Height control of greenhouse crops is an important practice to optimize efficient handling and rapid establishment in the field. Many techniques are available, but chemical height control has been the standard practice in commercial operations. Because of potential health risks to consumers and concerns of environmental pollution, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) have imposed restrictions on the use of growth regulating chemicals in agriculture. Use of daminozide (Alar), once the primary chemical used for controlling vegetable transplant height has been banned in the United States. As a result, no chemical growth regulators are currently labeled for height control of vegetable transplants in the United States. Growers in other countries are facing similar restrictions on using chemical growth regulators on food crops. Several research teams around the world are investigating alternative height control measures, such as gene manipulation, greenhouse temperature management, mechanical conditioning, and light quality manipulation.

Light and Plant Growth

Plants have specialized pigment systems that can capture radiant energy in different regions of the electromagnetic spectrum. For example, photosynthetically active radiation (400-700 nm), captured by chlorophyll pigments, provides the energy for photosynthesis, the process by which plants combine carbon dioxide and water to produce oxygen and carbohydrates. Carbon assimilated during photosynthesis provides the energy to sustain life on earth.

Light also acts as a signal of environmental conditions surrounding the plants. There are photoreceptors that function as signal transducers to provide information that controls physiological and morphological responses. Through these pigments, plants have the ability to perceive subtle changes in light composition for initiation of physiological and morphological changes. This ability of light to control plant morphology is independent of photosynthesis and is known as photomorphogenesis. In photomorphogenesis, photons in specific regions of the spectrum are perceived by the photoreceptors present in smaller quantities. Known photomorphogenic receptors include phytochrome (the red and far-red light sensor that has absorption peaks in red and far-red regions of the spectrum, respectively) and "cryptochrome" (the hypothetical UV-B and blue light sensor).

Phytochrome is the most intensively studied sensory pigment that controls photomorphogenesis. Phytochrome is capable of detecting wavelengths from 300 to 800 nm with maximum sensitivity in red (R, 600 to 700 nm with peak absorption at 660 nm) and far-red (FR, 700 to 800 nm with peak absorption at 730 nm) wavelengths of the spectrum. This pigment system consists of two interconvertible forms: the P_r form absorbs red light and upon absorption is transformed into the P_fr form which absorbs far-red light and is transformed into the P_r form. Of the two forms, the P_fr form is assumed to be the active form that controls signal transduction and plant response.

Photon ratios between the red and far-red region of the spectrum (R:FR ratio) and in vitro estimates of phytochrome photoequilibrium (f) [amount of phytochrome in the P_fr form relative to total phytochrome (P_fr:P_tot at photoequilibrium)] have been commonly used to quantitatively describe the phytochrome-mediated responses such as stem elongation. In general, f depends largely on the absorption of red and far-red wavelengths by the plant and therefore, f decreases with decreasing R:FR ratio. A hyperbolic relationship exists between R:FR ratio and f indicating that a small change in R:FR ratio can result in a large change in f in the natural environment. Stem elongation rate and height of a range of herbaceous plants have been shown to be inversely proportional to the f (i.e. higher the f shorter the plant). Therefore, by manipulating the red and far-red light in the greenhouse to establish a high f, height of greenhouse crops can be controlled with minimum chemical applications.

Greenhouse light quality manipulation can be achieved either with supplemental electric lighting systems with relatively high red and low far-red light or by spectral filters that can alter red and far-red light balance of sunlight. Incandescent lamps, which are low in R:FR ratio, frequently lead to stem elongation while fluorescent sources, which are high in R:FR ratio, produce short and compact plants. Radiation filters, both liquid and rigid, for improving greenhouse crop productivity and reducing greenhouse temperature gained attention in the 1970s and considerable progress has been made since then.

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Liquid Spectral Filters

Channeled, double-walled acrylic and polycarbonate plastic greenhouse glazings allow liquid dyes to be contained in hollow channels of the glazing as filtering materials. In the 1970s and 1980s, liquid filters were widely investigated for filtering out infrared radiation (heat) from sunlight as a mean to cool greenhouses. Van Bavel noted liquid radiation filters reduce energy requirements by 20%-40% and virtually eliminate the need for forced ventilation in greenhouses. The ability of various aqueous dye filters [red, green, yellow, blue, and copper sulfate (CuSO₄•5H₂O)] to selectively remove elongation-stimulating far-red light from the natural spectrum and to reduce plant height was investigated in the late 1980s in Norway and in the USA. Of the different liquid filters tested only liquid CuSO₄ filters were effective in removing elongation-stimulating far-red wavelengths from the sunlight (Fig. 1). The CuSO₄ liquid filter reduced both red and far-red wavelengths, but the reduction of far-red was greater than the reduction of red wavelengths thus, resulting in a high R:FR ratio and high f.

Figure 1. Spectral distribution under liquid red dye (#259), blue dye #171 (CIBA-GEIGY, Greensboro, NC), and CuSO₄ filters.

Plant Responses to Liquid Spectral Filters

Height and internode length

In addition to height reduction, plants grown under CuSO₄ filters had more leaf chlorophyll, darker green leaves, and were compact than control plants, similar to plants treated with chemical growth regulators. Subsequent studies revealed that a wide range of plants respond to CuSO₄ filtered light (Table 1). Azalea and bulbs such as tulip, hyacinth, and daffodil did not respond to CuSO₄ filtered light.

In chrysanthemums, CuSO₄ filtered light reduced height by Å30% in short photoperiod-grown (fall and spring) plants but in long photoperiods (summer), plant height reduction was Å20%. A similar response was observed with miniature roses grown in short and long photoperiods.

Water use

Flower development

Dry matter accumulation and partitioning

Carbohydrate status

Postharvest quality

Physiological Basis of Light Quality Responses

Gibberellins (GAs) are a group of plant growth hormones involved in a wide range of plant processes such as germination, cell division, cell elongation, flowering and fruit set and development. Endogenous gibberellins play an important role in the control of stem elongation and internode length. Chemical growth retardants reduce plant height by suppressing the production of natural gibberellins and there are similarities between the effects of chemical growth regulators and CuSO₄ spectral filters. Therefore, it is possible that GA biosynthesis or its action may be suppressed under CuSO₄ spectral filters.

Stem elongation in response to changes in light quality may be mediated by changes in GA level or sensitivity to GA. In efforts to understand the physiological basis for growth control by spectral filters, we applied 50 mg•L^-1 (ppm) GA₃ (Pro-Gibb) on the first day of spectral filter treatment or weekly to chrysanthemum plants grown under control or CuSO₄ spectral filters. Both single and weekly applications of GA₃ reversed the plant height reduction caused by CuSO₄ filters, but the weekly applications were more effective than the single application. We also applied 3500 mg•L^-1 of daminozide (B-Nine), a known gibberellin biosynthesis inhibitor, weekly to chrysanthemum plants grown under control and CuSO₄ filters. Daminozide treatment reduced plant height under both CuSO₄ and control filters but the effect was greatest under the control filter.

The level of GA-like substances in apical regions is known to be high in plants treated with far-red light. Exposure to end-of-day far-red light reversed the reduction of plant height and internode length caused by the CuSO₄ filters to a level comparable with plants that received no end-of-day far-red treatment under control filters. Exposure to end-of-day red light reduced height and internode length of chrysanthemum plants grown under control filters but had no effect under CuSO₄ filters. Exposure to end-of-day far-red did not significantly alter height and internode length under control filters. Observations with exogenous GA application and with end-of-day exposure to red or far-red light suggest that reduction of gibberellin levels by CuSO₄ filter may be, at least partially, responsible for plant height reduction.

Gibberellin biosynthesis is a complex process that involves several enzymes and intermediate gibberellins. The current research focuses on quantifying the endogenous gibberellin levels (GA₁₉, GA₂₀, and GA₁) and on investigating the responses of spectral-filter-grown chrysanthemum plants to intermediate gibberellins (GA₁₉ and GA₂₀) in the GA biosynthetic pathway. Our quantification studies indicate that GA₁₉ levels (inactive) were higher and GA₁ (active) levels were lower in CuSO₄ filter grown plants than in control plants. The response of CuSO₄ filter grown plants to exogenous GA₁₉ was lower than control plants. These preliminary observations suggest that the conversion of GA₁₉ to GA₂₀ may be reduced under the CuSO₄ filters.

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Commercialization of Spectral Filter Technology

Although our early research has demonstrated that light manipulation by liquid CuSO₄ filters have the potential for being a non-chemical alternative for height control of greenhouse plants, liquid spectral filter technology has limited value to commercial growers because of difficulties in material handling and high initial construction costs. In addition, CuSO₄ is hazardous and can be phytotoxic in the event of spills.

For spectral filter technology to be acceptable commercially, an easy-to-handle plastic greenhouse covering or shading material with the ability to filter out far-red light must be developed. Although a plastic material with far-red removing properties is not commercially available at present, several plastic and pigment manufacturers have shown interest in developing such material. The Clemson University researchers are currently collaborating with Mitsui Chemicals, Inc., Tokyo, Japan to develop photoselective greenhouse plastic films or rigid plastic panels.

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Development of photoselective covers

Both types of far-red light absorbing photoselective filters reduced height of all species tested in preliminary trails but the magnitude of height reduction varied with the species. (Table 2 and Figure 2). In general, watermelon seedlings showed the greatest height reduction followed by bell peppers, tomato, and chrysanthemum. Number of leaves was not affected, indicating that height reductions were caused by shorter internodes. The height reduction increased as the dye concentration in the panels increased but total shoot dry weight was reduced because of the severe light reduction as the dye concentration increased. Therefore, a dye concentration that gives a light transmission of 75% was selected for photoselective film production and further experimentation.

Table 2. Effect of dye concentrations in YBM-1 and YBM-10 photoselective chambers on height of chrysanthemum, watermelon, bell pepper, and tomato plants. The number followed by the film indicates the percent light transmission through the panels. Percentage height reductions compared to control plants are given in parentheses.

Based on initial findings, photoselective greenhouse films with red and far-red light absorbing films (SXE-4 and YXE-10 films, respectively) were produced with a dye concentration that results in a 75% light transmission (light spectrum of two types of films). Growth of several vegetable transplants and ornamental bedding plants was evaluated inside growth chambers covered with these films. The results are summarized in Table 3. Plants produced under the far-red light absorbing film were, in general, shorter (except snapdragon and miniature roses) than the control plants while plants produced under the red light absorbing film had similar or increased height compared to the control plants. The magnitude of height reduction varied with the species and cultivar.

We also evaluated flowering of selected ornamental crops inside the chambers under natural short day conditions. Flowering of miniature rose plants was not affected (Table 3). Flowering of cosmos, zinnia, and chrysanthemum (short day plants) was slightly delayed (by 1-2 days) under the far-red light absorbing film. Photoselective films had the greatest influence on flowering of snapdragon and petunia (long-day plants). Flowering of these species was delayed by 7-13 days under the far-red light absorbing films. Red light absorbing film did not significantly affect flowering of these species tested.

Table 3. Influence of red and far-red light absorbing plastic films (SXE-4 and YXE-10, respectively) on plant height and flower development (days to anthesis, DA) under natural short days of selected crops. Control is a clear polyethylene film.

Current Concerns with Photoselective Films

One draw back of the photoselective films that we tested is their short film life. We have evaluated the film life under both protected (in a greenhouse) and unprotected (exposed to full sun) conditions at Clemson University and at a nursery research site (Carolina Nursery, Monks Corner, SC). Films tested in the greenhouse lasted longer (over one year) than those films tested under natural conditions. The dye in the films tested under unprotected conditions began to degrade during the first year of exposure (10 to 12 months). Short film life is a limitation to the commercial applications of the photoselective films we tested but experiments are being conducted to increase the stability of the dyes in the films under natural environments. Using the photoselective film as the inner layer of a double layered poly house may help extend the life of the films. Experiments are currently underway to test the film in this type of situations.

Another concern of using photoselective films is that the reduction of light transmission may limit their use in low light seasons and in the northern latitudes where sunlight is limited. In a given day, the red:far-red ratio of sunlight is relatively constant (about 1.1) from sunrise to sunset; however, during a half-hour-period before sunrise or after sunset, red:far-red ratio is reduced due to the increase in far-red light. Therefore, exposing plants to far-red light absorbing photoselective films at the end of the day may help effectively exclude far-red light in the evening while maximizing the light during the daytime. We are currently testing the use of photoselective films as an end of the day curtain to block far-red light during the evening hours. Preliminary experiments were conducted with cucumber by exposing seedlings continuously to far-red light absorbing films or by exposing seedlings to films at the end of the day (from 3:00 PM to 9:00 AM or from 5:00 PM to 9:00 AM, in October to November). Treatments were terminated after 15 days. The shortest plants were those grown continuously in far-red light absorbing (YXE-10) chambers (Figure 3). End of the day exposure to YXE-10 film was also effective in height reduction. However, the height reduction by end of the day exposure (25% height reduction) was not as high as continuous exposure (44% height reduction). There was no difference in height between the two end of the day exposure treatments to YXE-10 film, indicating that later exposure to film was as effective.

Plants grown continuously in YXE-10 chambers had the lowest shoot dry weight. Dry weight of end of the day exposed plants was greater than continuous YXE-10 plants, suggesting that end of the day exposure can minimize the dry weight reduction. By using photoselective film as an end of the day curtain, film life may also be extended. Although effective with cucumbers, to make this strategy commercially useful, a wide range of crops must be tested. If effective with a range of crops, this will provide an opportunity to maximize the use of sunlight during the daytime and achieve a reasonable height reduction without using chemicals.

As the general public becomes more concerned with the chemical use, interest in using non-chemical alternatives to regulate plant growth and to control pests and diseases will increase. With the commercial development of photoselective greenhouse covers or shade material in the near future, nursery and greenhouse industry could reduce costs for growth regulating chemicals, reduce health risks to their workers and consumers, and reduce potential environmental pollution.

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