The light in nature comes from the sun, and the solar spectrum can be roughly divided into three parts: ultraviolet light <400nm (uv-a315-400nm, uv-b280-315nm, uv-c100-280nm), far red light and infrared light> 700nm ( Far-red light 700-780nm, infrared light 780nm-1000μm), photosynthetically active radiation 400-700nm (blue-violet light 400-500nm, green light 500-575nm, yellow-orange light 575-620nm, red light 620-700nm). Among them, the mid-ultraviolet UV-B and far-ultraviolet UV-C are mostly absorbed by the ozone layer above the earth, and the ultraviolet light reaching the ground is mainly near-ultraviolet UV-A.
Light is the basic environmental factor for plant growth and development. Light not only supplies the energy needed for plant growth through photosynthesis, it is also an important regulatory factor for plant growth and development.
1. Photoreceptors of plants
A series of responses of plants to the external light environment are all based on the absorption of light by photoreceptors. The main photoreceptors include photosynthetic pigments, phytochromes, cryptochromes and phototropins. They perform their respective duties in plants, affecting all aspects of plant photosynthesis physiology, metabolic physiology, and morphogenesis.
1.1 Photosynthetic pigment
Photosynthetic pigments are the basic building blocks of the light system. Photosynthetic pigments include chlorophyll a, chlorophyll b and carotenoids. Mainly responsible for photosynthetic processes such as light energy reception, energy transfer, and photoelectric conversion in photosynthesis. Experiments show that the main absorption wavelength of chlorophyll is 640-663nm, and there is a secondary absorption peak at 430-450nm. The carotenoids are more of a protective effect on the body. In photosynthesis, due to the existence of the two light systems Ps II and Ps I, it shows that when red light and far-red light are irradiated together, the photo-synthesis rate is much higher than the dual-light gain phenomenon of monochromatic irradiation.
Phytochromes are formed by the covalent bonding of chromophores and apoproteins, including far-red light absorption type (Pfr) and red light absorption type (Pr). They mainly absorb red light at 600-700nm and 700- The far-red light of 760nm regulates the physiological activities of plants through the reversible effects of far-red light and red light. In plants, phytochromes are mainly involved in the regulation of seed germination, seedling formation, establishment of photosynthetic system, shade avoidance, flowering time and circadian rhythm response. In addition, it also plays a regulatory role in the plant’s stress resistance physiology.
Cryptochrome is a blue light receptor, which mainly absorbs blue light and near-ultraviolet light UV-A at 320-500nm, with absorption peaks approximately at 375nm, 420nm, 450nm and 480nm. Cryptochromes are mainly involved in the regulation of flowering in plants. In addition, it is also involved in the regulation of plant tropism, stomata opening, cell cycle, guard cell development, root development, abiotic stress, apical dominance, fruit and ovule development, programmed cell death, seed dormancy, pathogen response And magnetic field induction.
luciferin is a blue light receptor discovered after phytochrome and cryptochrome. It can be phosphorylated after binding to flavin mononucleotide. It can regulate the phototaxis of plants, chloroplast movement, stomata opening, leaf extension and inhibit hypocotyl elongation of chlorophyll seedlings.
2. The effect of light quality on plants
Different light qualities or wavelengths of light have significantly different biological effects, including different effects on the morphological structure and chemical composition of plants, photosynthesis, and organ growth and development.
2.1 Red light
Red light generally inhibits internode elongation of plants, promotes tillering, and increases the accumulation of chlorophyll, carotenoids, soluble sugars and other substances. Red light promotes the growth of leaf area and β-carotene accumulation of pea seedlings; lettuce seedlings are pre-irradiated with red light and then applied with near-ultraviolet light. It is found that red light can enhance the activity of antioxidant enzymes and increase the content of near-ultraviolet absorbing pigments, thereby reducing near-ultraviolet light. Ultraviolet light damages lettuce seedlings; full-light experiments on strawberries found that red light is beneficial to increase the content of organic acids and total phenols in strawberries.
Blue light can significantly shorten the pitch of vegetables, promote the horizontal expansion of vegetables, and reduce leaf area. At the same time, blue light can also promote the accumulation of plant secondary metabolites. In addition, experiments have found that blue light can reduce the inhibition of red light on the photosynthetic system activity and photosynthetic electron transport capacity of cucumber leaves. Therefore, blue light is an important factor affecting the photosynthetic system activity and photosynthetic electron transport capacity. There are obvious species differences in the blue light needs of plants. After harvesting strawberries, it was found that 470nm in blue light of different wavelengths had obvious effects on the content of anthocyanins and total phenols.
2.3 Green light
Green light has always been a controversial light quality. Some scholars believe that it will inhibit the growth of plants, cause short plants and reduce vegetable yields. However, there are also many studies on the positive effects of green light on vegetables. A low proportion of green light can promote the growth of lettuce; adding 24% of green light on the basis of red and blue light can promote the growth of lettuce.
2.4 yellow light
Yellow light basically inhibits plant growth, and because many researchers incorporate yellow light into green light, there is very little literature on the effect of yellow light on plant growth and development.
2.5 UV light
Ultraviolet light is generally more manifested as a killing effect on organisms, reducing plant leaf area, inhibiting hypocotyl elongation, reducing photosynthesis and productivity, and making plants more susceptible to infection. However, appropriate supplementation of ultraviolet light can promote the synthesis of anthocyanins and flavonoids. By adding a small amount of UV-B to the postharvest cabbage to promote the synthesis of its polyphenols; postharvest UV-c treatment can slow down the fruit of red pepper Glue dissolution, quality loss and softening process, thereby significantly reducing the spoilage rate of red peppers, extending the shelf life, and can promote the accumulation of phenols on the surface of red peppers. In addition, ultraviolet light and blue light affect the elongation and asymmetric growth of plant cells, thereby affecting the directional growth of plants. UV-B radiation causes dwarf plant phenotypes, small and thick leaves, short petioles, increased axillary branches, and changes in the root/shoot ratio.
2.6 Far red light
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, the effects of red light and far-red light on plants can be mutually converted and mutually offset. When the white fluorescent lamp is the main light source in the growth room, LEDs are used to supplement far-red radiation (emission peak 734nm), 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 effect of supplementing FR on growth may be due to the increase in light absorption caused by the increase in leaf area. Arabidopsis thaliana treated with low R/FR has larger and thicker leaves, increased biomass, and accumulation of more soluble metabolites, which improves cold resistance than that treated with high R/FR.
3. The effect of light quality on plant tissue culture
In the process of plant tissue culture, seedling morphology and physiological and biochemical changes are regulated by many environmental factors (light, temperature, humidity, etc.). Among them, light plays an extremely important role in the growth and differentiation of plant cells, tissues and organs. In the process of plant tissue culture, each morphological stage from explant callus induction to formation of complete plants is affected by LED light quality, and different tissue culture stages of different plants have different responses to light quality.
3.1 The effect of LED light quality on callus induction, growth and differentiation
3.1.1 Effect on callus induction
callus culture is an important part of plant in vitro culture. The study found that 100% red light has the highest induction rate for orchid callus, and the growth effect of callus is the best when the ratio of red to blue light is 3:1. Monochromatic red LED promotes the formation of callus of Anthurium andraeanum, but as the proportion of blue light increases, the induction rate of leaf callus gradually decreases. Red light and white light promoted the induction of callus from pepper cotyledons, while green light and blue light showed an inhibitory effect. Yellow light is beneficial to the induction of callus from radish hypocotyls, while blue light promotes the induction of cotyledon callus. Red light significantly promotes the induction and proliferation of garlic callus, while blue light has the strongest effect on promoting the differentiation of callus of Chinese yam. Yellow light is most conducive to the proliferation of grape callus, followed by green light. Yellow light is beneficial to the induction of callus from the hypocotyl of radish, while blue light is beneficial to the induction of cotyledon callus, and red light is beneficial to the proliferation of callus. Red light is beneficial to the induction and proliferation of oncidium protocorm callus. Gladiolus protocorm callus had the highest proliferation rate under red light. The callus induction rate of orchids was the highest under red light. Blue light and yellow light obviously promoted the proliferation and growth of birch callus. It can be seen that the effect of different light quality on callus induction varies with plant species or explant types.
3.1.2 Effect on the growth of callus
The growth curve of callus under different light quality is “s” shape, but the effect of different light quality on callus growth is different due to different plant genotypes and matrix additives. Yellow LED promotes the growth of Vietnamese ginseng callus. Red and blue LEDs inhibit the growth of callus. Among them, red LED has the strongest inhibitory effect, while green and white LEDs have no significant effect on callus growth. The effect of different light quality on the proliferation of broccoli callus is white light>red light, blue light>green light>yellow light, and the soluble protein content and water content of callus under different light quality are related to callus proliferation. sex. The growth of callus of radish was the highest under red light, while the effect of yellow light on the growth of callus was the lowest. When cinnamic acid is not added, yellow light is most beneficial to the proliferation of callus of grape leaves. When cinnamic acid and light quality work together, green light is most suitable for the proliferation of callus, and the growth effect of yellow light on callus is obviously weakened. .
3.1.3 Effect on the differentiation of callus buds
Light quality plays an important role in bud differentiation. Red light obviously promoted the differentiation and emergence of sugarcane callus. The number of adventitious buds of Oncidium protogium under blue light is the largest. Both blue light and red blue light inhibited the differentiation of adventitious buds of lettuce explants. The germination rate of garlic callus is the highest under red light, reaching 25%, followed by white light. Blue light and mixed red and blue light have an inhibitory effect on the germination of callus. Among them, blue light has the strongest inhibitory effect, and the germination rate is only 3%. . Red light promotes the induction of adventitious buds from callus of Anthurium andraeanum leaves, while blue light is more conducive to the increase in the number of adventitious buds. The callus bud differentiation rate of Chinese yam was the highest under blue light, followed by red light and white light, while green light and yellow light were lower. The budding rate of maca callus under blue light is almost zero. Red and blue light are beneficial to the differentiation of adventitious buds of callus, blue light promotes the increase of adventitious buds through cryptochrome, and red light regulates the apical dominance through phytochrome to promote the growth of adventitious buds.
3.2 The effect of LED light quality on the proliferation of tissue culture seedlings
Research found that red light in monochromatic light can promote the proliferation of tissue culture seedlings. The number of single buds of Phalaenopsis under the pure red LED was significantly increased compared with the control fluorescent lamp, and the experiment of chrysanthemum and tobacco also reached similar conclusions. The single blue light is not conducive to the proliferation of tissue culture seedlings. In the study of eustoma and sugarcane tissue culture seedlings, it was found that the tissue culture seedlings under the monochromatic blue light treatment had the lowest proliferation coefficient among all the light quality treatments. But blue LED can effectively promote the formation of Phalaenopsis protocorm.
A large number of experiments have proved that, compared with monochromatic LEDs, different LED combinations are more conducive to the proliferation of tissue culture seedlings. LED red and blue combined light can promote the proliferation of sugarcane adventitious buds better than monochromatic light, and is better than fluorescent lamps and plant growth lamps. The regeneration effect of adventitious buds from the leaves of Rhododendron alpina under the combined treatment of red and blue light was significantly better than that of 100% red light and blue light. However, different plants or different varieties of the same species have different requirements for the light quality ratio during the tissue culture proliferation stage. Rhododendron alpina had the best leaf adventitious bud regeneration under the treatment of red and blue light (3:1), while sugarcane had the highest number of adventitious buds proliferated under the treatment of red and blue light (4:1).
3.3 The effect of LED light quality on the growth and development of tissue culture seedlings
3.3.1 The effect of LED light quality on the growth of tissue culture seedlings
Research has proved that the effect of LED monochromatic light on the growth of tissue culture seedlings is lower than that of different LED combined light, and the red and blue LED combined light can enhance plant photosynthesis to promote plant growth and development. The growth of the white palm tissue culture seedlings treated with a single red or blue LED is poor, and a certain ratio of red and blue LED composite light is beneficial to promote plant growth. The net photosynthetic rate of chrysanthemum tissue culture seedling leaves under the combination of red and blue LED light was significantly higher than that of monochromatic red and blue light, and the fresh weight and dry weight of the plant reached the maximum. The aboveground dry and fresh weight of strawberry sugar-free tissue cultured seedlings was the smallest under blue light. Dori Phalaenopsis has the highest fresh weight and dry weight under red and blue light.
The optimal ratio of red and blue LEDs for the growth of tissue culture seedlings, the research conclusions of different plants are not consistent. The growth of tissue culture seedlings of Japanese double butterfly and strawberry under 70% red light + 30% blue light is the best. However, the growth indexes of Anthurium andraeanum tissue culture seedlings under the treatment of 50% red light + 50% blue light were significantly higher than those of the control. Red light (R) treatment of white and tissue cultured seedlings grow, blue light (B) treatment of white and tissue cultured seedlings are low, complex light is conducive to the growth and morphology of white and white; 1RB light source treatment of white and tissue cultured seedlings soluble sugar The content was significantly higher than other treatments; red light and blue light (1:1) were most conducive to the accumulation of soluble sugar in white and tissue culture seedlings.
Therefore, in the application of tissue culture production, adjusting the best red and blue ratio is the key to producing good quality tissue culture seedlings.
3.4 The influence of LED light quality on tissue culture seedlings taking root and strong seedlings
The effect of light quality on the induction and growth of in vitro plant roots varies with different wavelengths, and the effect of light quality depends on the plant genotype and rooting material concentration. Red light promotes the formation of adventitious roots in tissue culture seedlings of anthurium, phalaenopsis, imperial flower and ground cover chrysanthemum, which is characterized by fast and dense rooting, and blue light has obvious inhibitory effect. Papaya tissue cultured seedlings have the shortest root length under blue light; red and blue mixed light can promote the growth of sweet potato tissue cultured seedlings to a certain extent. However, the root morphology of tissue culture seedlings under monochromatic red light is abnormal, and the survival rate of transplantation is low. Blue light helps to improve the vigor of the later roots, promotes dry matter accumulation, reduces water content, and prevents plant vitrification. The root vigor of chrysanthemums irradiated by monochromatic red light was low, and the survival rate of transplanting was only 75%, while the transplanted tissue culture seedlings under the combination of red and blue light all survived. The root length and root vitality of Phalaenopsis treated with LED combination with far red light increased significantly compared with the control. Oncidium tissue culture seedlings have the longest root length under the combination of red and blue light, and the shortest root length under fluorescent light.
The LED light source used in the tissue culture stage will affect the growth and survival of the tissue culture seedlings after transplanting outside the room. The red and blue LED combined light source used in the indoor tissue culture stage can increase the survival rate of tissue culture seedlings of strawberry, white palm and chrysanthemum and promote the growth of seedlings after transplantation. Therefore, for plant tissue culture seedlings that are difficult to root, the rooting rate and number of roots can be increased through the pretreatment of red light in the early stage, and then transferred to a certain ratio of red light, blue light and far-red light for cultivation to promote root growth and development And improve root vitality, thereby improving the adaptability of tissue culture seedling transplanting.
4. The effect of light quality on vegetable seedlings
Light quality has a significant effect on the growth, development and photosynthesis of plant seedlings. Red light is conducive to the elongation of vegetable seedling stems and dry matter accumulation. Blue light is conducive to protein accumulation and the activity of antioxidant enzymes. Combination light is more conducive to the photosynthesis and growth of vegetable seedlings than single light quality. Ultraviolet radiation can reduce the leaf area of a single plant, inhibit the elongation of hypocotyls, reduce photosynthesis and productivity, make plants vulnerable to pathogens, but can induce flavonoid synthesis and defense mechanisms; it can also significantly reduce the height and dryness of soybeans. Heavy weight and water content will damage the photosynthetic pigments of seedlings more seriously. Blue light can inhibit the elongation of the hypocotyl of red bean sprouts and the stems of tobacco seedlings, and reduce the relative growth rate; it has an extremely important effect on the growth of plant leaves and roots. It can reduce the leaf area and reduce the number of leaves of lettuce seedlings. It is beneficial to promote the synthesis of nutrients related to flower bud differentiation and flower formation. Green light is not an efficient absorption spectrum for photosynthesis, but supplementing green light can synergistically increase the synthesis of pigments with red and blue light, which can significantly increase the plant height and stem thickness of tomato seedlings, and promote the growth of pea sprouts. Red orange light is beneficial to the growth of stems, and promotes the flowering of plants and the formation of chlorophyll, shortens the growth cycle, and increases the soluble sugar content and yield. Far red light can increase the dry and fresh weight, stem length, leaf length and leaf width of the plant; but in many cases it will offset the red light effect and reduce the content of anthocyanins, carotenoids and chlorophyll. In the morning, cucumber seedlings were supplemented with low-intensity blue and red light for two hours, and it was found that supplemental light increased the fresh weight, leaf area and stem thickness of the seedlings. The use of red LEDs for night time delay supplement light can promote the early growth of cucumber seedlings, and the red and blue mixed light night delay supplement light can promote the later growth of cucumber seedlings and improve the seedling index. Using LED red and blue light as the light source can effectively promote the morphology of cowpea, bitter gourd, lettuce and pepper seedlings. With the enhancement of LED red and blue light, the morphological indicators of seedlings gradually increase, chlorophyll synthesis gradually increases, and root vitality gradually increases.
Different LED light quality has significant and different effects on the growth of seedlings of different varieties of cucumber, pepper and tomato. Supplementing red light or red and blue light in the seedling stage can promote the growth of seedlings, which is conducive to the cultivation of strong seedlings. Supplementing light can increase the content of flavonoids and total phenols in tomatoes and peppers, enhance the activity of the antioxidant enzyme system cAt and soD, and help improve the plant’s resistance to stress and adaptability to the environment. After being transplanted to the field, the growth stage, the growth rate and the agronomic characteristics of the maturity stage are better than the conventional seedlings in the greenhouse, and the difference in the number of leaves and the thickness of the stem reaches a very significant level; the length and width of the flue-cured tobacco leaves Ratio, single leaf weight, thickness, specific leaf weight, etc. are far better than the control.
As an important characteristic of the light environment, light quality directly or indirectly affects the synthesis and transportation of plant hormones. Irradiation of red and blue light in the seedling stage can significantly promote the growth of vegetable seedlings and improve the index of strong seedlings. Different wavelengths of light can regulate the growth of plant internodes by affecting the hormone levels in the plant. Phytochrome can regulate the growth of hypocotyls by affecting the level of endogenous GA of cowpea seedlings. Far-red light promotes significant elongation of the hypocotyls of tomato and lettuce seedlings, and the seedlings are severely elongated. Blue-violet light can increase the activity of auxin oxidase, by reducing the level of auxin in the plant, weakening the apical dominance, enhancing the tillering ability, and then inhibiting the elongation of plant nodes. The elongation of the hypocotyl of seedlings is related to the light quality of different wavelengths. White light and blue light can inhibit the elongation of the stem, while green light significantly promotes the elongation of the internodes. Exogenous application of IAA or GA can restore the hypocotyl lengthening of lettuce seedlings inhibited by blue light to a certain extent, indicating that blue light may inhibit the elongation of hypocotyls by reducing the level of endogenous GAs in lettuce seedlings. However, the levels of various hormones in the plant were reduced under different compound light quality treatments, and the hypocotyls showed a lower growth rate. In the light treatment, when the blue light ratio (R/B=7:3) is appropriately increased, the plant height of the seedlings is significantly reduced, and the strong seedling index is significantly increased.