The best bands of spectrum for plant growth Sep 25 , 2021

The sensitivity of plants to the spectrum is different from that of the human eye. The most sensitive spectrum of the human eye is 555nm, which is between yellow and green light. The sensitivity to the blue light zone and the red light zone is poor. This is not true for plants, which are most sensitive to the red light spectrum and less sensitive to green light, but the difference in sensitivity is not as great as the human eye. The most sensitive area of plants to the spectrum is 400-700nm. This section of the spectrum is usually called the effective energy region of photosynthesis. Approximately 45% of the energy of sunlight lies in this section of the spectrum. Therefore, if an artificial light source is used to supplement the amount of light, the spectral distribution of the light source should also be close to this range.

   The photon energy emitted by the light source varies with the wavelength. For example, the energy of a wavelength of 400nm (blue light) is 1.75 times the energy of 700nm (red light). But for photosynthesis, the effect of the two wavelengths is the same. The excess energy in the blue spectrum that cannot 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 absorb in 400-700nm, and is not related to the number of photons sent by each spectrum. However, most people generally believe that the color of light affects the rate of photosynthesis. Plants have different sensitivity to all spectra. This reason comes from the special absorption of pigments in the leaves. Among them, chlorophyll is the most known. But chlorophyll is not the only pigment useful for photosynthesis. Other pigments also participate in photosynthesis, so the photosynthesis efficiency cannot only consider the absorption spectrum of chlorophyll.

   The difference in photosynthesis path is also not related to color. Light energy is absorbed by chlorophyll and carotene in leaves. Energy is converted into glucose and oxygen by fixing water and carbon dioxide by two photosynthetic systems. This process uses all the visible light spectrum, so the effects of light sources of various colors on photosynthesis are almost the same.

   Some researchers believe that the orange-red light part has the greatest photosynthesis capacity. But this does not mean that plants should be cultivated in such a monochromatic light source. Regarding the morphological development and leaf color of plants, plants should receive a variety of balanced light sources.

   Blue light source (400-500nm) is very important for plant differentiation and stoma regulation. If the blue light is not enough and the ratio of far red light is too much, the stems will grow too much, which will easily cause the leaves to yellow. The ratio of the energy of the red light spectrum (655~665nm) to the energy of the far red light spectrum (725~735nm) is between 1.0 and 1.2, and the development of plants will be positive growth. But each plant has different sensitivity to these spectral ratios.

   High pressure sodium lamps are often used as artificial light sources inside the greenhouse. Take Philips Master SON-TPIA lamp source as an example, it has the highest energy in the orange-red spectral region. However, the energy of far-infrared light is not high, so the ratio of red light/far-red light energy is greater than 2.0. However, since the greenhouse still has natural sunlight, it did not cause the plants to become shorter. (If this light source is used in the growth box, it may have an impact.)

   In natural sunlight, blue light energy occupies 20%. For artificial light sources, such a high proportion is not required. For normal plants, most plants only need 6% blue light energy in the range of 400-700nm. In natural sunlight, there is enough blue light energy. Therefore, the artificial light source does not need to supplement more blue light spectrum. However, when the natural light source is insufficient (such as winter), the artificial light source needs to increase the blue light energy, otherwise the blue light source will become a limiting factor for plant growth. However, if the light source improvement method is not used, there are still other methods to remedy the problem of insufficient light source. For example, temperature regulation or the application of growth hormones.

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