Measurement of intracellular and extracellular free calcium in apple fruit cells using calcium-selective microelectrodes

Abstract. Intracellular and extracellular free calcium was measured in cortical cells from apple fruit ( Malus domestica Borkh cv. Cox's Orange Pippin) using calcium-selective microelectrodes. It was difficult to position microelectrodes in the cytoplasm, and thus most intracellular measurements reflected vacuolar concentrations of about 0.6mol m⁻³ free calcium. Extracellular free calcium was measured in wells (0.2mm³) that had been cut through the epidermis of the fruit, then filled with osmoticum. Concentrations of free calcium in the sampling well varied between 0.02 and 1.3 mol m⁻³, and were related to the calcium content of the tissue. Functioning of the plasma membrane, and perhaps the supply of calcium for intracellular needs during fruit ripening and storage, may require mol m⁻³ concentrations of extracellular free calcium. Thus, it is suggested that low levels of extracellular free calcium in calcium-deficient fruit may be partly responsible for development of deficiency disorders.