Gibberellin Physiology of Spectral Filter Grown Chrysanthemum Plants

Sonja L. Maki, Venkat R. Kambalapally, and Nihal C. Rajapakse
Department of Horticulture, Clemson University

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Plants develop differently under different light environments and detect changes in light quality through the phytochrome system. For example, when the red: far-red ratio is high, plants display a light-exaggerated phenotype with shortened internodes, increased anthocyanin production and darker green leaves. In contrast, plants grown in a low red:far-red light environment display a shade-avoidance growth pattern with elongated internodes and reduced leaf area. The change in plant form due to different light environments is thought to involve changes in plant hormone levels. Current research suggests that the plant hormone gibberellin is a mediator between environmental signals, such as light, and plant growth responses such as internode elongation and flowering. Research at Clemson has shown that plants grown under CuSO₄ filters are similar in appearance to plants treated with growth retardants. The reduction of growth observed under CuSO₄ filters can be reversed by gibberellin application. Since many of the growth retardants target enzymes involved in gibberellin production, it is of intrest to determine whether the reduction of growth under CuSO₄ filters is related to gibberellin physiology.

We have begun investigations comparing the gibberellin level of control and spectral filter grown 'Bright Golden Anne' chrysanthemum plants. Chrysanthemum has served as the model plant for characterization of plant grown under spectral filters. To date, there are approximately 100 different gibberellins which have been identified as natural plant products and different species possess a specific set of gibberellins. One major pathway results in the production of GA₁ (Fig. 1), a gibberellin important for stem elongation.

GA₁₂ --> GA₅₃ --> GA₁₉ --> GA₂₀ --> GA₁

Figure 1. A common pathway of gibberellin biosynthesis operating in plants. GA₁₂, a gibberellin synthesized early in the pathway, is successively oxidized to the biologically active GA₁.

Since not much is known about chrysanthemum gibberellins, an initial characterization of the naturally-occurring gibberellins was undertaken to determine which gibberellins were present in chrysanthemum plants. A bioassay of chrysanthemum leaf extract, which had been fractionated by high performance liquid chromatography, revealed gibberellin activity in fractions where a standard of GA₁₉ was found, suggesting the presence of the pathway which produces GA₁(Fig. 2). The presence of GA₅₃, GA₁₉, GA₂₀, and GA₁, was confirmed by subjecting purified samples of chrysanthemum apices to gas chromatography-mass spectrometry. Identification of this pathway in chrysanthemum makes it possible to measure levels of each gibberellin in different plant parts under control and CuSO₄ filters. Ongoing experiments are focused on determining gibberellin levels in the apices and young internodes of chrysanthemum plants growing under control and CuSO₄ filters since it had recently been shown that these are sites of the enzymes which synthesize gibberellins.

Acknowledgments

We are grateful to Yoder Brothers for donating plant material and the Clemson University Ornamentals Enhancement Program for financial support.

Last Updated 2/1/97