Influence of the Level of Nitrate Nutrition on Ion Uptake and Assimilation, Organic Acid Accumulation, and Cation-Anion Balance in Whole Tomato Plants

Abstract

Tomato plants (Lycopersicon esculentum L. var. Ailsa Craig) were grown in water culture in nutrient solution in a series of 10 increasing levels of nitrate nutrition. Using whole plant data derived from analytical and yield data of individual plant parts, the fate of anion charge arising from increased NO₃ assimilation was followed in its distribution between organic anion accumulation in the plant and OH⁻ efflux into the nutrient solution as calculated by excess anion over cation uptake. With increasing NO₃ nutrition the bulk of the anion charge appeared as organic anion accumulation in the plants. OH⁻ efflux at a maximum accounted for only 20% of the anion charge shift. The major organic anion accumulated in response to nitrate assimilation was malate. The increase in organic anion accumulation was paralleled by an increase in cation concentration (K⁺, Ca²⁺, Mg²⁺, Na⁺). Total inorganic anion levels (NO₃⁻, SO₄²⁻, H₂PO₄⁻, Cl⁻) were relatively constant. The effect of increasing NO₃ nutrition in stimulating organic anion accumulation was much more pronounced in the tops than in the roots.

It is suggested that increasing the level of NO₃ nutrition to tomato plants stimulates cation uptake and translocation as counter-ions are required to accompany NO₃⁻ ions to the upper plant parts, the major site of NO₃ reduction. On NO₃ reduction, the resulting stoichiometric accumulation of organic anions is balanced by the cations originally accompanying NO₃⁻ ions. Organic anions and cations are largely retained in the upper plant parts. The results suggest that only a small fraction of the total K absorbed by the roots can be translocated downward from the leaves to the roots in the phloem sap. The possible extent of K recirculation is thus low.