N2 Fixation (C2H2) and Ureide Content of Soybeans: Environmental Effects and Source‐Sink Manipulations1

Abstract

The use of ureides as an indirect index of symbiotic N2 fixation requires an understanding of how their relationship is influenced by environmental factors. Source‐sink manipulation (leaf or pod removal) and environmental factors (time of day, light intensity, root anaerobiosis, and soil nitrate) were tested for their effects on acetylene reductiona ctivity (ARA) and ureide concentration in the shoot. In soybeans [Glycine max (L.) Merr.] there was no significant diurnal variation of stem ureide concentration. Inhibition of ARA by root anaerobiosis reduced ureide concentrations in both xylem sap and stem tissue. Ureide concentrations in xylem sap reflected decreased N2 fixation more rapidly than concentrations in stem tissue. The ureide content of stem tissue apparently responds more slowly to a reduction in ARA. Partial leaf removal or a reduction in light intensity reduced ARA. Defoliation increased concentrations of ureides in the stem. Loweringli ght intensity had no effect on stem ureide concentrations, but increased leaf ureide concentration. Pod removaldid not affect ARA but increased the concentrations of ureides in the stem. In the field, addition of 30 kg N ha‐1 had little influence on ARA or ureide content of young stem or first stem internodes. Higher nitrate levels (100 and 200 kg N ha‐1) greatly reduced both ureide concentration and ARA.