Abstract: Vegetable seedlings are the first step in vegetable production, and the quality of seedlings is very important to the yield and quality of vegetables after planting. With the continuous refinement of the division of labor in the vegetable industry, vegetable seedlings have gradually formed an independent industrial chain and served vegetable production. Affected by bad weather, traditional seedling methods inevitably face many challenges such as slow growth of seedlings, leggy growth, and pests and diseases. To deal with leggy seedlings, many commercial cultivators use growth regulators. However, there are risks of seedling rigidity, food safety and environmental contamination with the use of growth regulators. In addition to chemical control methods, although mechanical stimulation, temperature and water control can also play a role in preventing leggy growth of seedlings, they are slightly less convenient and effective. Under the impact of the global new Covid-19 epidemic, the problems of production management difficulties caused by labor shortages and rising labor costs in seedling industry have become more prominent.

With the development of lighting technology, the use of artificial light for vegetable seedling raising has the advantages of high seedling efficiency, less pests and diseases, and easy standardization. Compared with traditional light sources, the new generation of LED light sources has the characteristics of energy saving, high efficiency, long life, environmental protection and durability, small size, low thermal radiation, and small wavelength amplitude. It can formulate appropriate spectrum according to the growth and development needs of seedlings in the environment of plant factories, and accurately control the physiological and metabolic process of seedlings, at the same time, contributing to pollution-free, standardized and rapid production of vegetable seedlings, and shortens the seedling cycle. In South China, it takes about 60 days to cultivate pepper and tomato seedlings (3-4 true leaves) in plastic greenhouses, and about 35 days for cucumber seedlings (3-5 true leaves). Under plant factory conditions, it takes only 17 days to cultivate tomato seedlings and 25 days for pepper seedlings under the conditions of a photoperiod of 20 h and a PPF of 200-300 μmol/(m2•s). Compared with the conventional seedling cultivation method in the greenhouse, the use of the LED plant factory seedling cultivation method significantly shortened the cucumber growth cycle by 15-30 days, and the number of female flowers and fruit per plant increased by 33.8% and 37.3%, respectively, and the highest yield was increased by 71.44%.

In terms of energy utilization efficiency, the energy utilization efficiency of plant factories is higher than that of Venlo-type greenhouses at the same latitude. For example, in a Swedish plant factory, 1411 MJ are required to produce 1 kg of dry matter of lettuce, while 1699 MJ are required in a greenhouse. However, if the electricity required per kilogram of lettuce dry matter is calculated, the plant factory needs 247 kW·h to produce 1 kg dry weight of lettuce, and the greenhouses in Sweden, the Netherlands, and the United Arab Emirates require 182 kW·h, 70 kW·h, and 111 kW·h, respectively.

At the same time, in the plant factory, the use of computers, automatic equipment, artificial intelligence and other technologies can accurately control the environmental conditions suitable for the seedling cultivation, get rid of the limitations of natural environment conditions, and realize the intelligent, mechanized and annual stable production of seedling production. In recent years, plant factory seedlings have been used in the commercial production of leafy vegetables, fruit vegetables and other economic crops in Japan, South Korea, Europe and the United States and other countries. The high initial investment of plant factories, high operating costs, and huge system energy consumption are still the bottlenecks that limit the promotion of seedling cultivation technology in Chinese plant factories. Therefore, it is necessary to take into account the requirements of high yield and energy saving in terms of light management strategies, establishment of vegetable growth models, and automation equipment to improve economic benefits.

In this article, the influence of LED light environment on the growth and development of vegetable seedlings in plant factories in recent years is reviewed, with the outlook of the research direction of light regulation of vegetable seedlings in plant factories.

1. Effects of Light Environment on Growth and Development of Vegetable Seedlings

As one of the essential environmental factors for plant growth and development, light is not only an energy source for plants to carry out photosynthesis, but also a key signal affecting plant photomorphogenesis. Plants sense the direction, energy and light quality of the signal through the light signal system, regulate their own growth and development, and respond to the presence or absence, wavelength, intensity and duration of light. Currently known plant photoreceptors include at least three classes: phytochromes (PHYA~PHYE) that sense red and far-red light (FR), cryptochromes (CRY1 and CRY2) that sense blue and ultraviolet A, and Elements (Phot1 and Phot2), the UV-B receptor UVR8 that senses UV-B. These photoreceptors participate in and regulate the expression of related genes and then regulate life activities such as plant seed germination, photomorphogenesis, flowering time, synthesis and accumulation of secondary metabolites, and tolerance to biotic and abiotic stresses.

2. Influence of LED light environment on photomorphological establishment of vegetable seedlings

2.1 Effects of Different Light Quality on Photomorphogenesis of Vegetable Seedlings

The red and blue regions of the spectrum have high quantum efficiencies for plant leaf photosynthesis. However, long-term exposure of cucumber leaves to pure red light will damage the photosystem, resulting in the phenomenon of “red light syndrome” such as stunted stomatal response, decreased photosynthetic capacity and nitrogen use efficiency, and growth retardation. Under the condition of low light intensity (100±5 μmol/(m2•s)), pure red light can damage the chloroplasts of both young and mature leaves of cucumber, but the damaged chloroplasts were recovered after it is changed from pure red light to red and blue light (R:B= 7:3). On the contrary, when the cucumber plants switched from the red-blue light environment to the pure red light environment, the photosynthetic efficiency did not decrease significantly, showing the adaptability to the red light environment. Through electron microscope analysis of the leaf structure of cucumber seedlings with “red light syndrome”, the experimenters found that the number of chloroplasts, the size of starch granules, and the thickness of grana in leaves under pure red light were significantly lower than those under white light treatment. The intervention of blue light improves the ultrastructure and photosynthetic characteristics of cucumber chloroplasts and eliminates the excessive accumulation of nutrients. Compared with white light and red and blue light, pure red light promoted hypocotyl elongation and cotyledon expansion of tomato seedlings, significantly increased plant height and leaf area, but significantly decreased photosynthetic capacity, reduced Rubisco content and photochemical efficiency, and significantly increased heat dissipation. It can be seen that different types of plants respond differently to the same light quality, but compared with monochromatic light, plants have higher photosynthesis efficiency and more vigorous growth in the environment of mixed light.

Researchers have done a lot of research on the optimization of the light quality combination of vegetable seedlings. Under the same light intensity, with the increase of the ratio of red light, the plant height and fresh weight of tomato and cucumber seedlings were significantly improved, and the treatment with a ratio of red to blue of 3:1 had the best effect; on the contrary, a high ratio of blue light It inhibited the growth of tomato and cucumber seedlings, which were short and compact, but increased the content of dry matter and chlorophyll in the shoots of seedlings. Similar patterns are observed in other crops, such as peppers and watermelons. In addition, compared with white light, red and blue light (R:B=3:1) not only significantly improved the leaf thickness, chlorophyll content, photosynthetic efficiency and electron transfer efficiency of tomato seedlings, but also the expression levels of enzymes related to the Calvin cycle, growth vegetarian content and carbohydrate accumulation were also significantly improved. Comparing the two ratios of red and blue light (R:B=2:1, 4:1), a higher ratio of blue light was more conducive to inducing the formation of female flowers in cucumber seedlings and accelerated the flowering time of female flowers. Although different ratios of red and blue light had no significant effect on the fresh weight yield of kale, arugula, and mustard seedlings, a high ratio of blue light (30% blue light) significantly reduced the hypocotyl length and cotyledon area of ​​kale and mustard seedlings, while cotyledon color deepened. Therefore, in the production of seedlings, an appropriate increase in the proportion of blue light can significantly shorten the node spacing and leaf area of ​​vegetable seedlings, promote the lateral extension of seedlings, and improve the seedling strength index, which is conducive to cultivating robust seedlings. Under the condition that the light intensity remained unchanged, the increase of green light in red and blue light significantly improved the fresh weight, leaf area and plant height of sweet pepper seedlings. Compared with the traditional white fluorescent lamp, under the red-green-blue (R3:G2:B5) light conditions, the Y[II], qP and ETR of ‘Okagi No. 1 tomato’ seedlings were significantly improved. Supplementation of UV light (100 μmol/(m2•s) blue light + 7% UV-A) to pure blue light significantly reduced the stem elongation speed of arugula and mustard, while supplementation of FR was the opposite. This also shows that in addition to red and blue light, other light qualities also play an important role in the process of plant growth and development. Although neither ultraviolet light nor FR is the energy source of photosynthesis, both of them are involved in plant photomorphogenesis. High-intensity UV light is harmful to plant DNA and proteins, etc. However, UV light activates cellular stress responses, causing changes in plant growth, morphology and development to adapt to environmental changes. Studies have shown that lower R/FR induces shade avoidance responses in plants, resulting in morphological changes in plants, such as stem elongation, leaf thinning, and reduced dry matter yield. A slender stalk is not a good growth trait for growing strong seedlings. For general leafy and fruit vegetable seedlings, firm, compact and elastic seedlings are not prone to problems during transportation and planting.

UV-A can make cucumber seedling plants shorter and more compact, and the yield after transplanting is not significantly different from that of the control; while UV-B has a more significant inhibitory effect, and the yield reduction effect after transplanting is not significant. Previous studies have suggested that UV-A inhibits plant growth and makes plants dwarfed. But there is growing evidence that the presence of UV-A, instead of suppressing crop biomass, actually promotes it. Compared with the basic red and white light (R:W=2:3, PPFD is 250 μmol/(m2·s)), the supplementary intensity in red and white light is 10 W/m2 (about 10 μmol/(m2·s)) The UV-A of kale significantly increased the biomass, internode length, stem diameter and plant canopy width of kale seedlings, but the promotion effect was weakened when the UV intensity exceeded 10 W/m2. Daily 2 h UV-A supplementation (0.45 J/(m2•s)) could significantly increase the plant height, cotyledon area and fresh weight of ‘Oxheart’ tomato seedlings, while reducing the H2O2 content of tomato seedlings. It can be seen that different crops respond differently to UV light, which may be related to the sensitivity of crops to UV light.

For cultivating grafted seedlings, the length of the stem should be appropriately increased to facilitate rootstock grafting. Different intensities of FR had different effects on the growth of tomato, pepper, cucumber, gourd and watermelon seedlings. Supplementation of 18.9 μmol/(m2•s) of FR in cold white light significantly increased the hypocotyl length and stem diameter of tomato and pepper seedlings ; FR of 34.1 μmol/(m2•s) had the best effect on promoting hypocotyl length and stem diameter of cucumber, gourd and watermelon seedlings; high-intensity FR (53.4 μmol/(m2•s)) had the best effect on these five vegetables. The hypocotyl length and stem diameter of the seedlings no longer increased significantly, and began to show a downward trend. The fresh weight of pepper seedlings decreased significantly, indicating that the FR saturation values of the five vegetable seedlings were all lower than 53.4 μmol/(m2•s), and the FR value was significantly lower than that of FR. The effects on the growth of different vegetable seedlings are also different.

2.2 Effects of Different Daylight Integral on Photomorphogenesis of Vegetable Seedlings

The Daylight Integral (DLI) represents the total amount of photosynthetic photons received by the plant surface in a day, which is related to the light intensity and light time. The calculation formula is DLI (mol/m2/day) = light intensity [μmol/(m2•s)] × Daily light time (h) × 3600 × 10-6. In an environment with low light intensity, plants respond to low light environment by elongating stem and internode length, increasing plant height, petiole length and leaf area, and decreasing leaf thickness and net photosynthetic rate. With the increase of light intensity, except for mustard, the hypocotyl length and stem elongation of arugula, cabbage and kale seedlings under the same light quality decreased significantly. It can be seen that the effect of light on plant growth and morphogenesis is related to light intensity and plant species. With the increase of DLI (8.64~28.8 mol/m2/day), the plant type of cucumber seedlings became short, strong and compact, and the specific leaf weight and chlorophyll content gradually decreased. 6~16 days after sowing of cucumber seedlings, the leaves and roots dried up. The weight gradually increased, and the growth rate gradually accelerated, but 16 to 21 days after sowing, the growth rate of leaves and roots of cucumber seedlings decreased significantly. Enhanced DLI promoted the net photosynthetic rate of cucumber seedlings, but after a certain value, the net photosynthetic rate began to decline. Therefore, selecting the appropriate DLI and adopting different supplementary light strategies at different growth stages of seedlings can reduce power consumption. The content of soluble sugar and SOD enzyme in cucumber and tomato seedlings increased with the increase of DLI intensity. When the DLI intensity increased from 7.47 mol/m2/day to 11.26 mol/m2/day, the content of soluble sugar and SOD enzyme in cucumber seedlings increased by 81.03%, and 55.5% respectively. Under the same DLI conditions, with the increase of light intensity and the shortening of light time, the PSII activity of tomato and cucumber seedlings was inhibited, and choosing a supplementary light strategy of low light intensity and long duration was more conducive to cultivating high seedling index and photochemical efficiency of cucumber and tomato seedlings.

In the production of grafted seedlings, the low light environment may lead to a decrease in the quality of the grafted seedlings and an increase in the healing time. Appropriate light intensity can not only enhance the binding ability of the grafted healing site and improve the index of strong seedlings, but also reduce the node position of female flowers and increase the number of female flowers. In plant factories, DLI of 2.5-7.5 mol/m2/day was sufficient to meet the healing needs of tomato grafted seedlings. The compactness and leaf thickness of grafted tomato seedlings increased significantly with increasing DLI intensity. This shows that grafted seedlings do not require high light intensity for healing. Therefore, taking into account the power consumption and planting environment, choosing an appropriate light intensity will help improve economic benefits.

3. Effects of LED light environment on the stress resistance of vegetable seedlings

Plants receive external light signals through photoreceptors, causing the synthesis and accumulation of signal molecules in the plant, thereby changing the growth and function of plant organs, and ultimately improving the plant’s resistance to stress. Different light quality has a certain promotion effect on the improvement of cold tolerance and salt tolerance of seedlings. For example, when tomato seedlings were supplemented with light for 4 hours at night, compared with the treatment without supplemental light, white light, red light, blue light, and red and blue light could reduce the electrolyte permeability and MDA content of tomato seedlings, and improve the cold tolerance. The activities of SOD, POD and CAT in the tomato seedlings under the treatment of 8:2 red-blue ratio were significantly higher than those of other treatments, and they had higher antioxidant capacity and cold tolerance.

The effect of UV-B on soybean root growth is mainly to improve plant stress resistance by increasing the content of root NO and ROS, including hormone signaling molecules such as ABA, SA, and JA, and inhibit root development by reducing the content of IAA, CTK, and GA. The photoreceptor of UV-B, UVR8, is not only involved in regulating photomorphogenesis, but also plays a key role in UV-B stress. In tomato seedlings, UVR8 mediates the synthesis and accumulation of anthocyanins, and UV-acclimated wild tomato seedlings improve their ability to cope with high-intensity UV-B stress. However, the adaptation of UV-B to drought stress induced by Arabidopsis does not depend on the UVR8 pathway, which indicates that UV-B acts as a signal-induced cross-response of plant defense mechanisms, so that a variety of hormones are jointly involved in resisting drought stress, increasing the ROS scavenging ability.

Both the elongation of plant hypocotyl or stem caused by FR and the adaptation of plants to cold stress are regulated by plant hormones. Therefore, the “shade avoidance effect” caused by FR is related to cold adaptation of plants. The experimenters supplemented the barley seedlings 18 days after germination at 15°C for 10 days, cooling to 5°C + supplementing FR for 7 days, and found that compared with white light treatment, FR enhanced the frost resistance of barley seedlings. This process is accompanied by Increased ABA and IAA content in barley seedlings. Subsequent transfer of 15°C FR-pretreated barley seedlings to 5°C and continued FR supplementation for 7 days resulted in similar results to the above two treatments, but with reduced ABA response. Plants with different R:FR values ​​control the biosynthesis of phytohormones (GA, IAA, CTK, and ABA), which are also involved in plant salt tolerance. Under salt stress, the low ratio R:FR light environment can improve the antioxidant and photosynthetic capacity of tomato seedlings, reduce the production of ROS and MDA in the seedlings, and improve the salt tolerance. Both salinity stress and low R:FR value (R:FR=0.8) inhibited the biosynthesis of chlorophyll, which may be related to the blocked conversion of PBG to UroIII in the chlorophyll synthesis pathway, while the low R:FR environment can effectively alleviate the salinity Stress-induced impairment of chlorophyll synthesis. These results indicate a significant correlation between phytochromes and salt tolerance.

In addition to the light environment, other environmental factors also affect the growth and quality of vegetable seedlings. For example, the increase of CO2 concentration will increase the light saturation maximum value Pn (Pnmax), reduce the light compensation point, and improve the light utilization efficiency. The increase of light intensity and CO2 concentration helps to improve the content of photosynthetic pigments, water use efficiency and the activities of enzymes related to the Calvin cycle, and finally achieve higher photosynthetic efficiency and biomass accumulation of tomato seedlings. The dry weight and compactness of tomato and pepper seedlings were positively correlated with DLI, and the change of temperature also affected the growth under the same DLI treatment. The environment of 23~25℃ was more suitable for the growth of tomato seedlings. According to temperature and light conditions, the researchers developed a method to predict the relative growth rate of pepper based on the bate distribution model, which can provide scientific guidance for the environmental regulation of pepper grafted seedling production.

Therefore, when designing a light regulation scheme in production, not only light environment factors and plant species should be considered, but also cultivation and management factors such as seedling nutrition and water management, gas environment, temperature, and seedling growth stage.

4. Problems and Outlooks

First, the light regulation of vegetable seedlings is a sophisticated process, and the effects of different light conditions on different types of vegetable seedlings in the plant factory environment need to be analyzed in detail. This means that to achieve the goal of high-efficiency and high-quality seedling production, continuous exploration is required to establish a mature technical system.

Secondly, although the power utilization rate of the LED light source is relatively high, the power consumption for plant lighting is the main energy consumption for the cultivation of seedlings using artificial light. The huge energy consumption of plant factories is still the bottleneck restricting the development of plant factories.

Finally, with the wide application of plant lighting in agriculture, the cost of LED plant lights is expected to be greatly reduced in the future; on the contrary, the increase in labor costs, especially in the post-epidemic era, the lack of labor is bound to promote the process of mechanization and automation of production. In the future, artificial intelligence-based control models and intelligent production equipment will become one of the core technologies for vegetable seedling production, and will continue to promote the development of plant factory seedling technology.

Authors: Jiehui Tan, Houcheng Liu
Article source: Wechat account of Agricultural Engineering Technology (greenhouse horticulture)