Free Ca2+in tissue 22+-selective electrodes

Calcium was measured with Ca2+ -selective electrodes in calciotrophic CAM plants. Several organic acid anions (citrate, isocitrate, malate, and malonate) were tested for their capacity to chelate Ca2+ in solutions at pH 4.8. Free Ca2+ was also calculated from the stability constants of the chelates at pH 4.0 and pH 6.0. The strongest chelator at pH 4.8 was citrate, reducing free Ca2+ from 10 to 0.5 mol m−3, while isocitrate and malate decreased ionized Ca2+ to 25% and 5O%, respectively. At pH 4.0 isocitrate is somewhat more effective than citrate. Malonate has only slight effects on free Ca2+ at pH 6.0. In tissue saps from field-grown species of Crassula sp., Crassula expansa, Aloe ramosissima, and Aloe pillansii, concentrations of total water-soluble Ca2+ ranged from 25 to 196 mol m−3. Measurements with Ca2+ -selective electrodes showed that ionized Ca2+ was reduced to 62–88% in the presence of isocitrate. Diurnal fluctuations in malate were less important for Ca2+ chelation, which was also true for the situation in greenhouse-grown plants of Kalanchoë daigremontiana, which were cultivated at different Ca2+ levels. Comparing the osmotic potentials measured in the tissue saps with those calculated from the concentrations of the different solutes gave further evidence for the chelation of Ca2+ by organic acid anions, since values for ψπusing only the free Ca2+ were much closer to the measured values of ψπ than those calculated with total water-soluble Ca2+.