Nitrogen assimilation and nitrate reductase activity in tomato seedlings. I. Comparative studies on the influence of the Ca:Mg ratio on nitrogen metabolism in relation to absorption of nitrates or ammonium salts

A change in the Ca:Mg ratio in the nutrient solution from the value optimal for tomato growth (3:1) to an unsuitable value (3:30) increases several times Mg2+ accumulation with simultaneous reduction of Ca2+ accumulation independently from the absorption of mineral nitrogen from, viz. NO3 - or NH4,- ions. Tomato seedlings receiving nitrogen in the form of NH4 show a complete tolerance to the unsuitable Ca:Mg ratio in the nutrient solution, whereas those supplied with NO3 react by growth inhibition, reduced dry weight increment and protein weight and inhibition of nitrogen reductase (NR) activity. It is suggested that the reaction of plants to an excess of Mg2+ in the nutrient solution in relation to Ca2+ depends above all on the form in which nitrogen is supplied to the plants. It was found that the reduction of dry weight increment and protein synthesis preceded in time NR activity inhibition. Accumulation of excess Mg2+ and decreased Ca2+ accumulation occur rather rapidly. It is, therefore, supposed that the excess of accumulated Mg2+ induces a physiological deficit of Ca2+ in the tissue of plants taking up NO3 nitrogen. This calcium deficit leads to inhibtion of nitrate assimilation