Regulation of Chrysanthemum Growth by Spectral Filters

Nihal C. Rajapakse and John W. Kelly
Department of Horticulture, Clemson University 

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Introduction


Plant growth can be altered by manipulating the amount of red (R) relative to far-red (FR) light without altering the amount of photosynthetic light. Red light has been reported to inhibit stem elongation, promote lateral shoot growth, prevent dark-induced leaf abscission and increase the number of tillers in grasses. The effects of FR light have been shown to be opposite to those of R light. These findings suggest that light quality manipulation may be used as an non-chemical alternative for growth regulation of greenhouse plants in the future. This aspect is being currently investigated by the light quality research team at Clemson University.

Alteration of greenhouse light quality can be achieved by the use of electric light sources with specific spectral qualities or by the spectral filters that transmit or reflect specific wavelengths of sun light. The Clemson University light quality research team initially investigated the use of liquid spectral filters using various dye solutions in double-layered polycarbonate panels used for greenhouse construction. Of the spectral filters tested, copper sulfate (CuSO4) filters were effective in reducing plant height and internode length of a wide range of horticultural crops in a manner similar to chemical growth regulators. In addition to height reduction, plants grown under CuSO4 filters were compact and had dark green leaves indicating that they contained higher chlorophyll levels.

Knowing that CuSO4 spectral filters are effectitive for height control of greenhouse plants we further investigated the effects of increased shading by increasing CuSO4 concentrations in the spectral filter on plant growth. This research will provide information on the lowest concentration of CuSO4 that can be effectively used in the filters for growth control purposes.

Materials and Methods

Uniformly rooted 'Bright Golden Anne' chrysanthemum 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 the greenhouse for 1 week before being subjected to the spectral filter treatments. All plants were fertilized throughout the experiment, once daily at irrigation, with 200 ppm nitrogen from Peter's 20-20-20 feritlizer.

After the establishment period, plants were transferred to six growth chambers roofed with double layered polycarbonate panels. Polycarbonate panels were sealed at one end and filled with a 0% (water), 4%, 8%, or 16% solution of CuSO4. All chambers were placed inside a geenhouse to receive natural sun light and photoperiod (average 10-h light and 14-h dark). Plants grown inside experiment chambers received only the light passed through the filter. Since CuSO4 filters increased shading as the concentration increased, a neutral shading material such as cheese cloth was used in each control chamber to ensure the same photosynthetic light level as in the corresponding CuSO4 chamber.

Plant height (height from soil level to plant apex) and number of fully expanded leaves were taken weekly for 4 weeks. Average internode length was calculated as plant height divided by the number of leaves. Total leaf area (LA), fresh dry weights of stems (SFW and SDW) and leaves (LFW and LDW) were measured at the end of 4 weeks.

Results and Discussion

Copper sulfate solutions reduced both R and FR portions of transmitted sun light compared to controls but the reduction of FR light was greater than that of R light. Copper sulfate at or above 8% eliminated all most all of FR from transmitted light. Because of the greater reduction of FR light compared to R light by the CuSO4 filter, plants grown under these filters received a higher amount of R relative to FR light.

Plant height in the controls (water filter) was similar for all three light levels (Table 1). Average height of plants grown under CuSO4 filters was about 40% less at the end of the experiment than that of plants grown under control filters. Plants grown under 8% CuSO4 filter were shorter than those grown under 4% CuSO4 filter but the height was similar for plants grown under the 8% or 16% CuSO4 filters. Plants grown under CuSO4 filters had slightly fewer leaves than control plants, but no differences were observed among CuSO4 control treatments. Average internode length of control plants was 50% longer than that of plants grown under CuSO4 filters at the end oth the experiment. There were no differences in internode length among plants within control or CuSO4 treatments. Stem elongation rate of plants grown under CuSO4 filters was about 50% less than that of control plants. Increased shading did not significantly reduce stem elongation rate of plants grown under the control filter, but under CuSO4 filters, increased shading ( at 8% CuSO4 compared to 4% CuSO4 ) reduced stem elongation rate.

Plants grown under CuSO4 filters had a lower total leaf area (32%) and average leaf size (24%) than control plants (Table 2). Shading did not significantly affect leaf area or leaf size in either control or CuSO4 treatment. Light transmitted through CuSO4 filters reduced leaf and stem fresh weight of plants. However, the reduction in stem fresh weight was greater than the reduction of leaf fresh weight; i.e. stem fresh weight of plants grown under CuSO4 fitlers was reduced by more than 50% while leaf fresh weight was reduced by about 30%.

Total shoot dry weight was reduced (38%) when plants were grown under CuSO4 filters (Table 3). Plants grown under CuSO4 filters had lower leaf (30%), stem (59%) and root (except in 4% CuSO4) (33%) dry weights than those of control plants. Low light resluted in decreased dry matter production in CuSO4 and control treatments. The partitioning of shoot dry matter into leaves and stems was affected by the light passing through CuSO4 filters. In control plants, about 73% of total shoot dry matter accumulated into leaves and 27% into stems. Light transmitted through CuSO4 filters reduced dry matter accumulation into stems from 27% to 18% and increased dry matter accumulation into leaves from 72% to 92%.

Height reduction under CuSO4 filters was mainly caused by the decreased average internode length and to a lesser extent, by a reduction of node count. Reduction in leaf area under CuSO4 filters could be attributed to the smaller leaves and, to a lesser extent, to the small reduction in leaf count. Reduced leaf size resulted in a more compact appearance to the plants. The reduced stem elongation and leaf size resulted in lower dry weights for plants grown under CuSO4 filters. Increased dry weight in control plants could also be a result of larger leaf area, enabling control plants to produce more photosynthate.

Plant perception of far-red light indicates shading from neighboring plants therefore, resulting in extension growth to avoid the competition for light among neighboring plants. By removing far-red light from the growing environment, plants can be manipulated to grow as if they have no neighboring plants. Height reduction of plants grown under CuSO4 filter was caused by alteration of the amount of red light relative to far-red light received by the plant. When sun light passes through the filter, the far-red portion of sun light is removed by the CuSO4 solution resulting in an increased amount of red light relative to far-red light received by the plant. Lack of far-red light in growth chambers with CuSO4 filters caused plants to grow short compared to plants in the control chamber which received far-red light.

Plants grown under CuSO4 filters were similar in many respects ( e.g. shorter plants and internodes, smaller leaves and leaf area, reduced dry weight, thicker leaves, and increased chlorophyll content) to plants treated with chemical growth retardants. We conclude that greenhouse light quality manipulation can be used as a non-chemical alternative means of controlling plant height and producing compact pot chrysanthemum plants. CuSO4 as low as effectively can be used as spectral filters. However, the difficulty in handling CuSO4 liquid filters is a major draw back for the application of this technology in the commercial opperations. With advances in plastic technology, it should be feasible to develop colored greenhouse covering or shading material with specific spectral qualities needed for plant growth regulation in the future. Currently, Clemson University light quality research team is collaborating with a plastic manufacturer (Klerk's Plastics) in efforts to develop easy to handle plastic spectral filters for commercial operations. Further experiments with various plant species would enhance our knowledge on the possible use of light quality as a mean for regulating plant growth in the ornamental plant industry.

Acknowledgements


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

Last Updated 7/16/98