Growth and Nitrogen Absorption of Wheat Seedlings as Influenced by the Ammonium: Nitrate Ratio and the Hydrogen-Ion Concentration

A study was made of the growth and nitrogen absorption of wheat seedlings initially 24 hr. old that were cultured in darkness in complete nutrient solutions containing either increasing concentrations of nitrate nitrogen, increasing concentrations of ammonium nitrogen or a constant total concentration of 0.01 M nitrogen in which the ammonium: nitrate ratios were varied from 0:100 to 100:0. Each culture was developed under small-volume and large-volume conditions at an initial pH of 4.3, 5.3, and 6.3. The culture solutions were renewed after every 10 and 14 hr. absorption period and the pH at the end of the final absorption period was recorded. Direct determinations of ammonium and nitrate absorption by the embryos were made on the small-volume solutions. At the end of a 4 day culture period, final root and shoot lengths were recorded and the separated embryos and endosperms were prepared for analysis of various nitrogen fractions. Under small-volume conditions it was not possible to maintain a constant pH throughout the absorption period. Solutions containing only ammonium nitrogen increased in acidity while nitrate nitrogen solutions decreased in acidity. Solutions containing ammonium-plus-nitrate had a final pH intermediate between that of ammonium alone and nitrate alone but always on the acid side of the initial pH. Changes in the pH of large-volume solutions were much smaller. Embryo dry weight was relatively uninfluenced by the nature of the culture solution except at pH 4.3, where embryos receiving all-ammonium under small-volume conditions were markedly inhibited. This was correlated with the low pH developed in these solutions. Root and shoot lengths were scarcely affected by the external concentration of nitrogen over the range investigated; but when nitrogen was lacking, there was a significantly greater root elongation and a decreased shoot length. Root length was constant as the ammonium: nitrate ratio was varied from 10:90 to 80:20 but under small volume conditions rose sharply at 0:100 and fell at 100:0. With ammonium as the sole external nitrogen source, root and shoot length decreased as the pH was reduced from 6.3 to 4.3. With nitrate alone, root length was inhibited at pH 4.3 under large-volume conditions. The influence of pH on root and shoot elongation could be partially correlated with the influence of pH on the dissociation of auxin. The absorption of ammonium and nitrate from solutions containing either nitrogen source rose with inc