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, w ere examined in response to a short-term drought (10 d) and to rewater ing in the CAM plant Aptenia cordifolia. Integrity of the isolated org anelles was always high (>90%) and further purification did not increa se activity or intactness. To evaluate the influence of mitochondrial activities on CAM, variations in malic and citric acid accumulation, a nd in diurnal gas exchange were concomitantly followed in a controlled environment chamber study While daytime CO2 uptake ceased within 2 da ys due to stomatal closure and nocturnal CO2 uptake gradually diminish ed, malic and citric acid accumulation increased slightly during droug ht. The latter could not be due to enhanced mitochondrial oxidative ac tivity, because its relative measure, the Cytochrome c (Cyt c) oxidase activity decreased reversibly during drought. In addition, complete m alate oxidation may occur to only a minor extent. Malate oxidation act ivity largely depended on malic enzyme (ME) activity which was high in A. cordifolia ii compared with other CAM species. Drought induced red uction of glycine oxidation activity may result in a larger relative c apacity for malate oxidation because competition for NAD would decreas e and changes of in vivo malate oxidation rates could then meet the ad ditional demand for oxidative capacity during drought. Mitochondrial s ensitivity to cyanide was always high, while that: to rotenone was low , in particular when malate was used as a substrate. Increased net cit ric acid accumulation in response to drought may result from the large r inhibition of the citrate degrading system relative to citrate synth esis, although the capacity for citric acid oxidation and the citrate synthetase (CS) activity declined during drought. Changes in mitochond rial oxidative properties and enzyme activities, observed during droug ht, were reversible upon rewatering.