- Photosynthesis of tomato plants (Lycopersicon esculentum (L.) Mill. cv. F144) was studied under conditions of CO2 supplementation and salinity. The purpose of the study was to elucidate the mechanisms underlying the effects of salinity on the acclimation of tomato plants to CO2 supplementation. Plants were grown under either low (355 μmol mol-1) or elevated (1200 ± 50 μmol mol-1) CO2 and were irrigated with low concentrations of mixed salts. The highest salinity level (E.C. 7 dS m-1) was that used to produce quality tomatoes in the Negev highlands, in Israel. During early development (three weeks after planting), the net photosynthetic rate of the leaves was much higher under elevated CO2, and other than a slight decrease in quantum yield efficiency as measured by fluorescence (ΔF/F'(m)), no signs of acclimation to high levels of CO2 were apparent. Clear acclimation to high CO2 concentration was evident ten weeks after planting when the net photosynthetic rate, photosynthetic capacity, and carboxylation efficiency of leaves of non-salinized plants were strongly suppressed under elevated CO2. This was accompanied by reductions in carboxylation efficiency, Rubisco activity and PSII quantum yield, and an increased accumulation of leaf soluble sugars. The reduction in photosynthetic capacity in the high CO2 plants was less in plants grown at the highest salinity level. This was correlated with an increase in the PSII quantum yield parameters (F(v)/F(m)) and ΔF/F'(m)) but not with Rubisco activity which was affected by the CO2 treatments only. These results explain the effects of high CO2 on yields in tomatoes grown at high levels of salt (Li et al., 1999).