Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch)

Abstract

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine.Pulse-feeding nodulated roots with`5N2 gas showed asparagine to be the principal nitorgen product exported from N2-fixing nodules. Maintaining root systems in an N2-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N2 fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of 'IN-nitrate, and a 'IN dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N2 or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N2. Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N2 and nitrate are being used in peanuts.Despite the importance of peanut (Arachis hypogaea L.) as an oil or protein seed crop, relatively little research has been directed at the assimilation and metabolism of nitrogen in the species.The classic study ofFowden (7) identified the unusual substituted amide, 4-methyleneglutamine, as the major form of nitrogen transported in xylem from roots to shoot and leaves of peanut seedlings, and a more recent study (23) confirmed this finding in showing that both 4-Megln2 and the free acid 4-methyleneglutamic acid were synthesized rapidly from cotyledon nitrogen following germination. Winter et al. (23)

Effects of total dietary nitrogen and nitrogen form on the development of xylophagous leafhoppers

“…Effects of secondary compounds and protein digestibility are thus minimized, as are confounding effects of variable water content (Slansky and Scriber, 1985). Xylem fluid not only contains the lowest nitrogen concentrations of any plant tissue (Mattson, 1980), but also has extremely unbalanced (high proportions of single amino acids) profiles of organic nitrogen (Peoples et al, 1986(Peoples et al, , 1991. Moreover, xylem fluid in relatively pure form can be easily extracted with a pressure bomb apparatus (Scholander et al, 1965, Andersen et al, 1989.…”

Carbon and nitrogen allocation in trees

“…Sugars, organic acids and amino acids are translocated from shoots to roots in the phloem, converted into organic nitrogen compounds and retranslocated back to shoots in the xylem (Dickson, 1979;Pate, 19130;. The amount and kind of organic nitrogen compounds translocated in xylem differ with plant species (Barnes, 1963;Pate, 1980), plant developmental stage, season of the year (Sauter, 1981;Tromp and Ovaa, 1985;Kato, 1986), the amount or kind of inorganic nitrogen available to roots (Weissman, 1964;Peoples et al, 1986) and perhaps other environmental factors. The two amides, asparagine and glutamine, are major transport compounds in trees and many other plants and move readily in both xylem and phloem (Bollard, 1958;Pate, 1980;Dickson et al, 1985;Schubert 1986).…”

Effects of Bradyrhizobium strain and host genotype, nodule dry weight and leaf area on groundnut (Arachis hypogaea L. ssp. fastigiata) yield

“…The second is that the combined nitrogen needed for reproductive growth should be provided in an appropriate form, which can only be provided by the nodules. Export of reduced nitrogen from groundnut nodules is mainly in the form of asparagine (Peoples et al, 1986). It could be Determinants of seed yield in groundnut 287 speculated that nodules are the specialised organelles for the provision of nitrogenous assimilates for reproductive growth.…”