Tomato plants (70 days old) were grown in hydroponic culture into a greenhouse, where supply of inorganic carbon, ammonium and calcium to saline nutrient solution, was investigated in order to reduce the negative effect of salinity. After 70 days, an ameliorating effect upon the decrease in growth observed under salinity was only observed with the treatments NaCl+Ca(2+) and NaCl+HCO(3)(-)+NH(4)(+)+Ca(2+). A large reduction of hydraulic conductance (L(0)) and stomatal conductance (G(s)) was observed with all treatments, compared with the control. However, the reductions were less when NaCl and Ca(2+) were added together. Organic acids (mainly malic acid) in the xylem were decreased with all treatments except with NaCl+NH(4)(+) and with all single treatments added together (NaCl+HCO(3)(-)+NH(4)(+)+Ca(2+)). Amino acid concentrations in the xylem (mainly asparagine and glutamine) decreased when plants were treated with NaCl and NaCl+Ca(2+), but there was a large increase in the plants treated with NaCl+NH(4)(+) or with all treatments together. As HCO(3)(-) is an important source of carbon for NH(4)(+) assimilation, the increase in the concentration of amino acids and organic acids caused by the treatments that contained NH(4)(+), support the idea that fixation of dissolved inorganic carbon was occurring and that the products were transported via the xylem to the shoot. The ameliorating effect of Ca(2+) on root hydraulic conductivity plus the increase of NH(4)(+) incorporation into the amino acid synthesis pathway possibly due to dissolved inorganic carbon fixation, could reduce the negative effect of salinity on tomato plants.