Influence of Gibberellic Acid and Copper Sulfate Spectral Filters on Growth of Chrysanthemums

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

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Introduction

Chemical growth regulators are extensively used in the ornamental horticulture industry to reduce plant height and maintain high quality plants during marketing. However, due to perceived risks to humans and the environment, the use of some chemical growth regualtors has been severely restricted and some are facing restrictions in the near future. These restrictions on the use of growth regulating chemicals on horticultural crops have tremendously increased the interest in the use of non-chemical alternatives for plant growth regulation. Both manipulation of day and night temperature and light quality in greenhouses have been proposed as non-chemical alternatives for controlling plant height. Clemson University research focusing on manipulation of greenhouse light quality as a non-chemical alternative for plant growth regulation indicated that copper sulfate (CuSO4) spectral filters induced a morphology similar to that of chemical plant growth regulators in a wide range of horticultural plants.

Chemical growth retardant effects are mostly mediated through the inhibition of gibberellin production, a plant hormone that induces stem elongation. Gibberellins play an important role in the control of stem elongation and internode length of many plant species. It has been suggested that the stem elongation in response to changes in light quality may be mediated by change in gibberellin level or sensitivity to gibberellins. Because of the similarities between the effects of chemical growth regulators and CuSO4 spectral filters, we hypothesized that the inhibition of gibberellin by CuSO4 spectral filters may be causing the height reduction. Therefore, we initiated experiments to investigate the influence of
Pro-Gibb, a gibberellin supplement, on CuSO4 spectral filter grown chrysanthemum plants to better understand the physiological basis of growth control by spectral filters. This research will provide vital information for crop management practices under altered light environments.

Material and Methods

Rooted 'Bright Golden Anne' chrysanthemum cuttings with 4 to 5 leaves were planted in 6 inch round plastic pots containing a commercial potting mixture and were allowed to establish for one week in a glass greenhouse before being transferred to the treatment chambers. Plants were fertilized once daily through irrigation with 200 ppm nitrogen from Peter's 20-20-20 fertilizer. The experiment was first conducted in spring (Feb.) and repeated once in summer (July) to evaluate the seasonal effects.

Growth chambers with sealed double layered polyacrylic panels as roofs were constructed in a glass greenhouse and the side walls of the chambers were covered with an inside-white and outside-black polyethylene sheet. The polyacrylic panels were filled with a 4% CuSO4 solution or water (control). Chrysanthemum plants were grown in the chambers for 4 weeks and received only the light filtered through water or CuSO4 solution.

Plants were sprayed to run-off with a solution containing 50 ppm GA3 from Pro-Gibb Plus and a surfactant on the day of transfer to the treatment chambers. The Pro-Gibb sprays were repeated on days 7, 14, and 21. The control plants in both spectral filter treatments were sprayed with water and the surfactant.

In another experiment, involvement of gibberellins in height reduction under CuSO4 filter was further investigated by treating plants with B-Nine, a compound known to inhibit gibberellin biosynthesis. Plants were sprayed to runoff with 3500 ppm B-Nine or 3500 ppm B-Nine followed by 50 ppm GA3 from Pro-Gibb prior to transfer to chambers. Leaves were allowed to dry for 30 min before Pro-Gibb application. Spray treatments were repeated after 14 days in the growth chambers.

Plant height and number of fully expanded leaves were recorded at weekly intervals for 4 weeks. The average internode length was calculated as height divided by the number of leaves. The leaf and stem dry weights were recorded at the end of the 4 week growing period.

Results and Discussion

Plants grown under the CuSO4 were 19% and 17% shorter during spring and summer seasons, respectively, than the plants grown under the control filter (Table 1). Weekly applications of Pro-Gibb increased plant height under control and CuSO4 filter, but the response under CuSO4 filter (76% increase in spring and 45% increase in summer) was greater that under control filter (54% increase in spring and 32% increase in summer). At the end of the 4 week period, the height of Pro-Gibb treated plants grown under CuSO4 filters was similar to Pro-Gibb treated control of plants.

The numbers of leaves per plant was not affected by the several filters or Pro-Gibb application in both seasons. The average internode length was reduced by 17% and 19% in spring and summer seasons, respectively, under the CuSO4 filters. Weekly applications of Pro-Gibb increased the internode length under control and CuSO4 filter (68% and 43% in spring and summer, respectively) was greater compared to control filter (51% and 31% in spring and summer, respectively). Plants treated weekly with Pro-Gibb in both control and CuSO4 chambers had similar internode lenghts at the end of the experiment. Light passing through CuSO4 filters reduced leaf dry weight (LDW) by 22% and 26% in spring and summer, respectively. Weekly Pro-Gibb applications in spring increased LDW by 20% under both CuSO4 filter and control filters. Pro-Gibb application did not signigicantly affect LDW during summer in both control and CuSO4 filter grown plants. The stem dry weight was reduced by 42% under CuSO4 filter both in spring and summer. Weekly Pro-Gibb applications increased the stem dry weight of plants grown under control and CuSO4 filters, but the response was greater under CuSO4 filter (115% in spring and 82% in summer) than control filter (94% in spring and 41% in summer).

B-Nine treatment reduced plant height under both control and CuSO4 filters, but the height reduction under control filter (32% ) was greater than that under CuSO4 filter (27% ) (Table 2). Average internode length was also reduced by B-Nine under both filters. The height reduction caused by B-Nine was fully prevented by the application of Pro-Gibb under control filter. However, under CuSO4 filter, plants treated with B-Nine followed by Pro-Gibb were taller than non-B-Nine-treated plants. Plants receiving B-Nine and Pro-Gibb in control chamber were taller (23% ) than the plants receiving the same treatment in CuSO4 chamber. Treatment with B-Nine reduced total leaf area and leaf size in plants grown under control and CuSO4 filters (Table 2). The reduction of leaf area caused by B-Nine could be alleviated by Pro-Gibb application.

B-Nine reduced total dry weight by 27% and 12% over non-treated plants grown under control and CuSO4 filters, respectively (Table 3). Treatment with B-Nine reduced leaf and stem dry weight of plants grown under CuSO4 and control filters, but the dry weight reduction caused by B-Nine was greater under control filter than that of the plants grown under CuSO4 filter. Pro-Gibb application increased leaf and stem dry weight of plants grown under control and CuSO4 filter.

Weekly Pro-Gibb applications reserved the height reduction caused by light passing through CuSO4 filter. Plants grown under CuSO4 filter responded more to Pro-Gibb application than the plants grown under control filter suggesting that sensitivity of plants to gibberellins was not lowered by light in CuSO4 chamber. It is possible that gibberellins became limiting in plants grown under CuSO4 filter due to reduced production or the decreased conversion from inactive to active forms even if plants were able to produce gibberellins. Currently we are investigating the changes in gibberellin levels of spectral filter grown plants to better understand how lightly quality manipulate plant growth and development.

Acknowledgments

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

Last Updated 2/1/97