Screening and Identification of Waterlogging Tolerance in Brassica napus Germplasm Resources

Yun LI Cunkou QI Sanxiong FU Feng CHEN Song CHEN

AbstractIn order to evaluate, screen and identify waterloggingtolerant Brassica napus resources and provide good germplasm materials for breeding of waterloggingtolerant rape, 608 B. napus germplasm materials from different sources were identified and screened for waterloggingtolerant germplasms. The identification results showed that tested materials had dead seedling rates in the range of 0-100%, indicating that these rape germplasm materials varied extensively in waterlogging tolerance. Among the 608 materials, waterloggingtolerant materials (grade I, relative dead seedling rate≤21%) accounted for 25.49%; nontolerant materials (grade Ⅳ+V, dead seedling rate≥61%) accounted for 29.43%; and other materials had waterlogging tolerance between the two. The identification results of waterlogging tolerance in the 349 rape germplasm resource from the upper, middle and lower reaches of Yangtze River and Huang-Huai Basin showed that materials from the lower Yangtze River had the lowest average dead seedling rate of 38.8%, which was remarkably lower than materials from other ecological regions, and waterloggingtolerant materials among them accounted for 33.6%, which was also remarkably higher than materials from other ecological regions. It could thus be seen that materials from the lower Yangtze River have stronger waterlogging tolerance than those from other ecological regions overall. In this study, 24 highlytolerant B. napus germplasms (dead seedling rate<1%) were selected from the 608 B. napus germplasm materials, which could be used for breeding of waterloggingtolerant rape and related fundamental research.

Key wordsBrassica napus; Identification of waterlogging tolerance; Germplasm screening

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Received: May 27, 2018Accepted: September 9, 2018

Supported by National Key Research and Development Program of China (2016YFD010020210).

Yun LI (1982-), female, P. R. China, master, devoted to research about Crop Genetics and Breeding.

*Corresponding author. Email: chensong1963@126.com.

Rape is one of the oil crops widely planted in the world. The planting area and yield of rape in China rank the first in the world［1］. The Yangteze River Basin is the main producing area in China, and has an area accounting for about 70% of rape planting area in China. Rape production in the middle and lower reaches of the Yangtze River is mainly conducted in the mode of ricerape rotation. Water in paddy field is hard to be drained after harvest, and causes wet damage to growth of rape planted later easily, and plus the wet and rainy weather in autumn and winter, waterlogging and damage or crop failure caused by waterlogging happen easily in seedling stage and thus seriously influence rape yield［2］. Researches have shown that waterlogging causes oxygen deficit in rhizosphere of rape, which leads to aggravated anaerobic metabolism, and harmful substances including ethanol, lactic acid and oxygen radical produced in glycolysis, alcoholic fermentation and lactic acid fermentation harm cells and cause greatlyreduced plant photosynthesis and inhibited growth［3］. Waterlogging also would influence plant height, stem diameter, root diameter, root length, green leaf number, leaf area and dry weight of rape, and cause decreases in number of effective branches, number of pods per plant and grains per pod, and grain yield might be reduced to 17.0%-42.4%［4-6］.

In order to reduce yield loss of rape caused by waterlogging, agronomic measures including furrowing and drainage, intertillage and increasing fertilizer are often taken in production［7-8］, or plant growth regulators are scientifically applied to improve the tolerance in rape seedlings to waterlogging［9］. However, these technical measures all require large labor cost, while the breeding of rape varieties relatively tolerant to waterlogging is a fundamental way to reduce damage caused by waterlogging. In recent years, in order to reduce rape production cost, notillage direct seeding and simplified shallowploughing cultivation technique have attracted more and more attention, and with the popularization and application of this technique in production, higher requirements have been put forward to waterlogging tolerance in rape varieties. Screening and breeding new waterloggingtolerant rape varieties has become an important target in rape breeding at this stage［9］.

Brassica napus is a main cultivated type for rape production in China. Therefore, evaluating, screening and identifying waterloggingtolerant B. napus resources is of great significance to the provision of good germplasm materials for breeding of waterloggingtolerant rape. In this study, according to "Identification method on waterlogging tolerance in rapeseed at seedling stage"［10］, 608 B. napus germplasm materials were indentified for waterlogging tolerance, and waterloggingtolerant germplasm materials were screened from them, aiming at providing valuable materials for breeding.

Materials and Methods

Experimental materials

608 B. napus germplasm resource materials from different sources were randomly selected from Rape Germplasm Bank, Institute of Economic Crops, Jiangsu Academy of Agricultural Sciences, including 349 Chinese varieties and 259 foreign varieties.

Experimental methods

Flooding tolerance test was carried out according to literature［10］. Full seeds were uniformly put on culture dishes laid with two layers of wet filter, and placed in an incubator at 25 ℃ for 36 h of germination. Normal seeds having a white bud (with a radicle length of 0.3 cm) were uniformly transplanted to plastic cups filled with culture medium, and cultured in an incubator (light 10 h, 25℃/dark 14 h, 20 ℃) to allow 36 h of normal growth. When the seedlings were about 3 cm long, weak seedlings were removed. 20 seedlings were selected from each cup and subjected to waterlogging treatment. Each treatment had three replicates. Seedlings in the waterlogging treatment were completely immersed (with water 2 cm higher than seedling top) for 6 d, and the control was free of waterlogging treatment and allowed to grow normally. Water was drained after the treatment, and number of dead seedlings was recorded on the 7th day after drainage for the calculation of dead seedling rate. 

Statistical analysis

Dead seedling rate and ecological distribution were analyzed with Excel 2003.

Results and Analysis

Performance of rape materials tolerant and nontolerant to waterlogging after waterlogging treatment and grading of waterlogging tolerance

Performance of tolerant and nontolerant B. napus materials after waterlogging treatment is shown in Fig. 1. Different materials differed greatly in waterlogging tolerance 6 d after waterlogging. For nontolerant materials, leaves of seedlings and plants gradually rotted over time. And after drainage, the plants could not grow and finally died. As to tolerant materials, leaves were intact without damage, but slightly withered. After drainage, seedlings recovered and then grew rapidly, and the leaves grew normally 2 d later. And on the 7th day after drainage, leaves of seedlings were remarkably larger than the very day of drainage, with significantly increased leaf area, and there were also new leaves grown normally in color of bright green (Fig. 1).

According to the frequency of average dead seedling rate after waterlogging treatment, waterlogging tolerance in the 608 materials were divided into five grades I-V, which were corresponding to 0-20.9%, 21.0%-40.9%, 41.0%-60.9%, 61.0%-80.9% and 81.0%-100% (Fig. 2), respectively. It could be seen from Fig. 2 that the 608 B. napus germplasm materials were distributed from tolerant to nontolerant, and the seedlings varied widely in waterlogging tolerance, indicating a good basis for selection of waterlogging tolerance. These materials were divided according to waterlodding tolerance into three types, i.e., waterlogging tolerant (I), moderately tolerant (grade II-IV) and nontolerant (grade V). Among the 608 rape materials, there were 155 tolerant materials (grade I), accounting for 25.49%, 87 nontolerant materials (grade V), accounting for 14.30%, and 366 moderatelytolerant materials (grade II-IV), accounting for 60.19%. Among the 349 Chinese materials, there were 99 tolerant materials, accounting for 28.36%, and 43 nontolerant materials, accounting for 14.30%; and among the 259 introduced materials, there were 56 tolerant materials, accounting for 15.05%, and 44 nontolerant materials, accounting for 16.99%. It could thus been seen that there were more tolerant germplasms among the Chinese germplasm materials than among the introduced foreign materials.



Fig. 1Performance of tolerant and nontolerant materials subjected to 6 d of waterlogging

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Fig. 2Waterlogging tolerance performance of tested materials

Difference in waterlogging tolerance of rape germplasm resources from different ecological regions

The identification results of waterlogging tolerance in the 349 rape germplasm resource from the upper, middle and lower reaches of the Yangtze River and Huang-Huai Basin (Table 1) showed that materials from different ecological regions differed greatly in waterlogging tolerance. Among the 220 materials from the lower Yangtze River, there were more tolerant materials (I), accounting for 33.6%, and the average dead seedling rate was the lowest, of 38.8%; the 50 materials from the middle Yangtze River included 9 tolerant materials, accounting for 18%, and the average dead seedling rate was 47.7%; among the 42 materials from the upper Yangtze River, there were 9 tolerant materials, accounting for 21.4%, and the average dead seedling rate was 47.8%; and the 37 materials from Huang-Huai region included 7 tolerant materials, accounting for 18.9%, and the average dead seedling rate reached 53.1%. Above results showed that the rape germplasms from the lower Yangtze River had overall higher waterlogging tolerance than materials from other ecological regions, which might be related to ecological adaptability and genetic selection.

Table 1Waterlogging tolerance performance of 349 B. napus germplasms from different ecological regions along with the Yangtze River

Ecological region

Grade of waterlogging tolerance

IIIIIIIVVSumTolerant material∥%Average dead seedling rate∥%

Lower Yangtze River744949291922033.6338.8

Middle Yangtze River915105115018.0047.7

Lower Yangtze River9910774221.4247.8

Huang-Huai Basin7751263718.9153.1

Sum998074534334928.3646.9

Screening of tolerant rape germplasms

In this study, 24 highlytolerant B. napus germplasms (with a dead seedling rate of 0) were selected from the 608 B. napus germplasm materials from different sources, including 11 Chinese germplasm materials and 13 foreign germplasm materials (Table 2). Among the 11 Chinese germplasm materials, there were 8 materials from the lower Yangtze River, accounting for 66.67%. These materials were mostly advanced lines or varieties, which can serve as parents for breeding of tolerant B. napus germplasms and molecular study on traits related to waterlogging tolerance.

Discussion

Waterlogging is a kind of abiotic stress agricultural production often faces, as well as kind of natural disaster. It often rains in the growth period of rape planted in main rape producing area (the middle and lower reaches of Yangtze River). Furthermore, rape production in this area mainly adopts rice fieldupland field rotation, especially the planting mode of notillage direct seeding rape following rice. As a result, wet damage often happens and causes certain loss of rape yield. Therefore, the breeding of rape varieties with strong waterlogging tolerance capable of growing in wet soil is of great significance.

In recent years, there have been many studies on screening of waterloggingtolerant rape resources. Zhang et al.［2］ studied the difference in waterlogging tolerance of 9 B. napus varieties (lines) having different genetic backgrounds through indoor waterlogging treatment on germinated seeds. The results showed that different rape varieties (lines) had larger genetic differences on wet damage. Chen et al.［15］ studied the 32 rape varieties from Yangtze River Basin through simulated wet damage environment, and the results showed that the wet tolerance in B. napus was controlled by genotypes and had greater genetic differences. And, specifically, there were 6 varieties with stronger wet tolerance, accounting for 18.75% of all the varieties, and 3 varieties with poorer wet tolerance, accounting for 9.38% of all the varieties, and other varieties had moderate wet tolerance. Xu et al.［14］ evaluated waterlogging tolerance in 270 rape resources through waterlogging treatment at germination stage according to qualified seedling percentage. The results showed that waterlogging tolerance in rape had extensive genetic variation, and different types of rape varieties differed remarkably in waterlogging tolerance. Generally, B. campestris has waterlogging tolerance stronger than B. napus; ordinary varieties are stronger in waterlogging tolerance than hybrid varieties; rape varieties from the lower Yangtze River have strong waterlogging tolerance than those from the middle and upper reaches of the Yangtze River, while the genetic variation of waterlogging tolerance in rape from the middle Yangtze River was richer than the lower Yangtze River; and the waterlogging tolerance in spring rape is stronger than that in winter rape. Many research results have consistently shown that there are differences in waterlogging tolerance between rape germplasms from different ecological regions, which provides a theoretical basis for the screening and breeding of waterloggingtolerant germplasms.

"Rapid screening method on waterlogging tolerance in rapeseed at seedling stage" is a method established by Li et al.［10］ for rapidly accurately identifying waterlogging tolerance in rapeseed seedlings. This method uses average dead seedling rate of rape subjected to 6 d of waterlogging treatment (totally immersed) on the 7th day after drainage as the identification index of waterlogging tolerance in rape seedlings. Their study showed that the method could identify tolerant rape with a dead seedling rate of 0 and rape very susceptible to waterlogging with a dead seedling rate of 100%, which had the morphologies and physiological and biochemical indices satisfying the characteristics of tolerance and nontolerant rape, respectively, indicating that method is simple, convenient and reliable［10］. In this study, 608 rape germplasms from different areas were identified by the method. The results showed that B. napus germplasm materials from different sources acted differently after the waterlogging treatment, and waterlogging tolerance varied extensively in the materials, which had the average dead seedlings in the range of 0-100%. The germplasms from different ecological regions differed remarkably in waterlogging tolerance, which accords with reported research results. In this study, the rape germplasms subjected to 6 d of waterlogging treatment were divided into five grades grade I-V (≤20.0%, 20.1%-40.0%, 40.1%-60.0%, 60.1%-80.0% and 80.1%-100.0%) according to average dead seedling rate, and further classified to three types, i.e., waterlogging tolerant: grade (I), moderately tolerant: grade II-IV, and nontolerant: grade (V), which is beneficial to the classification of experimental materials and could better reflect waterlogging tolerance in materials. The materials with a dead seedling rate<1% were deemed to be highly tolerant to waterlogging. These highlytolerant materials could serve as parents for breeding. In this study, 24 highlytolerant resource materials were identified, among which 13 were introduced foreign rape germplams, and 11 were Chinese germplasms. The 11 Chinese materials are mostly rape varieties bred in the lower Yangtze River including Jiangsu, which have very good utilization value in rape breeding for waterlogging.

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