Light and Lighting Control in Greenhouses
Light quality refers to the composition of light as to wavelengths (colors) that are effective in photosynthesis and other plant growth processes. The wavelengths of light are typically expressed nanometer (nm). The visible wavelengths of light lie from about 390 nm to 760 nm, which is just a small section of the entire electromagnetic spectrum of solar radiation. Visible light corresponds roughly to the Photosynthetically Active Radiation (PAR) from about 400 to 700 nm. PAR measures the intensity of light that directly affects photosynthesis; that is, the portion of the light spectrum that makes up the colors of the rainbow red, orange, yellow, green, blue, indigo, and violet. PAR is a much more accurate way to measure light intensity since it deals with the portion of the light spectrum that “drives” photosynthesis. Approximately half of the energy that comes from the sun falls within the photosynthetic waveband. The remaining amount of energy has shorter wavelengths (such as UV light) or longer wavelengths (such as infra-red radiation). Different wavelengths are used for specific plant functions, but all wavelengths in this range are absorbed in varying amounts. In regards to photosynthesis, red and blue light are by far the two most important colors concerning plant growth. PAR is measured using quantum sensors with filters that block light outside the PAR waveband.
The most important blue wavelengths are from 430 to 450 nm. This part of the spectrum is also known as cool light. These wavelengths encourage vegetative and leaf growth through strong root growth and intense photosynthesis.
The longer wavelengths of light are red in color. The most important wavelengths in the red spectrum are from 600 to 700 nm. These wavelengths encourage stem growth, tuber and bulb formation, flowering, and fruit production, and chlorophyll production. It also helps increase stem diameter and promotes branching.
This waveband (700 to 800 nm) is not considered photosynthetically active, but far-red light does influence growth. Plants under a canopy (such as under hanging baskets) or lower leaves of plants spaced closely receive a greater proportion of far-red than red radiation. Plants perceive this filtering of light and in response, typically elongate in an attempt to capture available light. This phenomenon is called the “shade-avoidance response.” In some situations, an elongation response is desirable but in the production of ornamentals, often it is not.
Green and Yellow Light
Some of the green (500 to 570 nm) and yellow (570 to 600 nm) light that reaches the plant is reflected, giving the plant a green color.
Distribution of Light from Common Light Sources
High-pressure sodium (HPS) fixtures provide full-spectrum light, with heavier representation of middle wavelengths (green, yellow) and red/far red light. This light appears yellowish in color. Metal halide (MH) fixtures also provide full-spectrum light, but with more, blue and less red and far red than HPS. This heavier percentage of blue light tends to reduce stem elongation and impacts production of secondary compounds (e.g., improved anthocyanin production for leaf color). Light from MH fixtures does appear bluer in color.
Full-Spectrum versus Partial-spectrum Lighting
In commercial greenhouses, photoperiodic and supplemental lighting are two strategies used to better meet plant growth needs throughout the day, grow cycle, and season. Depending on the season and individual characteristics of greenhouse operations, either full-spectrum or partial-spectrum lighting may be used to meet plant requirements for supplemental or photoperiodic lighting where either or both are lacking.
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