Xylem-solute technique to measure N2 fixation by Phaseolus vulgaris L.: Calibration and sources of error

Abstract

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 vulgaris L. 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%.