The primary objective of this study was to characterize the effects of calcium chloride spray on the formation of stone cells and peroxidase activity in 'Niitaka' pear fruit. Calcium chloride (0, 0.3, 0.5, and 1.0%) was sprayed 4 times on selected 15-year-old 'Niitaka' pear trees at 10-day intervals with a handgun until runoff, beginning 20 days after full bloom (DAFB). The calcium content and peroxidase activity were determined in flesh extracted from harvested fruit. The distribution of stone

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... cells in the fruit flesh was determined via light microscopy, using phloroglucinol dye. The calcium contents of the leaves and fruit increased significantly in the fruit sprayed with 0.5 and 1% CaCl 2 , as compared to the control, or the fruit sprayed with 0.3% CaCl 2 . The stone cell contents in the fruit flesh decreased significantly when the fruit were sprayed with 0.5 and 1.0% CaCl 2 . Fruit treated with calcium chloride showed an increase in the rate of small stone cell cluster formation (< 200 µm 2 ), whereas the rate at which medium (200-400 µm 2 ) and large (400 µm 2 <) clusters formed was significantly reduced. Peroxidase activity in the fruit flesh increased substantially at 60 DAFB, and decreased with fruit development. Peroxidase activity was much higher in the non-treated fruit than in those treated with 0.5% CaCl 2 . The activities of bound and soluble peroxidase in the cell wall were lower in the fruit treated with 0.5% CaCl 2 than in the non-treated fruit. In our experiment, the stone cell content and stone cell size decreased significantly in fruit treated with calcium chloride, as the result of a reduction in lignification due to low peroxidase activity (soluble and bound).