SEASONAL-CHANGES IN THE ACTIVITY OF MITOCHONDRIAL OXIDASES DETECTED BY THE TRADITIONAL INHIBITOR ANALYSIS IN DISKS CUT FROM MATURE SUGAR-BEET LEAVES

The rate of dark oxygen uptake by disks cut from the mature leaves of sugar beet (Bera vulgaris L.) during the first year of plant growth wa s measured by polarography in the presence of the respiration inhibito rs KCN and salicylhydroxamic acid (SHAM). Changes in the O-2 uptake by the leaf tissues during plant develop ment were insignificant and did not fall outside the range of 300-400 mu l O-2/(g fr wt h). At the ea rly stages of plant development (60-80-day-old plants), respiration wa s almost completely inhibited by KCN, and cytochrome oxidase was respo nsible for more than 80% of the total O-2 uptake by leaf disks during this period. Highly active cyanide-resistant and SHAM-sensitive respir ation brought about by the activation of a mitochondrial alternative o xidase (AO) was observed at the terminal stage of growth (120-day-old and older plants). As shown by inhibitor analysis, at the terminal sta ge of plant growth, potential AO activity amounted to 50-70%, and its contribution to respiration was 25-35% of total O-2 uptake by the tiss ue. There was no correlation between the activity of cyanide-resistant respiration and the ATP : ADP ratio in the tissue. The use of inhibit or analysis to evaluate AO activity and its participation in the respi ration of plant tissues in relation to recent data on the mechanisms r egulating the alternative oxidation pathway is discussed.