XVI International Botanical Congess

As environmental conditions influence plant growth, nutrient demand, and thus the N sink strength, is accordingly affected. There is evidence that the demand for symbiotically fixed N is involved in the regulation of symbiotic N2 fixation in legume nodules, including the observation that an extra supply of mineral N reduces the demand for symbiotically fixed N. An N feedback regulation is observed at the ecosystem and at the plant level. Plant physiological evidence for a feedback control of symbiotic N2 fixation are presented. The N feedback model of the regulation of symbiotic N2 fixation involves sensing of the plant>s N status through cycling amino acids in the nodules. Both nodulation and nitrogenase activity would be regulated in this way. Possible links to other mechanisms (e.g. variable nodule oxygen permeability) are discussed.