Atividade fotoquímica e proteção oxidativa em mudas de cajueiro expostas a seca e luminosidade elevada.

Abstract

In this study were characterized biochemical and physiological mechanisms arising from photoinhibition that act in the modulation of photochemical activity and oxidative protection, helping to reduce photo oxidative damage in response to drought and high light stresses in cashew plants. The seedlings were obtained from seeds and grown in substrates by mixing sand and vermiculite (ratio 1:1) in plastic bags, with a volume of 2 liters. Drought stress was imposed by withholding the water supply to the plants (± 20 days) in greenhouse conditions. For exposure to light treatments and recovery to the light effect, the plants were placed in a chamber with controlled conditions of relative humidity (60% ±5) and temperature (30°C ±2). The plants subjected to drought stress showed a marked decrease in photosynthesis when exposed to excess light (combination of drought with high luminosity), compared with those irrigated. This higher sensitivity of CO2 fixation to excess light on plants under drought was associated with a higher intensity of damage to the photochemical apparatus, as indicated by measures of effective quantum efficiency and apparent electron transport rate. The data show that the reduction of photosynthetic activity in response to excess light, has been attributed in part to process photoinhibition occurred under these conditions as shown by a drastic reduction in potential maximum quantum efficiency of PSII (given by Fv/Fm ratio). The results also suggest that the intense photoinhibition, triggered by excess light, may have assisted in photoprotection, because under these conditions there was no oxidative damage, as indicated by the absence of changes in H2O2 content and lipid peroxidation (TBARS content). This photochemistry protection, assigned to photoinhibition in this study, is enhanced due to non-occurrence of dissipation of excess energy through non-photochemical quenching (NPQ), because this mechanism did not show significant changes after long periods of exposure to excessive light. However, the energy dissipation through NPQ in response to short time of exposure to excessive light, observed here indicates that when the PSII activity is normal this mechanism is essential to the protection photochemistry. The role of photoinhibition for photo oxidative protection can be attributed to the effect of this process on reducing the activity of PSII and consequent lower photochemical activity, resulting in lower electron transfer. This suggestion is also reiterated in this study based on the reduction of the content of D1 protein, a major component of structural and functional PSII, and plastocyanin (PC), another major carrier of electrons. The modulation these proteins content may have contributed to restrict the formation of ROS and consequent oxidative damage under conditions inducing photo oxidatives stress such as drought associated with high luminosities, in this species.