Greenhouse Lighting
Light Intensity
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 that 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.
Light Units
The light intensity can be measured at any point in time (an instantaneous value), or it can be measured frequently over a period of time and integrated as a cumulative value referred to as the daily light integral (DLI). Traditionally, light was measured in lux and foot candles. However, these units of measurement focused on human perception of light. By approaching light from the plant perspective, light intensity is more often measured in photosynthetic photons.
Photometric: Foot-Candles and Lux
The foot-candle and lux are the most common units used to measure instantaneous light in the United States and other parts of the world, respectively. However, growers should understand the limitations of using these photometric units. First, photometric units are instantaneous readings that do not accurately represent the amount of light a plant has received during a 24-hour period.
Quantum: Moles of Photons
Quantum light measurements measure the amount of photosynthetically active radiation (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, i.e., DLI.
Radiometric: Watts
The units of energy employed in measuring this incoming and outgoing radiation are watts per square meter (W∙m–2). Light sensors that measure a wide range of radiation, from approximately 250 to 2,800 nm (depending on the sensor), are termed pyranometers. Environmental control systems often use units of W∙m–2 of total energy because all energy entering a greenhouse needs to be considered for heating and cooling, not just the energy for plant growth. One Watt = 1 Joule per second.
Effects of Light Intensity on Plant Growth
Below a minimum light intensity, the plant falls below the compensation point. The 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.
Light Requirements for Plants
Young plants need less light than 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.
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