Excretion of nitrogen assimilated from N2 fixed by nodulated roots of alfalfa (Medicago sativa)

Abstract

Root excretions into the medium of hydroponically grown N2-fixing alfalfa were studied by quantitative and qualitative measurement of nitrogenous compounds in frequently collected aliquots of the culture solutions. Considerable amounts of several nitrogenous compounds were excreted from the nodulated roots. Ammonia, glutamate, serine, alanine, and aspartate were the main compounds excreted and were similar in proportion to the soluble nitrogenous compounds in nodule and root tissues, with the exception of asparagine. Asparagine was present in very high concentrations in the plant tissues and also in the root xylem exudates destined for the shoots, but it was not detected in the solution bathing the roots. 15N2 was also supplied to the nodulated roots of alfalfa and the flow of the label was followed both in the plant tissues and in the culture solution. Results showed that the amounts of nitrogenous compounds excreted to the medium were related to their formation following N2 fixation and also indicated that the recently fixed nitrogen was the main source of the excreted N compounds.