End of Day Exposure to Far-Red Light Reverses
the Height Reduction of Chrysanthemum Induced
by Copper Sulfate Spectral Filters.
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Introduction
Our research indicated that copper sulfate (CuSO4) spectral
filters reduced plant height and internode length, increased leaf chlorophyll
content and produced compact plants similar to plants treated with chemical
growth regulators. The reduction in plant height and internode length under
CuSO4 filters was attributed to the removal of far-red (FR) wavelengths
of transmitted light and the increased amount of red (R) light received
by the plant relative to FR light. Stem elongation in response to the change
in light quality is regulated by phytochrome, a plant pigment that is capable
of recognizing R and FR light in the environment.
The pigment, phytochrome, has two light absorbing forms and is capable of
absorbing both R and FR light. The red absorbing (or Pr ) form
of phytochrome absorbs R light and is converted to the far-red absorbing
(or Pfr ) form which absorbs FR light. Upon absorption of FR
light, it can convert back to Pr form. The Pfr form
is assumed to be the active form in signaling the plant to respond to the
changes in the environment and, therefore, the amount of Pr at
the beginning of the dark period is thought to play a critical role in regulating
plant morphology such as height. End of the day (EOD) exposure to R or FR
light can therefore, influence plant morphology by altering the amount of
Pfr present at the beginning of the dark period. Similarly, light
quality in the growing environment can affect plant morphology by affecting
the amount of Pfr present. For example, plants grown under a
forest canopy receive high amounts of FR light due to the absoprtion of
R light by the tall plants. Mutual shading by adjacent plants can have a
similar effect under dense planting conditions in the field or in the greenhouse.
Under such conditions, plants tend to grow taller due to high FR light and
the resulting high amount of red absorbing (inactive) phytochrome in the
plant.
In the present investigation, we were interested in understanding the influence
of EOD exposure to R and FR light on plants grown under CuSO4
and control filters to gain knowledge on the involvement of phytochrome
in regulating plant response under these filters. This basic understanding
will provide useful information for the development of crop management practices
if/when light manipulation becomes a reality under commercial operations.
Materials and Methods
Uniformly rooted 'Bright Golden Anne' (BGA) and 'Spears' shoot cuttings
with 3-4 leaves were planted in 4.5 inch square plastic pots containing
a commercial potting mix. Plants were allowed to establish, as single-stem
plants, in a greenhouse under natural photoperiod (average 12 h light and
12 h dark) for ten days before being subjected to the treatments. All plants
were fertilized once daily with 200 ppm nitrogen from Peter's 20-20-20 fertilizer
through irrigation.
After establishment period, plants were transferred to growth chambers with
6% CuSO4 or water (control) "fluid roofs" (spectral
filters). The chambers were placed inside a glass greenhouse to receive
sun light. Plants received a 9 h light and 15 h dark period. At the end
of the light period, plants were exposed to R light (from six 40W cool-white
fluorescent bulbs filtered through a red acetate filter) or FR light (from
two internal-reflector incandescent bulbs filtered through polyacrylic sheet
of cast acrylic dark red filter for 15 min in specially designed light treatment
boxes). Following EOD-light treatment, plants were kept in these boxes for
the remainder of the night period. EOD light treatment was given for 21
days. Non-EOD-treated plants remained in growth chambers and were covered
with a black cloth at the end of the light period.
Plant height (height from soil level to apical bud) and number of fully
expanded leaves were recorded weekly (for three weeks for 'Spears' and four
weeks for 'BGA'). Average internode lenght was calculated as plant height/number
of leaves. Dry weights of stems and leaves were measured at the end (21
days and 28 days for 'Spears' and 'BGA', respectively) of the experiments.
Results and Discussion
Copper sulfate filter reduced final height of 'BGA' and 'Spears' plants
by 28% and 21%, respectively (Table
1).
EOD-R light significantly reduced height of 'BGA' plants (11%) grown under
control filter but had no effect on height under the CuSO4 filter.
EOD-FR light had no effect on height of 'BGA' plants grown under control
filter; but under CuSO4 filter EOD-FR light resulted in a 39%
height increase. Final height of EOD-FR light treated 'BGA' plants grown
under CuSO4 filter was similar to the height of non-EOD treated
plants grown under control filter. EOD-R light did not reduce height of
'Spears' plants grown under either filter. EOD-FR light increased height
of 'Spears' plants under both filters.
CuSO4 filter or EOD light treatment did not significantly alter
number of leaves in 'BGA' or 'Spears' plants (Table
1). EOD-FR light significantly increased internode lenght of 'BGA'
and 'Spears' plants grown under CuSO4 and control filters. However,
internode elongation of 'BGA' plants in response to EOD-FR light was greater
under CuSO4 filter (i.e. 11% increase under control and 36% under
CuSO4 filter). In 'Spears' plants the response was similar under
both filters. Average internode length of 'BGA' plants exposed to
EOD-FR under CuSO4 filter was similar to that of non-EOD treated
plants grown under control filter.
EOD-R light did not significantly reduce internode length of 'BGA' or 'Spears'
plants under either filter.
CuSO4 filter reduced leaf and stem dry weights of non-EOD treated
'BGA' plants (Table 2); but
in 'Spears' plants, only stem dry weight was reduced by CuSO4
filter. EOD-R and EOD-FR light significantly reduced leaf dry weight of
'BGA' plants grown under control filter, but had no effect on leaf dry weight
under CuSO4 filter. EOD-R light reduced stem dry weight of 'BGA'
plants grown under control filter, but did not reduce stem dry weight under
CuSO4 filter. In 'Spears' plants, EOD-R light reduced stem dry
weight under both filters. EOD-FR light did not affect stem dry weight of
'BGA' or 'Spears' plants grown under control filter, but it increased stem
dry weight of plants grown under CuSO4 filter to a level comparable
with non-EOD treated plants under control filter suggesting that EOD-FR
light altered the dry matter partitioning into leaves and stems. 'Spears'
plants exposed to EOD-R light and grown under control filter did not differ
in stem or leaf dry weight from plants grown under CuSO4 filters
that received no EOD.
Induction of a response by EOD exopsure to R or FR and reversal by subsequent
FR or R, respectively, has been used to demonstrate phytochrome mediation
of a growth response. In the present experiment, EOD-FR reversed the reduction
of plant height, internode length and the stem dry weight caused by the
CuSO4 filters (which remove FR) to a level comparable with plants
that received no EOD treatment under control filter. These observations
support our assumption that phytochrome is involved in the regulation of
morphological characteristics of plants grown under CuSO4 filters.
Further, exposure to EOD-R light reduced the height and stem dry weight
of 'BGA' plants grown under control filter but
EOD-R had no effect under CuSO4 filter. Similarly, exposure to
EOD-FR did not significantly alter height and stem dry weight under control
filter but under CuSO4 filter, EOD-FR increased height and stem
weight to a level comparable with non-EOD treated plants grown under control
filter suggesting that phytochrome may be involved.
'Spears' and 'BGA' responded differently to spectral filter and EOD light
treatments. 'Spears' is a cultivar that will flower eight weeks after the
start of short days. By the time of termination (3 weeks), flower buds were
visible in 'Spears' plants in all treatments. The initiation of flower buds
broke apical dominance and caused the start of lateral bud development.
'Bright Golden Anne' however, flowers ten weeks after the start of short
days and flower buds and lateral branches were not visible even after 4
weeks. Differences in plant form development may have been influenced by
the treatments which in turn could explain some of the differences observed
in leaf and stem dry weight and dry matter partitioning between cultivars.
The effects of light transmitted through CuSO4 filter were similar
to those seen in plants treated with chemical growth regulators which are
known to inhibit gibberellin production. Our research indicated that Pro-Gibb,
a gibberellin supplement, could alleviate the height reduction caused by
CuSO4 filters. It is possible that EOD-FR increased gibberellin
production because the effects of the EOD-FR are similar to those of Pro-Gibb
application (i.e. elongated stems, long internodes, and increased stem dry
weight).
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
