Dark-induced changes in legume nodule functioning

Exposure of nodulated leguminous plants to prolonged periods of continuous darkness has been used as a convenient tool to investigate host plant contr ol on nitrogen fixing systems in legume root nodules. Foliar dark treatment of plants results in a rapid decline in N-2-fixation in terms of acetylene reduction activity (ARA) and predisposes the nodules to metabolic and stru ctural senescence. After 2 d of darkness, a significant decrease is seen in nitrogenase (N-2-ase) proteins of common bean nodules. The effect of dark treatments on nodule respiration varies with plant species. A variable decr ease in nodule carbohydrates is observed in different plant species under d ark treatments, but no direct correlation between N-2-ase activity and gros s levels of carbohydrates present in the nodules has been detected. Usually nodule leghemoglobin (Lb) shows a decrease of variable intensity depending on plant species. The mRNA of Lb, sucrose synthase and glutamine synthetas e shows a significant decline within 24 h of complete darkness. Dark-induce d acceleration of proteolytic activity and decreased plant growth are refle cted in decreased nodule proteins and accumulation of free amino acids foll owing a drop in ARA. Antioxidants such as ascorbic acid and glutathione, al ong with the enzymes of their oxidation-reduction cycle, show a considerabl e decrease in their content and activity in nodules from dark-treated plant s. Among H2O2 scavengers, nodule catalase activity decreases in most of the plants studied, but peroxidase activity shows an increase. Dark-induced ad verse effects on N-2-fixation are completely or partially reversible on shi fting the plants back to a normal light/dark regime. Significant changes in nodule ultrastructure are induced by dark treatment. Attempts have been ma de to explain the mechanisms underlying dark-induced changes in nodule func tioning.