NITROGEN-FIXATION AND CARBON METABOLISM BY NODULES AND BACTEROIDS OF PEA-PLANTS UNDER SODIUM-CHLORIDE STRESS

Acetylene reduction activity (ARA) and leghemoglobin (Lb) content in nodules were significantly reduced when pea (Pisum sativum L. cv. Lincoln) plants were subjected to 50 mM sodium chloride stress for 3 weeks. C2H2 reduction activity by bacteroids isolated from pea nodules was drastically inhibited by saline stress, and malate appeared to be a more appropriate substrate than glucose or succinate in maintaining this activity. Salt added directly to the incubation mixture of bacteroids or to the culture medium of plants inhibited O-2 uptake by bacteroids. Nodule cytosolic phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) and bacteroid malate dehydrogenase (MDH; EC 1.1.1.37) activities were strongly enhanced by salt stress. Under these conditions, malate concentration was depressed in bacteroids and cytosol, whereas total soluble sugar (TSS) content slightly increased in both fractions. The effect of salt stress on TSS and malate content suggests that the utilization of carbohydrate within nodules could be inhibited during salt stress. The inhibitory effect of NaCl on Nz fixation activity of bacteroids from pea nodules may be related to the carbon deprivation of bacteroids and to the decrease in bacteroid respiration. The stimulation of fermentative metabolism induced by salinity suggests some reduction in O-2 availability within the nodule. Salt stress was also responsible for a decrease of the cytosolic protein content, specifically of leghemoglobin, in the nodules.