ABSTRACT 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 CO2 concentrations ranging from approximately 70 to 1500 mol CO2 mol-1 air. Salinity dramatically decreased net photosynthesis (ACO2) and stomatal conductance (gs). Net photosynthesis was plotted against computed leaf internal CO2 concentration (Ci), and the initial slope of this ACO2-Ci 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 CO2 compensation point. Leaves appeared healthy and leaf chlorophyll content increased with increasing salinity. Although partial stomatal closuer occurred with salinisation but reductions in photosynthesis 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.

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