ABSTRACT
Two japonica rice genotypes, INIA Tacuarí and L2825CA, were analyzed for tolerance to low temperature during early vegetative growth. Effect on photosynthesis, energy dissipation, pigment content, xanthophyll-cycle pool conversion, hydrogen peroxide accumulation, oxidative damage and antioxidant enzyme activities were determined to better understand potential mechanisms for cold tolerance. Photoinhibition was measured using chlorophyll fluorescence and oxidative damage by lipid peroxidation and electrolyte leakage. Both genotypes were demonstrated to be cold tolerant which was consistent with their reduced levels of photoinhibition and oxidative damage compared with a cold-sensitive genotype during chilling stress. The strategy for cold tolerance differed between the two genotypes, and involved different mechanisms for disposal of excess energy. The presence of high lutein concentrations and the existence of active non-harmful energy dissipation processes through the xanthophyll cycle appeared to be responsible for chilling tolerance in INIA Tacuarí. On the other hand, increased cold tolerance of L2825CA relative to INIA Tacuarí was related to the higher constitutive superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11) and catalase (CAT, EC 1.11.1.6).
