Relative ureide-N (%) = [4 x ureides/(4 x ureide
+ nitrate + amino-N)] x 100
Although direct measurements of these parameters were not conducted in this study, the significant reductions in ureides
between the PlusN and the ZeroN treatments (53.5% in the nonirrigated environment and 57.2% in the irrigated environment) clearly indicate a major effect on nodulation and/or nodule activity.
An earlier report suggested that chickpea exports ureides
as the main N compound resulting from [N.sub.2] fixation, but the exact concentrations were not reported (Pate and Atkins, 1983).
Other nonnodulating genotypes have subsequently been isolated and one study showed that, in fact, the nonnodulating mutants contained much lower concentration of ureide
in developing pods (Matsumoto et al., 1977).
An alternative method for estimating NDA has been developed for soybean (Herridge and Peoples, 1990) and other legumes (Herridge and Peoples, 2002) that use allantoin and allantoate (collectively referred to as ureides
) as their primary N-export product from nodules.
This sensitivity is not universal among grain legumes and appears to be a trait of those species that transport ureides
(allantoin and allantoic acid) from the nodules to the shoot (Sinclair and Serraj, 1995).
An increase in leaf ureides
(allantoin and allantoic acid), the major nitrogen compounds exported from soybean nodules, and a simultaneous decrease in nodule activity is a major feature of the drought-stress response.
It has been proposed that feedback inhibition of [N.sub.2] fixation may be in direct response to elevated concentrations of ureides
, the products of [N.sub.2] fixation, in the nodule (Streeter, 1993).
In the comparison of legume species, Sinclair and Serraj (1995) found that those species transporting N to the shoot in the form of ureides
, such as soybean, had [N.sub.2] fixation that was sensitive to soil drying while those transporting amides were quite tolerant.
Role of amides, amino acids and ureides
in the nutrition of developing soybean seeds.
During water deficit, ureides
accumulate in soybean shoots (deSilva et al., 1996; Serraj and Sinclair, 1996; Purcell et al., 1998; Serraj et al., 1999b).
Water deficit in soybean [Glycine max (L.) Merr.] results in the accumulation of the products of [N.sub.2] fixation (ureides
) in shoots, and this may lead to feedback inhibition of [N.sub.2] fixation.