Symbiotic N-2 fixation response to drought

Symbiotic nitrogen fixation is highly sensitive to drought, which results i n decreased N accumulation and yield of legume crops. The effects of drough t stress on N-2 fixation usually have been perceived as a consequence of st raightforward physiological responses acting on nitrogenase activity and in volving exclusively one of three mechanisms: carbon shortage, oxygen limita tion, or feedback regulation by nitrogen accumulation. The sensitivity of t he nodule water economy to the volumetric flow rate of the phloem into the nodule offers a common framework to understand each of these mechanisms. As these processes are sensitive to volumetric phloem flow into the nodules, variations in phloem flow as a result of changes in turgor pressure in the leaves are likely to cause rapid changes in nodule activity, This could exp lain the special sensitivity of N-2 fixation to drying soils. It seems like ly that N feedback may be especially important in explaining the response m echanism in nodules, A number of studies have indicated that a nitrogenous signal(s), associated with N accumulation in the shoot and nodule, exists i n legume plants so that N, fixation is inhibited early in soil drying. The existence of genetic variation in N-2 fixation response to water deficits a mong legume cultivars opens the possibility for enhancing N-2 fixation tole rance to drought through selection and breeding.