Determining the optimal flowering time in rainfed wheat under simultaneous heat and drought stresses in some regions of Iran as predicted by APSIM-Wheat model

Introduction: Drought and heat stress as the most important limiting factors in the production of crops and finally the food security, have been the result of changing climate which is due to human interventions in increasing greenhouse gas emissions. It is also simulated that each 1 â—¦C increase in temperature caused 6% decline in global wheat production (Asseng et al., 2011). The increase in temperature can further decline grain yield when occurred during reproductive stage (Gourdji et al., 2013). The simultaneous effects of drought and heat during the growth stages, especially the flowering and grain filling stages, which are the most sensitive stages, can be more harmful than the individual effects and lead to a significant reduction in yield. The current study aimed at determining the optimal cultivar × sowing date in different climates to coincide the flowering date with optimum climatic conditions (temperature and rainfall) using simulation modelling approach.
Materials and methods: The current study was conducted in eight locations with different climates according to the GYGA (Global Yield Gap Atlas) methods. Dezful and Shushtar had warm and dry climate with high fluctuation in average seasonal temperature, Hamedan and Nahavand had mild and dry climate with medium fluctuation in average seasonal temperature, Khorramabad and Aligodarz had mild and dry climate with high fluctuation in average seasonal temperature and Zanjan and Khodabendeh had cold and dry climate with medium fluctuation in average seasonal temperature. Choosing the study locations was based on both area under wheat cultivation and diversity in their climates. In this research, several management methods including 4 sowing dates, 4 initial soil water and 3 cultivars in 8 locations with different climates for 37 years (1980-2016) was investigated. A modified version of APSIM-Wheat model was used in which a heat stress module could capture the impacts of heat stress on grain number and weight. The simulations was conducted under drought stress alone (rainfed) as well as the simultaneous drought and heat stress.
Results and discussion: The average grain yield in drought conditions simulated as 2.99 tons/hectare while in the simultaneous conditions of drought and heat stress was 2.44 tons/hectare, indicating a decrease of about 500 kg/hectare in yield reduction due to the mutual effect of heat and drought. The mid-maturity cultivar had the highest yield reduction of 52% while the very early-maturity cultivar showed the lowest yield reduction of 0.16%. The wheat flowering time in cold regions occurred after the completion of flowering in warmer locations. In temperate climates, the occurrence of flowering dates was more extensive than in other locations. Overall, any delay in emergence under rainfed resulted in further yield reduction all cultivars and locations.
Conclusion: Under warm climates, an early-maturity cultivar along with early sowing could provide better environmental conditions for photosynthesis and plants could escaped from terminal drought and heat stresses. In contrast, in cold climates, any delay in flowering date increased yield. In mild regions, using a mid-maturity cultivar resulted in late flowering date and higher yields in spite of coinciding wheat with heat and drought for a short period of time.