Tolerance of pea (Pisum sativum L.) to long-term salt stress is associatedwith induction of antioxidant defences

Using two cultivars of Pisum sativum L. with different sensitivity to NaCl, the effect of long-term (15 d) NaCl (70 mM) treatments on the activity and expression of the foliar ascorbate-glutathione cycle enzymes, superoxide d ismutase isozymes and their mRNAs was evaluated and related to their ascorb ate anti 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 o f ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorb ate reductase (MDHAR), Mn-superoxide dismutase (Mn-SOD) and dehydroascorbat e reductase (DHAR) increased, while CuZn-SOD activity remained constant. In the NaCl-sensitive plants (cv Challis), salinity did not produce significa nt changes in APX, MDHAR and GR activities. Only DHAR activity was Induced in cv Challis, whereas soluble CuZn-SOD activity decreased by about 35%. To tal 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, chloropl astic 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 ind uction of antioxidant defences is at least one component of the tolerance m echanism of peas to long-term salt-stress.