Induction of Parthenocarpy by Bordeaux Mixture and Its Use for Cultivation in Japanese Pear ‘Kosui’

Abstract

Since both artificial pollination and fruit thinning are necessary for marketable fruit production of Japanese pears (Pyrus pyrifolia (burm. f.) Nakai) in Japan, about a 20% fruit set is ideal without pollination because only 5% of flowers are actually subjected to fruit production. In this study, copper (Cu²⁺) and ferrous (Fe²⁺) ions were shown to be effective for inducing a nearly-ideal fruit set of the Japanese pear ‘Kosui’. Fruit induced by ferrous sulfate (FeSO₄) solution or a Bordeaux mixture, which is a combination of copper sulfate (CuSO₄), lime, and water, were parthenocarpic, because 1) self-pollen tube growth was not promoted by Cu²⁺ and Fe²⁺, 2) almost no perfect seeds were observed at harvest, and 3) Cu²⁺ and Fe²⁺ acted as strong inhibitors of pollen tube growth in vitro. The effective stage for inducing parthenocarpy was sprouting time to 4 days after anthesis in the Bordeaux mixture, and sprouting time to 4 days before anthesis in the FeSO₄ solution. Annual changes in the effectiveness were found in both chemicals, and the Bordeaux mixture showed no inhibitory effect on the fruit set of cross-pollinated flowers. The growth of Bordeaux mixture-induced fruit was improved by gibberellin (GA) paste or GA paste mixed with N-(N-(2-chloro-4-pyridy1)-N'-phenylurea (CPPU) treatment of the fruit stalk; the treated fruit was about 100 g heavier than the untreated fruit. The GA paste treatment is currently conducted to promote fruit growth and maturation in ‘Kosui’ fruit production in Japan, and the Bordeaux mixture can be substituted for chemical fungicide generally used before anthesis to control scab and black spot disease. Accordingly, the Bordeaux mixture is a promising chemical for great labor-saving in ‘Kosui’ cultivation.