ABSTRACTThe activity of mitochondria and the pectic enzymes, pectinesterase (PE) and polygalacturonase (PG), isolated from tomato fruit was inhibited by the addition of high concentrations of calcium ions. These effects could explain the complete inhibition of ripening of whole tomato fruit obtained when infiltrated with substantial amounts of calcium. The addition of lower concentrations of calcium were found to stimulate mitochondrial and PE activity, but PG activity at pH 4.5 was inhibited by concentrations greater than 10−7