Free Ca2+ in tissue 22+ -selective electrodes

Calcium was measured with Ca²⁺ -selective electrodes in calciotrophicCAM plants. Several organic acid anions (citrate, isocitrate,malate, and malonate) were tested for their capacity to chelateCa²⁺ in solutions at pH 4.8. Free Ca²⁺ was also calculated fromthe stability constants of the chelates at pH 4.0 and pH 6.0.The strongest chelator at pH 4.8 was citrate, reducing freeCa²⁺ from 10 to 0.5 mol m^–3, while isocitrate and malatedecreased ionized Ca²⁺ to 25% and 5O%, respectively. At pH 4.0isocitrate is somewhat more effective than citrate. Malonatehas only slight effects on free Ca²⁺ at pH 6.0. In tissue sapsfrom field-grown species of Crassula sp., Crassula expansa,Aloe ramosissima, and Aloe pillansii, concentrations of totalwater-soluble Ca²⁺ ranged from 25 to 196 mol m^–3. Measurementswith Ca²⁺ -selective electrodes showed that ionized Ca²⁺ wasreduced to 62–88% in the presence of isocitrate. Diurnalfluctuations in malate were less important for Ca²⁺ chelation,which was also true for the situation in greenhouse-grown plantsof Kalancho daigremontiana, which were cultivated at differentCa²⁺ levels. Comparing the osmotic potentials measured in thetissue saps with those calculated from the concentrations ofthe different solutes gave further evidence for the chelationof Ca²⁺ by organic acid anions, since values for using onlythe free Ca²⁺ were much closer to the measured values of thanthose calculated with total water-soluble Ca²⁺. Key words: Calciotrophic plants, CAM, organic acids, free Ca²⁺, Ca²⁺ chelation