Young plants need less light than do older plants. When the light intensity is too low, plants develop long stems. It is low for plants that usually grow in the shade and high for plants that normally need full sunlight. The light compensation point varies for different plants of different species and genera. 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. Effects of Light Intensity on Plant Growthīelow a minimum light intensity, the plant falls below the compensation point. Light is quantified as micromoles per square meter per second (µmol?m–2?s–1) of PAR when measuring light instantaneously. There are two different ways to measure quantum light: instantaneously or cumulatively. 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. Quantum light measurements measure the amount of photosynthetically active radiation, or PAR, which is the light between 400 and 700 nanometers (nm). It is for this reason that the use of these meters is not recommended when evaluating the light environment for plant production. 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. There is an under representation of blue (400 to 500 nanometers) and red (600 to 700 nm) light in the measurement. While these units, especially fc, are some of the most commonly used for measuring light in greenhouses, they are biased toward measuring light. The foot candle (fc) and lux (LUX) are both types of photometric units of light measurement. This causes a slight variation in the amount of light and heat that the earth receives. Depending on the particular time of the year, the sun-to-earth distance varies it is closest in January and farthest in early July. Maximum intensity occurs at the equator, and gradually decreases with increasing distance from the equator to the south and north poles. 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. The intensity of light can change with the time of the day, season, geographic location, distance from the equator, and weather. In contrast to light quality, the description of the intensity of light does not consider wavelength or color. Light intensity drives photosynthesis which, in turn, produces the carbohydrates which serve as the building blocks for plant growth. Light intensity or light quantity refers to the total amount of light that plants receive. Chapter 7 Light and Lighting Control in Greenhouses Light Intensity
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