Author
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DENISON R FORD - UNIV. CALIFORNIA, DAVIS |
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Kinraide, Thomas |
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Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/10/1995 Publication Date: N/A Citation: N/A Interpretive Summary: Legumes (members of the pea family) are important to agriculture and the environment because they, unlike almost all other plants, can use nitrogen directly from the air for their nutrition. Other plants must receive nitrogen from other sources such as nitrate fertilizers. The atmospheric nitrogen is absorbed and converted to usable form in root structures known as nodules. When legumes encounter stressful conditions, such as drought, the nodules undergo changes that reduce the permeability of the nodules to nitrogen. These changes limit the uptake of nitrogen and, consequently, the growth of the plant. The factors that control the changes in nodule permeability are not understood. The present study indicates that the control of nodule permeability may be similar to the control of gas permeability in leaves, which is much better understood. An understanding of nodule permeability control will answer some interesting scientific questions and may allow the selection, breeding, or engineering of legume varieties that are better suited to some agronomic purposes. Technical Abstract: Such seemingly unrelated stresses as defoliation, drought, nitrate exposure, and elevated O2 all trigger a common response in legume root nodules -- rapid and reversible decreases in nodule O2 permeability (PO). Decreased PO may prevent inactivation of nitrogenase by O2 when respiratory O2 consumption in the nodule interior is limited by stress. Several possible mechanisms for permeability control have been proposed, but no supporting data have previously been presented that meet the requirements for both rapidity and reversibility. Stomatal regulation of gas permeability in leaves involves electrically driven fluxes of inorganic osmoticants, so we investigated the possibility of a somewhat similar mechanism in nodules. We used microelectrodes to monitor membrane potential in intact, attached nodules of Glycine max, Medicago sativa, Lotus corniculatus, and Trifolium repens, while controlling external O2 concentration and, in the case of Glycine max, measuring PO with a nodule oximeter. A 1 to 2 min exposure to 100 kPa O2 was found to induce rapid and reversible membrane depolarizations with exposure to elevated O2 in nodules of each species. This depolarization (which, to our knowledge, is unique to nodules) is accompanied by reversible decreases in PO in Glycine max nodules. An osmoelectrical mechanism for control of nodule gas permeability, consistent with these data, is presented. |
