Potassium Independence of Osmoregulated Oscillations of Malate2- Levels in the Cells of CAM -Leaves

Abstract

On an equivalent basis equal amounts of H + and malate 2- are stored and mobilized, respectively, in the diurnal rhythm of CAM (crassulacean acid metabolism). In intact plants of Bryophyllum daigremontianum this is independent of the levels of K + (between about 10 and 100 mM) and Cl - (between about 10 and 50 mM) in the leaves. Valinomycin, a K + ionophore, does not affect this rhythm. In salinity induced CAM in Mesembryanthemum crystallinum the stoichiometry of diurnal changes of H + : 1/2mal 2- of unity is even observed in the presence of very large levels of salt. Leaf slices of Bryophyllum daigremontianum obtained from leaves harvested at the end of the dark phase when leaves have large levels of malic acid, loose H + and malate 2- to external media, again with a stoichiometry of unity on an equivalent basis. The malic acid movement and the H + : 1/2mal 2- = 1 stoichiometry is independent of concomitant K + movements and of K + levels in the tissue. Valinomycin has no effect on these movements. Experiments with an uncoupler (FCCP) and with varied temperature suggest that loss to the medium is a passive process. It is concluded that the diurnal rhythm in H + and malate 2- levels and the associated movements of H + and malate 2- into and out of the vacuoles L ÜTTGE and B ALL , 1974a; L ÜTTGE et al., 1975), that 2H + + malate 2- transport at the tonoplast of CAM leaf cells is under osmoregulatory control. The results described here are compared with other osmoregulatory and turgor dependent processes where K + does play a role.