Assimilate allocation priority as affected by nitrogen compounds in the xylem sap of tomato

Experiments with tomato (Lycopersicon esculentum (L.) Mill. cv. F144) grown in a greenhouse show that salinity (electric conductivity 6-7 dS m^-1) and bicarbonate fertilization enhanced the accumulation of total solids in the fruits. Short term experiments with tomato seedlings were carried out simultaneously to study the effect of salinity (NaCl) and NO₃/^- concentrations on the activities of nitrate reductase (NR, EC 1.6.6.1), phosphoenolpyruvate carboxylase (PEPc, EC 4.1.1.31) and glutamine synthetase (GS, EC 6.3.1.2) in roots and leaves to study the mechanisms controlling assimilate transport to reproductive organs. The effect of bicarbonate fertilization on fruit quality was more pronounced under saline conditions which increased NO₃/^- reduction and assimilation in the roots. Salinity (100 mM NaCl) inhibited xylem loading of NO₃/^- and its subsequent transport to the shoot, shifting the main NO₃/^- reduction and assimilation in the plant from the shoot to the root. At the same time, addition of bicarbonate increased dark CO₂ fixation in the roots, producing oxaloacetate, aspartate and asparagine. The resulting amino acids were transported from the root to the shoot through the xylem. Low NO₃/^- concentration, salinity and bicarbonate in the medium produced a higher ratio of N(red)/(N(red) + NO₃/^- ) in the xylem sap. The ratio N(red)/(N(red) + NO₃/^-) in the xylem sap was related to the determination of assimilate allocation priorities in the tomato plant. A high ratio correlated with increased allocation to reproductive organs while a low ratio, generated by fertilization with high NO₃/^- concentrations, stimulated the vegetative growth at the expense of fruit quality.