Higher Grain Filling Rate in Inferior Spikelets of Tolerant Rice Genotype Offset Grain Yield Loss under Post-Anthesis High Night Temperature

Increased nighttime respiratory losses decrease the amount of photoassimilates available for plant growth and yield. We hypothesized that the increased respiratory carbon loss under high night temperature (HNT) can be compensated by increased photosynthesis during the day following HNT exposure. Two rice genotypes, Vandana (HNT-sensitive) and Nagina 22 (HNT-tolerant), were exposed to HNT (+4°C above the control) from anthesis to physiological maturity and were assessed for alterations in the carbon balance of the source (flag leaf) and its influence on grain filling dynamics and the quality of spatially differentiated sinks (superior and inferior spikelets). Both genotypes exhibited significantly higher night respiration rates. However, only in Nagina 22, high respiration rates were compensated by an increased photosynthetic rate, resulting in the steady production of total dry matter under HNT. Nagina 22 also recorded a higher grain filling rate, particularly at 5 and 10 d after anthesis, with 1.5- and 4-fold increases in translocation of 14C sugars towards superior and inferior spikelets, respectively. The ratio of photosynthetic rate to respiratory rate on a leaf area basis was highly correlated with the number of grains per plant and negatively with spikelet sterility, resulting in a higher grain number and grain weight per plant, particularly of inferior grains in Nagina 22. Grain quality parameters like head rice recovery, high-density grains, and gelatinization temperature were maintained in Nagina 22. An increase in the rheological properties of rice flour starch of Nagina 22 under HNT indicated the stability of starch and its ability to reorganize during the cooling process of product formation. Thus, our study showed that sink adjustments between superior and inferior spikelets favored the growth of inferior grains, which helped to offset grain weight reduction under HNT in the tolerant genotype Nagina 22.