INTERACTIONS BETWEEN NITRATE AND AMMONIUM DURING UPTAKE BY CAROB SEEDLINGS AND THE EFFECT OF THE FORM OF EARLIER NITROGEN NUTRITION

Seedlings of carob (Ceratonia siliqua L. cv. Mulata) were used in two sets of experiments in order to evaluate: (1) the reciprocal effects o f each nitrogen form on net uptake of nitrate and ammonium, and (2) th e effect of earlier nitrogen nutrition on ammonium versus nitrate upta ke. In the former group of experiments we studied the kinetics of nitr ate and ammonium uptake as well as the interference of each of the two forms with net uptake of ammonium and nitrate by both nitrogen deplet ed and nitrogen fed carob seedlings. On the whole, nitrogen depletion led to increase in both affinity and V-max of the system for both form s of nitrogen, at the same time as the effects of nitrate on uptake of ammonium and vice versa were concentration dependent. In the second g roup of experiments the effects of earlier nitrogen nutrition on nitra te and ammonium uptake were characterized, and in this case we observe d that: (a) if only one form of N was supplied, ammonium was taken up in greater amounts than nitrate; (b) the presence of ammonium enhanced nitrate uptake; (c) ammonium uptake was inhibited by nitrate; (d) the re was a significant effect of the earlier nitrogen nutrition on the r esponse of the plants to a different nitrogen source. The latter was e vident mainly as regards ammonium uptake by plants grown in ammonium n itrate. The interactions between nitrate and ammonium uptake systems a re discussed on the basis of the adaptation to the nitrogen source dur ing early growth.