Effect of moderately-high temperature stress and recovery on the photosynthetic characteristics of tomato (Lycopersicon esculentum L.)

Changes in the photosynthetic and fluorescence parameters of tomato (Lycopersicon esculentum L.) leaves were investigated after plants were treated to moderately high temperature stress, then allowed to recover under normal temperature conditions. Plants were treated at 35 degrees C for 2 h, 4 h, 6 h, or 8 h, then transferred to 25 degrees C for recovery. These heat treatment groups were identified as T1, T2, T3, and T4, respectively. Plants grown continuously at 25 degrees C served as the controls. Compared to the controls, photosynthetic activity decreased in plants exposed to 35 degrees C for >= 2 h. Photosynthetic activities in T1, T2, and T3 plants continued to decrease within the first 2 h after the plants were transferred to 25 degrees C, then increased gradually, but did not attain the level observed in control plants. This observation indicates that complete recovery of the rate of net photosynthesis (P-n) in tomato leaves was not possible within 6 h, even if the plants had been exposed to 35 degrees C for only 2 h. Taking stomatal conductance (g(s)), intercellular CO2 concentration (C-i), the rate of transpiration (T-r), and the limitation of stomatal conductance (L-s) into account, these results suggest that the decrease in P under moderately-high temperature stress was caused mainly by non-stomatal restriction. Stomatal restriction appeared to be the limiting factor causing the decrease in P-n when plants were returned to 25 degrees C. In parallel with the decline in P-n, the intrinsic photochemical efficiency (F-v/F-m), quantum yield of PS II (Phi(PSII)), and photochemical quenching (q(p)) of plants that had been exposed to 35 degrees C for 2 h also decreased. However, in T1,T2, and T3 plants, these parameters recovered almost to the levels recorded in control plants after they had been kept at 25 degrees C for 2 h.