Influence of Spectral Filters on
Water Use by Chrysanthemum
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Introduction
Chemical growth regulators are commonly used in horticulture industry
to reduce plant height. In addition to height reduction, chemical growth
regulators have been reported to increase leaf chlorophyll content, reduce
leaf area, reduce plant water use and improve plant establishment in the
field. However, recent restrictions on the use of certain growth regulating
chemicals on horticultural crops and increasing environmental awareness
have stimulated interest in the use of non-chemical alternatives for plant
growth regulation.
Our experiments with spectral filters to alter quality of sunlight indicated
that light transmitted through copper sulfate (CuSO4) filters
reduced plant height and internode length in a manner similar to chemical
growth regulators in a wide range of horticultural plants. In addition to
the growth regulating effects of CuSO4 spectral filters, reduced
water use may be an added benefit. Water loss from a plant mainly takes
place through stomata, and therefore, the number of stomata and their aperture
influence water loss by plants. Red light has been shown to induce stomatal
opening while far-red light has been shown to induce stomatal closure. In
previous experiments, we have shown that CuSO4 spectral filters
reduced red and far-red portion of transmitted sun light. The reduction
of red and far-red light by CuSO4 filter may reduce stomatal
aperture thus, reducing water use by the plants. Therefore, we evaluated
the influence of CuSO4 spectral filters on water use pattern
of chrysanthemum plants.
Materials and Methods
Uniformly-rooted 'Bright Golden Anne' chrysanthemum cuttings with three
to four leaves were planted in 4.5 inch square plastic pots containing a
commercial potting mix. Plants were grown, as single-stem plants, in a greenhouse
for 10 days before being subjected to the light treatments. All plants were
fertilized, once daily at irrigation, with 200 parts per million nitrogen
from Peter's 20-20-20 fertilizer.
After the 10 day establishment period, plants were transferred to growth
chambers with 6% copper sulfate or water (control) "fluid roofs"
(spectral filters). The chambers were placed inside a glass greenhouse.
Water loss was measured during two, 5-day dry-down cycles (7 to 11 days
and 21 to 25 days) after placement in the chambers. On the evening before
the beginning of a dry-down cycle, plants were watered to field capacity
and excess water was drained overnight. The following morning, pots were
covered with clear plastic film to prevent direct water loss from the media
surface. Weight measurements were taken daily at 0830 and 1730 HR during
each dry-down cycle. Plants were not watered during the 5-day drying cycles.
Chambers were covered with a black cloth after each 1730 HR weight measurement.
The black cloth was removed at the
0830 HR measurement giving a 9-h photoperiod. Water loss rate and cumulative
water use were calculated from the weight loss data.
Total leaf area was measured at the end of the experiment (28 days). Water-use
efficiency at the end of the second dry-down cycle was calculated as the
average units of water consumed for production of a unit of dry matter.
Results and Discussion
Cumulative water loss of plants grown under CuSO4 filters
was lower than that of control plants during both dry-down cycles (Fig 1a and 1b). The difference in water loss
between control and CuSO4 plants was greater during the second
dry-down cycle mainly due to greater leaf area of control plants (i.e. a
37% reduction in cumulative water
use at the end of the second dry-down cycle compared to a 13% reduction
at the end of first cycle).
The water loss rate per plant during light period was significantly higher
in plants grown under the control filter than the CuSO4 filter
in both dry-down cycles (Fig 2a and 2b).
However, the difference in water loss rate per plant between CuSO4
and control plants was small in the first dry down cycle (17% increase in
control over CuSO4 filter). The difference in water loss rate
per plant between the CuSO4 and control plants was greater during
the second dry down cycle (72% increase in control over CuSO4
filter). Water loss rate per plant during the night was similar between
plants grown under control and the CuSO4 had lower leaf area
than control plants. This indicates that plants grown under the CuSO4
filters lost more water per unit leaf area during night possibly due to
higher water loss from the cuticle or impaired stomatal closure. During
the second dry-down cycle, the day and night extremes were greater in plants
grown under the control filter.
Day time water loss rate, calculated on the leaf area basis, (at the end
of second dry-down cycle) was significantly higher (10%) in control plants
than in CuSO4 plants. Night water loss rate of plants grown under
CuSO4 filter was about 33% higher than that of control plants
during second dry-down cycle.
Water-use efficiency (WUE, estimated as units of water used to produce one
unit of dry matter) of plants grown under the control filter (394) was greater
than that of plants grown under the CuSO4 filter (515). Our previous
research showed that plants grown under CuSO4 filters had about
38% lower dry matter production compared to that of control plants. Reduction
of WUE under CuSO4 filter could be due to a greater reduction
of dry matter accumulation compared to control plants.
The difference in cumulative water loss and water loss rate per plant between
control and CuSO4 plants may be explained by plant and stomatal
characteristics. Number of stomata per unit leaf area of plants grown under
the C filter was slightly lower (10%) than that of control plants (Table 1). Stomatal size (length,
width or pore area) was similar between plants grown under control and CuSO4
filters suggesting that light transmitted through the CuSO4 filter
did not affect stomatal opening. Although the size of a stomate was similar
betweeen plants grown under CuSO4 and control filters, total
pore area and total number of stomata per plant was about 50% lower in plants
grown under the CuSO4 filter due to reduction in total leaf area.
Our results suggest that the quality of light transmitted through CuSO4
filters had the potential to reduce water use by chrysanthemum plants in
addition to reducing plant height. The reduction of water use was a result
of reduced plant size under CuSO4 filters. The reduced water
loss by spectral filters can be an added advantage in reducing water requirement
and fertilizer demand while controlling plant growth.
Acknowledgements
We are grateful to Yoder Brothers for donating plant material and Clemson
University Ornamentals Enhancement Program for financial support.
Last Updated 2/1/97
