Utilising the ¹⁵N dilution technique the relationship between the proportion of N derived from N₂ fixation and relative abundance of ureides in xylem sap was evaluated for Phaseolus vulgarisL. cv. Mokcham during vegetative and reproductive development. In order to establish calibration curves for time integrated estimates of N₂ fixation, plants were raised in sand culture during the dry season in northern Thailand and continuously supplied with a N-free nutrient solution or the same solution amended with 0, 3, 6 or 9 mol m⁻³ nitrate. Large changes in plant dependence on N₂ fixation were concomitantly reflected by corresponding alterations in N solutes in xylem sap. Regression analyses of the data suggested high correlations between relative ureide content and N₂ fixation, but different slopes and line intercepts indicated the requirement for the use of calibration curves established for different phases of the development of the plant. Largest age related differences were noted between vegetative and reproductive development. Judging from 95% confidence limits, utilisation of appropriate calibrations can reduce errors of the technique to close to ±5%.

A second experiment, involving similarly cultivated plants exposed to different sources of mineral N, indicated an effect of ammonium on xylem sap composition. This implies that calibrations, in which N₂ fixation is regulated only by applications of various concentrations of nitrate, may lead to errors in situations where a major proportion of the plant available soil nitrogen fraction is present in the form of ammonium.