Light and Lighting Control in Greenhouses
Light intensity or light quantity refers to the total amount of light that plants receive. Light intensity drives photosynthesis which, in turn, produces the carbohydrates which serve as the building blocks for plant growth. 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.
Foot Candle and Lux Light Measurements
The foot candle (fc) and lux (LUX) are both types of photometric units of light measurement. While these units, especially fc, are some of the most commonly used for measuring light in greenhouses, they are biased toward measuring light. There is an under representation of blue (400 to 500 nanometers) and red (600 to 700 nm) light in the measurement. Therefore, using a foot candle or LUX meters introduces some error when one is interested in only measuring the amount of light available to plants for photosynthesis. It is for this reason that the use of these meters is not recommended when evaluating the light environment for plant production.
Quantum Light Measurements
Quantum light measurements measure the amount of photosynthetically active radiation, or PAR, which is the light between 400 and 700 nanometers (nm). The light within this range is most effective for photosynthesis, which is why growers use units that quantify the light used for plant growth and not the human eye. There are two different ways to measure quantum light: instantaneously or cumulatively. Light is quantified as micromoles per square meter per second (µmol?m–2?s–1) of PAR when measuring light instantaneously.
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.
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.
Click on the following topics for more information on light and lighting control in greenhouses.