Growth, ion distribution, potassium and nitrate uptake of Leucaena leucocephala, and effects of NaCl

Three to six-week old plants of Leucaena leucocephala (Lam.) de Wit, cv K8, were grown hydroponically without root nodules. They showed much better growth response to nitrate than to ammonium. They were found to be tolerant to salinity of up to 50 mM NaCl. Effects of salinity in this range and moderate Na + accumulation in young organs could be alleviated by supplying 10 mM nitrate to the medium. Na + mainly accumulated in basal leaves and cotyledons as disposable organs. Distribution of Cl - was more uniform than of Na + , its exclusion from young organs by nitrate was less effective. K + distribution in the plants was almost uniform and little affected by NaCl, while nitrate accumulated only in the roots and at high external concentrations. Nitrate uptake of N-starved plants initially showed higher rates, a minimum after 2 h and then increased during several hours suggesting a constitutive and an inducible transport component. Induction of nitrate assimilation was shown by nitrate reductase activity (EC 1.6.6.2). Nitrate uptake was not affected by 20 mM NaCl. In K + - and N-starved plants uptake rates of K + roughly followed those of nitrate. In N-supplied plants nitrate reductase activity was found mainly in the roots, where in starved plants induction was also strongest. In the nutrient supplied culture salt treatment especially stimulated nitrate reductase activity in the roots. In general, counteraction of nitrate to salinity effects occurs in distribution of the ions within the plant but only at low salinity.