Light is the basic environmental factor of plant growth and development. Light not only supplies energy for plant growth through photosynthesis, but also plays an important role in plant growth and development.
1. Photoreceptors of plants
A series of responses of plants to external light environment are based on light absorption by photoreceptors. The main photoreceptors include photosynthetic pigment, photosensitive pigment, cryptochrome and phototropin. They play their respective roles in plants and affect the photosynthetic physiology, metabolic physiology and morphogenesis of plants.
1.1 photosynthetic pigment
Photosynthetic pigments are the basic components of the photosystem. They include chlorophyll a, chlorophyll b and carotenoids. It is mainly responsible for photosynthetic processes such as light energy reception, energy transfer, photoelectric conversion and so on. The results show that the main absorption wavelength of chlorophyll is 640-663 nm, and there is a secondary absorption peak at 430-450 nm. However, carotenoids are more protective to the body. Due to the existence of PS II and PS I in photosynthesis, the photosynthetic rate of red and far red light is much higher than that of monochromatic light.
Phytochrome is formed by covalent combination of chromogenic group and apolipoprotein, including far red light absorption type (PFR) and red light absorption type (PR). It mainly absorbs red light of 600-700 nm and far red light of 700-760 nm, and regulates physiological activities of plants through reversible action of far red light and red light. In plants, phytochrome is mainly involved in seed germination, seedling formation, establishment of photosynthetic system, shading, flowering time and circadian rhythm response. In addition, it also plays a role in regulating the stress resistance of plants.
Cryptochrome is a blue light receptor. It mainly absorbs 320-500 nm blue light and near ultraviolet UV-A. the absorption peaks are about 375 nm, 420 nm, 450 nm and 480 nm. Cryptochrome is mainly involved in the regulation of flowering in plants. In addition, it is involved in plant tropism, stomatal 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 induction.
Phototropin is a kind of blue light receptor discovered after photosensitive pigment and cryptochrome. It can be phosphorylated by binding with flavin mononuclear acid. It can regulate phototaxis, chloroplast movement, stomatal opening, leaf extension and inhibit hypocotyl elongation of etiolated seedlings.
2. The influence of light quality on plants
Light with different light quality or wavelength has different biological effects, including different effects on plant morphological structure and chemical composition, photosynthesis and organ growth and development.
2.1 red light
Red light generally inhibited internode elongation, promoted tillering and increased accumulation of chlorophyll, carotenoids and soluble sugar. Red light promoted the growth of leaf area and the accumulation of β - carotene in pea seedlings. It was found that red light could enhance the activity of antioxidant enzymes and increase the content of pigment absorbed by near ultraviolet light, so as to reduce the damage to lettuce seedlings by near ultraviolet light. The whole light experiment of Strawberry showed that red light was beneficial to increase the contents of organic acids and total phenols.
2.2 blue light
Blue light can significantly shorten the spacing between nodes, promote the lateral extension of vegetables and reduce the leaf area. At the same time, blue light also promoted the accumulation of secondary metabolites. In addition, blue light can reduce the inhibition of red light on photosynthetic system activity and photosynthetic electron transport capacity of cucumber leaves, so blue light is an important factor affecting photosynthetic system activity and photosynthetic electron transport ability. There are obvious species differences in the needs of plants for blue light. It was found that 470nm blue light of different wavelengths had obvious effect on anthocyanins and total phenols content in strawberry.
2.3 green light
Green light has always been a controversial light quality, some scholars believe that it will inhibit the growth of plants, resulting in plant dwarf and reduce the yield of vegetables. However, there are also many studies on the positive effect of green light on vegetables. Low proportion of green light can promote the growth of lettuce; adding 24% green light on the basis of red and blue light can promote the growth of lettuce.
2.4 yellow light
Yellow light basically inhibited the growth of plants, and because many researchers incorporated yellow light into green light, there were few literatures about the effect of yellow light on plant growth and development.
2.5 ultraviolet light
Generally speaking, UV light is more toxic to organisms, reducing leaf area, inhibiting hypocotyl elongation, reducing photosynthesis and productivity, and making plants more susceptible to infection. The results showed that the proper supplement of ultraviolet light could promote the synthesis of anthocyanins and flavonoids, and a small amount of UV-B could promote the synthesis of polyphenols; the post harvest UV-C treatment could slow down the pectin dissolution, mass loss and softening process of red pepper, significantly reduce the decay rate of red pepper, prolong the shelf life, and promote the phenolic compounds on the surface of red pepper Accumulation. In addition, ultraviolet light and blue light also affect the elongation and asymmetric growth of plant cells, thus affecting the directional growth of plants. UV-B radiation causes short plant phenotype, small and thick leaves, short petioles, increased axillary branches and root / shoot ratio.
2.6 far red light
Far red light and far red light are usually used in proportion to red light. Due to the structure problem of phytochrome in absorbing red light and far red light, the effects of red light and far red light on plants can be transformed and offset each other. When the white fluorescent lamp was used as the main light source in the growing room, the contents of anthocyanins, carotenoids and chlorophyll decreased, while the fresh weight, dry weight, stem length, leaf length and leaf width increased. The effect of FR supplementation on growth may be due to the increase of light absorption caused by the increase of leaf area. Compared with high R / FR treatment, low R / FR treatment had larger and thicker leaves, increased biomass, and more soluble metabolites accumulated, thus improving cold resistance.
3. The effect of light quality on plant tissue culture
In the process of plant tissue culture, the morphogenesis and physiological and biochemical changes of seedlings are regulated by many environmental factors (light, temperature, humidity, etc.). Among them, light plays an important role in the growth and differentiation of plant cells, tissues and organs. In the process of plant tissue culture, all the morphogenesis stages from callus induction to complete plant formation are affected by LED light quality, and the response of different plant tissue culture stages to light quality is also different.