Recent Advances in the Physiology of Drought Stress Effects on Symbiotic N2 Fixation in Soybean

Abstract

Oxygen limitation, feedback regulation by ureide accumulation, and carbon shortage, are considered in the present research as three key mechanisms affecting nitrogen fixation in response to drought stress. The interaction of nodule permeability to O2 and drought stress on N2 fixation was examined in soybean nodules, and led to the overall conclusion that 02 limitation seems to be involved only in the initial stages of water deficit stresses in decreasing nodule activity. The involvement of ureides in the drought response of N2 fixation was initially suspected by an increased ureide concentration in shoots and nodules under drought leading to a negative feedback response between ureides and nodule activity. A direct evidence for inhibition of nitrogenase activity by ureides supported this hypothesis. The carbon shortage hypothesis was investigated by 14C labeling and by studying the combined effects of CO2 enrichment and water deficits on nodulation and N2 fixation in soybean. Under drought, in a genotype with drought tolerant N2 fixation, approximately four times the amount of 14C was allocated to nodules compared to a drought sensitive genotype. It was found that an important effect of CO2 enrichment of soybean under drought was an enhancement of photoassimilation, an increased partitioning of carbon to nodules, whose main effect was to sustain nodule growth, which helped sustain N2 rates under soil water deficits. We conclude that all three mechanisms are important in understanding the response of N2 fixation to soil drying.