Effects of Nitrate and Plant Development on the Abundance of Nitrogenous Solutes in Root‐Bleeding and Vacuum‐Extracted Exudates of Soybean 1

In the experiments reported, the relative contents of ureides [(ureide‐N/ureide‐N + nitrate N + a‐amino‐N) × 100] ofxylary exudates and the relative symbiotic dependence of nodulated soybean [ Glycine max (L.) Merr. ‘Bragg’] were compared using glasshouse‐grown plants supplied with various concentrations of nitrate ranging from zero to 8.0 raM. Xylary exudates were sampled from the plants by vacuum extraction of detached shoots or as root‐bleeding exudate. Symbiotic dependence for each of the NO − 3 treatments was calculated on the basis of total‐plant N and time‐integrated rates of nitrogenase (C 2 H 2 reduction) activities. Analyses of all samples of the two exudates for ureides, NO − 3 and α‐NH 2 ‐N and of samples from R2 plants of individual amino compounds revealed only minor differences between the two exudates in the composition of these compounds. Amino acids and amides contributed slightly more and ureides and NO − 3 slightly less to total N of vacuum‐extracted exudate relative to root‐bleeding exudate. The photosynthetic amino acids, alanine and serine, were more evident in stem exudate. Effects of NO − 3 supply and plant age on the relative contents of ureides were similar for both exudates. These data were combined with the data on the effects of NO − 3 supply on N 2 (C 2 H 2 ) fixation to describe standard curve relationships between the symbiotic dependence of the plants and the relative contents of ureides for each of eight phases of growth.