Light is considered by many plant scientists as the single most important variable with respect to plant growth and development, and light is often the limiting factor in plant growth. Light essentially provides the energy required to run photosynthesis which is the process by which a plant utilizes carbon dioxide (CO₂) and water (H₂0) to form carbohydrates. In fact, this process is the basis for almost all terrestrial life on Earth.

Light also affects plant growth and development by its involvement in functions other than photosynthesis. Changes in plant growth and development that are controlled by light, but not necessarily a function of photosynthesis are referred to as being photomorphogenic responses (photo = light, morpho = change and genic = growth). For example, a poinsettia plant will only form leaves (vegetative growth) if the length of the night is less than approximately 11.5 hours. However, if the length of the night exceeds 11.5 hours, physiological changes occur in the meristematic tissues and the plant forms floral structures. This process is an example of a photomorphogenic response and more specifically would be referred to as a photoperiodic response (photo = light and periodic = duration).

Before discussing greenhouse light control, a few basic concepts regarding light should be reviewed. Light displays both particle and wave properties. This simply means that light behaves as particles (photons) when it is measured in certain ways and as waves (wavelength) when measured in other ways. Photons are discrete particles of visible light with varying energy levels. The wavelength of light is indicative of the energy level of the light and may be expressed as a specific wavelength or its corresponding color (as perceived by the human eye). The shorter the wavelength, the higher the energy level.

Three Important Attributes of Light Affect Plant Growth

Light Quality

Because light energy is transformed to chemical energy (and thus physiological responses) when photons interact with energy level-specific (or wavelength-specific) photoreceptors, different wavelengths of light affect different processes. Light with wavelengths of 400 to 700 nm is referred to as photosynthetically active radiation (PAR) and can be used by the plant to drive photosynthesis. Within this range, red (650 - 700 nm) and blue (460 - 480 nm) light are most efficiently used by plants for photosynthesis.

Light also affects plant growth through photomorphogenesis. Probably the most well known case where light quality affects photomorphogenesis is through the phytochrome photoreceptor. Phytochrome has been shown to be involved in many photomorphogenic responses including seed germination and photoperiodism. In addition to the example described above regarding flowering and photoperiodism in poinsettia, a common photomorphogenic response that is often observed is that plants grown under low light conditions like in a forest or under other plants tend to stretch and have longer internodes than plants grown in bright light (levaes filter red light more than far red light thus creating a far red light enriched environment within a leaf canopy). Additionally, plants grown under a light source high in blue light will be shorter and have darker colored leaves than plants grown under a light source high in far-red light. These are all examples of photomorphogenic responses by plants to light and are dependant upon or controlled by the light quality perceived by the plant.

Light Quantity

Light is the energy source that powers photosynthesis. For each plant species there is an optimal light level. Below this level, photosynthetic activity is reduced to a level below the maximum photosynthetic potential of the plant. Above the optimal level, photosynthetic rate becomes static. Further increases in light levels can cause damage the photosystems of the plant and actually reduce the photosynthetic rate. The goal in greenhouse crops production is to optimize light levels so as to maximize the photosynthetic rate.

Light levels may be measured and expressed in numerous ways:

Radiant flux is the energy (of all wavelengths) emitted by a light source and is measured in watts.

Radiant emittance is the energy received from a light source per square meter and is measured as watts per square meter.

Irradiance is a measure of the amount of light energy intercepted by a surface and is measured in watts per square meter.

Lumen is a unit of light energy output (in a directions) from a light source. A 100W incandescent bulb produces approximately 1,740 lumens.

Footcandle is equivalent to 1 lumen per square foot. Footcandles are often used to report light levels in greenhouses. However, devices designed to measure footcandles are designed to mimic the human eye (being more sensitive to yellow and green light) and are not the most appropriate for measuring or reporting light with respect to plant requirements.

Illumination intensity is measured as the number of footcandles striking a surface (100 lumens striking a 1 square foot area is equivalent to 100 footcandles).

Lux is equivalent to 1 lumen per square meter.

Quantum flux density is the number of quanta or photons (usually within the 400 - 700 nm band) intercepted by a surface over a period of time and is measured as Einsteins or moles per second per square meter (1 Einstein = 1 mole). This is the most appropriate way to measure and report light levels with respect to plant growth and development.

Photoperiod (light duration)

The duration that light is received by or provided to a plant (photoperiod) can significantly affect plant development. The major horticultural interest in controlling photoperiod is that photoperiod often controls when a plant shifts from a vegetative to a reproductive phase. Long-day plants (i.e. Easter lily) become reproductive when the length of the night is less than some critical value. Short-day plants (i.e. poinsettia, chrysanthemum) become reproductive when the length of the night exceeds some critical value.

Controlling Light in Greenhouses

Light Quality

In commercial greenhouse production, light quality is important when selecting a light source for supplemental photosynthetic lighting or photoperiod control. A broad emission spectrum within the 400 to 700 nm range is desirable especially when adding light to increase photosynthetic rate. A light source that provides a very narrow spectrum (i.e. low pressure sodium lamps) is generally not desirable since this will hinder plant growth and development and may result in undesirable photomorphogenic responses. Light sources being used to extend day length and create artificially long days must provide sufficient light in the red range in order to affect the phytochrome photoreceptor.

In some situations, it may be desirable to specifically alter the light quality experienced by the plant and increase the relative ratio of certain wavelengths experienced by the plant. This is generally done to manipulate plant growth and development. For example, reducing the far-red light and increasing the blue light experienced by the plant results in shorter, darker-colored and stronger plant. Light quality can also affect the development of certain foliar diseases such as Botrytis. Light-emitting diodes that emit in very narrow wavelengths might be used for this purpose. However, light-emitting diodes are expensive and primarily used for research purposes. Some commercially available light sources (i.e. low pressure sodium) have narrow emission spectra and may be used to increase the relative amount of selected wavelengths of light. Greenhouse glazings have also been developed with additives or pigments that filter certain wavelengths of light and allow for a shift in the relative ratios of wavelengths of light entering the greenhouse.

Light Quantity

The Light level might need to be increased or decreased to maintain optimal levels. Different plant species have different optimal light levels. However, for a given species, plant spacing, nutritional level and plant age can affect the optimal light level. For example, the optimal light level for a tomato seedling is lower than that for a well established and actively growing tomato.

Two methods are commonly used to reduce light levels in greenhouses. The first is the application of a shading compound to the glazing. There are several commercially available shading compounds. However, a mixture of 1 part white latex paint to 20 parts water works well. The shading compound is applied to the glazing (on the outside of the greenhouse) in the spring and washed off in the fall. The second method is to block out a portion of the light with some type of shading screen made of cloth, polypropylene, polyester, or aluminum-coated polyester. These systems may be placed on the exterior of the greenhouse or on the interior. They may be purchased in weaves that provide 10% to 90% light reduction. However, 30% to 60% is most commonly used.

A problem with these types of shading systems is that the shade remains in place in the mornings, afternoons, and on cloudy days when shading would not be needed. During these times, light levels fall below optimal levels. Retractable shade systems are being installed in many new greenhouses. These systems are placed in the gables of the greenhouses and are controlled by a computer that is in turn connected to a photometer (Light meter) . A desired light level can be programmed into a computer and the shade automatically pulled when light levels exceed the desired level. The shade will automatically be retracted when light levels fall below the desired level. There are several variations to these systems, but the end result is that they allow for a more uniform application of light as well as allowing for the optimal light levels to be maintained for a longer period of time during the day.

Often, particularly in northern climates in the fall and winter months, increasing light levels is required. Light levels that are too low can cause flower bud abortion, reduced growth rates, longer internodes, lower quality, and increased disease incidence. Selection of a glazing that allows maximum light transmittance, minimizing obstructions, keeping the glazing clean, and increasing plant spacing are all ways of increasing the amount of light reaching the plants. However, these measures may not be enough and supplemental lighting may be required to increase light levels.

Supplemental Lighting in Greenhouses

Before selecting a light source for greenhouse or growth chamber lighting, numerous factors should be considered. Among these are the total energy emitted by the lamp, efficiency (% of electrical energy converted to light energy), wavelengths emitted (especially in the 400 to 700 nm wavelengths), cost, life expectancy (of bulbs and fixtures), and the fixtures required (including ballasts).

Types of Lamps Used in Controlled Environments:

 

Incandescent lamps (tungsten-filament) - These lamps are generally not used for supplemental lighting in greenhouses for photosynthetic purposes. A large portion of the radiation given off by these lamps is in the form of infrared (heat). Because of this, their efficiency rating is only 7%. Lamps range from 40 to 500 watts. Life span ranges from 750 to 1000 hours. In order to produce enough light for effective photosynthetic lighting, a large number of these lights would be required. This would require a large number of fixtures and would result in large amounts of heat being produced. Further, most of the visible radiation that these lamps produce is in the red and far-red wavelengths that cause plants to become tall and to have weak stems. However, because relatively low light levels are required for photoperiodic lighting, incandescent plants are suitable and commonly used for this purpose.

Tungsten-halogen lamps - These lamps combine tungsten filament with iodine vapor. This allows for the output to remain constant throughout the life of the lamp. These lamps are available in up to 1,500 watts and have a have a 2000-hour lifespan.

Fluorescent lamps - These lamps are most commonly used in growth chambers and seed germination rooms. They are rarely used to produce crops in greenhouses. As with incandescent lamps, a large number of lamps would be required to produce enough light to benefit the crop. These fixtures cost money, require additional wiring and block natural sunlight. Fluorescent lamps are more efficient than incandescent lamps (20% efficiency) and provide their light over a broader spectrum (more in the blue region) than incandescent lamps. Both cool white and warm white lamps are used, but cool white lamps are most commonly used in controlled environments.

Cool white lamps have a lifespan of 12,000 hours, and come in lengths of 4 ft and 8 ft, and 75-watt bulbs are most commonly used for horticultural purposes. Fluorescent lamps require ballasts that provide adequate voltage to start operation and limits continuing current to the lamp. These ballasts can be heavy and thus increase the dead load on a structure. Additionally, ballasts generate significant amounts of heat.

High output (HO) and very high output (VHO) fluorescent lamps are also available. Whereas cool white lamps generate 91 watts of light energy, HO lamps generate 126 watts and VHO lamps generate 225 watts.

There are other types of fluorescent lamps designed for plant production. These are used in hobby situations since the same limitations apply as to other fluorescent lamps. These "plant lamps" or "grow lamps" usually have slightly broader emission spectra than standard cool white incandescent lamps. These lamps generally generate 46 watts of light energy and have a life span of 10,000 to 12,000 hours.

High Intensity Discharge (HID) Lamps - These are the most commonly used lamps for supplemental lighting in greenhouses. As with fluorescent lamps, these lamps require ballasts that can be very heavy and generate significant amounts of heat. Reflectors are used to direct the light generated downward and to improve uniformity of light distribution. Numerous types off bulbs are available for use in HID lamps.

High-pressure mercury bulbs have an emission spectrum similar to fluorescent lamps but with a greater concentration of their radiation being emitted in the red region. Light energy is produced by these lamps using a two-step process. First the filament gives off UV light. This UV radiation excites a phosphor powder in the tube. This powder fluoresces and gives off visible light. Because of this two-step process, these lamps have an efficiency rating of only 13% and have a lifespan of about 10,000 hours. These lamps are more commonly used along roadways and in parking complexes. They may be purchased in wattages of up to 1000W.

High-pressure metal-halide bulbs cost more than high-pressure mercury bulbs and have a shorter life span than high-pressure mercury bulbs. However, they have a broader emission spectrum than mercury bulbs. Their efficiency rating is 20%. These bulbs have a lifespan of 8,000 to 15,000 hours. They may be purchased in wattages of up to 2000W.

Low-pressure sodium bulbs have an efficiency rating of 27% and have a lifespan of 18,000 hours. However, these bulbs have very narrow emission spectrums (nearly all light is emitted around 590 nm). This narrow emission spectrum can cause adverse effects on crop development. These bulbs are available in wattages of 35, 55, 135 and 180W.

High-pressure sodium lamps are the most common type of bulb used for supplemental lighting in greenhouses. They have a broader emission spectrum than low-pressure sodium bulbs and are cheaper than mercury bulbs (approximately $150 -$250 per unit or $2.25 - $3.00/ft² to purchase and install). These bulbs have efficiency ratings of 25% and a lifespan of 24,000 hours. They are available in wattages of 250, 400 and 1000W. The 400 watt and 1000 watt bulbs are most common.

Using Supplemental Lighting in Greenhouses

Supplemental HID lighting is most often used on roses, vegetative stock plants, ornamental and vegetable plugs or seedlings, and greenhouse-grown vegetable crops. In some cases, HID lighting might be used on containerized crops.

It is best to work with the manufacturer to decide on the best placement of fixtures to achieve desired light levels with maximum efficiency. In some cases, 1000 watt lamps are used. However, tall greenhouses are required with these lamps to insure uniform light distribution.

When conducting supplemental HID lighting, the supplemental light is supplied for 16 - 18 hours per day. Providing supplemental light for 24 hours per day can be detrimental.

The reflector used with the light source is important. The goal is to achieve the most uniform distribution of light that is possible while using as few lamps as possible. HID lamps using the older style circular reflectors are generally spaced apart at 1.5 times the distance between the lamp and the plant material. New types of reflectors allow spacing to be up to 4.5 times the distance between the lamps and the plants.

Controlling Photoperiod in Greenhouses

The duration that light is perceived by the plant (photoperiod) is usually controlled in order to time flowering or to maintain plants in a vegetative condition. During long-day photoperiods (i.e. late spring and summer), shade cloth is pulled to artificially shorten the length of the photoperiod. In some cases, shade must be pulled over the crop even during short-day photoperiods because of light pollution. During short-day photoperiods, we may use supplemental lighting to increase photoperiod. This is usually accomplished using incandescent lamps. These lamps may be turned on for 4 - 8 hours at sunset or before sunrise. More commonly, the lights are turned on during the middle of the night (night interruption). Night interruption has been shown to be the most effective method of creating a long-day photoperiod. For this method, lights are usually turned on at 10:00PM and turned off at 2:00AM. Cyclic lighting may also be used to interrupt the night period. There are several potential lighting cycles that can be used, but a common method is to turn the lights on for 5 - 10 minutes per hour during the night. If HID lighting is being used on the crop, and long days need to be maintained, the lights can simply be turned on for 18 hours as usual. This period is sufficient to create long days for all greenhouse crops

EnviroCept Greenhouse
 

© 2003, M.R. Evans