Stromal and thylakoid‐bound ascorbate peroxidases in NaCl‐treated wheat

Salinity is one of the most widespread causes of plant growth inhibition and reduction of agricultural productivity. Activated oxygen species seem to be implicated in the injury mechanism induced by salt. In our work, the effect of NaCl was studied in chloroplasts from seedlings of two cultivars of Triticum durum Desf., differently sensitive to drought and to heavy metals (cv. Ofanto more tolerant than cv. Adamello). The seedlings were grown for 9 days in Hoagland's no. 2 solution, to which increasing concentrations of NaCl (0, 50 and 100 m M ) were added. The results indicate an involvement of activated oxygen species in the mechanism of cellular toxicity of NaCl and the capability of the seedlings to induce their antioxidant defences. In chloroplasts of both cultivars, NaCl treatment induced a significant increase in ascorbate, even if cv. Ofanto showed the higher response. Ascorbate peroxidases appear to behave differently in the two cultivars in response to NaCl‐induced oxidative stress of chloroplasts. In seedlings of Adamello treated with 50 m M NaCl, a sudden increase in thylakoid‐bound ascorbate peroxidase (tAPX) activity and a decrease in that of stromal ascorbate peroxidase (sAPX) occurred. Differently, in Ofanto, a slight increase of both sAPX and tAPX was induced by NaCl. In both cultivars, three well‐resolved bands of sAPX were found. In chloroplasts of both cultivars, monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR) behaved similarly, even if in Adamello a greater decrease of DHAR was shown. The experimental evidence suggested a more marked effect of NaCl stress on Adamello as compared with Ofanto.