Evidence for carbon flux shortage and strong carbon/nitrogen interactions in pea nodules at early stages of water stress

Symbiotic N₂ fixation in legume nodules declines under a widerange of environmental stresses. A high correlation betweenN₂ fixation decline and sucrose synthase (SS; EC 2.4.1.13) activitydown-regulation has been reported, although it has still tobe elucidated whether a causal relationship between SS activitydown-regulation and N₂ fixation decline can be established.In order to study the likely C/N interactions within nodulesand the effects on N₂ fixation, pea plants (Pisum sativum L.cv. Sugar snap) were subjected to progressive water stress bywithholding irrigation. Under these conditions, nodule SS activitydeclined concomitantly with apparent nitrogenase activity. Thelevels of UDP-glucose, glucose-1-phosphate, glucose-6-phosphate,and fructose-6-phosphate decreased in water-stressed nodulescompared with unstressed nodules. Drought also had a markedeffect on nodule concentrations of malate, succinate, and