Manganese application alleviates the water deficit‐induced decline of N2 fixation

Abstract

ABSTRACTWater deficit is a very 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 mol m−3 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. At the plant level, allantoic acid application in the nutrient solution of hydroponically grown soybean resulted in a higher accumulation of ureide in leaves and lower acetylene reduction activity (ARA) by plants grown with 0 mol m−3 Mn than those grown with 6·6 mol m−3 Mn. Those plants grown with 6·6 mol m−3 Mn in comparison with those grown with 52·8 mol m−3 Mn had, in turn, higher accumulated ureide and lower ARA. To determine if Mn level also influenced N2 fixation sensitivity to water deficit, a dry‐down experiment was carried out by slowly dehydrating plants that were grown in soil under four different Mn nutritions. Plants receiving no Mn had the lowest leaf Mn concentration, 11·9 mg kg−1, and had N2 fixation more sensitive to water deficit than plants treated with Mn in which leaf Mn concentration was in the range of 21–33 mg kg−1. The highest Mn treatments increased leaf Mn concentration to 37·5 mg kg−1 and above but did not delay the decline of ARA with soil drying, although these plants showed a significant increase in ARA under well‐watered conditions.