Uptake and assimilation of nitrogen from solutions containing multiple N sources

We assessed the extent to which plants can acquire amino acids when supplied as single N-sources or when plants have access to a mixture of aminoand inorganic N sources. Because the uptake of different N-sources is temperaturedependent, the effects of temperature on amino-N uptake were also tested. Lolium perenne (perennial rye-grass) was grown hydroponically at 11 ∞ C or 21 ∞ C. Uptake of N was determined using 15 N tracers at the growth temperature from solutions containing either nitrate, ammonium or glycine as single N sources and from a mixture containing all three N-forms. Estimates of the relative importance of amino acids such as glycine to the total N budget of plants will have been underestimated in studies where uptake was determined in single source solutions compared with those from solutions containing a mixture of N-forms. The proportion of total N acquired from the mixed N source as ammonium increased as temperature was reduced. Regarding the uptake and initial metabolism of glycine, uptake was probably the rate limiting step at 11 ∞ C whilst it was the metabolism of glycine to serine at 21 ∞ C. Although 15 N incorporation into the plant amino-N pool was generally in proportion to the abundance of individual amino acids, its incorporation into the glycine pool was sometimes significantly less than predicted. Key-words : nitrate, ammonium, glycine, amino acids, temperature, uptake, mixed N solutions.