Sonja L. Maki, Venkat
R. Kambalapally, and Nihal C. Rajapakse
Department of Horticulture, Clemson University
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
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.
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.
We are grateful to Yoder Brothers for donating plant material and the
Clemson University Ornamentals Enhancement Program for financial support.