Response of antioxidant systems and leaf water relations to NaCl stress in pea plants

Abstract

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.