The maximum intensity of sunlight is generally less than 2000umol/m2·s. Sunlight is a continuous spectrum with wavelengths ranging from 100nm to 100m. 99% of the energy radiation is concentrated in the range from 280 to 500nm. The wavelength of light greater than 760nm is called infrared light. Different spectra in sunlight contribute differently to plant photosynthesis, and only the range of 400-700nm is the photosynthetic active radiation of plants.
According to the perception ability of human eyes,there are two type of light including visible light and invisible light. The wavelength of visible light is 380-780nm, which is consist of red light (625-760nm), orange light (595-626nm), Yellow light (575-595nm), green light (470-575nm), blue light (4356-470nm) and purple light (380-450). Invisible light includes infrared light and ultraviolet light, and the wavelength of light less than 380nm is called ultraviolet light. ultraviolet light is divided into four part: long-wave ultraviolet (UV-A) with a wavelength of 320～380mm, medium-wave ultraviolet (UV-B) with a wavelength of 280～320nm, short-wave ultraviolet (UV-C) with a wavelength of 100～280mm and vacuum ultraviolet (<100nm).
Visible light shares for about 50% in the total radiation from solar, infrared light shares for 48%-49%, and the rest ultraviolet light shares for 1%-2%.
In the natural environment, the composition of the spectrum reaching on plants is constantly changing. There is more blue light on cloudy days, and more red light in the morning and evening,the strong and white light at noon on sunny days.
Light quality and wave length has become an important factor affecting plant photosynthesis. Therefore, the photosynthesis of crops can be improved by adjusting light quality. So,what color is the needed for plant grow?
What color light is needed for plant growth?
The sensitivity of plants to the wavelength of light is different from that of the human eye. The most sensitive wavelength of light to the human eye is 555nm between yellow and green light,and the less sensitive to the blue light and the red light range. However, the most sensitive wavelength of light to plant is the red light and less sensitive to green light, The most sensitive light range to plants is 400-700nm,which is usually called PAR. About 45% of the energy of sunlight in this range. Therefore, if an artificial light source is used to grow light, it should also be close to this range.
The energy of the photons emitted by the light source varies with the wavelength. For example, the energy of wavelength 400nm (blue light) is 1.75 times that of 700nm (red light). But for photosynthesis, the excess energy in the blue light that does not be used for photosynthesis is converted into heat. In other words, the photosynthesis rate of plants is determined by the number of photons that the plant can be absorbed in 400-700nm, and is not related to the number of photons emitted by each light.
Plants have different sensitivity to different wavelength of light. This reason comes from the special absorption of pigments in the leaves,such as: chlorophyll, carotenoids and phycobilidin etc.
Among them, chlorophyll is the most known. But chlorophyll is not the only pigment useful for photosynthesis. Other pigments also take in plant growth.
There are two types of Phytochromes,including far-red light absorption type (Pfr) and red light absorption type (Pr). They mainly absorb red light( 600-700nm) and far light (600-700nm) to regulate the physiological activities of plants. Phytochromes are mainly taken in the regulation of seed germination, seedling formation, establishment of photosynthetic system, shade, flowering time etc.
Cryptochrome is a blue light receptor, which mainly absorbs blue light(320-500nm) and near-ultraviolet light UV-A,their absorption peaks approximately at 375nm, 420nm, 450nm and 480nm. Cryptochromes are mainly taken in the regulation of flowering in plants. In addition, it is also involved in the regulation of phototropic growth , stomata opening, cell cycle, protecting cell development, root development etc.
Phototropin is other blue light receptor discovered after phytochrome and cryptochrome. It can regulate the phototaxis, chloroplast movement, stomata opening, leaf extension and hypocotyl elongation of chloroplast seedlings.
The photosynthesis path is also not related to color light,the light is absorbed by chlorophyll and carotene in leaves,then converted into glucose and oxygen by photosynthetic systems. This process to various colors on photosynthesis are almost the same.
The blue light (400-500nm) is very important for plant differentiation and stoma regulation. If the blue light is not enough and the too much ratio of far red light, the stems will grow excessively, resulting in yellowing of the leaves. The ratio of the red light (655~665nm) to the far red light (725~735nm) is between 1.0 and 1.2, it will be positive growth. But each plant has different sensitivity to red to blue ratio,click here to know more different red to blue ratio for plants.
In sunlight, blue light shares for 20%. For artificial light sources, it is not required high proportion. Most plants only need 6% blue light for normally developing.
The specific spectrum of LED grow light for indoor plants
UV light (100–400 nm)
The ultraviolet light is invisible to the human eye,the wavelength range fro 100nm to 400nm. Approximately 10% of sunlight is ultraviolet light. Just like humans, plants can be harmed by excessive ultraviolet light.
Although the benefits of ultraviolet light used in indoor plants are still being researched, a small amount of ultraviolet light is beneficial for increasing color, nutritional value, taste and aroma.
Studies have shown that using a amount of ultraviolet light can also reduce fungi and pests to protect plant grow better.
Blue light (400–500 nm)
The blue light is beneficial for improving plant quality—especially in leaf crops. It promotes stomata opening and allow more carbon dioxide to reach the leaves.
Blue light can promote plant to produce more chlorophyll, help seedlings better absorb and use the energy in photosynthesis, making plant grow and mature faster.
This is essential for seedlings in the early stage, as they will grow a healthy root and stem structure.
Green light (500–600 nm)
Compared with the red or blue light, the green light is less important for plant photosynthesis because it is not easily absorbed by chlorophyll. Nevertheless, green is still absorbed and used for photosynthesis.Only 5-10% is actually reflected-the rest is absorbed or reached to a lower place because of green light can penetrate the canopy of plants.
Red light (600–700 nm)
As we all know, red light is the most effective light for promoting photosynthesis because it is highly absorbed by chlorophyll pigments. Red light (especially around 660nm) is good to promote the growth of stems, leaves and general vegetative growth.
Although it is the most effective light to plant grow,it should be kept right red to blue light ratio to grow the best.
Far-red light (700-850 nm)
Far-red is very useful for promoting flowering and can increase fruit yield in some plants. For some short-day plants.far-red light(around 730nm) can be used to promote flowering.
However,Far-red light is generally used in proportion to red light. Due to the structure of the photosensitive pigment that absorbs red light and far-red light, red light and far-red light on plants is opposite.
If the white fluorescent light is the main light source in the growth room , LEDs are used to supplement far-red radiation, the content of anthocyanin, carotenoid and chlorophyll is reduced, while the fresh weight, dry weight, stem length, leaf length and leaf width of the plant increase .
The reason for supplementing far red light to promote plant to grow may be due to absorbing more light caused by the increase in leaf area.
Through the above introduction, we know that red light and blue light have the greatest impact on promoting plant growth, but it does not mean that other light is not needed. As for which color light is best for plant growth? There are too many factors to consider, such as plant species, red light ratio, total light, photoperiod, etc., All that will affect plant growth.
As photosynthesis,the color of the light does not affect the rate of photosynthesis, and therefore does not affect the fresh weight or dry weight. The main factors affecting the rate of photosynthesis are still the amount of light and temperature.
Light quality affects the morphology of tissue cultured seedlings, such as the height of the seedlings, the chlorophyll content of the leaves etc.