Research Progress on Improving the Production Capacity of Brassica napus

Zhenxian GAO　Dongliang TIAN　Qiao CAO　Lipeng LIANG　Yali FU

AbstractDuring the past 30 years， Brassica napus L. has grown rapidly worldwide， and the emergence of a large number of innovative rape varieties has provided tremendous support for the food， feed， biofuel and ornamental markets. This paper summarized the research on B. napus variety screening， supporting cultivation techniques and breeding and utilization of fine varieties， and discussed the great potential of screening， planting and cultivating B. napus according to production goals in improving production efficiency， providing a basis for better development and utilization of B. napus.

Key wordsBrassica napus; Screening; Matching; Breeding

China has a long history of rape cultivation. With plant morphological classification as the main body， combined with cytological classification and the evolution of chromosomes， rape is divided into three types： Brassia campestris （chromosome number n=10）， Brassica juncea （chromosome number n=18）， and Brassica napus （chromosome number n=19）[1]. Among them， B. campestris and B. juncea originated in the high mountains and hilly areas of western China， and B. napus was introduced from Europe in the 1930s[1-2]. Due to the high yield and oil content of B. napus， it gradually replaces Chinese B. campestris and B. juncea after the 1970s and has became the main cultivar of rape in China[2].

B. napus is an important oilseed crop in both tuber （rutabaga） and leaf （feed rape and kale） forms， mainly for human consumption and animal feed[3]. According to the different requirements for vernalization time， B. napus is divided into such three ecotypes as winter， semi-winter and spring， and can be planted nationwide. However， for a long time， the rape production areas in China are mainly concentrated in the Yangtze River Basin， and its planting area and total output account for 90% of China[4]. B. napus fully reflects its economic value in terms of "oil， vegetable， flower， honey， feed and fertilizer"， and has a wide range of uses. The most important economic value， rapeseed oil， accounts for more than 41% of China's self-produced vegetable oil[5].

Due to the large population in China， the current self-sufficiency rate of edible oil is only 40%， while rapeseed oil accounts for more than 55% of domestic edible oil[6]. In addition， the main rapeseed producing areas in China have always adopted the double-cropping rice planting model. Due to the long growth period of the currently planted rapeseed varieties and the difficulty in stubble connection[4]， the rape planting area in these areas is also shrinking[7]. There is a big gap between the quality of rapeseed oil in China and foreign countries， only the oil content of rapeseed is 4%-6% lower than that of foreign countries[8]. Therefore， how to improve the production efficiency and quality of B. napus in China is of great significance， and it is also the main task for our future work.

Screening Suitable Rape Varieties

On the one hand， there are many varieties in the traditional rape planting regional seed market， and there are obvious differences in yield and adaptability. On the other hand， with the adjustment of China's planting structure and the reform of the supply-side structure， many regions have begun to introduce rape. China has a vast territory， complex and diverse climate， and diverse topography. In order to promote the healthy development of the rape industry and improve the economic benefits of farmers， rape varieties suitable for local environment should be screened before large-scale promotion and planting.

The necessity of variety screening in traditional planting areas

The southern double-cropping rice area is the area with the largest potential resources of rape in China. In order to make full use of the winter idle land resources， Hu et al.[9] screened three early-maturing rape varieties from 10 rape varieties， and applied them to the three-cropping mode of rape-middle rice-ratoon rice. Geng et al.[10] screened five rape varieties with good comprehensive traits such as yield， plant height， seed setting and disease resistance from 14 B. napus varieties through field comparison experiments in Funan County， Anhui Province， which are suitable for local planting. The experimental results showed that the differences in the highest yield between different varieties were 1 019.25 and 1 256.25 kg/hm2 for seedling transplanting and field direct seeding， respectively. With the reduction of rural labor， mechanized rape harvesting will become mainstream. Rape varieties that meet the requirements of mechanized harvesting require moderate height， few branches， especially as few secondary branches as possible， concentrated pod growth on the main stem， the same density resistance， lodging resistance and maturity， and strong anti-cracking ability[11]. Cui et al.[11] screened rape varieties suitable for mechanized harvesting in Eryin District， southern Gansu Province， and found that the yield difference between the nine spring rape varieties tested was as high as 1 065.00 kg/hm2.  The above experiments showed that， first of all， there is a significant difference in the yield of rape varieties currently circulating on the market， and finding rape varieties suitable for local climate environment by screening multiple rape varieties in the field can significantly improve economic benefits. Secondly， traditional rape planting areas are faced with the adjustment of farming methods and planting structure， and the selection of rape varieties suitable for the new mode is conducive to reducing costs， making full use of light and heat resources and increasing farmers' income.

The necessity of variety screening in introduced areas

There are two situations in the introduction of B. napus in China. One is the introduction represented by Tibet. Song et al.[12] analyzed the factors of light and temperature characteristics of spring-sown semi-winter B. napus introduced in Tibet and found that the selection of semi-winter B. napus varieties insensitive to light and temperature could give consideration to both early maturity and high yield. Luo et al.[13] conducted a comparative test on the adaptability of 18 B. napus varieties in Linzhi， Tibet， and found that 4 varieties with early maturity were poor in yield traits， and 5 varieties with good agronomic and yield traits were late-maturing. The other is the impact of the characteristics of climate change and the development of the tourism industry driven by the rape flower sea on the rapeseed production area in China in recent years. For example， Gao et al.[14] tested 48 B. napus varieties from different regions at home and abroad in Shijiazhuang， where the traditional wheat-maize double-cropping system is the main system. The harvesting periods of different varieties were distributed in 68-96 d， the yield distribution with 750 000 plants/hm2 basic seedlings was 687.50-3 375.00 kg/hm2; and Yuan et al.[15] screened out two fodder rape varieties suitable for planting in neutral soil， saline-alkali land and sandy land in the western part of Jilin Province， which is a half agriculture-half herding area. Previous experiments have shown that there are significant differences in agronomic traits and yields during the introduction of different varieties. In addition to the selection of winter， semi-winter and spring rape varieties in non-traditional rape planting areas based on local climatic characteristics， it is also necessary to screen high-yield and high-quality rape varieties for promotion and application according to soil texture， farming system， purpose and use.

Establishing Supporting Cultivation Techniques

It has long been known that genotype and environment influence oil content and fatty acid composition in B. napus rapseeds， and it is this awareness that has led to the pursuit of the optimal combination of genotype and environment to breed and produce higher quality rape. Because of the uncontrollable environment， research in this area has been slow. Faraji[16] reported that grain oil content was affected by the duration of grain filling， and oil content was negatively correlated with temperature during grain filling， but high temperature promoted development and shortened grain filling time. Higher temperature （above 17 ℃） stimulated the biosynthesis of oleic acid in grains， and the content increased by 60% compared with the control[17]. Changes in temperature and light alter the levels of saturated fatty acids， monounsaturated fatty acids （C18∶1）， and linolenic acid （C18∶3）[18]. At present， it is still necessary to extensively investigate the influence of environmental factors such as light and temperature on the oil production of B. napus， so that human beings can utilize natural resources more effectively and rationally.

Humans can change the yield and quality of rape to a certain extent by taking corresponding cultivation measures. Common cultivation measures are as follows， controlling sowing date and sowing amount， fertilization， irrigation， mulching， etc. Xue et al.[19] compared the picked bolt amount and sugar content of Youtai 929 with different sowing dates， and found that earlier sowing dates had earlier bolting and higher yield， but lower soluble sugar content. Similarly， Wang et al.[20] also found that an early sowing date led to a long growth period and high yield; and different varieties in different regions differed in planting density to achieve high yield. For example， the planting densities of Fengyou 737 in Liuyang， Hunan and Fengyou 10 in Yuanyang， Henan differed by 180 000 plants/hm2 when reaching the highest yields. In terms of fertilizers， B. napus needs to be applied with boron fertilizer in addition to nitrogen， phosphorus and potassium fertilizers. Increasing nitrogen fertilizer to 180 kg/hm2 can prolong the flowering period of rape， increase the total number and density of flowers in full bloom， and improve the ornamental value. Meanwhile， the number of branches and the number of pods per plant can be significantly increased， and the grain yield can be improved. Increasing the application of phosphorus fertilizer can increase the oil content and erucic acid content， and reduce the contents of glucosinolate， protein and oleic acid[21]. Phosphorus fertilizer can also promote the absorption and utilization of phosphorus， potassium， magnesium and molybdenum nutrients in rape， which significantly increases the accumulation of nutrient elements in various parts of the plant[22]. The growth of B. napus can be significantly inhibited after 14 d of potassium deficiency[23]， and the lack of boron causes the rape to be "flowery and not fruitful"， resulting in a large reduction in yield[24]. Boron can increase the oleic acid content of B. napus， reduce the erucic acid content， and improve the quality[25]. At present， due to drought and water shortages in rapeseed cultivation in northern China， micro-spraying and film mulching are adopted to effectively improve water use efficiency and yield[26]. The previous cultivation experiments showed that providing reasonable nutrients and water for B. napus can effectively improve the yield and quality. Meanwhile， it was found that different planting management modes and different varieties have differences in the absorption and use efficiency of nutrients and water. According to local soil fertility and variety characteristics， the selection of supporting cultivation techniques will help to give full play to the potential of varieties.

Cultivating High-quality Rape Varieties

Rapeseed oil is the main product of B. napus， as well as the third largest vegetable oil after palm oil and soybean oil[27]. The physical and chemical properties of rapeseed oil depend on the fatty acid composition of the seed oil[28]. Compared with other types of vegetable oils， rapeseed oil from B. napus has a unique fatty acid characteristic， that is， it is low in saturated fatty acids （SFAs）， typically 7%.

With the advancement of modern molecular genetics and genetic technology， as well as the understanding of the genetic genes that control the major biological and biochemical processes in rape， the acquisition of knowledge will facilitate the targeted improvement of the genetic resources of B. napus. For cooking and biofuel regeneration， rapeseed oil with high oleic acid （C18∶1） and low linolenic acid （C18∶3） is preferred， which can provide high stability and longer shelf life[29]. Around 1970， intensive breeding programs for B. napus began in many countries.   In 2007， Maher et al.[30] defined high oleic acid-low linolenic acid （HOLL） rapeseed oil in new Australian spring rape varieties， that is， oil with an oleic acid content more than 65% and a linolenic acid content less than 3%， and HOLL is an important goal of the B. napus breeding program. Subsequently， an HOLL spring B. napus variety was obtained through open pollination and hybridization in Canada and Australia， and the seed oil contained 68% of oleic acid and 3% of linolenic acid; and in Europe， the seed oil of newly registered winter hybrid varieties contained about 80% of oleic acid and less than 3% of linolenic acid[29]. The specific breeding target of B. napus varies from country to country. For example， Canada expanded the planting area of B. napus with high oleic acid， and further reduced SFA to less than 6.8%， especially the contents of palm oil and stearic acid， which were less than 4%[31].

Erucic acid extracted from rapeseed oil is a raw material for the production of films， nylons， lubricants and elasticators， and is an excellent renewable and sustainable resource[32]. Low-erucic acid rape （<2%） is used for human consumption and high-erucic acid rape （45%-60% erucic acid） is used for biofuel and feedstock oil supplementation[33]. The first batch of low-erucic acid rape varieties were successfully released in 1968 by backcrossing with the canola line Liho[31]. Simultaneous overexpression of the fatty acid elongase gene （fae1） and the lysophosphatidic acid acyltransferase gene （Ld-LPAAT） resulted in an increased erucic acid content as high as 72% and a decreased polyunsaturated fatty acid content as low as 6% in transgenic lines[34]. It can be seen that， according to different production purposes， traditional breeding combined with modern molecular genetics and gene technology， such as transgenic technology， molecular markers and gene editing， can effectively improve the content of a specific fatty acid.

Prospect

With the progress of society， people's living standards has been improved. Rapeseed oil has attracted much attention because of its very small amount of saturated fatty acids and high content of mono- and polyunsaturated fatty acids， which are beneficial to human nutritional needs and physical health[35]. In addition to containing bioactive compounds and antioxidants， rapeseed oil has important components of functional foods， including natural biofortification[36]， and these oils are as valuable as salad oils and dressings. In addition， with the increasing awareness of environmental protection， rapeseed oil with high oleic acid and low linolenic acid has attracted more and more attention as a renewable raw material for biofuels[36]. The development of biodiesel in the future can further promote the increase of rapeseed yield and planting area， which can benefit local farmers. It can be seen that according to different production purposes， it is of great practical significance and application value to cultivate and plant suitable rapeseed varieties， improve oil content， and optimize the ratio of different fatty acid components in rapeseed oil.

Previous studies have shown that the yield and quality of B. napus have a great relationship with genotype and environment[29]， so this paper emphasized that the matching of improved varieties and good methods will significantly increase the yield of B. napus and improve its quality.  It is recommended that people conduct small-scale screening tests before large-scale promotion and sowing to find excellent varieties suitable for local climate， and take reasonable cultivation and management measures such as cultivation， fertilization and irrigation to give full play to the potential of the varieties. The food， feed and biofuel industries are still the main markets for rape crops， and it is necessary to give priority to oil yield and quality while enhancing tolerance to major pathogens and their resistance to abiotic stresses and herbicides. Modern genetics and genetic engineering technology will play a crucial role in the breeding of Brassica napus in the future.

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