Phenylalanine Ammonia-Lyase and Source-Flow-Sink Related Attributes in Rice Genotypes Subjected to High Night Temperatures

Phenylalanine ammonia-lyase (PAL) which is considered to be one of the main lines of cell acclimation against stress in plants, non-structural carbohydrates (NSC) accumulation and chlorophyll fluorescence parameters were quantified in two rice genotypes as a function of two temperature regimes: 22/30 °C (control) and 28/30 °C night/day (high night temperatures - HNT), imposed from heading to milk stage. The rice cultivars chosen were Nagina22 (N22) and BRS Querência (BRS-Quer), which are genotypes tolerant and sensitive to high temperatures, respectively. BRS-Quer genotype highlighted more sensitive responses maintaining higher PAL and peroxidase levels on seventh and twenty-first days after stress imposing. On the other hand, this genotype showed levels of fructose, glucose and sucrose decreasingly across stress period whether compared to N22. Both genotypes showed similarity for most of the chlorophyll fluorescence parameters. However, the photosynthesis induction curve highlighted that HNT caused decreases in some photochemical quenching of fluorescence as well as increases of non-phochemical quenching, affecting more prominently BRS-Quer genotype. N22 maintained unaltered the spikelet sterility and 1000-grain weight across temperature regimes showing a consistent trend with its stem NSC accumulation during stress period. The higher availability of soluble sugars shown by N22 at the end of stress period could be unloaded in spikelet formation and grain fillings contributing in their lower sterility rate and greater 1000-grain weight stability across the environments. These results indicate that selecting genotypes with higher capacity to stem NSC translocation beyond accumulation at HNT could lead to more grain yield stability in future climate scenarios.