THE NITROGEN-SINK IS INVOLVED IN THE REGULATION OF NITROGENASE ACTIVITY IN WHITE CLOVER AFTER DEFOLIATION

In an attempt to manipulate plant nitrogen-sink strength, various defo liation treatments were applied to white clover (Trifolium repens L. c v. Ladino) grown in a controlled environment. Nitrogenase activity and its oxygen limitation were measured as H-2 evolution in Ar:O-2 using a flow-through gas exchange system. An experiment to monitor the respo nse of nitrogenase activity to various degrees of defoliation showed t hat the removal of up to 50% of the leaf area had no effect on nitroge nase activity within 6 h. If more than 50% of the leaf area was remove d, the nitrogenase activity decreased in relation to the loss of leaf area. This was accompanied by a corresponding increase in the O-2 limi tation of nitrogenase activity. In the experiment to determine the N-s ink strength after defoliation, the dry weight increase was initially unaffected by the removal of 40% 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 af ter 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 nitrog en assimilation in the still fully intact symbiotic system had to be d own-regulated. Consequently, to avoid ammonia toxicity, nitrogenase ac tivity had to be reduced. Such an interpretation is supported by the f act that complete defoliation of nitrogen-starved plants caused a much milder decrease in nitrogenase activity compared to the decrease in p lants well supplied with nitrogen. The present data are consistent wit h the hypothesis that after defoliation nitrogenase activity is adjust ed in response to the reduced demand for symbiotically fixed nitrogen (nitrogen-sink strength). It is proposed that such an adjustment could be made by a nitrogen feedback mechanism that regulates a Variable ox ygen-diffusion barrier in the nodules.