Abstract
Summary Isolated phyllodia of Kalanchoe tubiflora were treated in various ways to elucidate the involvement of cytoplasmic pH in crassulacean acid metabolism (CAM). Solutes were applied via the cut ends of phyllodia and distributed in the tissue by transpiration. Relative bulk cytoplasmic pH was determined using the 14 C-DMO technique. Large concentrations of the weak acid DMO (50 mM) reduced cytoplasmic pH and inhibited malic-acid mobilization during the light period. Conversely, weak bases increased cytoplasmic pH and inhibited malic-acid accumulation during the dark period. Fusicoccin had similar effects as the weak bases. Treatments inhibiting or preventing malate mobilization during the normal light period, such as prolonged darkness, DCMU or reduced turgor due to application of 200 mM mannitol, eliminated the changes of cytoplasmic pH characteristic of CAM leaf cells during the light period. The uncoupler and protonophore CCCP stimulated passive malic-acid efflux from slices of phyllodia, but other metabolic inhibitors were without effect. The results are discussed in terms of the influence of cytoplasmic pH on the control of the CAM rhythm.
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