Nihal C. Rajapakse and John W. Kelly
Department of Horticulture, Clemson University
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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.
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.
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: Influence of CuSO4 or water (control) filters on cumulative water loss of chrysanthemum A: first dry down cycle B: second dry down cycle. Boxes on the time axis indicate night periods.). 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).
Figure 2: Influence of CuSO4 or water (control) filters on day and night water loss rate of chrysanthemum A: first dry down cycle B: second dry down cycle. Boxes on the time axis indicate night periods.) 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.
We are grateful to Yoder Brothers for donating plant material and Clemson University Ornamentals Enhancement Program for financial support.
Last Updated 2/1/97