Effects of short-term drought and rewatering on the activity of mitochondrial enzymes and the oxidative capacity of leaf mitochondria from a CAM plant, Aptenia cordifolia

Changes in the oxidative properties of isolated leaf mitochondria and in the activities of several mitochondrial enzymes, involved in CAM, were examined in response to a short-term drought (10 d) and to rewatering in the CAM plant Aptenia cordifolia. Integrity of the isolated organelles was always high (>90%) and further purification did not increase activity or intactness. To evaluate the influence of mitochondrial activities on CAM, variations in malic and citric acid accumulation, and in diurnal gas exchange were concomitantly followed in a controlled environment chamber study While daytime CO(2) uptake ceased within 2 days due to stomatal closure and nocturnal CO(2) uptake gradually diminished, malic and citric acid accumulation increased slightly during drought. The latter could not be due to enhanced mitochondrial oxidative activity, because its relative measure, the Cytochrome c (Cyt c) oxidase activity decreased reversibly during drought. In addition, complete malate oxidation may occur to only a minor extent. Malate oxidation activity largely depended on malic enzyme (ME) activity which was high in A. cordifolia ii compared with other CAM species. Drought induced reduction of glycine oxidation activity may result in a larger relative capacity for malate oxidation because competition for NAD would decrease and changes of in vivo malate oxidation rates could then meet the additional demand for oxidative capacity during drought. Mitochondrial sensitivity to cyanide was always high, while that: to rotenone was low, in particular when malate was used as a substrate. Increased net citric acid accumulation in response to drought may result from the larger inhibition of the citrate degrading system relative to citrate synthesis, although the capacity for citric acid oxidation and the citrate synthetase (CS) activity declined during drought. Changes in mitochondrial oxidative properties and enzyme activities, observed during drought, were reversible upon rewatering.