Effects of Water and Turgor Potential on Malate Efflux from Leaf Slices of Kalanchoë daigremontiana

Malate efflux from leaf cells of the Crassulacean acid metabolism plant Kalanchoë daigremontiana Hamet et Perrier was studied using leaf slices submerged in experimental solutions. Leaves were harvested at the end of the dark phase and therefore contained high malate levels. Water potentials of solutions were varied between 0 and -5 bar using mannitol (a slowly permeating solute) and ethylene glycol (a rapidly permeating solute), respectively. Mannitol solutions of water potentials down to -5 bar considerably reduced malate efflux. The slowly permeating solute mannitol reduces both water potential and turgor potential of the cells. The water potential of a mannitol solution of -5 bar is just above plasmolyzing concentration. Malate efflux in ethylene glycol at -5 bar was only slightly smaller than at 0 bar, and much higher than in mannitol at -5 bar. Tissues in rapidly permeating ethylene glycol would have turgor potentials similar to tissues in 0.1 mm CaSO(4). The results demonstrate that malate efflux depends on turgor potential rather than on water potential of the cells.