Env Exp Biol (2014) 12: 187–197

Xylem-mediated channeling of nitrogen in broad bean (Vicia faba)

Abstract

Nitrogenous compounds in leguminous plants translocate in the form of ureides, allantoin and allantoic acid, the oxidation products of de novo purine synthesis, from the nodules to the aerial parts. The nodules are the main sites of ureides synthesis through the coordination of the plant-bacteria association. However, aspects related to the occurrence, localization and properties associated with the enzymes involved in the assimilation of ureides in shoot tissues have not yet been fully resolved. In this study, a modified and simplified automated analysis was used to determine allantoin concentration in plant xylem exudates. The total amount of ureides translocated to the aerial parts of faba bean (Vicia faba L.) plants was quantified by the stem sap extraction method using allantoin as the internal standard. Other parameters measured at different time intervals (from sowing to harvest) included shoot and root length, symbiotic parameters, plant biomass, and the nitrogen (N) status of the stem, leaves and nodules. Two rhizobial isolates (KR1 and MR2), isolated from Pisum sativum L. var. ‘Macrocarpon’ and Phaseolus vulgaris L. (cv. ‘Carioca 29’) plants, respectively, were selected from entirely different agro-climatic regions. MR2 accumulated more ureides (587.28 mg L^–1) than KR1 (573.33 mg L^–1) when assessed at harvest. Plants were harvested at regular intervals for dry matter and stem-extracted exudates. Results were insignificant (P > 0.05) for different inocula, shoot and root length and nodule N, but were significant (P < 0.05) for both rhizobial isolates during nodulation. The concentration of ureides, which were compared with total N concentrations in nodules, stems and leaves, were significantly different (P < 0.01). We conclude that the percentage of N in the form of ureides, however, does not always indicate the ability of the plant to symbiotically fix N₂.