In an attempt to manipulate plant nitrogen‐sink strength, various defoliation treatments were applied to white clover (Trifolium repent L. cv. Ladino) grown in a controlled environment. Nitrogenase activity and its oxygen limitation were measured as H; evolution in Ar:O2 using a flow‐through gas exchange system. An experiment to monitor the response of nitrogenase activity to various degrees of defoliation showed that the removal of up to 50% of the leaf are a had no effect on nitrogenase activity within 6 h. If more than 50% of the leaf area was removed. the nitrogenase activity decreased in relation to the loss of leaf area. This was accompanied by a corresponding increase in the O2 limitation of nitrogenase activity. In the experiment to determine the N‐sink strength after defoliation, the dry weight increase w as initially unaffected by the removal of 4% of the leaf area, whereas removal of 85% or 100% of the leaf area resulted in a dry weight loss for several days. The time course of nitrogen assimilation was similar to that of dry weight increase. This study provides substantial evidence that after a severe defoliation the nitrogen demand is temporarily restricted due to the lack of dry weight increase. Since the plant's ability to store organic nitrogen is very limited. it seems plausible that nitrogen assimilation in the still fully intact symbiotic system had to be down‐regulated. Consequently, to avoid ammonia toxicity. nitrogenase activity had to be reduced. Such an interpretation is supported by the fact that complete defoliation of nitrogen‐Starved plants caused a much milder decrease in nitrogenase activity compared to the decrease in plants well supplied with nitrogen. The present data are consistent with the hypothesis that after defoliation nitrogenase activity is adjusted in response to the reduced demand for symbiotically fixed nitrogen (nitrogen‐sink strength), It is proposed that such an adjustment could he made by a nitrogen feedback mechanism that regulates a variable oxygen‐diffusion barrier in the nodules.

Previous studies have shown that nitrogenase activity decreases dramatically after defoliation, presumably because of an increase in the Oz diffusion resistance in the infected nodules. It i s not known how this Oz diffusion resistance is regulated. l h e aim of this study was to test the hypothesis that current Nz fixation (ongoing flux of Nz through nitrogenase) is involved in the regulation of nitrogenase activity i n white clover (Trifolium repens 1.cv Ladino) nodules. We compared the nitrogenase activity of plants that were prevented from fixing Nz (by continuous exposure of their nodulated root system to an Ar:Oz [80:20] atmosphere) with that of plants allowed to fix Nz (those exposed to Nz:OZ, 80:20

No abstract

To test the hypothesis of an indirect or direct involvement of carbon metabolites i n the short-term regulation of nitrogenase activity, nodule O , permeability was manipulated either by defoliation or by varying rhizosphere O , partial pressure. In contrast to defoliation, a 50% reduction of the nodule O , permeability, due to adapting nodules to 40 kPa O,, had no effect on nodule sucrose concentration. Likewise, total concentrations of other carbon metabolites such as fructose, starch, i-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 i n nodules decreased significantly after the removal of 40% of the leaf area, which is known to have no effect on nitrogenase activity and O , permeability. During regrowth after a 1 nn% defoliation, nitrogenase activity could be increased at any time by elevating rhizospheric O , 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 O, permeability.

No abstract