Tolerance of pea (Pisum sativum L.) to long‐term salt stress is associated with induction of antioxidant defences

ABSTRACT Using two cultivars of Pisum sativum L. with different sensitivity to NaCl, the effect of long‐term (15 d) NaCl (70 m M) treatments on the activity and expression of the foliar ascorbate–glutathione cycle enzymes, superoxide dismutase isozymes and their mRNAs was evaluated and related to their ascorbate and glutathione contents. High‐speed supernatant (soluble) fractions, enriched for cytosolic components of the antioxidant system, were used. In this fraction from the NaCl‐tolerant variety (cv Granada), the activities of ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), Mn‐superoxide dismutase (Mn‐SOD) and dehydroascorbate reductase (DHAR) increased, while CuZn‐SOD activity remained constant. In the NaCl‐sensitive plants (cv Challis), salinity did not produce significant changes in APX, MDHAR and GR activities. Only DHAR activity was induced in cv Challis, whereas soluble CuZn‐SOD activity decreased by about 35%. Total ascorbate and glutathione contents decreased in both cultivars, but the decline was greater in NaCl‐sensitive plants. This difference between the two cultivars was more pronounced when the transcript levels of some these enzymes were examined. Transcript levels for mitochondrial Mn‐SOD, chloroplastic CuZn‐SOD and phospholipid hydroperoxide glutathione peroxidase (PHGPX), cytosolic GR and APX were strongly induced in the NaCl‐tolerant variety but not in the NaCl‐sensitive variety. These data strongly suggest that induction of antioxidant defences is at least one component of the tolerance mechanism of peas to long‐term salt‐stress.