration of grain filling Gf (Table 2). Modifying the duration and timing of crop growth cycle in relation to seasonal variations of solar radiation and water availability may have significant effects on yield (; ). An optimal flowering time has been the single most important factor to maximise yield in dry environments (). The phyllochron Ph is the thermal time required for the appearance of successive leaves, and is a major driver of phenological development (; ; ). Details of the response of final leaf number to daylength Pp could be found in (; ). By modifying phyllochron Ph and daylength response Pp we alter the rate of crop development and, therefore, the date of flowering and maturity. Increasing the duration of the grain filling period Gf has been suggested as a possible trait for increasing grain yield in wheat (). In Sirius, Gf is defined as a cultivar-specific amount of
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