Manganese application alleviates the water deficit-induced decline of N-2 fixation

Water deficit is a very serious constraint on N-2 fixation rates and grain yield of soybean (Glycine max Merr,), Ureides are transported from the nodu les 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, i s manganese (Mn) dependent. As Mn deficiency can occur in soils where soybe an is grown, this deficiency may aggravate soybean sensitivity to water def icit. 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 i n which remnant ureide was measured over time, In situ ureide breakdown was dramatically decreased in leaves from plants grown without Mn. At the plan t level, allantoic acid application in the nutrient solution of hydroponica lly 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 in fluenced N-2 fixation sensitivity to water deficit, a dry-down experiment w as carried out by slowly dehydrating plants that were grown in soil under f our different Mn nutritions. Plants receiving no Mn had the lowest leaf Mn concentration, 11.9 mg kg(-1), and had N-2 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 c oncentration to 37.5 mg kg(-1) and above but did not delay the decline of A RA with soil drying, although these plants showed a significant increase in ARA under well-watered conditions.