A proposed role for copper ions in cell wall loosening

The measurement of N₂ fixation by legumes is necessary for gaining an understanding of their contributions to the N economies of agricultural and forestry systems and for their management in those systems. We report research to determine whether N₂ fixation of four of the commonly-grown ureide-producing legumes, soybean (Glycine max), cowpea (Vigna unguiculata), mungbean (V. radiata) and black gram (V. mungo), could be quantified from a single sampling and N-solute analysis of xylem sap. Data were derived from a previously-published experiment involving six genotypes of soybean at five field sites and from a second, irrigated experiment in which two genotypes of soybean, and one each of cowpea, mungbean and black gram were assessed in low- and high-nitrate soils for nodulation, yields of shoot and grain dry matter and N, and N₂ fixation using xylem solute (ureide) and ¹⁵N methods. Regression analysis of the published soybean data set indicated that the early pod-fill (R3.5 and R4) samplings for xylem sap gave estimates of percentage of plant N derived from N₂ fixation (%Ndfa) which agreed well with %Ndfa for the entire growing season obtained from ¹⁵N analysis of the shoots at R6-7. There was a marginal benefit in combining the R3.5 and R4 samplings and using the average of the two, with regression coefficients (r ²) increasing from 0.86 (R3.5 or R4 alone) to 0.92 (average of R3.5+R4). There was no additional benefit in combining R3, R3.5 and R4. In the second experiment, agreement between ¹⁵N-determined %Ndfa and either measured (R4 sampling) or calculated ureide-determined %Ndfa (R3.5 sampling) was also good (r ² of 0.73 (R4) and 0.79 (R3.5)). We conclude that seasonal %Ndfa can be accurately estimated using the xylem solute (ureide) method from a single sampling of xylem sap during early pod-fill (R3.5) and that this simplification of the protocol of the technique may encourage expanded use.