Mudanças foto-oxidativas em plantas de arroz com reduzida atividade da oxidase do glicolato submetidas ao estresse hídrico combinado ao excesso de luz e calor

Abstract

In this study we characterized the role of photorespiration glycolate oxidase enzyme (GO) in photosynthesis and the generation of oxidative damage in exposed rice plants to isolated and combined stress of drought and excessive heat. The seeds were germinated in vermiculite substrate in a greenhouse, and 60 days after planting, were subjected to the effects of drought stress and excess heat in growth chambers. In this condition, the rice plants (Oryza sativa L.). Nippombare were subjected to the treatment of partial inhibition of GO imposed by the specific inhibitor, α-hidroxisulfonato (HPMS). The results show that the plants subjected to drought, reducing photosynthesis (PN) was attributed to stomatal limitations, lower stomatal conductance (gs) and transpiration (E), as well as metabolic restrictions, lower efficiency instant carboxylation (PN/Ci). On the other hand, in plants exposed to excess heat insulated occurred photochemical restrictions, less effective quantum efficiency (ΔF/Fm'), photochemical quenching (qP) and electron transport rate (ETR). The chlorophyll content was reduced in response to drought, which did not occur in plants exposed to excess heat (42°C). In drought combined stress and excess heat was observed greater stomatal restriction than in the dry individually, indicating a negative interaction of these factors generating a thermal dissipation failure via transpiration stream. The reduction of GO activity did not stimulate the PN, but increased photorespiration (Pr) followed by decreases in maximum speed of carboxylation (Vcmax) and maximum rate of electron transport (J). Moreover, metabolites downstream of the GO activity as the amino acids glycine and serine were significantly accumulated in an attempt to reduce photorespiration pathway. These results suggest that the Pr can be adjusted by other distinct pathways apart from that exerted by the GO and their relationship with the PN is very complex. This combination of stress also limited oxidative protection under CAT-APX and ASA system, which resulted in significant accumulation of H2O2 which was related to increased lipid peroxidation (TBARS), suggesting the involvement of other pathways in the generation of H2O2 in addition to the oxidation glycollate by GO. Together, the results show that the combination of dry, excess heat, severely restricts the photosynthetic activity by causing stomata and metabolic limitations. Moreover, the reduction of the GO activity was not stimulated PN did not reduce photorespiration. In addition metabolites via photorespiration showed no direct relationship with the activity of GO and photorespiration does not seem to be the main generator of oxidative damage to the rice in this condition.