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Knowledge of Plant Lighting: Five Kinds of Monochromatic Light Affecting Plant Growth


Light is the basic environmental factor of plant growth and development. It is not only the basic energy of photosynthesis, but also an important regulator of plant growth and development. The growth and development of plants are not only restricted by the amount or intensity of light (photon flux density, PFD), but also affected by the light quality, i.e. different wavelengths of light and radiation and their different proportions of composition.

The solar spectrum can be roughly divided into three parts: ultraviolet radiation (UV < 400nm, including uv-a320 ~ 400nm; uv-b280 ~ 320nm; UV-C < 280nm, 100 ~ 280nm), visible light or photosynthetically effective radiation (PAR, 400 ~ 700nm, including blue light 400 ~ 500nm; green light 500 ~ 600nm; red light 600 ~ 700nm) and infrared radiation (700 ~ 800nm). Due to the absorption of ozone in the stratosphere (stratosphere), UV-C and most of UV-B cannot reach the earth's surface. The intensity of UV-B radiation reaching the ground changes due to different geographical (altitude and latitude), time (daily time, seasonal change), meteorological (whether there is cloud, thickness, etc.) and other environmental factors such as air pollution.

Plants can detect the subtle changes of light quality, light intensity, light duration and direction in the growing environment, and initiate the physiological and morphological changes necessary for survival in this environment. Blue light, red light and far red light play a key role in controlling plant light morphogenesis. Photoreceptors, such as phytochrome (PHY), cryptochrome (cry) and phototropin (phototropin, photo), receive light signals, and initiate changes in plant growth and development through signal transduction.

The monochromatic light here refers to the light in a specific band. The band range of the same monochromatic light used in different experiments is not exactly the same, and it often overlaps with other monochromatic light with similar wavelength in different degrees, especially before the appearance of the LED light source with good monochromaticity. In this way, it will naturally produce different or even contradictory results.

Red light

Red light (R) inhibited internode elongation, promoted lateral branching and tillering, delayed flower differentiation, and increased anthocyanin, chlorophyll and carotenoid. Red light can cause the positive light movement of Arabidopsis root system. Red light plays an active role in plant resistance to biotic and abiotic stresses.

Far red light (FR) can counteract the red light effect in many cases. The low R / FR ratio led to the decrease of photosynthetic capacity. In the growth chamber, the white fluorescent lamp was used as the main light source, and led was used to supplement far red radiation (emission peak 734nm) to reduce the content of anthocyanin, carotenoid and chlorophyll, while the fresh weight, dry weight, stem length, leaf length and leaf width of the plant increased. The growth promoting effect of FR supplementation may be due to the increase of light absorption caused by the increase of leaf area. Arabidopsis plants grown at low R / FR were larger and thicker than those grown at high R / FR, with large biomass and strong cold adaptability

Obviously, red light is not enough for photosynthesis and growth of plants. Under a single red LED light source, wheat can complete its life cycle, but in order to obtain large plants and a large number of seeds, an appropriate amount of blue light must be added (Table 1). The yield of lettuce, spinach and radish grown under single red light was lower than that under red blue light, while that under red blue light with proper amount of blue light was higher than that under cold white fluorescent light. Similar to this, Arabidopsis can produce seeds under a single red light, but compared with the plants grown under a cold white fluorescent lamp, with the decrease of the blue light proportion (10% ~ 1%), the plants grown under the red blue combination light are delayed in bolting, flowering and bearing. However, the seed yield of plants grown under red blue combination light with 10% blue light was only half of that of plants grown under cold white fluorescent light. Excessive blue light inhibited the growth of plants, resulting in shorter internode, fewer branches, smaller leaf area and lower total dry weight. There are obvious species differences in the need of blue light.

It should be pointed out that although some studies with different types of light sources have shown that the differences in plant morphology and growth and development are related to the different proportion of blue light in the spectrum, the conclusion is questionable because the composition of non blue light emitted by different types of light sources is also different. For example, although the dry weight and net photosynthetic rate per unit leaf area of soybean and sorghum plants growing under the same light intensity fluorescent lamp are significantly higher than those growing under the low pressure sodium lamp, these results can not be completely attributed to the lack of blue light under the low pressure sodium lamp, I'm afraid that they are also related to too much yellow and green light and too little orange and red light under the low pressure sodium lamp.

Green light

The dry weight of tomato seedlings growing under white light (including red, blue and green light) was significantly lower than that under red and blue light. The results of spectral analysis of growth inhibition in tissue culture showed that the most harmful light quality was green light with a peak value of 550nm. The plant height, fresh and dry weight of marigold growing under the light without green light were 30% ~ 50% higher than that under full spectrum light. Full spectrum light complementing green light resulted in plant dwarf and reduced dry and fresh weight. Removing green light can enhance the flowering of marigold, while adding green light can inhibit the flowering of carnation and lettuce.

However, there are also reports of green light promoting growth. Kim et al. (2006) summarized the experimental results of red blue combined light (LEDs) supplementing green light and concluded that: when green light exceeds 50%, plant growth is inhibited, while when green light ratio is lower than 24%, plant growth is enhanced. Although the green light added by green fluorescent light on the red blue combination light background provided by LED can increase the aboveground dry weight of lettuce, the conclusion that adding green light to enhance growth and produce more biomass than that under cold white light is problematic: (1) the dry weight of biomass they observed is only the dry weight of aboveground, if including the dry weight of underground roots, the results may be different; (2) red and blue light The results showed that the above ground dry weight of lettuce growing under green light and green light was greater than that of the plants growing under cold white fluorescent light, which was probably the result that the green light (24%) contained by the three-color light was far less than that of the cold white fluorescent light (51%), that is, the green light inhibition effect of the cold white fluorescent light was greater than that of the three-color light; (3) the photosynthetic rate of the plants growing under red blue combination light was significantly higher than that of the plants growing under green light Measurement.

Green light effect is usually opposite to red and blue light effect. Green light can reverse the stomatal opening promoted by blue light. However, the seeds treated with green laser can make radish and carrot grow twice as big as the control. A dim green pulse can accelerate the elongation of seedlings growing in the dark, that is, promote the elongation of stems. Arabidopsis albino seedlings were treated with a single green light pulse (525nm ± 16nm) (11.1 μ mol · m-2 · s-1, 9S) from LED light source, resulting in the decrease of plastid transcripts and the increase of stem growth rate.

Based on the research data of plant photobiology in the past 50 years, this paper discusses the role of green light in plant development, flowering, stomatal opening, stem growth, chloroplast gene expression and plant growth regulation. It is believed that green light sensing system and red and blue light sensors can harmoniously regulate plant growth and development. It should be noted that in this review, green light (500-600nm) is extended to include the yellow part of the spectrum (580-600nm).

Yellow light

Yellow light (580 ~ 600nm) inhibited the growth of lettuce. The results showed that only yellow light (580 ~ 600nm) could explain the difference of growth effect between high pressure sodium lamp and metal halide lamp, that is, yellow light inhibited growth. In addition, yellow light (peak at 595nm) inhibited the growth of cucumber more than green light (peak at 520nm).

Some conflicting conclusions about the Yellow / green effect may be due to the different wavelength ranges used in those studies. Moreover, because some researchers classify the light of 500-600nm as green light, there are few literatures about the effect of yellow light (580-600nm) on plant growth and development.

Ultraviolet radiation

UV radiation can reduce leaf area, inhibit hypocotyl elongation, reduce photosynthesis and productivity, make plants vulnerable to pathogens, but it can induce flavonoids synthesis and defense mechanism. UV-B can reduce the content of ascorbic acid and β - carotene, but can effectively promote anthocyanin synthesis. UV-B radiation results in short plant phenotype, small and thick leaves, short petioles, increased axillary branches, and changes in root / crown ratio.

16 rice cultivars from 7 different regions including China, India, Philippines, Nepal, Thailand, Vietnam and Sri Lanka were investigated in greenhouse. The results showed that adding UV-B increased the total biomass of 4 cultivars (only 1 of which reached significant level, from Sri Lanka), and 12 cultivars (6 of which reached significant level) were rare- The leaf area and tiller number of B sensitive cultivars decreased significantly; 6 cultivars increased chlorophyll content (2 of which reached a significant level); 5 cultivars decreased leaf photosynthetic rate significantly, and 1 cultivars increased significantly (its total biomass also increased significantly).

The ratio of UV-B / par is an important determinant of plant response to UV-B. For example, UV-B and par jointly affect the shape and oil yield of peppermint, and the production of high-quality oil requires high levels of unfiltered natural light.

It should be pointed out that although the laboratory research of UV-B effect has a role in identifying transcription factors and other molecular and physiological factors, the results can not be mechanically extrapolated to the natural environment due to the use of higher UV-B level, no UV-A accompanying and often low background par. In field research, UV lamp is usually used to improve or filter to reduce UV-B level.

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