Cotyledon expansion, effect

Pistillate flowers are usually formed earlier (at a lower node) when plants are grown under a short photoperiod. A similar but less striking effect can be induced by a brief (2- to 3-week) exposure of cucumber seedlings, beginning at cotyledon expansion, to a short photoperiod, followed by an extended photoperiod (3). 

Cotyledon Expansion. Cotyledon expansion is markedly stimulated by GA and by BR as well, but to a lesser extent the activity is about 1/100 that of GA4. On the other hand, IAA has practically no effect Dark-grown cucumber cotyledons have also been shown to expand when subjected to BR treatment (6). Since expansion of cotyledons and leaves of intact seedlings is a normal effect of gibberellin, in this case BR acts like gibberellin. 

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