Plants and light signaling
Energy is transported through the air by electromagnetic waves. Microwaves, radio or television waves, X-rays, ultraviolet rays or visible light are examples of electromagnetic waves, which are characterized by having different frequencies and wavelengths. The electromagnetic spectrum represents different frequencies and wavelengths that are known under different names (microwave, radio waves, visible light, etc.).
It was Albert Einstein who proposed in 1905 for the first time that light has both particle and wave nature. A beam of light includes a set of particles, called photons. Photons corresponding to longer wavelengths (lower frequencies) carry less energy than photons from short wavelength areas. Human eye captures visible light between 400 and 700 nanometer (nm) wavelength area, which corresponds approximately to the region of the spectrum that plants use for photosynthesis.
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Light between 400 and 700 nm is therefore referred to as PAR; photosynthetically active radiation. Sunlight has a continuous spectrum within and beyond the visible wavelengths. Human eye transforms different wavelengths into colors in human brain. Short wavelengths close to 400 nm are perceived as blue color and longer wavelengths in the 600nm area are seen as red light. Human eye has the most sensitive region in the yellow-green wavelength area.
Plant pigments, photoreceptors, and photosynthesis
Plants absorb the light spectrum in an almost similar range as the human eye, but unlike humans, they absorb best red and blue light.
One of the main molecules enabling plants to absorb light and use its energy to transform water and carbon dioxide into oxygen and complex organic molecules is called chlorophyll and the process is known as photosynthesis. Chlorophyll is a plant pigment found in the intracellular chloroplasts, they are green in color and are in fact responsible of the green coloration of leaves and stems. There are two main types of chlorophyll found in the higher plants; chlorophyll a and b, which differ from each other slightly by their light absorption curves.
The small difference allows them to capture different wavelengths, catching more of the sunlight spectrum. Chlorophylls absorb mainly red and blue light and reflect green wavelengths, which is why we see plants green.
However, chlorophyll is not the only plant pigment; the so-called accessory pigments (carotenoids, xanthophylls, etc.) and phenolic substances (flavonoids, anthocyanins, flavones and flavonoids) capture wavelengths other than only red and blue. The accessory pigments are yellow, red and violet in color. These colors attract insects and birds, as well as help protect tissues from environmental stress, such as high light irradiation.
There are also other particles absorbing light; photoreceptors. The main photoreceptor groups are phytochromes, phototropins and cryptochromes. In addition, there is a specific photoreceptor for ultraviolet light; the UVR8. All photoreceptors capture light in different wavelength areas and are responsible of different responses in plants as described below:
- Phototropins affect the location of the chloroplasts and the stomatal opening. They absorb blue light.
- Cryptochromes capture external stimuli related to light and control the internal clock of plants. In addition, they are related to morphological responses, such as inhibition of stem elongation, expansion of cotyledons, production of anthocyanins and photoperiodic flowering. Cryptochromes absorb UVA (ultraviolet), blue, and green wavelengths.
- Phytochromes are responsible for flowering induction and seed development. Phytochromes regulate stem elongation, leaf expansion, and “shade avoidance syndrome”. The responses regulated by phytochromes are mediated by the ratio of surrounding red and far-red light, which affects the photostationary state of the phytochrome molecule.