EFFECTS OF SALT STRESS ON GROWTH AND NITROGEN-FIXATION BY PEA, FABA-BEAN, COMMON BEAN AND SOYBEAN PLANTS

The effects of salinity on growth, nodulation, acetylene reduction activity (ARA), nodule leghemoglobin (Lb) content and respiratory capacity of bacteroids from pea (Pisum sativum cv. Lincoln), faba-bean (Vicia faba L. var. minor cv. Alborea), bean (Phaseolus vulgaris cv. Contender) and soybean (Glycine max L. var. Williams) were determined. The depressive effect of saline stress (50 and 100 mm NaCl) on dry weight and ARA of nodules was directly related to the salt-induced decline in dry weight and N content in shoots. The results indicate that P. savitum was the legume most severely affected by salinity, whereas G. max was the most salt-tolerant species. Saline stress was also responsible for a decrease in cytosolic protein of nodules, specifically Lb; this effect was more pronounced in pea and bean nodules than in soybean and faba-bean nodules. Bacteroids isolated from soybean and bean nodules exhibited higher respiratory activity in the absence of energy-yielding substrates than those isolated from pea and faba-bean plants. Salt added directly to the incubation mixture of bacteroids inhibited 02 uptake of bacteroids isolated from nodules of each legume. However, salt treatment of plants decreased respiratory capacity only in pea and faba-bean bacteroids. Inhibition of ARA under moderate saline stress may be related to the drop in bacteroid respiration. Under severe stress both the reduction in Lb content and inhibition of bacteroid respiration may be involved in salt-induced inhibition of nitrogen fixation.