Abstract
A difference in the dawn and dusk concentrations of free sulphate in leaf-cell sap, similar to the typical oscillation of titratable acidity in CAM plants, was observed in Fourcroya humboldtiana (Agavaceae) sampled in the succession of a cloud forest in Venezuela. Similar results were found in Clusia rosea and Clusia sp. (Clusiaceae), Tillandsia flexuosa (Bromeliaceae) and Subpilocereus ottonis (Cactaceae) growing in a natural oil spill. The concentration of free sulphate was related to the pH of the cell sap. The leaf-cell sap of F. humboldtiana dusk samples was titrated with malic acid to obtain the low pH usually present in the morning, and the sulphate concentrations were compared. An increase in the initial value of sulphate was observed, as found in the field in the dawn samples where the acidity is high. The presence of K 2 SO 4 stored mainly in the vacuole and its dissociation at low pH almost completely explains the daily oscillation of sulphate in F. humboldtiana. The salts MgSO 4 and CaSO 4 can also be dissociated, but the daily oscillations of soluble Mg and Ca are much lower than that of soluble K. In C. rosea, Clusia sp. and S. ottonis the fluctuation of sulphate is higher than the oscillation of cations and in C. rosea the oscillation of Ca is twice as much as that of K.
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