Photosynthesis is improved by exogenous calcium in heat-stressed tobacco plants

Effects of exogenous calcium chloride (CaCl2) (20 mM) on photosynthetic gas exchange, photosystem II photochemistry, and the activities of antioxidant enzymes in tobacco plants under high temperature stress (43 °C for 2 h) were investigated. Heat stress resulted in a decrease in net photosynthetic rate (Pn), stomatal conductance as well as the apparent quantum yield (AQY) and carboxylation efficiency (CE) of photosynthesis. Heat stress also caused a decrease of the maximal photochemical efficiency of primary photochemistry (Fv/Fm). On the other hand, CaCl2 application improved Pn, AQY, and CE as well as Fv/Fm under high temperature stress. Heat stress reduced the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), whereas the activities of these enzymes either decreased less or increased in plants pretreated with CaCl2; glutathione reductase (GR) activity increased under high temperature, and it increased more in plants pretreated with CaCl2. There was an obvious accumulation of H2O2 and O2− under high temperature, but CaCl2 application decreased the contents of H2O2 and O2− under heat stress conditions. Heat stress induced the level of heat shock protein 70 (HSP70), while CaCl2 pretreatment enhanced it. These results suggested that photosynthesis was improved by CaCl2 application in heat-stressed plants and such an improvement was associated with an improvement in stomatal conductance and the thermostability of oxygen-evolving complex (OEC), which might be due to less accumulation of reactive oxygen species.