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 tot al 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, E C 1.6.6.1), phosphoenolpyruvate carboxylase (PEPc, EC 4.1.1.31) and gl utamine synthetase (GS EC 6.3.1.2) in roots and leaves to study the me chanisms controlling assimilate transport to reproductive organs. The effect of bicarbonate fertilization on fruit quality was more pronounc ed under saline conditions which increased NO3- reduction and assimila tion in the roots. Salinity (100 mM NaCl) inhibited xylem loading of N O3- 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 resu lting amino acids were transported from the root to the shoot through the xylem. Low NO3- concentration, salinity and bicarbonate in the med ium produced a higher ratio of N-red/(N-red + NO3-) in the xylem sap. The ratio of 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 org ans while a low ratio, generated by fertilization with high NO3- conce ntrations, stimulated the vegetative growth at the expense of fruit qu ality.