Reprogramming assimilate partitioning in the second half of the night supports grain filling in inferior spikelets under high night temperature stress in rice

High night temperature (HNT) stress disrupts key physiological processes like respiration, assimilate partitioning, and grain filling, challenging crop production. While the impact of HNT on grain growth and yield is known, the role of sink strength and starch biosynthesis in inferior or superior spikelets, as well as the effects of temporal variations on assimilate distribution, remain underexplored. We hypothesized that a tolerant genotype reallocates sugars to inferior spikelets under HNT stress by enhancing sink strength and starch biosynthesis, with the second half of the night playing a critical role in these processes. Two rice genotypes, Nagina 22 (HNT-tolerant) and Vandana (HNT-sensitive), were subjected to HNT (4 degrees C above the control) from anthesis to physiological maturity. Assimilate movement and sink enzyme activity were investigated during peak grain-filling. Results revealed differential 14C partitioning to starch synthesis in spikelets, with superior spikelets maintaining higher synthesis rates under HNT. Under HNT, Vandana showed reduced sucrose synthase and ADP-glucose pyrophosphorylase (AGPase) activities (up to 63 % in inferior spikelets), while Nagina 22 exhibited increased sucrose synthase (up to 2.7-fold) and AGPase (up to 31 %) activities in inferior spikelets. Under HNT, Vandana showed reduced starch and sugar levels, while Nagina 22 maintained or increased starch content and exhibited varied sugar responses. Overall, our results confirm that Nagina 22 reallocates sugars to inferior spikelets under HNT stress, driven by enhanced sink strength and starch biosynthesis in the second half of the night. This highlights a novel dimension for developing rice genotypes with improved resilience to HNT, ensuring stable yield under changing climate.