Greenhouse Lighting
Light Quality and Photomorphogenesis
Light quantity (intensity and photoperiod) and quality (spectral composition) affect plant growth and physiology and interact with other environmental parameters and cultivation factors in determining plant behavior. More than providing the energy for photosynthesis, light also dictates specific signals that regulate plant development, shaping, and metabolism in the complex phenomenon of photomorphogenesis, driven by light colors. The key to photomorphogenesis lies in the plant's ability to perceive light. This is achieved through specialized proteins called photoreceptors.
Light Wavelengths Impact on Plant Development
The light spectrum, specifically important for plants, spans from ultraviolet (UV) to infrared, including the entire range of visible light, which humans see as the colors in a rainbow. Plants can perceive and respond to a slightly wider range than humans, from UV-B (280–315nm), UV-A (315–400nm), the entire visible spectrum (400–700nm), all the way to near-infrared (700–750nm), thanks to specialized photoreceptors.
UV Light
There are three kinds of UV radiation. UV-A has the longest wavelengths (315–400nm), is the least energetic, and is the most abundant form from sunlight. UV-B (280–315nm) and especially UV-C (100-280nm) are dangerous to people as well as plants, but fortunately, the ozone layer absorbs most UV-B and all UV-C.
Blue Light
The most important blue wavelengths are from 400 to 500 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. Blue light will prevent excessive internode (the space between branches along the main stem of a plant) elongation and will increase the number of chloroplasts in a plant, making them turn dark green.
Red Light
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, fruit production, and chlorophyll production. It also helps increase stem diameter and promotes branching. Seed germination is favored by red light, too. Red is usually the dominant color for photosynthetic and photoperiodic lighting. In short-day plants, the delivery of red light during the night can prevent flowering.
Far-Red Light
This waveband (700–800nm) is not considered photosynthetically active, but far-red light does promote extension growth and, in some crops, also flowering. Like UV, far-red light is outside the PAR waveband, and so has not been considered effective at increasing plant biomass. In contrast to PAR, leaves do not absorb far-red light well. A majority of far-red light that strikes a leaf is reflected back or transmits through the leaf. However, this light then becomes available to other leaves and plants nearby.
Green and Yellow Light
This waveband, from 500 to 600 nm, is nearly or as effective as blue and red light at increasing plant growth. Plants appear green to us because leaves reflect more green light than other visible wavelengths.
Plant Shading
Plant shading is typically caused by low light intensity, close plant spacing, overhead hanging baskets, greenhouse structures, and equipment. Plant shading decreases the absolute amount of light, but the magnitude of that decrease varies by the color, or waveband, of light. When a light particle (a photon) strikes a leaf, it can be absorbed, transmitted, or reflected.
Greenhouse Supplemental Lighting
Supplemental lighting in greenhouses alters the light spectrum by increasing the intensity of specific wavelengths and, thus, modifying the ratio of other wavelengths. These changes in light quality are more evident during low solar DLIs, such as in the winter months and cloudy days or when the supplemental lighting becomes sole-source lighting (i.e., before dawn and after dusk).
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.
Full-Spectrum Lighting
Full-spectrum lighting systems (light that covers the electromagnetic spectrum) are often designed to provide a closer equivalent to daily light integral (DLI) values offered by full daylight.
Partial-Spectrum Lighting
More commonly, partial-spectrum lighting is used to improve plant productivity, health, or other characteristics through the targeted addition of light at wavelengths or across spectral regions that are not sufficiently available under baseline lighting conditions.
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