It has been reported that parthenocarpic cucumbers with many female flowers and heavy fruit load often show fruit abortion and reduced fruit yield. To achieve the stable fruit production, it is necessary to elucidate the mechanism involved in fruit abortion in cucumbers via fruit load and endogenous plant hormones. In Exp. 1, the effects of exogenous plant growth regulators (PGRs) on yield and fruit growth were examined in a gynoecious, parthenocarpic cucumber (Cucumis sativus
L.). Four types of PGR [indole-3-acetic acid (IAA), 2,3,5-triiodobenzoic acid (TIBA), benzyl adenine (BA), and gibberellic acid (GA3
)], which have been shown to enhance fruit growth in non-parthenocarpic cucumbers, were applied using lanolin paste to the peduncles at each node (6–25) of cucumber plants at anthesis. TIBA and BA applications significantly decreased the rate of fruit abortion, thereby increasing yield. IAA and GA3
treatments increased the rate of fruit abortion at the middle and upper nodes, thereby reducing fruit yield. In Exp. 2, to clarify whether PGRs affected the concentrations of endogenous plant hormones, and whether IAA of high concentration increased fruit abortion under low-fruit-load conditions, we applied these PGRs (two concentrations of IAA, BA, and TIBA) to cucumber plants bearing only one fruit at node 11 or 12. No fruit abortion occurred following any PGR or control treatment, although both IAA and BA treatments inhibited fruit growth. BA application increased the production of cytokinins (Z, ZR, iP, and iPR) to a level similar to that after TIBA application one day after anthesis. However, compared with the response to TIBA treatment, after BA treatment, the peak in endogenous cytokinin production occurred at an earlier stage of fruit development. Additionally, both IAA and TIBA treatments increased endogenous IAA and cytokinin concentrations in fruit. These results suggest that exogenous PGRs affect the overall concentration of PGRs, as well as that of other endogenous plant hormones. However, PGRs may not be associated with fruit abortion when the fruit load is low. In conclusion, fruit load may have a greater influence on fruit abortion than that of PGRs.
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