A pea (Pisum sativum cv. Puget) cultivar was grown on a medium containing different NaCl concentrations (0–160 mol m⁻³) in order to study the effects of salt stress on leaf water relations and on the activity of antioxidant enzymes. NaCl stress caused a rapid decline in chlorophyll content. Both leaf water (ψ_l) and osmotic potentials (ψ_s) decreased progressively with the severity of the stress (from 90–160 mol m⁻³ NaCl) whereas leaf turgor pressure (ψ_p) increased in treated plants. Pea leaves contained an iron‐containing superoxide dismutase (Fe‐SOD) isozyme in chloroplasts alongside a copper‐zinc‐containing (CuZn‐SOD) form (CuZn‐SOD II). The lowest NaCl concentration (70 mol m⁻³) had no effect on the activity of these antioxidant enzymes while higher concentrations (110–130 mol m⁻³) enhanced the activity of cytosolic CuZn‐SOD I and chloroplastic CuZn‐SOD II as well as that of mitochondrial and/or peroxisomal manganese‐containing superoxide dismutase (Mn‐SOD). These inductions were matched by increases in the activity of ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR). The increased activities coincided with decreased stomatal conductance and were unaffected by the severity of stress except in the case of CuZn‐SOD II which fell to control values under the highest stress conditions (140–160 mol m⁻³ NaCl), when a concomitant increase in chloroplastic Fe‐SOD activity was observed. Glutathione reductase (GR) and dehydroascorbate reductase (DHAR) activities were only induced under severe NaCl stress (130–160 mol m⁻³) and were accompanied by losses in the ascorbate and glutathione pools, lower ASC/DHA and GSH/GSSG ratios and increases in GSSG. Electron microscopy showed that the thylakoidal structure of the chloroplasts became disorganized and their starch content decreased in plants treated with 160 mol m⁻³ NaCl. Overall, the results suggest that salt stress is accompanied by oxidative stress, perhaps at the cell compartment level. The capacity of Puget cultivar to ensure cell turgor and to enhance the activity of enzymes involved in the defence against oxidative stress seems to be important in determining adaptation to moderate NaCl stress conditions. In plants exposed to severe NaCl stress (130–160 mol m⁻³) it seems that such resistance to oxidative stress is overcome, which might contribute to the deleterious effects of salt and significant growth reduction in these conditions.

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.