Photosynthetic changes and protective mechanisms against oxidative damage subjected to isolated and combined drought and heat stresses in Jatropha curcas plants

Photosynthetic changes and protective mechanisms against oxidative damage were evaluated in Jatropha curcas leaves subjected to drought and heat stresses, both individually and combined, in order to elucidate the synergistic and antagonistic mechanisms involved with these abiotic factors. Both the drought and heat stresses caused significant damage to the leaf membrane integrity and lipid peroxidation, and the combination of these stresses greatly enhanced these physiological disturbances. The leaf CO2 assimilation rate, stomatal conductance and instantaneous carboxylation efficiency (PN/CI) were significantly decreased in all plants subjected to stressful conditions in comparison to unstressed plants (reference). In contrast, a reduction in photochemical activity was observed only in plants exposed to drought and drought + heat conditions. Catalase (CAT), ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities were stimulated only under heat stress, whereas APX activity was increased in all treated plants in comparison to the references. Moreover, the leaf H2O2 content was increased similarly under all studied stresses. However, the balance of reduced and oxidized ascorbate did not show significant differences between reference and stressed plants. Although J. curcas plants acclimated to the studied stresses, they did not present an efficient mechanism for protection against drought-induced oxidative stress, especially when at high temperatures. However, heat-treated plants triggered an efficient enzymatic antioxidant system of reactive oxygen species scavenging and an effective protection against photochemical damages. The combination of drought and heat most significantly impaired the photosynthetic assimilation of CO2 and the photochemical activity. These results indicate that drought greatly disturbs photosystem II activity and oxidative metabolism and that these negative effects are strongly stimulated by heat stress. The data also evidence that the combination of heat and drought triggers an intricate response involving antagonistic and synergistic interactions.