ABSTRACT

Rich and diverse oilseed mustard (Brassica juncea) germplasm resources are available for successful programmes in Yunnan province. Though field collection for native mustard in remote areas remains incomplete, broad-based germplasm collections have been conserved. The systematic evaluation had been made and germplasm accessions with stable resistance to various diseases were identified and utilized in breeding. The cytoplasm male-sterile (CMS) Ouxin was found in 1973 and a set of tree lines was accomplished in 1975. Since 1975, 15 high-yielding hybrids in mustard were released and widely adopted in Yunnan. Presently, other 3 highly diverse CMS lines and restorers are being developed and utilized. In this article an attempt is made to review the progress of research in mustard during the last 4 decades.

Introduction

Mustard (Brassica juncea) is an important crop of upland in Yunnan province and is predominantly grown as a oilseed crop also valued for vegetable. It is grown in most adverse soil and climatic conditions but still responds well to irrigated and improvement conditions. The area under oilseed mustard during the last 40 years has shrunk from 1.0 million ha in 1950s to around 0.33 million ha in 1990s. However, the reduction of acreage mostly occurred in irrigated areas and in the upland areas the overall production has not affected.

Yunnan has the distinction of having abundant and diverse mustard germplasm resources in China. Natural and semi-natural selection of mustard adapted to varying ecological conditions and cropping systems has resulted in a broad array of landraces. The diversity of both indigenous landraces and wild relatives is very rich. Research achievements have made but seldom reported. In this article an attempt is made to review the progress during the last 45 years.

Germplasm Collection and Documentation

Landrace collection

Mustard germplasm collection and conservation activities in Yunnan province began in the mid-1950s. Since 1955, the Oil Crop Research Institute of Yunnan Academy of Agricultural Sciences (OCRI) collaborated with local agricultural communities and supported by national research funds has assembled more than 200 indigenous landraces and wild relatives. There were two periods of exploration to assemble the province-wide diversity of mustard: the 1950s and the 1980s. From 1955 to 1959, the field collection project led to the acquisition of 120 samples of native cultivars and wild relatives that came from 63 counties. In 1987, an extensive exploration for landraces and relatives, which covered 38 counties, yielded 77 seed samples and 162 specimens. Some sporadic efforts resulted in increasing the number of accessions around 30. However, many of the target areas for collecting are in remote regions that are difficult and expensive to reach.

New germplasm

More than 3000 new germplasm, such as mutants, isolines, tissue cultures, hybrids, have been generated and used in breeding programmes. The acquisition (rather than collecting) of the germplasm and its appropriate conservation will further stretch the genebank’s resources.

Documentation

As early as 1965, the OCRI had constructed its mustard germplasm data base to serve as a guideline to users of the information stored. Each record in the germlpasm base data file consisted of Accession number, Accession name, Former designation, Seed source, and Morphoagronomic traits. The standardized descriptor list for characterization of mustard cultivars was developed in 1980s.

EVALUATION AND CULTIVAR DEVELOPMENT

Ecotype research

Systematic evaluation of the entire collection of germplasm was conducted in the early 1960s.The OCRI launched ‘the Yunnan native mustard ecotype research project’ in 1960. According to climatic conditions, origin areas of germplasm were divided into 6 ecotypes: south, mid-south, south-east, north-east, central, and north-west types. Because of a lack of knowledge of the collecting site, it took two years to observe and accumulate information. Simultaneously, preliminary evaluation work that included a basic morphoagronomic characterization was carried out in the course of first multiplication of accessions. Further characterization and evaluation, which carried out from 1962 to 1965,were a range of other characters of interest to breeders. Those traits included drought tolerance, disease resistance, quality characters etc. Research results showed that there were more ecotypes in Yunnan than expected and maturity, plant length, and branch height increased from south to north areas. Breeding target for different areas were various.

In order to measure genetic polymorphism, isoenzyme variability was used in detecting variants in landraces and wild relatives in 1980s. But not enough information obtained to provide clear discrimination within the accessions.

Core collection

To utilize effectively and reduce genetic vulnerability in future agriculture development, a active collection (core collection) was established in the mid-1980s. The accessions in the core were chosen on the basic of representative variability, as described by passport, characterization and evaluation data, plus additional rare types. Active collection was maintained to meet the day-to-day research needs of breeders and agronomists. The base collections acted as reserve stocks to prevent loss of germplasm and erosion of genetic variability, and most of them were sent to the national long-term genebank by 1989.

Cultivar development

The original source populations for breeding programmes were landraces in core collection. The level of genetic variability with the populations was adequate to permit response to selection. Research efforts made in 1960s and 1970s towards varietal improvement concentrated on the improvement of yield in the locally adapted materials through simple mass selection. As a result, a few varieties like Songming Huangzi, Kungao, Yimeng Fengwei etc were developed and released (Table 1). Those cultivars had already demonstrated many desirable agronomic traits but could not became popular because of their limited productivity and narrow range of adaptability.

Table 1 Varieties of mustard released in Yunnan since 1965

───────────────────────────────────────
Variety Maturity(day) Oil content(%) Special features
───────────────────────────────────────
Songming Huangzi 179 45.67
Jiangchuan Aijiao 166 41.44 drought tolerance
Jiangchuan Zhongke 167 40.34
Yimeng Fengwei 175 41.66
Kungao 180 48.30
Yunmou Youcai 174 45.34
Wuding Huangzi 183 45.48
Lijiang Youcai 177 46.72 cold tolerance
───────────────────────────────────────

Since 1972, interbreed lines development was emphasized in applied breeding programmes and several F₂ and backcrossing populations were created. Semi-dwarf, drought tolerance, virus resistance, and low erucid acid traits which from different germplasms were incorporated into one or two promising lines and the wide-adapted cultivars, 88-122 and 88-242, were developed and released in 1980s. They outyielded local varieties by 15.0-84.2%.

Expliotation of Heterosis

Cultivar development received a major impetus when cytoplasmic male sterility (CMS) was found in a native landrace in 1973. After several years of study, a male-sterile line OuxinA, a maintenance line OuxinB, and a restoring line 74243-6 were developed successfully and a set of ‘three lines’ was accomplished in 1975 (Shi Huaqing 1991). Yunnan enjoyed the privilege of possessing the unique gremplasm in China and a series of studies on the male-sterile material were conducted during last 25 years.

Sterility and its restoring

Breeders have paid much attention to the male-sterile line and its restorers. Sterility was transferred into several elite lines by backcrossing and the developed male-sterile lines still maintained self-pollinated seed-setting rate at 0.000-0.103% for 14 generations (Shi Huaqing 1991). Meanwhile, 4 landraces with restoring genes were selected. Restorers with rates of restored plant 97.92-100% in F₁ were developed (Shi Huaqing 1990).

Cytoplasm genetic effects

The genetic effects and their expression specialty in cytoplasm OuxinA on some agronomic characters of F₀ and F₁ were studied by comparison sterile-line hybrids with their maintenance counterparts in 1990s. Both the universality and specialty of the expression of the cytoplasm were found. The cytoplasm of OuxinA showed a trend of positive effect on the most of the agronomic traits of F₁. The study also showed negative effects on seed setting of F₀ and plant height of F₁. The genetic effects appeared combining specialty obviously.

Hybrids

There was strong heterosis at the vegetative stage with the F₁ hybrids of OuxinA. Heterosis also showed in seed yields and seed oil contents (Shi Huaqing 1910,1991). OuxinA was extensively utilized because of its high sterility, uniform flowing and good combining ability, and 15 hybrids based on it were released from 1991 to 1996 (Table 2). Among those combinations, ‘Jiangai D1’ became most popular because of its high seed yields and adaptability to drought-stress conditions. Adoption of those hybrids led to 22.8-110.4% increase of seed yields over local varieties. However, intensive cultivation of hybrids based on a single male-sterile line resulted in cytoplasmic and genetic homogeneity. Attempts to find new sterile resources were made in 1990s and 3 highly diverse CMS lines YongpingA, YimengA, and YunmouA were developed and utilized in breeding.

Table 2 Mustard hybrids released in Yunnan since 1991

Discussion

Despite several constraints to the expansion of growing, mustard will continue to have a significant role as a traditional oil crop in Yunnan agriculture. Continued cultivation and enhanced production is important from the point of regional economic security. Use as a regular supply of plant oil will continue to be of particular importance in mountainous areas of its production. There is a large but bridgeable gap existing in the seed yields that could be realized by adoption of improved cultivars or hybrids. Intensification of research for genetic improvements in breeding is urgently required. Present development programmes mostly pay attention to high seed yields. However, accelerated research to develop high drought-tolerance lines may be more necessary to increases or at least stabilizes seed yields.

There is also a lack of sufficient information of the germplasm, though accessions have been evaluated. The total volume of collections poses particular difficulties in exploitation for breeders because of meager information in their potential usefulness owing to deficiencies in evaluation, related research and information dissemination. The plant breeder often stands at the end of the line until other disciplines have ascertained the useful genes in unimproved or row germplasm. Thus, multidisciplinary and international collaboration is urgently needed to fill the gaps.

Acknowledgments

Research was supported by the Natural Science Research Fund of Yunnan Province.

References

1. Shi Huaqing et al. (1991) Studies on the utilization of heterosis in mustard (Brassics juncea). ACTA AGRONOMICA SINICA 17: 32-41

2. Shi Huaqing et al. (1990) Breeding for mustard hybrids of high quality and vigorous heterosis. Crop Research 4: 41-46(in Chinese)