FACTORS AFFECTING ASSIMILATION OF AMMONIUM AND NITRATE NITROGEN, PARTICULARLY IN TOMATO AND APPLE

Abstract

A continuation of work previously reported shows that ammonium ions, when absorbed, may be assimilated by plants without further change in a few min. and may be immediately available for synthesis of amino acids. When ammonium salts were supplied plants in soil, some ions were oxidized to nitric acid and were absorbed as nitrate ions and were then reduced back to the ammonium ion by the action of reducase in the plant. The activity of reducase is determined by external and internal factors. For these reasons the ammonium ion was assimilated as soon as absorbed and did not accumulate in the plant whereas the nitrate ion was much more slowly assimilated and in many cases accumulated in comparatively high concs. in the plant. The pH of the nutrient medium had a controlling influence on the assimilation, particularly of the ammonium ion. The nitrate ion was most satisfactorily assimilated in the tomato and apple when absorbed from an acid nutrient medium of approx. pH 4.0 as compared with pH 6.0 for the ammonium ion. The volume of growth obtained with nitrate or ammonium depended on the conc. of the nitrogenous salt in the nutrient solution, the quantity of ammonium ions (nitrate reduced or ammonium directly absorbed) and the available carbohydrates in the plant. A much higher conc. of nitrate than ammonium ions was necessary for a given volume of growth. Plants containing large reserves of carbohydrates assimilated ammonium ions more rapidly than nitrate ions. Plants grown with ammonium N contained a much higher conc. of soluble organic N than those supplied with an equal quantity of nitrate N.