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RESUMOEstudou-se o efeito da aplicação exógena de peróxido de hidrogênio (H 2 O 2 ) na aclimatação ao estresse salino com base nas alterações fotossintéticas, indicadores de estresse oxidativo e atividade de enzimas antioxidativas em folhas de plantas de arroz. Utilizaram-se duas diferentes concentrações de H 2 O 2 (1 e 10 M) para o pré-tratamento, concentrações essas aplicadas na solução nutritiva dois dias antes da indução do estresse salino (100 mM de NaCl). A limitação fotossintética e estomática resultante da exposição ao NaCl foi amenizada quando as plantas foram pré-tratadas com 1 M de H 2 O 2 . Plantas expostas ao pré-tratamento com H 2 O 2 e expostas ao NaCl apresentaram dano de membrana menor quando comparadas com as plantas submetidas ao NaCl isoladamente. O conteúdo de TBARS e H 2 O 2 foi reduzido sensivelmente nas plantas pré-tratadas com 1 M de H 2 O 2 e expostas ao NaCl em relação às que não foram pré-tratadas. O sistema antioxidativo enzimático nas plantas expostas ao NaCl foi induzido principalmente quando ocorreu o pré-tratamento com 1 M de H 2 O 2 . Os dados sugerem que exposição prévia ao H 2 O 2, pode resultar numa aclimatação mais efetiva às condições de estresse salino. Palavras-chave: peróxido de hidrogênio, estresse oxidativo, salinidadeSalt stress acclimation in rice plants induced by H 2 O 2 pretreatment ABSTRACT The effect of exogenous application of hydrogen peroxide (H 2 O 2 ) in the acclimation to salt stress was studied on the basis of photosynthetic changes, indicators of oxidative stress and activity of antioxidant enzymes in leaves of rice plants. Two different concentrations of the H 2 O 2 (1 and 10 M) were used for the pre-treatment, these concentrations were applied in the nutrient solution two days before the induction of salt stress (100 mM NaCl). The photosynthetic and stomatal limitations after exposure to NaCl were alleviated when the plants were pretreated with 1 M H 2 O 2 . Plants pretreated with H 2 O 2 and submitted to salt stress showed membrane damage lower in compared to plants exposed to NaCl alone. TBARS and H 2 O 2 content was reduced appreciable in plants pretreated with 1 M H 2 O 2 and exposed to NaCl in relation to not pretreated. The enzymatic antioxidative system in plants exposed to NaCl was mainly induced when there was the 1 M H 2 O 2 pretreatment. Data suggest that an anticipated exposure to H 2 O 2 may result in more effective acclimation to salt stress.

We tested the hypothesis that effective antioxidant and photoprotective mechanisms are able to avoid photodamage induced by prolonged water deficit (WD) followed by high light (HL). We employed cashew plants (Anacardium occidentale L.), a semiarid adapted species, as a model plant. WD-plants exposed to HL did not show alterations in maximum quantum efficiency of photosystem II (Fv/Fm), cellular integrity, H 2 O 2 and thiobarbituric acid reactive species (TBARS) contents, evidencing that they did not suffered photoinhibition and oxidative stress. These responses were associated with increases in ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities, glutathione (GSH) oxidation and ascorbate (ASC) synthesis. This effective oxidative protection occurred in parallel to strong decrease in photosystem II and I (PSII and PSI) activities, increase in heat dissipation (qE), which was related to enhancement in cyclic electron flux. These favorable photoprotective changes were associated with efficient water use in response to HL, all contributing to avoid excess energy in chloroplasts of drought-exposed leaves. These protective features were associated with a tight regulation in D1 protein accumulation during HL, contributing to avoid reactive oxygen species over-accumulation and a subsequent effective PSII recovery during darkness. Our results indicate that cashew plants are able to avoid photoinhibition, tolerating extreme conditions of drought combined with HL. The displayed mechanisms involve essentially integrated responses to balance energy input and output, avoiding oxidative stress.

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