As an important branch of modern agriculture, the concept of plant factories has become a hot spot. In the indoor planting environment, plant lighting is an essential energy source for photosynthesis. LED plant lights have overwhelming advantages that traditional supplementary lights do not have, and have become the first choice for main or supplementary lights in large commercial applications such as vertical farms and greenhouses.

    Plants are one of the most complex life forms in the world. Growing plants is both simple and difficult. In addition to plant lighting, many variables influence each other. Balancing these variables is a superb art that growers need to understand and master. But in terms of plant lighting, there are still many factors that need to be carefully considered.  

    First of all, you should know the spectrum of the sun and the absorption of the spectrum by plants. As can be seen from the figure below, the solar spectrum is a continuous spectrum, in which the blue and green spectrum are stronger than the red spectrum, and the visible spectrum ranges from 380 to 780 nm. However, there are several key absorption factors in plant growth, and several key auxins that affect plant growth have significant differences in light absorption spectra. Therefore, the application of LED plant growth lights is not a simple matter, but very targeted. Here, it is necessary to introduce the concept of two main photosynthetic plant growth factors.      
    Plant photosynthesis depends on the chlorophyll in leaf chloroplasts, and chlorophyll is the most important pigment related to photosynthesis. It exists in all organisms that can produce photosynthesis, including green plants, prokaryotic cyanobacteria and eukaryotic algae. Chlorophyll absorbs light energy and hydrates carbon dioxide into hydrocarbons.

    Chlorophyll a is blue-green and mainly absorbs red light; chlorophyll b is yellow-green and mainly absorbs blue-violet light. Mainly to distinguish shade plants from sun plants. The ratio of chlorophyll b to chlorophyll a of shade plants is small, so shade plants can use blue light strongly and adapt to growing in shade. There are two strong absorptions of chlorophyll a and chlorophyll b: the red light region with a wavelength of 630 to 680 nm, and the blue-violet light region with a wavelength of 400 to 460 nm.      
    Carotenoids are the general term for a class of important natural pigments, which are generally found in the yellow, orange or red pigments of animals, higher plants, fungi and algae. So far, more than 600 natural carotenoids have been discovered. The carotenoids produced in plants can not only absorb and transfer energy, help photosynthesis, but also protect cells from the destruction of excited single-electron bond oxygen molecules. Carotenoids absorb light in the range of 303~505 nm, provide the color of food, and affect people's food intake. In algae, plants, and microorganisms, the color is covered by chlorophyll and cannot be displayed.

    In the process of designing and selecting LED plant lights, there are several misunderstandings that need to be avoided, including the following aspects.      
   1.The ratio of red to blue wavelengths of light wavelengths

   As the two main absorption regions of the two kinds of photosynthesis, LED plant lights should have red and blue emission spectra. But it cannot be simply measured by the ratio of red to blue. For example, the ratio of red to blue is 4:1, 6:1, 9:1, and so on.      
   Plant species are very diverse and have different habits, and different growth stages also have different light focus needs. The spectrum required for plant growth should be a continuous spectrum with a certain distribution width. It is obviously inappropriate to use a light source made of two specific wavelength chips of red and blue with a very narrow spectrum. In the experiment, it was found that the plants will be yellowish, the leaf stems are very light, and the leaf stems are very thin. Many countries have conducted a large number of studies on the response of plants to different spectra, such as the role of infrared light on photoperiod, the role of yellow and green light on shading, and the role of violet light on resisting pests and nutrients, and so on.

    2. Ordinary white light and full spectrum

    The light effect "seen" by plants is different from the human eye. Your commonly used white light can not replace the sun's light, such as the three-primary white light tube widely used in Japan, etc. The use of these spectrums has a certain effect on the growth of plants, but the effect is not as good as the light source made by LED .

    For fluorescent tubes with three primary colors commonly used in previous years, although white is synthesized, the red, green, and blue spectra are separated, and the width of the spectrum is very narrow, and the continuous part of the spectrum is relatively weak. At the same time, the power is still relatively large compared to LEDs, 1.5 to 3 times the energy consumption. The full spectrum of LEDs designed specifically for plant lighting optimizes the spectrum. Although the visual effect is still white, it contains the important part of the light required for plant photosynthesis.    
    3. Photosynthetic Photon Flux Density（PPFD）

    photosynthesis flux density （PPFD) is an important parameter to measure the light intensity of plants. It can be expressed either by light quanta or by radiant energy. It refers to the effective radiant flux density of light in photosynthesis, which represents the total number of light quanta incident on the plant leaf stem in the wavelength range of 400 to 700 nm per unit time and unit area. The unit is μE·m-2·s-1 (μmol·m-2·s-1). The photosynthetically active radiation (PAR) refers to the total solar radiation with a wavelength in the range of 400 to 700 nm.    
    The light compensation saturation point of plants, also called light compensation point, means that the PPFD needs to be higher than this point for photosynthesis to be greater than respiration, and the growth of plants is greater than consumption before plants can grow. Different plants have different light compensation points, and it cannot be simply regarded as reaching a certain index, such as PPFD greater than 200μmol·m-2·s-1.

    The light intensity reflected by the illuminance meter used in the past is brightness, but because the spectrum of plant growth changes due to the height of the light source from the plant, the coverage of the light, and whether the light can pass through the leaves, etc., it is used as light when studying photosynthesis. Strong indicators are not accurate enough, and PAR is now mostly used.    
    Generally, positive plant PPFD> 50 μmol·m-2·s-1 can start the photosynthesis mechanism; while shade plant PPFD only needs 20 μmol·m-2·s-1. Therefore, when installing LED plant lights, you can install and set according to this reference value, select the appropriate installation height, and achieve the ideal PPFD value and uniformity on the leaf surface.

    Light formula is a new concept recently proposed, which mainly includes three factors: light quality, light quantity and duration. Simply understand that light quality is the spectrum most suitable for plant photosynthesis; light quantity is the appropriate PPFD value and uniformity; duration is the cumulative value of irradiation and the ratio of day to night time. Dutch agriculturists have discovered that plants use the ratio of infrared to red light to judge the day-night change. The infrared ratio increases significantly at sunset, and plants respond quickly to sleep. Without this process, it would take several hours for the plant to complete this process.    

    In practical applications, it is necessary to accumulate experience through testing and select the best combination.
    So today our sharing is over. If you want to know more about plant cultivation, you can contact the author of this article, Li Yang 15362904900.