Nitrite Toxicities in Vegetable Crops

II. Nitrite Toxicities as Compared with Ammonium Toxicities

Abstract

Cucumber, pepper, celery, and lettuce were grown in solution culture in order to investigate nitrite toxicities in comparison with ammonium toxicities. Nitrite or ammonium was supplied in combination with nitrate in various proportions, with the total nitrogen concentration being held constant at 12me/l. The pH of the solution was adjusted to two levels, 7.5 or 5.0. Plants were grown under different treatments for three to four weeks.
1. Nitrite toxicity symptoms, such as leaf wilting and root browning, occurred much more severely at pH 5.0 than at pH 7.5. On the other hand, ammonium was. less injurious than nitrite, but induced slight root browning in some species at pH 7.5.
2. At pH 7.5 the relative tolerance of vegetable crops to nitrite evaluated on the basis of growth response was in the following order: cucumber>celery>pepper> lettuce. At pH 7.5 each vegetable crop showed better growth with ammonium than with nitrite, and the growth of lettuce was stimulated by ammonium treatments. The order of relative preference or tolerance of vegetable crops to ammonium at pH. 7.5 was as follows: lettuce>cucumber>celery>pepper. At pH 5.0 nitrite inhibited the plant growth much more markedly than at pH 7.5 and caused the death of plants in nitrite sensitive species. Ammonium treatments at pH 5.0, however, did not affect the plant growth so markedly.
3. At pH 7.5 the concentration of ammonium nitrogen and amide nitrogen in. plants generally increased with the increasing proportion of nitrite or ammonium in the solution. In nitrite treatments, nitrite sensitive species tended to accumulate more ammonium nitrogen and amide nitrogen in both leaves and roots. On the other hand, the more the species was sensitive to ammonium, the more the accumulation of these constituents in roots increased in ammonium treatments. However, the difference of growth response of a crop to nitrite and ammonium treatments could not be explained solely on the basis of accumulation of these constituents in plants.
4. As to inorganic constituents in leaves of plants grown at pH 7.5, ammoniumm treatments decreased the concentration of cation elements such as K, Ca, and Mg, while nitrite treatments decreased that of various elements, P, K, Ca, Mg, and Fe.
5. In another solution culture experiment, the increasing concentration of nitrate in the solution at a given pH level did not alleviate the growth inhibition of vegetable crops caused by nitrite, and the absolute concentration of nitrite appeared to be the dominant factor in the growth inhibition.
6. From these results, it seems that at a relatively high pH level there is a similarity between nitrite and ammonium toxicities with respect to the accumulation of ammonium nitrogen and amide nitrogen in plants. Compared with ammonium, however, nitrite induces serious injury to roots, which may be the main cause of nitrite toxicities especially at low pH.