Sap was collected at the pod-fill stage by Vacuum Extraction Method and stored in the freezer at -15degC, then concentrations of ureides
, nitrate and amino-N were determined to calculate the relative ureide- N (%) and percent nitrogen derived from atmosphere (% Ndfa) by the following formula:
Soybean [N.sub.2] fixation estimates, ureide
concentration, and yield responses to drought.
However, a recent study showed that ureide
metabolism was also involved, because urea was found to be a product of ureidoglycolate degradation in different organs of chickpea (Munoz et al., 2001).
The procedure for ureide
analysis was as described by Young and Conway (1942) with minor modifications (Streeter, 1979).
From a plant-breeding standpoint, the leaf color method used in this study for identifying promiscuous genotypes would be most rapid and least expensive compared with other frequently used methods such as acetylene reduction (Sinclair and Serraj, 1995; Giller, 1987; Chowdhury and Doto, 1981) or xylem ureide
assay (Herridge and Rose 1994; Kueneman et al., 1984; McClure et al., 1980).
concentration is multiplied by four because there are four N atoms in the ureide
On the basis of responses to Mn, Vadez and Sinclair (2000) subsequently concluded that these two cultivars, in fact, employed differing catabolic enzymes as originally reported, and that there was genetic variation in the Mn requirement involved in ureide
Several studies have confirmed the tolerance of [N.sub.2] fixation in Jackson to water deficits and have shown that leaf ureide
concentrations in Jackson are relatively low under water deficits (Serraj and Sinclair, 1996a; Purcell et al., 1997).
We tested the hypothesis that large nodules in combination with lower leaf ureide
levels would confer drought-tolerant [N.sub.2] fixation in the cultivar Jackson relative to the drought-sensitive cultivar KS4895.
In their study, Jackson also was found to have the lowest level of xylem-sap ureide
concentration among eight cultivars that were compared.
The increased shoot ureide
concentration during drought (deSilva et al., 1996; Serraj and Sinclair, 1996; Purcell et al., 1998) may serve as a feedback signal to nodules to decrease [N.sub.2] fixation (Purcell and Sinclair, 1995).
[plant.sup.-1] mg [nodule.sup.-1] Jackson 93 3.71 ** KS4895 158 ** 2.18 avg ([dagger]) 132ns 3.40 * avg 119 2.50 Shoot ureide
Acetylene reduction Cultivar Conc.