Leaf gas exchange, photochemical responses and oxidative damages in assai (Euterpe oleracea Mart.) seedlings subjected to high temperature stress

In this work, we aimed to examine the magnitude of changes in leaf gas exchange, chlorophyll fluorescence, and antioxidant responses in assai (Euterpe oleracea Mart.) seedlings subjected for 14 days to different temperatures (28 degrees C, control treatment, and 36 and 40 degrees C, high-temperature stress). Exposure to 36 degrees C decreased stomatal conductance (g(s)), photochemical quenching (q(L)), and non- photochemical quenching (NPQ) by 27, 53, and 20%, respectively. Further, it increased the photosynthetically active radiation (PAR) fraction that was absorbed by photosystem (PS)-II-antenna (but neither used in photochemistry nor dissipated thermally) by 14%. Moreover, incubation of seedlings at 36 degrees C induced a 53% increase in ascorbate peroxidase (APX) activity and a 16% decrease in hydrogen peroxide (H2O2); however, there were neither photooxidative damages to the PS nor lipid peroxidation. In contrast, seedlings incubated at 40 degrees C exhibited significantly decreased net CO2 assimilation rate (93%), g(s) (83%), transpiration (75%), and water use efficiency (71%). The maximum efficiency of PS II photochemistry did not differ compared to control plants, but the actual quantum yield of PS II electron transport, qL, and NPQ were decreased by 74, 60 and 19%, respectively. At 40 degrees C, there was increased photorespiration and elevated superoxide dismutase and APX activities, but the seedlings exhibited increased lipid peroxidation and general chlorosis in the youngest leaves. Taken together, our findings support that assai seedlings were tolerant to 36 degrees C but very sensitive to 40 degrees C.