Unscrambling confounded effects of sowing date trials to screen for crop adaptation to high temperature

Against the backdrop of climate change, genotypes with improved adaptation to elevated temperature are required; reliable screening methods are therefore important. Sowing date experiments are a practical and inexpensive approach for comparison of large collections of lines. Late-sown crops usually experience hotter conditions and phenotypes thus partially capture this environmental influence. Two sets of confounded factors, however, limit the value of sowing date trials. First, daily mean temperature correlates with both minimum and maximum temperature, photoperiod, radiation and vapour pressure deficit, and it may also correlate with rainfall. Second, temperature alters the genotype-dependent phenology of crops, effectively shifting the timing and duration of critical periods against the background of temperature and other environmental variables. Here we advance a crop-level framework to unscramble the confounded effects of sowing date experiments; it is based on four physiological concepts: (1) annual crops accommodate environmental variation through seed number rather than seed size; (2) seed number is most responsive to the environment in species-specific critical windows; (3) non-stressful thermal effects affecting seed set through development and canopy size can be integrated in a photothermal quotient relating intercepted photosynthetically active radiation (PAR) and mean temperature during the critical window; (4) stressful temperature reduces yield by disrupting reproduction. The framework was tested in a factorial experiment combining four chickpea varieties with putatively contrasting adaptation to thermal stress and five environments resulting from the combination of seasons and sowing dates. Yield ranged from 13 to 577 gm. Shifts in phenology led to contrasting photothermal conditions in the critical window between flowering and 400 degrees C d after flowering that were specific for each variety-environment combination. The photothermal quotient ranged from 2.72 to 6.85 Mi m(-2)degrees C-1; it explained 50% of the variation in yield and maximum temperature explained 32% of the remaining variation. Thus, half of the variation in yield was associated with developmental, non-stressful thermal effect and (at most) 16% of the variation was attributable to thermal stress. The photothermal quotient corrected by vapour pressure deficit accounted for by 75% of the variation in yield and provided further insight on photosynthesis-mediated responses to temperature. Crop adaptation to non-stressful, developmental thermal effects and stressful temperatures disrupting reproduction involve different physiological processes and requires partially different agronomic and breeding solutions. Our analytical approach partially separates these effects, adds value to sowing date trials, and is likely to return more robust rankings of varieties