These studies were conducted to investigate the effect of plant growth regulators (PGRs) on the level of endogenous hormones such as IAA, ABA, gibberellin-like substances, t-ZR and DHZR for elucidating the mechanism of parthenocarpy induced by exogenous growth regulators in cucumber. Application of auxin transport inhibitors, N-1-napthylphthylamic acid (naptalam) and 2,3,5-triiodobenzoic acid (TIBA) to the ovary or peduncle significantly increased the IAA content in the ovary. The rate of IAA/ABA in pollinated or naptalam and TIBA treated ovaries was also higher than that in unpollinated control. The unpollinated ovaries of genetically parthenocarpic ´Pandex´ showed 92% fruit set, whereas the non-parthenocarpic ´Khira´ failed to set fruit and their hybrid set only 8%. Application of auxins, NAA and 4-CPA, GA₃, cytokinins, BA and N-(2-chloro-4-pyridyl)-N´-phenylurea (CPPU) to the ovary at anthesis, however, induced over 60% parthenocarpic fruit set in ´Khira´ and F, hybrid. Exogenously applied all PGRs such as NAA, 4-CPA, GA₃, BA and CPPU increased the IAA content in the ovary, but did not increase the content of endogenous gibberellin-like substanse, trans-zeatin riboside (t-ZR) and dihydrozeatin riboside (DHZR).
From these results we assume that exogenously applied PGRs, not only synthetic auxins but also gibberellin and synthetic cytokinins, increase the content of IAA in the ovary and its accumulated IAA may trigger the parthenocarpic fruit set in cucumber.
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