The assimilation of ammonium and nitrate nitrogen by detached potato sprouts

Abstract

Detached potato sprouts have been grown successfully in ammonium and nitrate culture solutions for 5 weeks. The pH and concentration of each solution were such that a similar rate of nitrogen absorption took place from both. The general appearance, increase in fresh and dry weight and the nitrogen distribution in the shoots and roots of sprouts removed at intervals for analysis have been compared. Sprouts, previously grown in nitrate or ammonium culture solutions, have been transferred and grown for 8 days on a minus‐nitrogen culture solution, and the changes in appearance, growth rate and nitrogen distribution followed in a similar manner.The importance of aerating the culture solution, of frequent solution renewal and of an adequate supply of soluble iron for successful solution culture of potato sprouts is stressed.The ammonium sprouts developed more evenly and ultimately more rapidly than the nitrate sprouts. The nitrate sprouts were a lighter green, showed a tendency for the lower leaves to yellow and dry up and, when transferred to the minus‐nitrogen solution, rapidly developed symptoms of nitrogen deficiency. Evidence is presented that, in young potato sprouts, the rate of nitrate reduction and assimilation are a serious limiting factor to the availability of nitrate nitrogen for protein synthesis.Ammonium assimilation takes place actively and mainly in the roots and stem bases. Nitrate assimilation takes place in both shoot and roots, but is more active in the shoot. Absorption of nitrate ions can take place even when a much higher concentration of nitrate ions is present in the root cells than in the external solution. The initial nitrate nitrogen present in the sprouts is not appreciably utilized during culture on ammonium solution.The ammonium shoots as compared with the nitrate shoots have a higher content of total nitrogen, protein nitrogen, non‐protein organic nitrogen, amide and amino‐acid nitrogen, but a lower content of inorganic nitrogen. The nitrate roots as compared with the ammonium roots have a higher content of total nitrogen, protein nitrogen and inorganic nitrogen, but a lower content of non‐protein organic nitrogen, amide and other nitrogen.The higher protein content of the nitrate roots is associated with a higher percentage dry weight and a lower content of ‘other’ nitrogen. The significance of this relationship is discussed.During ammonium culture both asparagine and glutamine accumulate, particularly in the shoot.Evidence is presented that asparagine is the amide primarily concerned in ammonia storage and also that it is formed during nitrate assimilation.Knowledge of the absorption and assimilation of ammonium and nitrate ions by plants is of fundamental importance for an understanding of fertilizer practice, and the present investigation, in which detached potato sprouts have been successfully grown in ammonium and nitrate culture solutions, forms part of a study of this problem.