EFFECTS OF SALINITY ON N-2 FIXATION, NITROGEN-METABOLISM AND EXPORT AND DIFFUSIVE CONDUCTANCE OF COWPEA ROOT-NODULES

N-2 fixation and nodule functioning were assessed following applicatio n of 40 or 80 mol m(-3) NaCl to root systems of an established cowpea symbiosis (21-22 day, Vigna unguiculata L. Walp cv. Vita 3: Bradyrhizo bium strain CB 756). Liquid culture techniques were used which preclud ed the initiation and development of new nodules and prevented direct exposure of the surface of nodules to salt during the period of study. Both levels of NaCl inhibited photosynthetic CO2 fixation (75 and 88% ), decreased stomatal conductance, increased substomatal CO2 level and decreased the total water potential of leaves. Despite these severely negative effects of NaCl on photosynthetic parameters of the plant, t he respiration of nodulated roots increased for up to 12 d following N aCl treatment and, although nitrogenase activity was depressed by NaCl , substantial rates of acetylene reduction, N-15(2) fixation and N exp ort from nodules were maintained. Data are interpreted to indicate tha t the continued high ''sink strength'' of the root system in attractin g substrates to maintain an enhanced level of respiration also serves to maintain a supply of oxidizable substrates to the nodules. NaCl tre atment caused significant changes in the relative concentrations of ni trogenous solutes of xylem; ureides were reduced and asparagine was in creased.