Submitted to: Journal of Experimental Botany
Publication Type: Review Article
Publication Acceptance Date: 10/1/1998
Publication Date: N/A
Citation: N/A Interpretive Summary: Legumes are a key component of most agricultural systems because of their ability to symbiotically fix atmospheric nitrogen. Unfortunately, it has been discovered that nitrogen fixation activity in some legumes is very sensitive to soil drying. This paper, which is coauthored by an USDA-ARS scientist located at Gainesville, FL, is a review of various hypotheses that might explain this high sensitivity to decreasing soil moisture. It is concluded that water flow into and out of the nodules where the nitrogen fixation occurs, is the key to understanding this limitation on nitrogen fixation. Encouragingly, however, there is evidence of genetic variability for this trait so that improved cultivars might be developed that are not so constrained by decreasing nitrogen fixation activity when soils dry.
Technical Abstract: Symbiotic nitrogen fixation is highly sensitive to drought, which results in decreased N accumulation and yield of legume crops. The effects of drought stress on N2 fixation usually have been perceived as a straightforward physiological process acting on nitrogenase activity and involving exclusively one of three mechanisms: carbon shortage, oxygen limitation, or feedback regulation by nitrogen accumulation. These hypotheses are considered in the present review, together with the nodule water economy as putative mechanisms affecting N2 fixation in response to drought stress. The sensitivity of the nodule water economy to the volumetric flow rate of the phloem into the nodule offers a common framework to understand each of these mechanisms. It is argued that an integrated perspective based on the water economy of the plants and nodules is needed to understand the response of N2 fixation to drought. The existence of genetic variation in N2 fixation among legume species and cultivars opens the possibility for enhancing N2 fixation tolerance to drought through selection and breeding.