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Photocontrol of Gibberellin Metabolism in Situ in Maize ¹

Department of Biological Sciences, University of Lethbridge, Lethbridge, Alberta TIK 3M4 Canada, Plant Physiology Research Group, Department of Biology, University of Calgary, Calgary, Alberta T2N 1N4 Canada

Mature maize seeds were labeled with 10 to 100 pg per seed of [³H] gibberellins (GA) and [³H]GA glucosyl conjugate-like substances by feeding [³H]GA₂₀ of high specific activity (2.3 Curies per millimole) during seed maturation. The dry seeds, which contained 14% [³H]GA₂₀, 7% putative [³H]GA₁ and 78% [³H]GA glucosyl conjugate-like metabolites, were imbibed and germinated in the dark and under incandescent light. In both light and dark the proportion of [³H]GA conjugate-like metabolities declined (relative to that in the mature dry seeds) during imbibition and up to germination at hour 36. This decline was accompanied by increases in the proportions of [³H]GA₂₀ and putative [³H]GA₁ thereby indicating hydrolysis, which was greater in the dark than in the light. The proportions of [³H]GA conjugate-like substances in light-grown germinants were higher (121 and 141% of dark-grown) at 24 and 48 hour harvests and this statistically significant pattern was sustained up to 120 hours after imbibition. Conversely, the proportions of [³H]GA₂₀ and putative [³H]GA₁ were lower in the light-grown seedlings. Thus, during imbibition, hydrolysis (de-conjugation) of [³H]GA glucosyl conjugate-like substances apparently occurred, and occurred more rapidly in the dark than in the light. Subsequently, during germination the reformation of [³H]GA conjugate-like substances was less rapid in the dark than in the light. The observation that dark-imbibed seeds and dark-grown seedlings have higher proportions of putative free [³H]GAs, relative to [³H]GA conjugate-like substances, is consistent with the increased shoot elongation (etiolation) that occurs in dark-grown maize seedlings, and may indicate a homeostatic role for GAs and their conjugates in shoot elongation of maize germinants.

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