Translocation of Nitrogenous Compounds in Symbiotic and Nitrate-Fed Amide-Exporting Legumes

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Abstract
Peoples, M. B., Sudin, M. N. and Herridge, D. F. 1987. Translocation of nitrogenous compounds insymbiotic and nitrate-fed amide-exporting legumes.–J. exp. Bot. 38: 567–579. The transport of nitrogen from the roots and nodules of chickpea (Cicer anetinum L.), lentil (Lens culinaris Medic), faba bean (Vicia faba L.) and pea (Pisum sativum L.) was examined in glasshouse-grown plants supplied either with nitrate-free nutrients or with nutrients supplemented with 1,2,4 or 8 mol m-3153N-nitrate. A sixth treatment comprised uninoculated plants supplied with 8–0 mol m-315 13N-nitrate. For each species, more than 75% of the nitrogen was exported from the nodules as the amides, asparagine and glutamine. In fully symbiotic plants, the amides also dominated nitrogen transport to the shoot When N2 fixation activity was decreased by the addition of nitrate to the rooting medium, the N-composition of xylem exudate and stem solutes changed considerably. The relative concentrations of asparagine tended to increase in the xylem whilst those of glutamine were reduced; the levels of nitrate increased in both xylem exudate and the soluble nitrogen pool of the stem with a rise in nitrate supply. The changes in relative nitrate contents reflected generally the contributions of root and shoot to overall nitrate reductase activity at the different levels of nitrate used. The relationships between the relative contents of xylary or stem nitrate and amino nitrogen and the plants' reliance on N2 fixation (determined by the 15N isotope dilution procedure) were examined. Data suggest that compositional relationships based on nitrate may be reasonable indicators of symbiotic dependence for all species under study except faba bean when greater than 25% of plant nitrogen was derived from N2 fixation.