Increase in the concentrations of amino acids in the vascular tissue of white clover and white lupin after defoliation: an indication of a N feedback regulation of symbiotic N-2 fixation

摘要

Defoliation, particularly experienced by pasture and forage legumes, is associated with a fast and dramatic decrease in the nitrogenase activity (C2H2 reduction, H-2 evolution in Ar/O-2). Since the plant's N demand is temporarily reduced after defoliation, the nitrogenase activity may be down-regulated through a N feedback mechanism. To test this hypothesis, the concentrations of free amino acids in the xylem of white clover and in the phloem of white lupin were measured before and after defoliation. The concentrations of free asparagine, glutamine and aspartate were the highest of all the amino acids before defoliation. The concentrations of these amino acids increased three-fold in the xylem of white clover and two-fold in the phloem of white lupin within one to three hours after defoliation. The results are consistent with the concept of a N feedback mechanism that regulates nitrogen fixation in legume plants, in which free amino acids, cycling in the vascular tissue, act as trigger molecules.