Abscisic acid induces a decline in nitrogen fixation that involves leghaemoglobin, but is independent of sucrose synthase activity

Sucrose synthase (SS) activity has been suggested to be a key point of regu lation in nodule metabolism since this enzyme is down-regulated in response to different stresses which lead to decreased nitrogen fixation. In soybea n, a dramatic decline of SS transcripts has been observed within 1 d from t he onset of drought. Such a quick response suggests mediation by a signal t ransduction molecule. Abscisic acid (ABA) is a likely candidate to act as s uch a molecule as it mediates in a significant number of plant responses to environmental constraints. The hypothesis of ABA controlling nodule metabo lism was approached in this work by assessing nodule responses to exogenous ABA supply in pea. Under the experimental conditions, ABA did not affect p lant biomass, nodule numbers or dry weight, However, nitrogen fixation rate was reduced by 70% within 5 d and by 80% after 9 d leading to a reduced pl ant organic nitrogen content. Leghaemoglobin (Lb) content declined in paral lel with that of nitrogen fixation, SS activity, however, was not affected by ABA treatment, and neither were the activities of the enzymes aspartate amino transferase, alkaline invertase, malate dehydrogenase, glutamate synt hase, uridine diphosphoglucose pyrophosphorylase, isocitrate dehydrogenase, and glutamine synthetase, Nodule bacteroid-soluble protein content was red uced in nodules only after 9 d of ABA treatment. These results do not suppo rt the hypothesis that ABA directly regulates SS activity. However, they do suggest the occurrence of at least two different control pathways in nodul es under environmental constraints, which include ABA being involved in a L bl oxygen-related control of nitrogen fixation.