Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/1999
Publication Date: N/A
Citation: Interpretive Summary: A key advantage of soybean is that the crop does not need to receive nitrogen fertilizer. Instead, soybean obtains nitrogen by means of symbiotic fixation of atmospheric nitrogen. Unfortunately, this fixation has been found to be very sensitive to soil water deficits resulting in lost fixation activity by virtually all soybean crops. The sensitivity to water deficits appears to be linked to an enzyme that breakdown the products of nitrogen fixation in the leaves. Low rates of enzyme activity are associated with decreased nitrogen fixation rates. The activity of this enzyme is dependent on the presence of the micronutrient manganese. The research by ARS-USDA scientists at Gainesville, FL shows that supplying the plant with abundant manganese results in increased enzyme activity, increased nitrogen fixation rates, and importantly, decreased sensitivity of nitrogen fixation to soil water deficits. This research opens the possibility that improved fertility of soybean with manganese can increase nitrogen fixation rates, and ultimately crop yield.
Technical Abstract: Water deficit is a serious constraint on N2 fixation rates and grain yield of soybean (Glycine max Merr.). Ureides are transported from the nodules and they accumulate in the leaves during soil drying. This accumulation appears responsible for a feedback mechanism on nitrogen fixation, and it is hypothesized to result from a decreased ureide degradation in the leaf. One enzyme involved in the ureide degradation, allantoate amidohydrolase, is manganese (Mn) dependent. As Mn deficiency can occur in soils where soybean is grown, this deficiency may aggravate soybean sensitivity to water deficit. In situ ureide breakdown was measured by incubating soybean leaves in a 5 mM allantoic acid solution for 9 h before sampling leaf discs in which remnant ureide was measured over time. In situ ureide breakdown was dramatically decreased in leaves from plants grown without Mn. To determine if Mn level also influenced N2 fixation sensitivity to water deficit, a dry-down experiment was done with plants grown under four different Mn nutritions. Plants receiving no Mn had N2 fixation more sensitive to water deficit than plants treated with Mn in which leaf Mn concentration was in the range of 21.0 - 32.7 mg kg 1.