Action mode of calcium formate as a chemical thinner on Japanese pear flower

Abstract

The mechanism of flower thinning by calcium formate was investigated in the Japanese pear cv. Kosui (Pyrus pyrifolia Nakai). Calcium formate thinned 30-40% flowers when a 1% solution was applied to the pistils just before or 24 hours after pollination, whereas 1% calcium acetate and calcium lactate were ineffective. Both adhesion of pollen grains to stigmatic surface and tube elongation in the style were reduced by calcium formate application. Calcium formate at 0.05% or higher concentrations was a much stronger inhibitor on pollen germination in vitro compared to calcium acetate or calcium lactate. When the inhibitory effect of formic acid, acetic acid, and lactic acid in vitro on pollen germination was compared, formic acid was the strongest inhibitor, suggesting that the compound induces floral abscission by preventing fertilization through the action of formate. At harvest, there was little or no difference in size and sugar content between the treated and control fruits. Thus, calcium formate may be a promising chemical thinner for Japanese pear flowers by reducing the cost of hand thinning.