Temporal chlorophyll fluorescence signals to track changes in optical properties of maturing rice panicles exposed to high night temperature

High night temperature (HNT) significantly influences rice grain filling dynamics. A novel phenotyping approach using chlorophyll fluorescence was employed to track changes in the optical properties of maturing rice panicles exposed to control and HNT. Two contrasting rice genotypes, Gharib (HNT sensitive) and N22 (highly tolerant), were exposed to control (23 °C) and HNT (29 °C), from panicle initiation till maturity. Changes in the optical properties of rice panicles throughout maturity were evaluated under field conditions by measuring (i) effective quantum yield of photosystem II efficiency (ΦII), (ii) steady-state chlorophyll fluorescence (FS) and (iii) ratio of emitted chlorophyll fluorescence at 690 and 735 nm, under excitation at 435 nm (F690/F735). Numerous vegetative indices (Vis) were correlated with fluorescence measurements to prove the accuracy of the phenotyping method. ΦII was selected as the most potent fluorescence parameter (i) to track changing optical properties of maturing rice panicles under both control and HNT and (ii) to estimate the elusive change point initiating rice panicle senescence. Detection of ΦII change point allows for larger genetic diversity scans under field conditions and for identifying novel donors for increasing rice yields and incorporating resilient strategies to reduce impact of HNT stress on grain-filling.