Light is the basic environmental factor for plant growth and development. It is not only the basic energy source for photosynthesis, but also an important regulator of plant growth and development. Plant growth and development are not only restricted by light quantity or light intensity (photon flux density, photon flux density, PFD), but also by light quality, ie different wavelengths of light and radiation and their different composition ratios.
The solar spectrum can be roughly divided into ultraviolet radiation (ultraviolet, UV<400nm, including UV-A320~400nm; UV-B280~320nm; UV-C<280nm, 100~280nm), visible light or photosynthetically active 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 (the stratosphere), UV-C and most of the UV-B do not reach the Earth's surface. The intensity of UV-B radiation reaching the ground changes due to geographic (altitude and latitude), time (day time, seasonal variation), meteorological (cloud presence, thickness, etc.) and other environmental factors such as air pollution.
Plants can detect subtle changes in light quality, light intensity, length of light, and direction in the growing environment, and initiate the physiological and morphological changes necessary to survive in this environment. Blue light, red light and far red light play a key role in controlling the photomorphogenesis of plants. Photoreceptors (phytochrome, Phy), cryptochrome (Cry), and photoreceptors (phototropin, Phot) receive light signals and induce growth and development of plants through signal transduction.
Monochromatic light as used herein refers to light in a specific wavelength range. The range of wavelengths of the same monochromatic light used in different experiments is not completely consistent, and other monochromatic lights that are similar in wavelength often overlap to different extents, especially before the appearance of a monochromatic LED light source. In this way, naturally, there will be different and even contradictory results.
Red light (R) inhibits internode elongation, promotes lateral branching and tillering, delays flower differentiation, and increases anthocyanins, chlorophyll and carotenoids. Red light can cause positive light motion in Arabidopsis roots. Red light has a positive effect on plant resistance to biotic and abiotic stresses.
Far red light (FR) can counteract the red light effect in many cases. A low R/FR ratio results in a decrease in photosynthetic capacity of kidney beans. In the growth chamber, the white fluorescent lamp is used as the main light source, and the far-red radiation (the emission peak of 734 nm) is supplemented with LEDs to reduce the anthocyanin, carotenoid and chlorophyll content, and the fresh weight, dry weight, stem length, leaf length and leaf are made. The width is increased. The effect of supplemental FR on growth may be due to an increase in light absorption due to increased leaf area. Arabidopsis thaliana grown under low R/FR conditions were larger and thicker than those grown under high R/FR, with large biomass and strong cold adaptability. Different ratios of R/FR can also alter the salt tolerance of plants.
In general, increasing the fraction of blue light in white light can shorten internodes, reduce leaf area, reduce relative growth rates, and increase nitrogen/carbon (N/C) ratios.
High plant chlorophyll synthesis and chloroplast formation as well as chloroplasts with high chlorophyll a/b ratio and low carotenoid levels require blue light. Under the red light, the photosynthetic rate of the algae cells gradually decreased, and the photosynthetic rate rapidly recovered after going to blue light or adding some blue light under continuous red light. When the dark-growing tobacco cells were transferred to continuous blue light for 3 days, the total amount and chlorophyll content of rubulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) increased sharply. Consistent with this, the dry weight of the cells in the volume of the unit culture solution also increases sharply, while it increases very slowly under continuous red light.
Obviously, for photosynthesis and growth of plants, only red light is not enough. Wheat can complete its life cycle under a single red LEDs source, but to obtain tall plants and large numbers of seeds, an appropriate amount of blue light must be added (Table 1). The yield of lettuce, spinach and radish grown under a single red light was lower than that of the plants grown under the combination of red and blue, while the yield of plants grown under the combination of red and blue with appropriate blue light was comparable to that of plants grown under cool white fluorescent lamps. Similarly, Arabidopsis thaliana can produce seeds under a single red light, but it grows under the combination of red and blue light as the proportion of blue light decreases (10% to 1%) compared to plants grown under cool white fluorescent lamps. Plant bolting, flowering and results were delayed. However, the seed yield of plants grown under a combination of red and blue light containing 10% blue light was only half that of plants grown under cold white fluorescent lamps. Excessive blue light inhibits plant growth, shortening internodes, reduced branching, reduced leaf area, and reduced total dry weight. Plants have significant species differences in the need for blue light.
It should be noted that although some studies using different types of light sources have shown that differences in plant morphology and growth are related to differences in the proportion of blue light in the spectrum, the conclusions are still problematic because the composition of the non-blue light emitted by the different types of lamps used is different. For example, although the dry weight of soybean and sorghum plants grown under the same light fluorescent lamp and the net photosynthetic rate per unit leaf area are significantly higher than those grown under low pressure sodium lamps, these results cannot be completely attributed to blue light under low pressure sodium lamps. Lack, I am afraid it is also related to the yellow and green light under the low-pressure sodium lamp and the orange red light.
The dry weight of tomato seedlings grown under white light (containing red, blue and green light) was significantly lower than that of seedlings grown under red and blue light. Spectral detection of growth inhibition in tissue culture indicated that the most harmful light quality was green light with a peak at 550 nm. The plant height, fresh and dry weight of marigold grown under the light of green light increased by 30% to 50% compared with plants grown under full spectrum light. Full-spectrum light-filled green light causes the plants to be short and dry, and the fresh weight is reduced. Removing green light strengthens the flowering of marigold, while supplementing green light inhibits the flowering of Dianthus and lettuce.
However, there are also reports of green light promoting growth. Kim et al. concluded that the red-blue combined light (LEDs) supplemented green light results in the conclusion that plant growth is inhibited when green light exceeds 50%, while plant growth is enhanced when the green light ratio is less than 24%. Although the dry weight of the upper part of the lettuce is increased by the green light added by the green fluorescent light on the red and blue combined light background provided by the LED, the conclusion that the addition of green light enhances the growth and produces more biomass than the cool white light is problematic: (1) The dry weight of the biomass they observe is only the dry weight of the aboveground part. If the dry weight of the underground root system is included, the result may be different; (2) the upper part of the lettuce grown under the red, blue and green lights Plants that grow significantly under cold white fluorescent lamps are likely to have the green light (24%) contained in the three-color lamp far less than the result of the cool white fluorescent lamp (51%), that is, the green light suppression effect of the cool white fluorescent lamp is greater than the three colors. The results of the lamp; (3) The photosynthesis rate of the plants grown under the combination of red and blue light is significantly higher than that of the plants grown under green light, supporting the previous speculation.
However, treating the seeds with a green laser can make radishes and carrots twice as large as the control. A dim green pulse can accelerate the elongation of the seedlings growing in the dark, that is, promote stem elongation. Treatment of Arabidopsis thaliana seedlings with a single green light (525 nm ± 16 nm) pulse (11.1 μmol·m-2·s-1, 9 s) from an LED source resulted in a decrease in plastid transcripts and an increase in stem growth rate.
Based on the past 50 years of plant photobiology research data, the role of green light in plant development, flowering, stomatal opening, stem growth, chloroplast gene expression and plant growth regulation was discussed. It is believed that the green light perception system is in harmony with the red and blue sensors. Regulate the growth and development of plants. Note that in this review, green light (500~600nm) is extended to include the yellow portion of the spectrum (580~600nm).
Yellow light (580~600nm) inhibits lettuce growth. The results of chlorophyll content and dry weight for different ratios of red, far red, blue, ultraviolet and yellow light respectively indicate that only yellow light (580~600nm) can explain the difference in growth effects between high-pressure sodium lamp and metal halide lamp. That is, yellow light inhibits growth. Also, yellow light (peak at 595 nm) inhibited cucumber growth more strongly than green light (peak at 520 nm).
Some conclusions about the conflicting effects of yellow/green light may be due to the inconsistent range of wavelengths of light used in those studies. Moreover, because some researchers classify light from 500 to 600 nm as green light, there is little literature on the effects of yellow light (580-600 nm) on plant growth and development.
Ultraviolet radiation reduces plant leaf area, inhibits hypocotyl elongation, reduces photosynthesis and productivity, and makes plants susceptible to pathogen attack, but can induce flavonoid synthesis and defense mechanisms. UV-B can reduce the content of ascorbic acid and β-carotene, but can effectively promote anthocyanin synthesis. UV-B radiation results in a dwarf plant phenotype, small, thick leaves, short petiole, increased axillary branches, and root/crown ratio changes.
The results of investigations on 16 rice cultivars from 7 different regions of China, India, the Philippines, Nepal, Thailand, Vietnam and Sri Lanka in the greenhouse showed that the addition of UV-B resulted in an increase in the total biomass. Cultivars (only one of which reached a significant level, from Sri Lanka), 12 cultivars (of which 6 were significant), and those with UV-B sensitivity were significantly reduced in leaf area and tiller size. There are 6 cultivars with increased chlorophyll content (2 of which reach significant levels); 5 cultivars with significantly reduced leaf photosynthetic rate, and 1 cultivar with significantly improved (its total biomass is also significant) increase).
The ratio of UV-B/PAR is an important determinant of plant response to UV-B. For example, UV-B and PAR together affect the morphology and oil yield of mint, which requires high levels of unfiltered natural light.
It should be noted that laboratory studies of UV-B effects, although useful in identifying transcription factors and other molecular and physiological factors, are due to the use of higher UV-B levels, no UV-A concomitant and Often low background PAR, the results are usually not mechanically extrapolated into the natural environment. Field studies typically use UV lamps to raise or use filters to reduce UV-B levels.