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 n odules were significantly reduced when pea (Pisum sativum L. cv. Linco ln) plants were subjected to 50 mM sodium chloride stress for 3 weeks. C2H2 reduction activity by bacteroids isolated from pea nodules was d rastically inhibited by saline stress, and malate appeared to be a mor e 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 conce ntration was depressed in bacteroids and cytosol, whereas total solubl e sugar (TSS) content slightly increased in both fractions. The effect of salt stress on TSS and malate content suggests that the utilizatio n 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 bacteroid s and to the decrease in bacteroid respiration. The stimulation of fer mentative 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 leghemog lobin, in the nodules.