Analysis of factors controlling dinitrogen fixation and nitrogen release in soybean using pod removal, stem girdling, and defoliation

Abstract

Abstract We studied the factors controlling nitrogen (N) release from roots and nodules of soybean (Glycine max L. Merr. cv. Kurosengoku). Plants were subjected to pod removal, stem-girdling and defoliation at the pod-filling stage and the effects on nitrogen fixation and release of nitrogenous compounds from the roots were monitored under water culture conditions. Nitrogen fixation was significantly reduced in all the treated plants. In spite of the reduction in the concentration of sugar in the roots and nodules of defoliated and stem-girdled plants, only the latter treatment promoted sustained exudation of total N and ureides. Pod removal did not affect the root and nodule sugar concentration, nor promote the release of nitrogenous compounds. Based on these results, it is suggested that low nitrogen fixation activity as well as low sugar concentration in root and nodules may not be the only factors responsible for N exudation.