Light is the basic environmental factor for plant growth and development. It is not only the basic energy source 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 of light or light intensity (photon flux density, PFD) but also by the quality of light, that is light and radiation of different wavelengths and their different composition ratios.
The solar spectrum can be roughly divided into ultraviolet radiation (ultraviolet, UV <400nm, including UV-A 320 ~ 400nm; UV-B 280 ~ 320nm; UV-C <280nm, 100 ~ 280nm), visible light or photosynthetically effective Radiation (photosynthetically active radiation, PAR, 400 ~ 700nm, of which 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 varies depending on the geographic (altitude and latitude), time (daily time, seasonal changes), meteorological (cloud presence, thickness, etc.) and other environmental factors such as atmospheric pollution. . Plants can detect subtle changes in light quality, light intensity, duration and direction of light in the growing environment, and initiate changes in the physiological and morphological structures necessary for survival in this environment. Blue light, red light, and far red light play a key role in controlling the establishment of plant light morphology.Photoreceptors (phytochrome, Phy), crypto-chrome (Cry) and phototropin (phototropin, Phot) These light receptors accept light signals, and trigger plant growth and development through signal transduction .
Plant biological effects of different light qualities
Different light qualities or wavelengths of light have significantly different biological effects, including different effects on plant morphology and chemical composition, photosynthesis and organ growth and development. The dry matter production of plants is the total result of these effects. The size of the dry weight of the plant is the most important and persuasive indicator of the positive and negative effects of light quality and size. Unfortunately, due to the limitations of their specific experimental purposes, many studies have not observed this indicator.
1.1 Monochromatic light
Monochromatic light here refers to light in a specific wavelength range. The wavelength range of the same monochromatic light used in different experiments is not exactly the same, and it often overlaps with other monochromatic lights with similar wavelengths to varying degrees, especially before the emergence of good monochromatic LED light sources. In this way, different or even contradictory results will naturally occur.
1.1.1 Red Light for Plant Growth
Red light (R) inhibits internode elongation, promotes lateral branching and tillering, delays flower differentiation, and increases anthocyanins, chlorophyll, and carotenoids. Red light can cause forward light movement in Arabidopsis roots. Red light has a positive effect on plant resistance to biotic and abiotic stress.
Far-red light (FR) can counteract the red light effect in many cases. A low R / FR ratio leads to a decrease in the photosynthetic capacity of kidney beans. In the growth room, using white fluorescent lamps as the main light source and supplementing far-red radiation with LEDs (emission peaks at 734 nm) reduce the anthocyanin, carotenoid, and chlorophyll content, and make the plant fresh weight, dry weight, stem length, leaf length, and Leaf width increases. The growth promotion effect of supplemental FR may be the increase of light absorption due to the increase of leaf area. Arabidopsis thaliana grown under low R / FR conditions has larger and thicker leaves, larger biomass and stronger cold adaptability than plants grown under high R / FR. Different ratios of R / FR can also change the salt tolerance of plants.
1.1.2 Blue Light for Plant Growth
In general, increasing the blue light share in white light can shorten internodes, reduce leaf area, reduce relative growth rates, and increase nitrogen/carbon (N / C) ratios. Chlorophyll synthesis and chloroplast formation in higher plants, as well as positive chloroplasts with high chlorophyll a / b ratios and low carotenoid levels, require blue light. Under red light, the photosynthetic rate of Umbellifera cells gradually decreased, and the photosynthetic rate quickly recovered after switching to blue light or adding some blue light under continuous red light. When dark-growing tobacco cells were transferred to continuous blue light for 3 days, the total amount of rubulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) and chlorophyll content were sharp. increase. Consistent with this, the dry weight of cells per unit volume of culture medium also increased sharply but increased very slowly under continuous red light.
Obviously, only red light is not enough for photosynthesis and growth of plants. Wheat can complete its life cycle under a single red LED light source, but in order to obtain tall plants and a large number of seeds, an appropriate amount of blue light must be added. The yield of lettuce, spinach, and radish grew under a single red light was lower than that of plants grown under a combination of red and blue light, and the yield of a plant grown under a combination of red and blue light with a proper amount of blue light was comparable to that of plants grown under a cold white fluorescent light.
Similarly, Arabidopsis can produce seeds under a single red light, but compared with plants grown under cold white fluorescent light, with the reduction of the blue light ratio (10% ~ 1%), Plants are mottled, flowering and bearing delayed. However, the seed yield of plants grown under the combination of red and blue light containing 10% blue light is only half that of plants grown under cold white fluorescent light. Excessive blue light inhibits plant growth, shortens internodes, reduces branching, reduces leaf area, and reduces total dry weight. There are significant species differences in the plant’s need for blue light.
1.1.3 Greenlight for Plant Growth
The dry weight of tomato seedlings grown under white light (including red, blue and green light) was significantly lower than that of red and blue light. Spectral results of growth inhibition in tissue culture indicate that the most harmful light quality is a green light with a peak at 550 nm. The height, freshness, and dry weight of marigolds grown under the light with the green light removed increased by 30% to 50% compared with the plants grown under the full spectrum light. Full-spectrum light supplementation with green light resulted in shorter plants and reduced dry and fresh weight. Removing green light enhances marigold flowering while adding green light suppresses flowering dianthus and lettuce.
The green light effect is usually opposed to the red and blue light effects. Greenlight can reverse the stomata opening promoted by blue light. However, treating the seeds with a green laser made radishes and carrots twice as large as the control. A dim green light pulse can accelerate the elongation of seedlings growing in the dark, that is, promote stem elongation. Treatment of Arabidopsis albino seedlings with a single green (525 nm ± 16 nm) pulse (11.1 µmol · m-2 · s-1, 9 s) from an LED light source resulted in a decrease in plastid transcripts and a decrease in stem growth rate. Increase.
1.1.4 Yellow light for Plant Growth
Yellow light (580 ~ 600 nm) inhibited the growth of lettuce. The chlorophyll content and dry weight were plotted against different proportions of red, far-red, blue, ultraviolet, and yellow light. The results showed that only yellow light (580 ~ 600 nm) could explain the difference in growth effects between high-pressure sodium and metal halide lamps That is, yellow light inhibits growth. Moreover, yellow light (peak at 595 nm) inhibited the growth of cucumber stronger than green light (peak at 520 nm).
1.1.5 UV radiation for Plant Growth
Ultraviolet radiation reduces plant leaf area, inhibits hypocotyl elongation, reduces photosynthesis and productivity, makes plants susceptible to pathogen attack, but can induce flavonoid synthesis and defense machinery. UV-B can reduce the content of ascorbic acid and β-carotene, but can effectively promote anthocyanin synthesis. UV-B radiation causes dwarf plant phenotypes, small and thick leaves, short petioles, increased axillary branches, and changes in root/cap ratio. A survey of 16 rice cultivars grown in greenhouses from 7 different regions in China, India, the Philippines, Nepal, Thailand, Vietnam, and Sri Lanka showed that the addition of UV-B led to an increase in total biomass Cultivated species (only 1 of which reached a significant level, from Sri Lanka), 12 decreased (of which 6 reached a significant level); those with UV-B sensitivity had significantly reduced leaf area and tiller number; 6 cultivars with increased chlorophyll content (of which 2 reached a significant level); 5 cultivars with significantly reduced leaf photosynthetic rate, and 1 cultivar with significantly improved increase).
UV-B / PAR ratio is an important determinant of plant response to UV-B. For example, UV-B and PAR together affect the morphology and oil production of peppermint. The production of high-quality oil requires high levels of unfiltered natural light.
It should be pointed out that although laboratory research on the effects of UV-B is useful in identifying transcription factors and other molecular and physiological factors, but because of the use of higher UV-B levels, there is no UV-A concomitant and Often with very low background PAR, the results usually cannot be mechanically extrapolated to the natural environment. Field studies often use UV lamps to raise or lower UV-B levels using filters.
1.2 Comparison of the effects of different wavelengths of light
For plant biological effects of light quality, many studies have compared the effects of light in two or more different bands on photosynthesis and growth.
1.2.1 Comparison of the two lights red and blue
Blue light is almost 10 times more efficient than red light in causing stomata to open. Compared with red, dry weight per unit time chlorella grown under blue productivity, high soluble protein content, low in unit dry weight of the chlorophyll content, but the high concentration of cytochrome f, chlorophyll photosynthesis meter per molecule high rate.
Compared with plants grown under blue and white light, plants grown under red light have less chlorophyll content (in terms of leaf area), but photosystem II (PSII) reaction centers, cytochrome f, and coupling The level of cofactor activity is high, the rate of CO2 assimilation in terms of unit leaf area is low, and the rate of CO2 assimilation in terms of unit chlorophyll is high.
Chlorophyll content and photosynthetic rate per unit leaf area of birch tissue culture seedlings grown under blue light were the highest, while those of seedlings grown under red light were the smallest. The photosynthetic rate of the leaves of barley seedlings grown under the same light intensity and measured under the same light intensity were all grown under blue light (400 ~ 700 nm, peak about 470 nm) significantly higher than red light (600 ~ 700 nm, peak (Approximately 670 nm). This may be partly because the so-called “blue light” has a wider spectral range (including red light) and is more suitable for the normal growth and development of photosynthetic mechanisms.
Tomato, lettuce and chrysanthemum plants grown under blue light have a lower dry weight than plants under red light. However, blue light promoted the development of storage organs of the radish (hypocotyl enlargement), while the red light was conducive to the growth of petioles and stems, but delayed the formation of roots and reduced the weight of roots. Plants growing under red light.
It can be seen that blue light seems to be more conducive to the formation of high photosynthetic capacity than red light, but this advantage of photosynthetic capacity does not necessarily lead to more dry matter accumulation due to the reduction in leaf area. However, a recent study using LED light sources and celery showed that at the same photosynthetic effective photon flux density (PPFD), increasing the proportion of red light compared to white background light significantly improves the net Photosynthetic rate, total sugar content, and plant dry weight, but significantly reduced protein content, while increasing the blue light ratio was the opposite, significantly reducing net photosynthetic rate, total sugar content, and plant dry weight, but significantly increased protein content.
1.2.2 Comparison of Multiple Lights
The order of the dry weight of tomato seedlings grown under different color fluorescent lights is: blue light (410 ~ 480 nm)> peach light (580 ~ 660 nm)> red light (630 ~ 700 nm)> golden light (550 ~ 620 nm)> Greenlight (510 ~ 570 nm); light use efficiency (the linear slope of the linear increase in dry weight accumulation with increasing light intensity under weak light) is: red + blue> blue> red> warm white> green fluorescent light Growing tomato seedling.
Red light (660 nm), red light LED + blue fluorescent light (350 ~ 550 nm), red light LED + far-red light (735 nm) and broad-spectrum metal halide light source. · M-2 · s-1) The results of pepper culture showed that the dry weight and leaf area of plants grown under red LED were significantly lower than those grown under a blue light source. Therefore, red light LED combined with blue light is suitable for controlling the environment such as a space plant cultivation system.
The order of photosynthetic oxygen evolution rate of cucumber leaves grown under the same light intensity and different light quality was red light> white light> blue light.
The comparison of different LEDs light sources with white fluorescent lamps shows that the net photosynthetic rate of chrysanthemum leaves grown under red-blue combined light (RB) LEDs is the highest, followed by white light (W), blue light (B) and blue light + far red (BFR) had the lowest growth; plants grown under W and RB had the largest dry weight and leaf area, while those grown under BFR had the smallest; plants are grown under R and RFR (red light + far-red light) had the longest stems, but the most vulnerable; plants are grown under RB and W were the largest above ground.
Photosynthesis of tomatoes grown under the same light intensity and different light qualities (blue, green, yellow, red, and white light and the spectral ranges are 360 ~ 500, 500 ~ 600, 520 ~ 620, 600 ~ 680, and 400 ~ 700 nm), Growth and fruit yield are significantly different: photosynthetic rate, chlorophyll content, sugar content, and dry weight per plant is highest under red light; photosynthetic rate, dry weight and fruit yield per plant are lowest under green light; growth under blue light The photosynthetic rate, dry weight, and fruit yield, as well as stem thickness, leaf area, root vigor, sugar content, and protein content of the plant, were higher than those grown under white light.
Compared with cucumbers grown under the same intensity (PPFD of 350 µmol · m-2 · s-1) under white light, the LEDs are purple (395 nm), blue (453 nm), green (523 nm), and yellow (595) The dry weight, net photosynthetic rate, quantum efficiency of PSII electron transfer, and chlorophyll content of plants grown under monochromatic light such as red) (nm) and red (629 nm) all decreased, and the growth of plants grown under green, yellow, and red light decreased more significantly. However, the stomatal conductance, the initial and total activity of Rubisco, the transcription level of key photosynthetic genes, the total soluble sugar and sucrose, the starch content, and the chlorophyll a / b ratio of plants grown under purple and blue light were higher than those grown under white light, and These parameters are reduced for plants grown under green, yellow and red light.
The total dry weight of red lettuce plants grown under several lights with the same light intensity (PPFD 100 µmol · m-2 · s-1) and different wavelengths is in the order of blue light (470 nm)> blue light + red light (660 nm )> Red light> White light, and the order of total chlorophyll content is white light> red light> blue light + red light> blue light, but blue light is beneficial to root growth, and promotes anthocyanin accumulation and antioxidant activity.
The types of light sources, plant types, and experimental conditions used in many of the above studies, especially the light quality wavelength range and the growth and development indicators examined, are quite different, and it is difficult to compare them strictly. However, there seems to be the following general trend: When comparing multiple wavelengths of light with each other, the most favorable is the combination of red and blue light with appropriate proportions, followed by white light, and the most inappropriate is a yellow light and green light. However, some studies have shown that several plants grow best under white light. Under the same light intensity (300 µmol · m-2 · s-1), spinach, radish, and lettuce grew for 21 d, and the photosynthetic rate, stomatal conductance and total stem of the plants grown under white light (cold white fluorescent light) grew. The weight is higher than that of plants grown under red light (LED), red light (LED) + blue light (fluorescent light, 10% blue light). It seems that red light supplementation with blue light is not enough to achieve maximum growth. It seems that the right combination of red and blue light and full-spectrum white light is more conducive to plant photosynthesis and growth and development. In addition, compared with blue light, which one is more conducive to plant photosynthesis and growth is also worthy of further discussion.