ISHS Acta Horticulturae 624: XXVI International Horticultural Congress: Elegant Science in Floriculture
UNLOCKING THE HORMONAL AND MOLECULAR CONTROLS OF FLOWERING |
| Author: | R.W. King |
| Keywords: | daylength, flowering, gibberellin, nutrient diversion, plant growth retardants, stem elongation |
| Abstract:
Highly sensitive measurements of gibberellin (GA) levels in the minute shoot apex in combination with studies of GA synthesis and catabolism has begun to unlock one of the oldest mysteries in plants, namely the regulation of flowering. At the onset of long day(LD)-induced floral evocation of the grass Lolium temulentum, levels of GA5 and GA6 double at the shoot apex. Days later there are larger (many-fold) increases in GA1 and GA4 content at the apex which, coupled with evidence from GA4 applications, shows these latter GAs act as secondary, late-acting LD stimuli involved with early inflorescence development. How GAs differ over time in their floral effectiveness reflects structure–specific inactivation by a GA 2-oxidase enzyme. However, the structure of a GA can also be chemically-hindered so that it is no longer a candidate for hydroxylation, a sparing role also evident on treatment with the synthetic plant growth retardant, Trinexapac ethyl. In many ornamental horticultural species, inactivating enzymes could be particularly important for preventing the promotion of growth and flowering by GA. There are however, some ornamentals and some tree species where GA inhibits flowering apparently by diverting nutrients from the shoot apex. For example, in the long-day plant Fuchsia x hybrida sucrose supply regulates its flowering because apex sucrose content and flowering increase in parallel with increase in light intensity in non-inductive short days (SD). However, applied GA inhibits flowering whether it is induced in LD or by high light intensities. By measuring shoot apex sucrose content we have now shown that GA inhibits early flowering response in LD by diverting assimilates from the apex and to the growing stem. Clearly, enhanced GA catabolism would be an advantage in these latter species and less catabolism would be an advantage in other species. Thus, a complete understanding of hormone–regulated flowering must accommodate such multiple and sometimes conflicting actions of GAs. | |
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