Light and Lighting Control in Greenhouses
Light intensity or light quantity refers to the total amount of light that plants receive. It is also described as the degree of brightness that a plant is exposed to. In contrast to light quality, the description of the intensity of light does not consider wavelength or color. The intensity of light can change with the time of the day, season, geographic location, distance from the equator, and weather. It gradually increases from sunrise to the middle of the day and then gradually decreases toward sunset; it is high during summer, moderate in spring and fall, and low during wintertime. Maximum intensity occurs at the equator, and gradually decreases with increasing distance from the equator to the south and north poles. Depending on the particular time of the year, the sun-to-earth distance varies; it is closest in January and farthest in early July. This causes a slight variation in the amount of light and heat that the earth receives.
Understanding Light Units
Growers either use foot-candles (fc) or lux (lx) when referring to the intensity of light. One foot-candle means the degree of illumination 1 foot away from a lighted standardized wax candle; 100 foot-candles is 1 foot away from 100 candles that are lighted simultaneously. Lux (pl. luces) is the unit of illumination that a surface receives one meter away from a light source. One foot-candle is equal to 10.8 luces and 1 lux is approximately equal to 0.093 foot-candle. Most horticultural researchers measure instantaneous light in micromoles (μmol) per square meter (m-2) per second (s-1), or: μmol·m-2·s-1 of Photosynthetically Active Radiation (PAR) because this unit represents the light spectrum used by plants in photosynthesis.
Estimating Photosynthetically Available Light
The amount of PAR light added by high pressure sodium or metal halide lamps can be calculated from Table 6.2. Multiply the number of foot-candles x the hourly correction factor (from Table 6.2) x the hours lit to get the moles/day. For example, if you run HPS at 575 foot-candles for 11 hours, then 575 x 0.00047 x 11 = 3 moles/day
Effects of Light Intensity on Plant Growth
Below a minimum light intensity, the plant falls below the compensation point. Compensation point is the metabolic point at which the rates of photosynthesis and respiration are equal so that leaves do not gain or lose dry matter. The light compensation point varies for different plants of different species and genera. It is low for plants that usually grow in the shade and high for plants that normally need full sunlight. When the light intensity is too low, plants develop long stems. The distance between the leaves, the internode distance, stretches. The stems become weak and are unable to support the flower heads. As the leaves of foliage plants develop under reduced light levels, each one becomes smaller and smaller. Flowering is delayed or ceases completely.
Light Requirements for Plants
Young plants need less light than do older plants. Seedlings can be started successfully under either low-level natural light or artificial light. Very shortly after the seedlings have germinated, and even before the first true leaf is visible, the plant starts responding to light levels. As plants grow and increase the number of leaves, the need for light increases. Part of this is probably a result of the newer leaves on the plant tending to shade the older leaves at the lower levels.
Maximizing Light Intensity
Low light is the main limiting factor for growth in greenhouses during the winter in temperate climates. It is important to ensure the highest light intensity possible during the dark portion of the year, from mid-fall through early spring, for all crops except the low-light group already mentioned. In this way, growth is maximized.
Maximization of light begins in the planning stage of the greenhouse range. The simpler the frame and the farther apart the sash bars, the greater the light intensity inside. Light incidence angle refers to the angle that sunlight strikes the glazing of a greenhouse.
The single glass layer transmits 90 percent of light, while the double-layer polyethylene cover transmits only 78 percent of light.
Plants tend to proliferate within a bench until the available light energy is fully utilized.
Reducing Light Intensity
Shade adapted plants require varying degrees of light reduction during different times of the year. Many can withstand full sun during the winter but need protection during other seasons.
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