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How Does Colored Light Affect Plants?

By D.C. Winston ; Updated September 21, 2017

Visible Light Spectrum

A plant, like the human eye, makes use of the visible light spectrum to fuel photosynthesis and other life processes. Visible light is a representation of radiant energy in the form of light waves that measure in intensity between 400 and 700 nanometers (nm). This range extends from violet (400nm) to red (700nm). Plants, which absorb different wavelengths at varying rates and reflect some, use the red- and blue-light wavelengths to manufacture food, grow foliage and produce flowers. Plants' surface cells capture the light energy, which fuels the breakdown of nutrients and water molecules into elements to power the plant's life functions, including transpiration, respiration and reproduction.

Red Spectrum Light

Photosynthesis relies principally on a large amount of energy absorbed from the high end of the visible light spectrum. In this respect, it is the most important portion of the light spectrum to plant life The red-light-wavelength energy absorbed by the plant cells combines with carbon dioxide and water to manufacture simple plants sugars in mesophyll cells that are embedded near the top and bottom surface layers of leaves and, in a few plants, the green stalks or stems as well. The more red light energy applied to the chloroplast cells the more intensely and quickly those cells carry out photosynthesis.

Blue Spectrum Light

The blue and violet portion of the visible light spectrum is the most readily absorbed by plants. More important than red light to the plants foliage growth and bloom, blue and violet light fuels vegetative and root growth along with a small, but not significant, portion of photosynthesis. When blue light wavelengths bombard the plant cells, several process occur. It spurs phototropism, which forces directional growth of the plant toward the light source. It also agitates chloroplast cells to grow and reproduce, resulting in leaf and vegetative growth. Blue light plays a role in modulating the opening and closing of the stomata, which aid in the plant's respiration, the process by which waste oxygen is expelled and carbon dioxide drawn in. Blue light is also significant in photoperiodism and triggering seasonal bloom. With only blue light, and no red-spectrum light for photosynthesis, plants would grow for a little while but produce weak, pale growth. When a plant's nutrient stores become depleted, the plant will die despite the application blue light energy.

Green Spectrum Light

Green light exists in the middle of the visible light spectrum (500nm). The green chlorophyll that populate each chloroplast plant cell in every green plant do not metabolize or make use of the green light spectrum. This is largely thought to be a evolutionary adaptation in plants. Theories on why this adaptation developed vary but there are two main thoughts. First, as sunlight is mostly made of blue and red spectrum light, plants evolved to run photosynthesis most efficiently with blue and red light energy. Second,most plants were once aquatic plants, and green light cannot penetrate water as well as blue and red light can. As a result, green light is not needed or absorbed, so it is reflected back, making plants appear green to our eyes.