Effects of salt stress on the growth, ion content, stomatal behaviour and photosynthetic capacity of a salt-sensitive species, Phaseolus vulgaris L

Abstract

Phaseolus vulgaris (cv. Hawkesbury Wonder) was grown over a range of NaCl concentrations (0-150 mM), and the effects on growth, ion relations and photosynthetic performance were examined. Dry and fresh weight decreased with increasing external NaCl concentration while the root/shoot ratio increased. The Cl(-) concentration of leaf tissue increased linearly with increasing external NaCl concentration, as did K(+) concentration, although to a lesser degree. Increases in leaf Na(+) concentration occurred only at the higher external NaCl concentrations (≧100 mM). Increases in leaf Cl(-) were primarily balanced by increases in K(+) and Na(+). X-ray microanalysis of leaf cells from salinized plants showed that Cl(-) concentration was high in both the cell vacuole and chloroplast-cytoplasm (250-300 mM in both compartments for the most stressed plants), indicating a lack of effective intracellular ion compartmentation in this species. Salinity had little effect on the total nitrogen and ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39) content per unit leaf area. Chlorophyll per unit leaf area was reduced considerably by salt stress, however. Stomatal conductance declined substantially with salt stress such that the intercellular CO2 concentration (C i) was reduced by up to 30%. Salinization of plants was found to alter the δ(13)C value of leaves of Phaseolus by up to 5‰ and this change agreed quantitatively with that predicted by the theory relating carbon-isotope fractionation to the corresponding measured intercellular CO2 concentration. Salt stress also brought about a reduction in photosynthetic CO2 fixation independent of altered diffusional limitations. The initial slope of the photosynthesis versus C i response declined with salinity stress, indicating that the apparent in-vivo activity of RuBP carboxylase was decreased by up to 40% at high leaf Cl(-) concentrations. The quantum yield for net CO2 uptake was also reduced by salt stress.