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

A pea (Pisum sativum cv. Puget) cultivar was grown on a medium containing d ifferent NaCl concentrations (0-160 mol m(-3)) ill order to study the effec ts of salt stress on leaf water relations and on the activity of antioxidan t enzymes. NaCl stress caused a rapid decline in chlorophyll content. Both leaf water (psi(1)) and osmotic potentials (psi(s)) decreased progressively with the severity of the stress (from 90-160 mol m(-3) NaCl) whereas leaf turgor pressure (psi(p)) increased in treated plants. Pea leaves contained an iron-containing superoxide dismutase (Fe-SOD) isozyme in chloroplasts al ongside 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 an tioxidant 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 acti vity of ascorbate peroxidase (APS) and monodehydroascorbate reductase (MDHA R). The increased activities coincided with decreased stomatal conductance and were unaffected by the severity of stress except in the case of CuZn-SO D 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 ac tivity was obserl ed. Glutathione reductase (GR) and dehydroascorbate reduc tase (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, lon-er ASC/DHA and GSH/GSSG ratios and increases in GSSG. Electron m icroscopy 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 accompanie d by oxidative stress, perhaps at the cell compartment level. The capacity of Puget cultivar to ensure cell turgor and to enhance the activity of enzy mes involved in the defence against oxidative stress seems to be important in determining adaptation to moderate NaCl stress conditions. In plants exp osed to severe NaCl stress (130-160 mol m(-3)) it seems that such resistanc e to oxidative stress is overcome, which might contribute to the deleteriou s effects of salt and significant growth reduction in these conditions.