SENSITIVITY OF N2 FIXATION TRAITS IN SOYBEAN CULTIVAR JACKSON TO MANGANESE.

Authors: VADEZ, VINCENT - UNIVERSITY OF FLORIDA; Sinclair, Thomas

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2002
Publication Date: N/A
Citation: N/A

Interpretive Summary: Soybean is an important component of crop rotations because the plants can fix atmospheric nitrogen, and therefore, do not need to be fertilized with nitrogen. Unfortunately, nitrogen fixation in soybean has been found to be very sensitive to water deficits and there is a loss in activity with only modest soil drying. The cultivar Jackson was previously identified as having nitrogen fixation tolerance to soil drying. The objective of this research involving a USDA-ARS scientist at the Center for Medical, Agricultural and Vetrinary Entomology in Gainesville, FL, was to understand the basis of the tolerance of Jackson. Previous research indicated that the breakdown of the products of nitrogen fixation was a key in contributing to the sensitivity to soil drying. In most soybean, the nitrogen fixation products are broken down using an enzyme that requires manganese. While Jackson accumulates manganese at higher concentrations than most soybean cultivars, this did not explain its behavior. It was discovered that Jackson uses an enzyme to breakdown the nitrogen fixation products that does not require manganese. This alternate enzyme appears to be crucial to the nitrogen fixation tolerance of Jackson.

Technical Abstract: There is a large increase in leaf ureides, allantoin and allantoic acid, upon water deficit in N2 fixing soybean, which is likely to trigger a feedback inhibition of nodule activity. The degradation of ureide in the leaves appears to be a major factor associated with N2 fixation tolerance to water deficit. Since one of the possible enzymes responsible for allantoic acid degradation depends on Mn as a co-factor, we investigated the possibility that the tolerance of the cultivar Jackson may result from a superior ability to accumulate Mn. Indeed, Jackson was found to have higher leaf Mn concentrations than other genotypes. Ureide degradation rates in Jackson leaves with differing Mn concentrations were, however, insensitive to Mn concentration. These results indicate that ureide degradation likely did not involve Mn as a co-factor and that ureide degradation in Jackson appears to be catalyzed by an enzyme not requiring Mn as a co-factor. It was concluded that the capacity to maintain a high leaf Mn concentration in Jackson under sufficient Mn availability was a trait with no causal relation with N2 fixation tolerance to water deficit.