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To test the hypothesis of an indirect or direct involvement of carbon metabolites in the short-term regulation of nitrogenase activity, nodule O2 permeability was manipulated either by defoliation or by varying rhizosphere O2 partial pressure. In contrast to defoliation, a 50% reduction of the nodule O2 permeability, due to adapting nodules to 40 kPa O2, had no effect on nodule sucrose concentration. Likewise, total concentrations of other carbon metabolites such as fructose, starch, L-malate, and succinate tended to be differentially affected by the two treatments. Upon defoliation, carbon metabolites in roots responded in a manner similar to those in nodules. Sucrose concentration in nodules decreased significantly after the removal of 40% of the leaf area, which is known to have no effect on nitrogenase activity and O2 permeability. During regrowth after a 100% defoliation, nitrogenase activity could be increased at any time by elevating rhizospheric O2 partial pressure. Thus, during the entire growing cycle nitrogenase activity seems primarily oxygen limited. Changes in whole nodule sucrose pools after defoliation have to be viewed as secondary effects not necessarily linked to nodule activity. Whole-nodule carbon metabolites appear not to be determinants of nodule activity, either through direct metabolic involvement or through indirect effects such as triggering O2 permeability.

Whole-Nodule Carbon Metabolites Are Not Involved in the Regulation of the Oxygen Permeability and Nitrogenase Activity in White Clover Nodules