Effect of Exogenous Plant Hormones on Parthenocarpy and Fruit Quality in Tropical Squash (Cucurbita moschata L.)

Abstract

Artificially occurring parthenocarpy can be induced by exogenous application of plant hormones and is useful in the fruit production of many fruit crops and fruit trees. In cucumber (Cucumis sativus L.), a model species in the Cucurbitaceae family, the plant hormones auxin, gibberellin, cytokinin, and brassinosteroids are known to induce parthenocarpy. In tropical squash (Cucurbita moschata L.), synthetic auxins are known to induce parthenocarpy, but the effects of gibberellin, cytokinin, and brassinosteroids are still unknown. In addition, there are few published reports on the quality of parthenocarpic fruits induced by plant hormones in tropical squash, and the effects of these hormones remain largely unknown. In this study, we examined the effects of gibberellin, cytokinin, and brassinosteroids on the parthenocarpy of tropical squash and characterized the parthenocarpic fruits induced by the plant hormones. First, we evaluated fruit set and development in unpollinated fruits of ‘Kogiku’, a tropical squash cultivar, treated with gibberellic acid (GA₃), a synthetic cytokinin-like substance, N-(2-chloro-4-pyridyl)-N′-phenylurea (CPPU), and brassinolide (BL). CPPU promoted parthenocarpy, but GA₃ and BL did not, showing that cytokinin works as an important factor for parthenocarpy in tropical squash. Next, we investigated the quality of parthenocarpic fruits from ‘Kogiku’ induced by a synthetic auxin, 1-naphthylacetic acid (NAA), and CPPU. Total soluble solids and myo-inositol of NAA-treated parthenocarpic fruit were higher than those of pollinated fruits, whereas fructose of NAA-treated parthenocarpic fruit and fructose and glucose of CPPU-treated parthenocarpic fruits were significantly lower than those of pollinated fruits. These results showed that parthenocarpy of tropical squash induced by auxin and cytokinin had differing effects on fruit quality.