|Allen, Leon - Hartwell|
Submitted to: Plant Cell and Environment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/25/1998
Publication Date: N/A
Citation: Interpretive Summary: Several studies have demonstrated that symbiotic nitrogen (N2) fixation in legumes, and particularly soybean, is highly sensitive to soil drying. Early in the soil drying cycle, the rate of N2 fixation has been found to decrease. Since a major advantage of growing legumes is the N2 fixation capability, this loss in activity is of considerable concern. This study by scientists with the USDA, Agricultural Research Service in Gainesville, FL was designed to look at the possibility that the decline in activity might be related to the supply of photosynthate in the plant. Increasing photosynthesis rate by enriching the atmosphere around the plants with carbon dioxide (CO2) was the experimental approach used to increase photosynthate levels. As a result of the increased CO2, N2 fixation became very drought tolerant. The balance of carbohydrate and N compounds in the plant were changed as a result of the CO2 treatment. Not only do these results give important insights about the regulation of N fixation in response to soil drying, they have important implications about the relative advantage of legumes under global climate change. Legumes may be at even greater advantage under global climate change than originally hypothesized because of the shift in N2 fixation tolerance to soil drying.
Technical Abstract: The combined effects of carbon dioxide (CO2) enrichment and water deficits on nodulation and N2 fixation were analyzed in soybean [Glycine max (L.) Merr.]. Two experiments, were conducted in greenhouses, with plants exposed simultaneously to soil drying and to 360 or 700 mmol CO2 mol-1. Under drought-stressed conditions elevated [CO2] resulted in a delay in the decrease in N2 fixation rates associated with soil drying. The elevated [CO2] also allowed the plants under drought to sustain significant increases in nodule number and mass relative to those under ambient [CO2]. Drought stress was associated with, ureide concentration increase in all plant tissues. However, exposure to elevated [CO2] resulted in substantially less drought-induced ureide accumulation in leaf and nodule tissues. The large effect of elevated [CO2] on the decrease of ureide accumulation in the leaves indicated the importance of ureide breakdown in the response of N2 fixation to drought and of feedback inhibition by ureides on nodule activity. We suggest that future [CO2] increases are likely to benefit soybean production by increasing the drought tolerance of N2 fixation.