Physiological effects of cotyledons on gibberellin-induced cucumber hypocotyl elongation

The cotyledon requirement for gibberellin-induced hypocotyl elongation of light-grown cucumber (Cucumis sativus L. cv. Aonaga-jibae) seedlings was studied. Gibberellin A7 was used. At the saturation dosage, gibberellin-induced elongation was roughly proportional to the amount of the cotyledon retained. The cotyledon dependence of gibberellin-induced elongation was most marked when decotylization was done at the time of gibberellin treatment or at 24 hr after it. However, when the time of decotylization was delayed by more than 24 hr after gibberellin treatment, the effect was not immediate, and gibberellin-induced elongation continued for a while at a higher rate than that of the control. Young hypocotyls were more sensitive to gibberellin than older hypocotyls and their cotyledon requirement for gibberellin-induced elongation was less. Covering the cotyledons with aluminum foil or treating them with N'-(3,4-dichlorophenyl)-N-methoxy-N-methylurea, a photosynthetic inhibitor, reduced the gibberellin effect. There was a quantitative relationship between the magnitude of the covered or the inhibitor-treated area of the cotyledon and gibberellin-induced elongation. This quantitative relationship paralleled that for decotylization. These results suggested that the cotyledon requirement of the hypocotyl for gibberellin-induced elongation is at least partly related to the photosynthetic activity of the cotyledons.