Nitrogen assimilation and nitrate reductase activity in tomato seedlings. II. Effect of increasing calcium doses in the presence of high magnesium concentrations on nitrate reduction and protein content

Abstract

A 3:30 Ca:Mg ratio in the nutrient solution produces in tomato seedlings symptoms of Ca²⁺ deficit owing to excessive accumulation of Mg²⁺ ions and the depressed Ca²⁺ accumulation. As a result of this a decrease in dry weight increment and protein content is observed together with inhibition of nitrate reductase activity. A doubled Ca²⁺ dose in the nutrient solution, that is a change in the Ca:Mg ratio to 6:30 abolishes the external symptoms of Ca²⁺ deficit and reduces Mg²⁺ accumulation, that of Ca²⁺ ions remaining unchanged. At the same time an enhanced activity of nitrate reductase appears, reaching values close to those in control plants. Tomato seedlings grown on a 3-fold increased Ca²⁺ dose (Ca:Mg = 9:30) did not differ at all from the control ones. An in-crease in calcium concentration in the nutrient solution, the high magnesium dose remaining unchanged, causes enhanced K⁺ accumulation, and this may affect nitrate absorption and reduction. It would seem that Ca²⁺ deficit in plant tissues induced by excessive Mg²⁺ accumulation with unsuitable Ca:Mg ratio in the nutrient solution in cause of disorders in NO₃ nitrogen assimilation.