Greenhouse horticultural agricultural engineering technology Published at 17: 30 on October 14, 2022 in Beijing  

With the continuous increase of the global population, people’s demand for food is increasing day by day, and higher requirements are put forward for food nutrition and safety. Cultivating high-yield and high-quality crops is an important means to solve food problems. However, the traditional breeding method takes a long time to cultivate excellent varieties, which limits the progress of breeding. For annual self-pollinating crops, it may take 10~15 years from the initial parent crossing to the production of a new variety. Therefore, in order to speed up the progress of crop breeding, it is urgent to improve the breeding efficiency and shorten the generation time.

Rapid breeding means to maximize the growth rate of plants, accelerate flowering and fruiting, and shorten the breeding cycle by controlling environmental conditions in a fully closed controlled environment growth room. Plant factory is an agricultural system that can achieve high-efficiency crop production through high-precision environmental control in facilities, and it is an ideal environment for rapid breeding. The planting environment conditions such as light, temperature, humidity and CO2 concentration in the factory are relatively controllable, and are not or less affected by the external climate. Under controlled environmental conditions, the best light intensity, light time and temperature can accelerate various physiological processes of plants, especially photosynthesis and flowering, thus shortening the generation time of crop growth. Using plant factory technology to control crop growth and development, harvesting fruits in advance, as long as a few seeds with germination ability can meet the breeding needs.

Photoperiod, the main environmental factor affecting crop growth cycle

Light cycle refers to the alternation of light period and dark period in a day. Light cycle is an important factor that affects the growth, development, flowering and fruiting of crops. By sensing the change of light cycle, crops can change from vegetative growth to reproductive growth and complete flowering and fruiting. Different crop varieties and genotypes have different physiological responses to photoperiod changes. Long-sunshine plants, once the sunshine time exceeds the critical sunshine length, the flowering time is usually accelerated by the prolongation of photoperiod, such as oats, wheat and barley. Neutral plants, regardless of photoperiod, will bloom, such as rice, corn and cucumber. Short-day plants, such as cotton, soybean and millet, need photoperiod lower than the critical sunshine length to bloom. Under the artificial environment conditions of 8h light and 30℃ high temperature, the flowering time of amaranth is more than 40 days earlier than that in the field environment. Under the treatment of 16/8 h light cycle (light/dark), all seven barley genotypes bloomed early: Franklin (36 days), Gairdner (35 days), Gimmett (33 days), Commander (30 days), Fleet (29 days), Baudin (26 days) and Lockyer (25 days).

Under the artificial environment, the growth period of wheat can be shortened by using embryo culture to obtain seedlings, and then irradiating for 16 hours, and 8 generations can be produced every year. The growth period of pea was shortened from 143 days in field environment to 67 days in artificial greenhouse with 16h light. By further prolonging the photoperiod to 20h and combining it with 21°C/16°C(day/night), the growth period of pea can be shortened to 68 days, and the seed setting rate is 97.8%. Under the condition of controlled environment, after 20 hours photoperiod treatment, it takes 32 days from sowing to flowering, and the whole growth period is 62-71 days, which is shorter than that in field conditions by more than 30 days. Under the condition of artificial greenhouse with 22h photoperiod, the flowering time of wheat, barley, rape and chickpea is shortened by 22, 64, 73 and 33 days on average, respectively. Combined with early harvest of seeds, the germination rates of early harvest seeds can reach 92%, 98%, 89% and 94% on average, respectively, which can fully meet the needs of breeding. The fastest varieties can continuously produce 6 generations (wheat) and 7 generations (wheat). Under the condition of 22-hour photoperiod, the flowering time of oats was reduced by 11 days, and 21 days after flowering, at least 5 viable seeds could be guaranteed, and five generations could be continuously propagated every year. In the artificial greenhouse with 22-hour illumination, the growth period of lentils is shortened to 115 days, and they can reproduce for 3-4 generations a year. Under the condition of 24-hour continuous illumination in artificial greenhouse, the growth cycle of peanut is reduced from 145 days to 89 days, and it can be propagated for 4 generations in one year.

Light quality

Light plays a vital role in the growth and development of plants. Light can control flowering by affecting many photoreceptors. The ratio of red light (R) to blue light (B) is very important for crop flowering. The red light wavelength of 600~700nm contains the absorption peak of chlorophyll of 660nm, which can effectively promote photosynthesis. The blue light wavelength of 400~500nm will affect plant phototropism, stomatal opening and seedling growth. In wheat, the ratio of red light to blue light is about 1, which can induce flowering at the earliest. Under the light quality of R:B=4:1, the growth period of middle and late-maturing soybean varieties was shortened from 120 days to 63 days, and the plant height and nutritional biomass were reduced, but the seed yield was not affected, which could satisfy at least one seed per plant, and the average germination rate of immature seeds was 81.7%. Under the condition of 10h illumination and blue light supplement, soybean plants became short and strong, blossomed 23 days after sowing, matured within 77 days, and could reproduce for 5 generations in one year.

The ratio of red light to far red light (FR) also affects the flowering of plants. Photosensitive pigments exist in two forms: far red light absorption (Pfr) and red light absorption (Pr). At a low R:FR ratio, photosensitive pigments are converted from Pfr to Pr, which leads to the flowering of long-day plants. Using LED lights to regulate the appropriate R:FR(0.66~1.07) can increase plant height, promote the flowering of long-day plants (such as morning glory and snapdragon), and inhibit the flowering of short-day plants (such as marigold). When R:FR is greater than 3.1, the flowering time of lentils is delayed. Reducing R:FR to 1.9 can get the best flowering effect, and it can bloom on the 31st day after sowing. The effect of red light on flowering inhibition is mediated by photosensitive pigment Pr. Studies have pointed out that when R:FR is higher than 3.5, the flowering time of five leguminous plants (pea, chickpea, broad bean, lentil and lupin) will be delayed. In some genotypes of amaranth and rice, far-red light is used to advance flowering by 10 days and 20 days respectively.

Fertilizer CO₂

CO₂ is the main carbon source of photosynthesis. High concentration CO₂ can usually promote the growth and reproduction of C3 annuals, while low concentration CO₂ may reduce the growth and reproduction yield due to carbon limitation. For example, the photosynthetic efficiency of C3 plants, such as rice and wheat, increases with the increase of CO₂ level, resulting in the increase of biomass and early flowering. In order to realize the positive impact of CO₂ concentration increase, it may be necessary to optimize the water and nutrient supply. Therefore, under the condition of unlimited investment, hydroponics can fully release the growth potential of plants. Low CO₂ concentration delayed the flowering time of Arabidopsis thaliana, while high CO₂ concentration accelerated the flowering time of rice, shortened the growth period of rice to 3 months, and propagated 4 generations a year. By supplementing CO₂ to 785.7μmol/mol in the artificial growth box, the breeding cycle of soybean variety ‘Enrei’ was shortened to 70 days, and it could breed 5 generations in one year. When the CO₂ concentration increased to 550μmol/mol, the flowering of Cajanus cajan was delayed for 8~9 days, and the fruit setting and ripening time were also delayed for 9 days. Cajanus cajan accumulated insoluble sugar at high CO₂ concentration, which may affect the signal transmission of plants and delay flowering. In addition, in the growth room with increased CO₂, the number and quality of soybean flowers increase, which is conducive to hybridization, and its hybridization rate is much higher than that of soybeans grown in the field.

Future prospects

Modern agriculture can speed up the process of crop breeding by means of alternative breeding and facility breeding. However, there are some shortcomings in these methods, such as strict geographical requirements, expensive labor management and unstable natural conditions, which can’t guarantee successful seed harvest. Facility breeding is influenced by climatic conditions, and the time for generation addition is limited. However, molecular marker breeding only accelerates the selection and determination of breeding target traits. At present, rapid breeding technology has been applied to Gramineae, Leguminosae, Cruciferae and other crops. However, plant factory rapid generation breeding completely gets rid of the influence of climatic conditions, and can regulate the growth environment according to the needs of plant growth and development. Combining plant factory rapid breeding technology with traditional breeding, molecular marker breeding and other breeding methods effectively, under the condition of rapid breeding, the time required to obtain homozygous lines after hybridization can be reduced, and at the same time, the early generations can be selected to shorten the time required to obtain ideal traits and breeding generations.

The key limitation of plant rapid breeding technology in factories is that the environmental conditions required for the growth and development of different crops are quite different, and it takes a long time to obtain the environmental conditions for rapid breeding of target crops. At the same time, because of the high cost of plant factory construction and operation, it is difficult to carry out large-scale additive breeding experiment, which often leads to limited seed yield, which may limit the follow-up field character evaluation. With the gradual improvement and improvement of plant factory equipment and technology, the construction and operation cost of plant factory is gradually reduced. It is possible to further optimize the rapid breeding technology and shorten the breeding cycle by effectively combining the plant factory rapid breeding technology with other breeding techniques.

END

Cited information

Liu Kaizhe, Liu Houcheng. Research progress of plant factory rapid breeding technology [J]. Agricultural Engineering Technology, 2022,42(22):46-49.