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 the plant behavior. More than providing the energy for photosynthesis, light also dictates specific signals which 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. Photoreceptors absorb specific wavelengths of light and convert them into signals that alter cellular processes, triggering changes in the plant's growth and development.
Light Wavelengths Impact on Plant Development
he 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. UV typically inhibits extension growth (shorter stems and smaller leaves) and increases leaf thickness, waxiness, and coloration.
Blue Light
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.
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, and fruit production, and chlorophyll production. It also helps increase stem diameter and promotes branching.
Far-Red Light
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.
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. Most photosynthetic light (blue, green, and red light) is absorbed by leaves, and a relatively small percentage is reflected by or transmitted through them. In contrast, leaves absorb less far-red light and more of it is reflected or transmitted to other leaves. Therefore, plant shading decreases the ratio of red light to far-red light.
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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