DARK TREATMENT EFFECTS ON NITROGEN-FIXATION AND ENZYMES ASSOCIATED WITH SCAVENGING HYDROGEN-PEROXIDE IN CLUSTERBEAN NODULES

Six-week-old clusterbean (Cyamopsis tetragonaloba Taub.) plants grown in pot culture house were subjected to a dark period of 18-66 h starti ng after 9 h exposure to natural light. Dark treatment brought about a rapid decline in N2-fixation with acetylene reduction activity (ARA) showing 54% decrease after 18 h and 80% after 66 h. This was accompani ed by a 12-21% decrease in nodule leghemoglobin and 11-15% decrease in total nodule soluble protein. Endogenously occurring reducing agents like ascorbic acid (ASC) and glutathione (GSH+GSSG) showed a decrease of 25-54% and 12-28%, respectively in the nodules under the dark treat ments. Enzymes associated with ascorbate-glutathione cycle like ASC-pe roxidase and GSSG-reductase also showed a decrease. Catalase showed a decrease parallel to the decrease in ARA but the reverse was true in t he case of peroxidase. Parallel decrease invoked by dark treatments in N2-fixation and enzymes involved in the defence against toxic O2 spec ies suggests a positive correlation between the two.