STOMATAL AND NONSTOMATAL COMPONENTS TO INHIBITION OF PHOTOSYNTHESIS IN LEAVES OF SUGAR BEET PLANTS UNDER SALT STRESS

摘要

Sugar beet (Beta vulgaris L.) plants grown in sand culture were gradually exposed to different levels of salinity (0, 50, 150, 250, and 350 mM, NaCl + CaCl2 in 5:1 molar ratio) and photosynthetic rates of individual attached leaves were measured during salinisation period at external CO_2 concentrations ranging from approximately 70 to 1500 ujnol CO_2 mol~(-1) air. Salinity dramatically decreased net photosynthesis (A_(CO2)) and stomatal conductance (g_s). Net photosynthesis was plotted against computed leaf internal CO_2 concentration (Q), and the initial slope of this A_(co2)-C curve was used as a measure of photosynthetic ability. Leaves from plants exposed to 50 mM salinity showed little change in photosynthesis, whereas those treated to high levels of salinity had up to 91.5% inhibition, with increase in CO_2 compensation point. Leaves appeared healthy and leaf chlorophyll content increased with increasing salinity. Although partial stomatal closuer occurred with salinisation but reductions inphotosynthesis were partly non-stomatal at high levels of salt treatment. Photosynthetic ability was inversely related to the concentration of either Na~+ and Cl~- in the leaf laminas sampled at the end of experimental period.