Changes in photosynthetic parameters and antioxidant activities following heat-shock treatment in tomato plants

Seedlings of two tomato genotypes, Lycopersicon esculentum Mill. var. Amalia and the wild thermotolerant type Nagcarlang, were grown under a photoperiod of 16 h light at 25°C and 8 h dark at 20°C. At the fourth true leaf stage, a group of plants were exposed to a heat-shock temperature of 45°C for 3 h, and measurements of chlorophyll fluorescence, gas-exchange characteristics, dark respiration and oxidative and antioxidative parameters were made after releasing the stress. The heat shock induced severe alterations in the photosynthesis of Amalia that seem to mitigate the damaging impact of high temperatures by lowering the leaf temperature and maintaining stomatal conductance and more efficient maintenance of antioxidant capacity, including ascorbate and glutathione levels. These effects were not evident in Nagcarlang. In Amalia plants, a larger increase in dark respiration also occurred in response to heat shock and the rates of the oxidative processes were higher than in Nagcarlang. This suggests that heat injury in Amalia may involve chlorophyll photooxidation mediated by activated oxygen species (AOS) and more severe alterations in the photosynthetic apparatus. All these changes could be related to the more dramatic effect of heat shock seen in Amalia than in Nagcarlang plants.Peer reviewe

Response of superoxide dismutase isoenzymes in tomato plants (Lycopersicon esculentum) during thermo-acclimation of the photosynthetic apparatus

Seedlings of Lycopersicon esculentum Mill. var. Amalia were grown in a growth chamber under a photoperiod of 16 h light at 25°C and 8 h dark at 20°C. Five different treatments were applied to 30-day-old plants: Control treatment (plants maintained in the normal growth conditions throughout the experimental time), heat acclimation (plants exposed to 35°C for 4 h in dark for 3 days), dark treatment (plants exposed to 25°C for 4 h in dark for 3 days), heat acclimation plus heat shock (plants that previously received the heat acclimation treatment were exposed to 45°C air temperature for 3 h in the light) and dark treatment plus heat shock (plants that previously received the dark treatment were exposed to 45°C air temperature for 3 h in the light). Only the heat acclimation treatment increased the thermotolerance of the photosynthesis apparatus when the heat shock (45°C) was imposed. In these plants, the CO2 assimilation rate was not affected by heat shock and there was a slight and non-significant reduction in maximum carboxylation velocity of Rubisco (Vcmax) and maximum electron transport rate contributing to Rubisco regeneration (Jmax). However, the plants exposed to dark treatment plus heat shock showed a significant reduction in the CO2 assimilation rate and also in the values of Vcmax and Jmax. Chlorophyll fluorescence measurements showed increased thermotolerance in heat-acclimated plants. The values of maximum chlorophyll fluorescence (Fm) were not modified by heat shock in these plants, while in the dark-treated plants that received the heat shock, the Fm values were reduced, which provoked a significant reduction in the efficiency of photosystem II. A slight rise in the total superoxide dismutase (SOD) activity was found in the plants that had been subjected to both heat acclimation and heat shock, and this SOD activity was significantly higher than that found in the plants subjected to dark treatment plus heat shock. The activity of Fe-SOD isoenzymes was most enhanced in heat-acclimated plants but was unaltered in the plants that received the dark treatment. Total CuZn-SOD activity was reduced in all treatments. Darkness had an inhibitory effect on the Mn-SOD isoenzyme activity, which was compensated by the effect of a rise in air temperature to 35°C. These results show that the heat tolerance of tomatoplants may be increased by the previous imposition of a moderately high temperature and could be related with the thermal stability in the photochemical reactions and a readjustment of Vcmax and Jmax. Some isoenzymes, such as the Fe-SODs, may also play a role in the development of heat-shock tolerance through heat acclimation. In fact, the pattern found for these isoenzymes in heat-acclimated Amalia plants was similar to that previously described in other heat-tolerant tomato genotypes.This work was supported by grants from the 'Convenio de Cooperación Científica Hispano-Cubano del CSIC/CITMA' (2001CU0015) and the MEC-FEDER (BFU 2005-02051/BFI)Peer reviewe