Warmer nights predominantly alter yield and biomass in wheat experiencing heat waves at the terminal growth stage

Heat waves at the regional level could be devastating if they coincide with the sensitive growth stage of the crop. The sensitivity of wheat (Triticum aestivum L.) to heat stress is a major constraint limiting wheat production. Nevertheless, there are limited field studies where the impact of heat waves has been systematically studied and quantified. We phenotyped a mini panel of thirty diverse wheat genotypes under optimum and low planting densities. Staggered sowing was done to expose the late-sown genotypes to a higher temperature at the terminal growth stage. Heat wave occurred during March–April, 2022, resulting in late sown genotypes being exposed to a staggering 6.6 °C/4 °C higher day/night temperature during flowering and early grain filling stage, as compared to normal sown genotypes. Heat stress caused significant loss in biomass (34.46%), grains spike⁻¹ (22.34%), test weight (15.52%), and grain yield (28.67%) in late-sown genotypes as compared to normal-sown genotypes. Conversely, genotypes grown under low planting density showed substantial variations in growth parameters including plant height, tillering, thicker leaves, and canopy cover. Interestingly, genotypes with denser canopy and thicker leaves could avoid high day temperature-induced losses by maintaining lower canopy and spike temperatures, which eventually resulted in higher grains spike⁻¹. However, the impact of higher night temperature on biomass and seed weight was not affected by denser canopy and lower spike temperature. Hence, yield loss was majorly confined to high night temperature. The study suggests that higher night temperature could be a major determinant of yield under naturally occurring heat waves.