Photosynthetic and Antioxidant Responses of Jatropha curcas Plants to Heat Stress: On the Relative Sensitivity of Shoots and Roots

This study aimed to evaluate the photosynthetic and antioxidant responses of Jatropha curcas plants exposed to heat stress applied simultaneously in both shoots and roots and solely in each tissue. Thirty-day-old plants were transferred to two growth chambers, where they were subjected to 27 or 42 A degrees C. In each growth chamber, the root system was isolated and then subjected to 27 or 42 A degrees C. Then, four groups of plants were formed according to shoot/root temperature: 27/27; 27/42; 42/27; and 42/42 A degrees C. Plants under 27/27 A degrees C were considered as control and plants were exposed to each thermal treatment for 12 h. The leaf CO2 assimilation rate, stomatal conductance, and mesophyll conductance were decreased by high temperature, regardless of plant tissue. However, primary photochemistry given by effective quantum yield of PSII, electron transport rate, photochemical and non-photochemical quenchings, and maximum electron transport rate driving RuBP regeneration revealed that high shoot temperature was more deleterious for photosynthesis than high root temperature. J. curcas presented efficient mechanisms to avoid photo-damage and photo-inhibition through increases in non-photochemical quenching, maintenance of the maximum quantum yield of PSII, and increases in total superoxide dismutase, ascorbate peroxidase, and catalase activities when shoots were exposed to high temperature, regardless of root temperature. In general, our data suggest that J. curcas present an efficient antioxidant protection system in photosynthetic tissues and acclimation of PSII avoiding photo-damage and photo-inhibition under heat stress. On the other hand, high root temperature inhibits antioxidant metabolism, leading to higher oxidative damage in root tissues and affecting primary photochemistry in leaf tissues.