Abstract
Symbiotic nitrogen fixation in soybean has been reported to be highly sensitive to salt stress. We describe here differences among soybean genotypes for nitrogen fixation under salt stress and relate this genotypic difference, to ionic concentration m stems, leaves, roots and nodules, shoot and root growth, and symbiotic nitrogen fixation. Among the 16 soybean cultivars screened for salt (NaCl, 80 mM) tolerance, eleven were found to be sensitive to salt stress. Foliar injury in these cultivars was severe. Leaves became chlorotic and necrotic and the plants lost all their foliage by six weeks. Root weight was markedly reduced. Although a few cultivars formed nodular swellings on roots, nitrogen fixation (as measured by acetylene reduction) was undetectable under salt stress. In contrast, the other five cultivars were tolerant to salt stress. They continued to grow and fix nitrogen with little evidence of foliar injury. The extent of shoot growth, nodulation and nitrogen fixation in salt-stressed tolerant plants was, however, less than that of non-stressed tolerant plants. Also in contrast to the sensitive cultivars, there were no significant differences in root weight between the stressed and non-stressed tolerant cultivars. During the 4-week growth period the leaves of salt-stressed sensitive cultivar showed a linear accumulation of large quantities of Cl-, accompanied by an accumulation of Ca2+, Mg2+ and K+. These cations were present in relatively small amounts in the non-stressed sensitive cultivars. In comparison with the sensitive cultivars, concentrations of Cl-, Ca2+, Mg2+ and K+ in the leaves of salt-stressed tolerant cultivars were low. The levels of Na+ in the leaves of both sensitive and tolerant cultivars remained low under salt stress. Scions of tolerant cultivars grafted onto the root stock of sensitive cultivar Williams showed a typical sensitive response in terms of growth, nitrogen fixation and ionic content. In contrast, scions of sensitive cultivars grafted onto the root stocks of tolerant cultivars showed a typical tolerant response, sustaining growth and nitrogen fixation under salt stress. This confirms the primary role for the root in the regulation of ion uptake, as well as growth and nitrogen fixation, under salt stress.
