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
