Impact of Heat Stress on Rainfed Wheat Growth and Yield Under Semi-arid, Semi-humid and Mediterranean Climates in Iran Condition

Assessing crop yield in response to heat and drought stress is important in implementing the best adaptation strategies to mitigate the effects of climate change. For this aim, observations from 105 agricultural/meteorological experiments in the semi-arid (Maragheh, Qamlou and Sararoud), Mediterranean (Hashem Abad and Oltan) and semi-humid (Gharakhil) regions of Iran were used to investigate the response of the reproductive growing duration (RGD) and grain yield of rainfed winter wheat to certain climatic and agro-climatic indices consisted of precipitation (mm), growing degree days (GDDs), and cumulative extreme temperatures above wheat tolerance threshold level of >= 34 degrees C (TAT). Accordingly, multiple linear regression was applied under baseline (1998-2012) and future increasing temperature (by 1 degrees C and 2 degrees C). Results indicated that the average of wheat RGD and yield were 37.2 +/- 0.71 d and 2.3 +/- 0.05 t ha(-1) in semi-arid, 25.7 +/- 0.8 d and 2.9 +/- 0.11 t ha(-1) in semi-humid, and 21.7 +/- 0.59 d and 5.25 +/- 0.17 t ha(-1) in Mediterranean regions, respectively. The main findings showed that, on average during 1998-2012, wheat RGD and yield changed by - 0.26 d yr(-1) and - 0.93% (0.02 t ha(-1) yr(-1)) in semi-arid, + 0.25 d yr(-1) and - 1.27% (0.04 t ha(-1) yr(-1)) in semi-humid, and - 0.01 d yr(-1) and - 0.27% (0.01 t ha(-1) yr(-1)) in Mediterranean regions, respectively. Precipitation and TAT had substantial positive and negative impacts on RGD by + 0.1 d yr(-1) and - 0.03 d yr(-1), and crop yield by + 0.04% and - 1.14% in all study locations. An increase in GDDs, however, significantly shortened RGD (- 0.06 d yr(-1)) and consequently reduced grain yield (- 0.04%) in semi-arid regions, while in semi-humid and Mediterranean regions, increasing GDDs had a positive impact on RGD (+ 0.07 d yr(-1)) and yield (+ 0.19%). Among the indices, TAT showed significantly greater detrimental effects on RGD and grain yield particularly when accompanied by less precipitation (i.e. drought stress). Our results highlighted that any increase in temperatures even by 1 degrees C or 2 degrees C would lead to drastic increases in TAT and GDDs in all study regions, most especially in semi-arid regions. Under these conditions, any benefits from precipitation would be neutralized by the negative impacts of increased GDDs and TAT in all study locations. The insights into crop response to weather variations and climate extremes provide excellent evidence and a basis for reducing crop yield damage by designing for improved heat tolerance for the future.