DIFFERENTIAL SENSITIVITY OF STOMATAL AND NONSTOMATAL COMPONENTS TO NACL OR NA2SO4 SALINITY IN HORSEGRAM, MACROTYLOMA-UNIFLORUM (LAM.)

(CO2)-C-14 assimilation rate (P), leaf diffusive conductance (g(s),), photosynthetic electron flow, and activities of enzymes of Calvin cycl e were studied in a horsegram [Macrotyloma uniflorum (Lam.)] in respon se to salinity induced by NaCl or Na2SO4. A significant reduction in P and g(s) by both salt treatments was registered. Na2SO4 caused a grea ter reduction in g(s) than the NaCl salinity. Studies with isolated ch loroplasts confirmed a greater sensitivity to NaCl than to Na2SO4. Sal inity inhibited the photosynthetic electron transport. The activity of ribulose-1,5-bisphosphate carboxylase (E.C.4.1.1.39) was under salini ty inhibited more than the activities of other three enzymes of the Ca lvin cycle, ribulose-5-phosphate kinase (E.C.2.7.1.19), ribose-5-phosp hate isomerase (E.C.5.3.16), and NADP-glyceraldehyde-3-phosphate dehyd rogenase (E.C.1.2.13). These inhibitions lead to a reduced capacity fo r ribulose-1,5-bisphosphate regeneration. Isolated chloroplasts extrac ted from salt stressed plants and supplemented with the substrates of Calvin cycle could elevate P, but the P was always lower than in the c ontrols. Decreased P in horsegram exposed to high salinity can be attr ibuted to both stomatal and nonstomatal components, however, the sensi tivity to the salt source, NaCl or Na2SO4, was different.