Expression of Characteristics of Ammonium Nutrition as Affected by pH of the Root Medium

Abstract

To study the effect of root-zone pH on characteristic responses of $NH4+$-fed plants, soybeans (Glycine max {L.} Merr. cv. Ransom) were grown in flowing solution culture for 21 d on four sources of N (1.0 mol m⁻³$NO3-$, 0.67 mol m⁻³$NO3-$ plus 0.33 mol m⁻³$NH4+$, 0.33 mol m⁻³$NO3-$ plus 0.67 mol m⁻³$NH4+$, and 1.0 mol m⁻³$NH4+$) with nutrient solutions maintained at pH 6.0, 5.5, 5.0, and 4.5. Amino acid concentration increased in plants grown with $NH4+$ as the sole source of N at all pH levels. Total amino acid concentration in the roots of $NH4+$-fed plants was 8 to 10 times higher than in $NO3-$-fed plants, with asparagine accounting for more than 70% of the total in the roots of these plants. The concentration of soluble carbohydrates in the leaves of $NH4+$-fed plants was greater than that of $NO3-$-fed plants, but was lower in roots of $NH4+$-fed plants, regardless of pH. Starch concentration was only slightly affected by N source or root-zone pH. At all levels of pH tested, organic acid concentration in leaves was much lower when $NH4+$ was the sole N source than when all or part of the N was supplied as $NO3-$. Plants grown with mixed $NO3-$ plus $NH4+$ N sources were generally intermediate between $NO3-$ - and $NH4+$-fed plants. Thus, changes in tissue composition characteristic of $NH4+$ nutrition when root-zone pH was maintained at 4.5 and growth was reduced, still occurred when pH was maintained at 5.0 or above, where growth was not affected. The changes were slightly greater at pH 4.5 than at higher pH levels.