MEASUREMENTS OF THE ENGAGEMENT OF CYANIDE-RESISTANT RESPIRATION IN THE CRASSULACEAN ACID METABOLISM PLANT KALANCHOE-DAIGREMONTIANA WITH THE USE OF ONLINE OXYGEN ISOTOPE DISCRIMINATION

Discrimination against O-18 during dark respiration in tissues of Kalanchoe daigremontiana, Medicago sativa, and Glycine max was measured using an on-line system that enabled direct measurements of the oxygen fractionation of samples in a gas-phase leaf disk electrode unit. Discrimination factors for cytochrome pathway respiration were 18.6 to 19.8 parts per thousand for all tissues. However, discrimination in cyanide-resistant respiration was significantly higher in green tissues (30.4-31.2 parts per thousand) compared with nongreen tissues (25.3-25.9 parts per thousand). Using these discrimination factors, the partitioning of electron transport to these pathways was calculated from measurements of discrimination in the absence of inhibitors. Changes in flux through the alternative pathway were measured during the light and dark phases of Crassulacean acid metabolism in leaf disks of K. daigremontiana. The flux of electrons through the alternative pathway was higher during deacidification than during the other phases of Crassulacean acid metabolism. The increase in alternative pathway electron flux accounted for all of the increased respiration in the light phase. Despite this increase, simultaneous measurements of malate concentration and respiratory flux confirm that only a small proportion of the total malate decarboxylation occurs in the mitochondria.