CORRELATION BETWEEN XYLEM UREIDE LEVELS AND NODULATION IN FIELD-GROWN PHASEOLUS-VULGARIS

The relationships between the relative abundance of ureide-N in xylem sap and nodulation characteristics of navy beans (Phaseolus vulgaris) were examined in a sandy field site that enabled good nodule recovery. Three management treatments of N fertiliser, inoculation and no inoculation were used in the irrigated trial. Five cultivars were sown, and they were sampled 48 and 83 days later. Plant roots were scored visually for nodule size, colour, abundance and mass. Xylem sap was extracted from shoots under mild vacuum and assayed for ureide, nitrate and amino-N contents. Main treatment effects on nodulation were detected in the first but not the second sampling. Cultivar differences were found in the second sampling only. The relative abundance of ureide-N in xylem sap, measured as 100 x 4 x ureides/(4 x ureides + nitrate + amino-N), was low (< 20%) in the first sampling, with no differences between cultivars. Fertiliser N depressed the ureide levels. At the second sampling, relative ureide-N ranged from 30 to 40% but there were no differences between management treatments or cultivars. Correlations between relative ureide-N and nodulation characteristics (abundance, size, colour, mass) were significant for each sampling (48 days, r = 0.29-0.33; 83 days, r = 0.72-0.76). The results indicate that it is possible to use a ureide assay as an alternative to nodule scoring where nodule recovery is impracticable in field studies of P. vulgaris.