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 is divided into: violet (380 to 430 nm), blue (430 to 500 nm), green (500 to 570 nm), yellow (570 to 590 nm), orange (590 to 630 nm), and red (630 to 760). Visible light corresponds roughly to the photosynthetically active radiation (PAR) from about 400 to 700 nm. These wavelengths contain the right amounts of energy for the biochemical processes and their relative proportion in the available light is of prime importance in determining light quality. 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).
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 640 to 680 nm. These wavelengths encourage stem growth, tuber and bulb formation, flowering, and fruit production, and chlorophyll production.
Far-red light can cause plant elongation and trigger flowering in some long-day plants. Plant elongation results when this far-red ratio is low. In other words, plants are more exposed to far red than red. This can become a problem with greenhouse crops that are shaded by overhead baskets or are planted too close together.
Green and Yellow Light
Some of the green and yellow light that reaches the plant is reflected, giving the plant a green color. While most of the absorbed wavelengths are in the red and blue ranges, recent research shows that plants do absorb some green and yellow light and use it in the photosynthesis process. A light source that provides light in the entire visible range will better meet the needs of the plant.
Distribution of Light from Common Light Sources
The distribution of light for the most common light sources is summarized in Table 6.1. The radiant yield refers to the percentage of energy that is in the form of photosynthetic light. High-pressure sodium lamps for supplemental greenhouse lighting are used widely primarily because they have the highest radiant yield.
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 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.
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