Metabolic acclimation of rice cultivars via cold stress-induced carbohydrate accumulation.

This study was conducted to comparatively investigate the accumulation of carbohydrates and transcript levels of genes encoding relevant enzymes in two rice (Oryza sativa L.) cultivars 'Hashemi' (cold-sensitive) and 'Kouhsar' (cold-tolerant) at 1, 3 and 6 days after cold stress (DAS) (4℃). The sinusoidal pattern of H2O2 and MDA in Hashemi under cold, compared with the steadily decreasing levels in Kouhsar indicates differing oxidative damage, reflected in 2.28- and 2.39-fold higher contents, respectively, at 6 DAS. Kouhsar had higher sucrose than Hashemi on all stress days, with a peak increase of 24% at 6 DAS. Glucose and fructose values increased more in Hashemi but changes ratio in fructose were more prominent and was accompanied by higher expression of sucrose transporters and invertase genes. In both cultivars, raffinose and trehalose values reached their highest levels at 6 DAS with no significant difference between them. During stress, higher expression of galactinol synthase, trehalose-6-phosphate phosphatase and phosphoenol pyruvate carboxylase genes was observed in Kouhsar, with the most increases of 23%, 35% and 30%, respectively, at 6 DAS. Despite chlorophyll deceasing and carotenoids increasing under cold, Kouhsar exceeded Hashemi by 69% in chlorophyll and 31% in carotenoids at 6 DAS. The results suggest that cold tolerance is more effectively conferred by the maintenance of sucrose level at the beginning of stress and its continued increase along with Raf and Treh accumulation throughout the stress period. Enhancing these features may pave the way for breeding programs to increase production sustainability in cold regions.