Thermotolerance and heat stress responses of Douglas-fir and ponderosa pine seedling populations from contrasting climates.Temperature and the frequency and intensity of heat waves are predicted to increase throughout the 21st century.  We quantified leaf thermotolerance and whole-plant physiological responses to heat stress in first-year germinant seedlings in two populations each of Pinus ponderosa P. and C. Lawson (PIPO) and Pseudotsuga menziesii (Mirb.) Franco (PSME) from climates with contrasting precipitation and temperature regimes.  Thermotolerance of detached needles was evaluated using chlorophyll fluorescence (FV/FM, FO) and electrolyte leakage.  PSME was more heat tolerant than PIPO according to both independent assessments of thermotolerance.  Thermotolerance and heat stress responses appeared to be phenotypically plastic and representative of the environment in which plants were grown, whereas intrinsic water-use efficiency appeared to reflect ecotypic differentiation and the climate of origin.
THERM