Comparisons of ureide and acetylene reduction methods for estimating biological nitrogen fixation by glasshouse-grown cowpea

Abstract

In dry field conditions, obtaining estimates of biological nitrogen fixation using either the acetylene reduction assay or measurements of ureide concentrations in xylem sap is limited by the difficulties involved in digging out root-nodule systems or obtaining xylem sap. This study was conducted to determine whether ureide concentrations in extracts from stem tissues are as reliable for estimating rates of biological nitrogen fixation by cowpea [ Vigna unguiculata (L.) Walp.] as acetylene reduction assays or ureide concentrations in xylem sap. Glasshouse-grown cowpeas were subjected to different levels of NO 3 − (0, 2, 10 mM) in a basal nutrient solution to achieve different levels of nitrogen fixation. Both levels of added nitrogen (2 and 10 mM) reduced nodulation and nitrogen fixation as measured by the ureide methods and acetylene reduction assay. All methods used to estimate rates of nitrogen fixation exhibited similar seasonal trends but some differences were apparent among treatments. Hot-water extraction of ureides from dried stem tissue appears promising as a rapid technique that could be effective under dry field conditions where the other techniques are not effective. Relative ureide abundance, expressed as 100 × [ureide (N)]/[ureide (N) + NO 3 − (N) + NH 4 + (N)], in either xylem sap or stem tissue, provided approximate estimates of the extent to which cowpeas rely on biological nitrogen fixation. Values of 85–95% were obtained for plants relying totally on nitrogen fixation, whereas non-nodulated plants grown on NO 3 − had values of 3–12%.