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Direct test for altered gas exchange rates in water-stressed soybean nodules

Davis, L.C., Imsande, J.
Annals of botany 1988 v.61 no.2 pp. 169-177
Glycine max, gas exchange, root nodules, water stress, acetylene
Water stress usually lowers the nitrogenase activity of soybean root nodules. This loss in activity might result from an increased barrier to nodular gas exchange, from a general reduction in biochemical function, or both. To test for the possibility of an increased barrier to gas diffusion, we measured the apparent lag time for initiation of acetylene reduction by intact soybean plants, both before and after water stress. Mild nodular water loss (i.e. 10% of fresh weight or less) seldom altered the apparent lag time, whereas severe water stress (20–40% f. wt loss) frequently produced a small increase in apparent lag time. Severe water stress also produced a large decrease (24%) in the external diameter of the nodules and a loss of the white lenticel traces. Water stress usually caused a decrease in the apparent Km for acetylene. The data do not suggest a large change in the diffusive resistance to acetylene of nodules subjected to water stress. Thus, the observed decrease in nitrogenase activity may result primarily from biochemical, rather than physical, changes. However, because of the relatively greater importance of gas-phase diffusion for oxygen entry, we cannot exclude the possibility of a large change in a small gas pathway that affects oxygen influx more than acetylene influx.