First, let's understand the spectrum of the sun, as shown in Figure 1. It can be seen that the solar spectrum is a continuous spectrum, in which the blue and green spectrum is stronger than the red spectrum, and the visible light spectrum ranges from 380 to 780 nm. The growth of organisms in nature is related to the intensity of the spectrum. For example, most plants in the area near the equator grow very fast, and at the same time, the size of their growth is relatively large. However, the sun's irradiation intensity is not as high as possible, and there is a certain degree of selectivity for the growth of animals and plants.
Figure 1 The characteristics of the solar spectrum and its visible light spectrum
Secondly, the second spectrum diagram of several key absorption elements of plant growth is shown in Figure 2.
Figure 2 Schematic diagram of absorption spectra of several auxins in plant growth
It can be seen from Figure 2 that the light absorption spectra of several key auxins that affect plant growth are significantly different. Therefore, the application of LED plant growth lights is not a simple matter, but very targeted. Here it is necessary to introduce the concepts of the two most important photosynthetic plant growth elements.
Chlorophyll is one of the most important pigments related to photosynthesis. All organisms can create photosynthesis, including green plants, prokaryotic blue-green algae (cyanobacteria), and eukaryotic algae. Chlorophyll absorbs energy from light, which is then used to convert carbon dioxide into carbohydrates.
Chlorophyll mainly absorbs red light, and chlorophyll b mainly absorbs blue-violet light, to distinguish shade plants from solar 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. Chlorophyll a is blue-green, and chlorophyll b is yellow-green. There are two strong absorptions of chlorophyll a and chlorophyll b, one in the red region with a wavelength of 630-680 nm, and the other is in the blue-violet region with 400-460 nm.
Carotenoids (carotenoids) are a general term for a class of important natural pigments, which are commonly found in yellow, orange-red, or red pigments in animals, higher plants, fungi, and algae. So far, more than 600 natural carotenoids have been discovered.
The light absorption of carotenoids covers the range of OD303~505 nm. It provides the color of food and affects the human body's intake of food; in algae, plants, and microorganisms, its color cannot be presented because it is covered by chlorophyll. In-plant cells, the carotenoids produced not only absorb and transfer energy to help photosynthesis but also have the function of protecting cells from being destroyed by excited single-electron bond oxygen molecules.
Some conceptual misunderstandings
Regardless of the energy-saving effect, the selectivity of light, and the coordination of light, semiconductor lighting has shown great advantages. However, from the rapid development of the past two years, we have also seen a lot of misunderstandings in the design and application of light, which are mainly reflected in the following aspects.
①As long as the red and blue chips of a certain wavelength are combined in a certain ratio, they can be used in plant cultivation, for example, the ratio of red to blue is 4:1, 6:1, 9:1, and so on.
②As long as it is a white light lamp, it can replace the sun's light, such as the three-primary white light tube widely used in Japan. 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.
③As long as the PPFD (light quantum flux density), an important parameter of illumination, reaches a certain index, for example, PPFD is greater than 200 μmol·m-2·s-1, but when using this index, be sure to pay attention to it. Living plants are still sun plants. You need to search or find the light compensation point of these plants, which is also called the light compensation point.
In actual applications, seedlings are often burned or withered. Therefore, the design of this parameter must be designed according to the plant species, growth environment, and conditions.
Regarding the first aspect, as introduced in the introduction, the spectrum required for plant growth should be a continuous spectrum with a certain distribution width, using two specific red and blue spectra with very narrow spectra (as shown in Figure 3(a)). The light source made of the wavelength chip is obviously inappropriate. In the experiment, it was found that the plant will be yellowish, the leaf stem is very light, and the leaf stem is very thin.
For fluorescent tubes with three primary colors commonly used in previous years, although white is synthesized, the red, green, and blue spectra are separated (as shown in Figure 3(b)), and the width of the spectrum is very narrow. The spectral intensity of the following continuous part is relatively weak, and the power is still relatively large compared to LEDs, 1.5 to 3 times the energy consumption. Therefore, the use effect is not as good as LED lights.
Figure 3 Red and blue chip LED plant light and three-primary color fluorescent light spectrum
PPFD is the light quantum flux density, which refers to the effective radiation light flux density of light in photosynthesis, which represents the total number of light quanta incident on plant leaf stems in the wavelength range of 400 to 700 nm per unit time and unit area. Its unit is μE·m-2·s-1 (μmol·m-2·s-1). Photosynthetically active radiation (PAR) refers to the total solar radiation with a wavelength in the range of 400 to 700 nm. It can be expressed either by light quanta or by radiant energy.
The intensity of light 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 plants with PPFD>50 μmol·m-2·s-1 can start the photosynthesis mechanism; while shade plants with PPFD only need 20 μmol·m-2·s-1. Therefore, when purchasing LED plant lights, you can choose the number of LED grow lights based on this reference value and the type of plants you plant. For example, if a single LED plant bulb PPFD is 20 μmol·m-2·s-1, more than 3 LED plant bulbs are required to grow positive plants.
Several design schemes of semiconductor lighting
Semiconductor lighting is used for plant growth or planting, and there are two basic reference methods.
• At present, the indoor planting model is very hot in China. This model has several characteristics:
①The role of LED lights is to provide the full spectrum of plant lighting, and the lamps are required to provide all the lighting energy, and the production cost is relatively high;
②The design of LED grow lights needs to consider the continuity and integrity of the spectrum;
③It is necessary to effectively control the lighting time and lighting intensity, such as letting the plants rest for a few hours, the intensity of the irradiation is not enough or too strong, etc.;
④The whole process needs to imitate the conditions required by the actual optimal growth environment of plants outdoors, such as humidity, temperature and CO2 concentration.
• Outdoor planting mode with a good outdoor greenhouse planting foundation. The characteristics of this model are:
① The role of LED lights is to supplement light. One is to enhance the light intensity in the blue and red areas under the condition of sunlight during the day to promote photosynthesis of plants, and the other is to compensate when there is no sunlight at night to promote plants. Growth rate;
② The supplementary light needs to consider which growth stage the plant is in, such as the seedling period or the flowering and fruiting period.
Therefore, the design of LED plant boosting lights should first have two basic design modes, namely, all-weather lighting (indoor) and plant growth supplement light (outdoor). For indoor plant cultivation, the design of LED plant booster lights needs to consider three aspects, as shown in Figure 4. It is not possible to package the chips with three primary colors in a certain proportion.
Figure 4 The design idea of using indoor LED plant booster lights for all-weather lighting
For example, for a spectrum in the nursery stage, considering that it needs to strengthen the growth of roots and stems, strengthen the branching of leaves, and the light source is used indoors, the spectrum can be designed as shown in Figure 5.
Figure 5 Spectral structures suitable for LED indoor nursery period
The design of the second type of LED plant booster lamp, it is mainly aimed at the design plan of supplementary light boosting for the base planted in the outdoor greenhouse. The design idea is shown in Figure 6.
Figure 6 Design ideas of outdoor plant boosting lights
The author suggests that more planting companies adopt the second option to use LED lights to promote plant growth.
First of all, China’s outdoor greenhouse planting has decades of experience, with a large amount and a wide range, both in the south and the north. It has a good base of greenhouse planting technology and provides a large number of fresh fruits and vegetables on the market for surrounding cities, especially where there is soil, water, and fertilizer. A wealth of research results have been achieved in planting.
Secondly, this kind of supplemental light solution can greatly reduce unnecessary consumption of energy, and at the same time can effectively increase the yield of fruits and vegetables. In addition, my country has a vast geographical area, which is very convenient for promotion.
As the scientific research of LED plant lighting, also provides a broader experimental base for it. Figure 7 is a kind of LED booster lamp developed by this research team suitable for growing light in greenhouses, and its spectrum is shown in Figure 8.
Figure 7 A kind of LED plant growth promotion light
Figure 8 A kind of LED plant growth promoting light spectrum
According to the above design ideas, the research team conducted a series of experiments, and the experimental results are very significant. For example, the nursery booster lamp originally used a fluorescent lamp with a power of 1000 W and a nursery cycle of 40 days.
We provide a 640 W LED light, which shortens the seedling cycle to 30 days, effectively reduces the influence of the temperature of the lamps in the seedling workshop and saves the power consumption of the air conditioner. The thickness, length, and color of the seedlings are better than the original seedling raising scheme.
This article introduces the latest developments in the application of LED grow lights in plant cultivation and cultivation and points out some misunderstandings in the application of LED grow lights in plant cultivation and cultivation. Finally, the technical ideas and schemes for the development of LED facilitating lights used for plant cultivation and planting are introduced. It should be pointed out that there are also some factors that need to be considered in the installation and use of the light, such as the distance between the light and the plant, the irradiation range of the light, and how to cooperate with normal water, fertilizer and soil, etc.