Chemical plant growth regulators (PGRs) are the standard practice in the greenhouse industry for controlling plant height. However, growers are showing increased interest in potential alternatives to the standard chemical products.
One obvious reason is the cost of chemical PGRs. (Growers are looking to cut costs in all facets of their businesses.) Another reason ties into the interest among growers for any non-chemical or reduced-chemical product that achieves a production function comparable to that achieved by a chemical product.
A third reason is the concern among growers that regulating agencies will continue placing restrictions on greenhouse chemicals. For example, the U.S. ban on daminozide (Alar), once the primary chemical used for controlling vegetable transplant height, has left this segment of the industry without a chemical growth regulator labeled for height control of vegetable transplants.
Several research teams around the world are investigating alternative height control measures such as genetic manipulation; temperature, water and nutrient management; mechanical conditioning; and light quality manipulation. Our intent with this article is to summarize the recent developments in greenhouse light manipulation.
The role of light
The term "light" is often described as what is perceived by the human eye, or more specifically, as the wavelengths in the electromagnetic spectrum that activate light receptors in the human eye. However, plants respond to all the wavelengths of the electromagnetic spectrum including the wavelengths that the human eye can (visible light) and cannot see (ultra-violet and infra-red light).
Light in human or animal vision acts mainly as an informational medium to distinguish the location, movement, shape, size, color and other aspects of material objects. Plants use light for a similar purpose in that variations in light can signal variations in environmental conditions surrounding the plant. Photoreceptors within plants function as light sensors that provide plants with information on subtle changes in light composition in the growing environment, and also control physiological and morphological responses independent of photosynthesis. This process is technically known as photomorphogenesis.
But plants also use light for the production of food through photosynthesis, the process by which plants convert carbon dioxide and water into carbohydrates and oxygen (in the presence of light).
Regarding plant development, the most important regions of the light spectrum can be broadly divided into (also see chart below):
• ultra-violet (below 400 nm)
• the visible (approximately 400-700 nm)
•far-red (approximately 700-800 nm).
[note: nm is the abbreviation for nanometer, which is one 1,000,000,000th of a meter].
Ultraviolet radiation (UV) accounts for less than 2 percent of the total light energy on the earth’s surface. In fact, increased UV radiation has been reported to cause detrimental effects to sensitive plants. The visible region, which provides the energy for photosynthesis, is often called the photosynthetically active radiation (PAR).
Photomorphogenesis, on the other hand, involves the activation of several sensory pigment systems by specific wavelength regions (i.e. blue, red, and far-red light). Phytochrome is the widely studied sensory pigment that controls photomorphogenesis in response to changes in red and far-red light in the growing environment. This pigment system consists of two forms: an active, far red light-absorbing form (Pfr); and an inactive, red light-absorbing form (Pr). These two forms have peak absorption in the far-red region at 730 nm and the red region at 660 nm, respectively.
Commercial applications of photomorphogenesis
In laboratory experiments, exposure to red light has enhanced seed germination, reduced seedling stem elongation, and promoted lateral shoot growth of many species.
The effects of far-red light have generally been at a direct variance to the effects of red light. In general, environments high in red light relative to far-red light are favorable for the production of short and compact plants. Therefore, an increase in red light and/or a reduction in far-red light in the greenhouse could be used to reduce plant height.
In a greenhouse, light quality manipulation can be achieved two ways:
• supplemental electric lighting systems with relatively high red light, and low far-red light.
• spectral filters that can alter the red and far-red balance of sunlight.
Incandescent lamps, which have high far-red light relative to red light, frequently lead to stem elongation. Fluorescent sources, which are high in red light relative to far-red light, produce short, compact plants.
More than 10 years ago, researchers at the Agricultural University of Norway and at The Clemson University began testing spectral filters to manipulate greenhouse light environments. The early research demonstrated that liquid spectral filters could be developed by placing various dye solutions in double-layered acrylic or polycarbonate sheets used in greenhouse glazing.
The liquid filled panels served as selective spectral filters and altered the quality of the light reaching greenhouse-grown plants. Of the liquid filters tested, only copper sulfate (CuSO4) was effective in removing far-red light from sunlight and in reducing height and internode length of a wide range of dicotyledonous plants (Table 1).
Although liquid CuSO4 filters were shown to be effective in reducing plant height and producing compact plants without chemical use, liquid filter technology has limited value to commercial growers because of difficulties in liquid handling and high initial costs.
This suggests that spectral filter technology can be commercially acceptable if a manufacturer can develop an easy-to-handle plastic greenhouse covering or shading material with the ability to filter out far-red light. Several greenhouse film manufacturers and chemical companies in Europe and Japan are working together to develop such material. The Clemson University and The OSU researchers are currently collaborating with Tokyo-based Mitsui Chemicals, Inc., to develop and test photoselective greenhouse plastic films capable of removing far-red light for effective height control.
Photoselective Plastic greenhouse covers
Mitsui Chemicals has identified two pigments that absorb far-red light from the natural spectrum and that are stable in polyethylene films or rigid plastic panels. Initial trials focused on identifying a suitable dye in a concentration that can effectively filter out far-red light from sunlight and reduces plant height. Another criterion of this dye concentration is also that it minimize the reduction in light transmission.
Rigid plastic panels containing five dye concentrations from each dye were produced by Mitsui Chemicals. (These were identified as control, YBM-1/YBM-10 #85, YBM-1/YBM-10 #75, YBM-1/YBM-10 #65, and YBM-1/YBM-10 #55. (The number following the YBM indicates the code of the dye.)
As the dye concentration in the panels increased, the absorption of far-red light increased – but the light transmission decreased. The number following the YBM-1 or YBM-10 indicates the percentage light transmission through each panel. Growth chambers (1 m x 0.8 m x 0.8 m) were built with each of these materials. The growth of bell pepper, tomato, and watermelon seedlings, along with chrysanthemum cuttings, were evaluated inside each chamber. Chambers were kept inside a greenhouse; to ensure uniform light in all chambers, the amount of light inside each chamber was adjusted with cheesecloth.
In preliminary trials, both types of far red light-absorbing photoselective filters reduced height in all species tested. However, the magnitude of height reduction varied with the species (see photos on p. 19).
In general, watermelon seedlings showed the greatest height reduction, followed by bell pepper, tomato, and chrysanthemum seedlings. Number of leaves was not affected, indicating that height reductions were caused by shorter internodes.
Height reduction increased as the dye concentration in the panels increased, but total shoot dry weight was reduced because light was reduced significantly in proportion to the increase in the dye concentration. With this finding, researchers selected a dye concentration that provides 75 percent light transmission for photoselective film production and further experimentation.
Based on initial findings, photoselective greenhouse films with red or far red light-absorbing films (SXE-4 and YXE-10 films, respectively) were produced with a dye concentration that results in a 75-percent light transmission. 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 generally 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.
Concerns with Photoselective Films
One drawback to the photoselective films we tested (at Clemson) 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 and at a nursery research site (Carolina Nursery, Monks Corner, S.C.). Films tested in the greenhouse lasted longer (more than one year) than did 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 films.
Another concern is that the reduction of light transmission may limit the use of photoselective films in low-light seasons and in the northern latitudes where sunlight is limited. On 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 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. We did preliminary experiments 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 and November). The shortest plants were those grown continuously in far red light-absorbing (YXE-10) chambers (pictures above). End-of-the-day exposure to YXE-10 film was also effective in height reduction. However, the height reduction obtained by end-of-the-day exposure (25-percent height reduction) was not as high as that obtained by continuous exposure (44 percent).
Photoselective film life also may be extended by using it as an end-of-the-day curtain. Although effective with cucumbers, this strategy must be tested with a wide range of crops to ensure that it is commercially useful. If proven effective with a range of crops, growers will have an opportunity to maximize the use of sunlight during daytime and achieve a reasonable degree of height reduction without using chemicals.