End of Day Exposure to Far-Red Light Reverses the Height Reduction of Chrysanthemum Induced by Copper Sulfate Spectral Filters

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

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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