Combined drought and heat stress impact during flowering and grain filling in contrasting rice cultivars grown under field conditions

Combined drought and heat stress is the most common abiotic stress occurring under field conditions that negatively affects rice productivity. Systematic evaluation of the response of rice cultivars to this combined stress under field conditions has not been attempted. To fill this major knowledge gap, three rice cultivars (N22, Dular, Anjali) were exposed to combined drought and heat stress during flowering and early grain-filling stages, using rainout shelters and natural summer conditions in 2013, 2014 and 2015. By employing staggered sowing, stress was imposed at the same time across both stages and between cultivars, which helped capture temporal soil and canopy-air temperature and soil water potential without being confounded by other climatic conditions. Across experiments, soil water potential under drought reached up to -61 and -57 kPa during flowering and grain filling, resulting in up to 1.75 °C and 1.17 °C higher canopy-air temperature, respectively. Across years and cultivars, 50.0% and 74.5% lower panicle conductance under combined stress during flowering and grain filling led to an increase in panicle tissue temperature by up to 3.94 °C and 3.27 °C, respectively. The range in combined stress-induced yield reduction between flowering and grain filling was similar (approximately 20-80%), while there were clear differences among cultivars. Dular had the highest reduction in yield (73.2%) with stress exposure during flowering, while N22 recorded a similar reduction (77.6%) during grain filling. A similar differential cultivar response in parameters related to grain quality was recorded with stress imposed at both developmental stages. Our findings demonstrate the potential of using existing rainout shelters to systematically characterize rice and other crops for combined drought and heat stress impact under field conditions, and to identify novel multi stress-tolerant donors to support abiotic stress breeding programs.