Detection of Soil Organic Nitrogen in Xylem Sap Collected from Nonmycorrhizal Plants using an Immunological Technique

Abstract

Phosphate buffer–extractable organic nitrogen (PEON) is considered to be a ubiquitous high-molecular-weightnitrogen (N) compound in soil and may be an important N source for some plant species. We examined whether nonmycorrhizal plants qing-geng-cai (Brassica rapa var. chinensis) and spinach (Spinacia oleracea L. cv. Atras) could uptake PEON from the soilusing animmunoassay that was previously developed from a PEON determination method using anti-PEON IgG. Xylem sap analysis of spinach using an enzyme-linked immune sorbent assay (ELISA) showed that this plant hadhigh capabilities to take up PEON from the soil. Spinach and qing-geng-cai exuded substantial amounts of oxalate or citrate under inorganic N-deficient conditions, indicating exudation of organic acids is probably involved in PEON mobilization. Further results of immunohistochemical localization of PEON in the root section of qing-geng-cai revealed that intact or at least a fraction of PEON absorbed by the epidermis passes through the cortex cell and intercellular space. Our findings indicated that some specific plants have the ability to mobilize PEON by root exudation and take up mineralizable organic N by endocytic invagination.