2. Conservation of mulberry genetic resources in Korea

2.1. Present status of mulberry germplasm maintenance

Mulberry genetic resources represent the basis for sericultural development and a reservoir of genetic adaptability that acts as a buffer against environmental change.

At the Department of Sericulture and Entomology, NIAST, RDA., we have collected more than 615 accessions of both indigenous and exotic origin of mulberry species, which are maintained both in field and greenhouse and a large number of morphological and agronomical characters are being evaluated for utilization in breeding programs.

2.1.1 Mulberry Species maintained at NIAST

The species of Morus, being highly heterozygous and as unisexual plants produce a number of natural hybrids with many intermediate forms which crate difficulties with regard to their correct delimitation. A critical taxonomic study of the germplasm collections is very important to correctly identify and circumscribe the species, to know the intraspecific variability and the distribution patterns of the species.

All the accessions have been classified according to taxonomic groups by classification system of the mulberry species which was established by Koizumi(1917).

According to Koizumi’s system that classified the genus Morus into 24 species and one subspecies, we classified the mulberry accessions preserved at the Department of Sericulture and Entomology of NIAST into 8 species.

The distribution of the mulberry accessions is as follows; Korea has 4 endemic species out of 8, namely M. bombycis Koidz., M. tiliaefolia Makino, M. mongolica(Bureau) Schneider and M. alba L. Majority of mulberry accessions available are of M. alba L., M. latifolia(M. lhu) and M. bombycis, which are widely cultivated in Korea.

Less number of accessions are from the genus M. tiliaefolia Makino, M. indica, M. nigra, M. acidosa(M. australis), M. kagayamae, M. laevigata. 259 accessions out of 615 are remained unclassified and needed further research for classification.

2.1.2 Origin of mulberry species

Genetic resources of mulberry are classified into five groups based on their origin: wild type, domestic type, bred type, others and unknown. The number of genotypes belonging to each group is as follows:

Most of bred mulberry accessions are crossed hybrids including 40 artificial tetraploid created by the method of polyploidy.

2.1.3 The place of origin

Mulberry cultivars or strains had been introduced to Korea from various countries, including Japan, Iran, China, Turkey, Uzbekistan, Pakistan, India, Canada, Taiwan, America, Philippines and Lebanon. In all, there are 615 accessions comprising of 205 indigenous, 151 exotic and 259 unclassified strains being preserved in Korea.

Among the exotic strains majority of them are from Japan (129) followed by Iran(17) and China(16). The major portion of indigenous accessions comprises of local selection, F1 hybrids, polyploid lines. These collected mulberries are now preserved in the field or on pots for non-dormant accessions after multiplication by grafting, cutting or tissue culture.

We expected them to be utilized in the breeding program in future, since they may genetically possess useful potentials in terms of growing habitude, leaf quality, resistance to diseases/pests and fruits etc.

The distribution of the mulberry accessions by their geographical origin is as follows:

The data shows that there are accessions from nearly all countries practicing sericulture from the temperate and sub-tropical belt like Japan, Iran, China, Uzbekistan, Pakistan, India, Canada, Taiwan, America, Philippines and Lebanon.

2.2 Management and utilization of mulberry germplasm resources

2.2.1 Methods employed in preservation

Mulberry genetic resources are preserved by the following methods: field cultivation, pot cultivation and greenhouse cultivation.

In case of field cultivation, 10 plants of each genotype are planted, and pruned at bottom part once a year in March (spring pruning) or in June (summer pruning). The stump is maintained by low-cut training method, and spacing is 1.2m x 0.6m.

Whereas accessions known to be susceptible to dwarf disease, 5 plants of each type are planted, and pruned by middle stemmed pruning with 3 fists, and spacing is 3.0m x 2.0m

Control of pest and disease are being carried out routinely. New saplings are planted to compensate for the stumps that are poorly growing or withered to death because of unknown reasons. In Korea, propagation of mulberry is generally carried out by bud grafting methods, because most of the domestic genotypes have poor rooting ability.

Seedlings are only used for the stocks for grafting and selections after hybridization.

Almost genotypes of non-dormancy type, such as the accessions introduced from Southeast Asia and created by crossing with non-dormancy type, are managed on the pots in the greenhouse.

2.3 Development of mulberry germplasm resources.

The overall objective of plant breeding is to combine as many desirable traits as possible into a commercially acceptable variety. The requirements for mulberry varieties are: high yielding ability, outstanding leaf quality and durable disease/pest resistance.

The purpose of mulberry breeding is to develop varieties that are efficient in nutrients so that they give high yield of quality leaves per unit area, varieties that can withstand extreme conditions (especially, freezing injury) and resist to diseases and pests.

Recently, on response of social needs, new purpose of breeding is added, namely suitability for the production of mulberry fruits. At present the main attention is paid to the development of mulberry varieties suitable for the production of mulberry fruits.

To breed mulberry varieties effectively, it is important to set a clear goals of mulberry breeding.

Our actual breeding strategy follows two lines. Firstly we are breeding new varieties that are efficient in nutrients so that they give high yield of quality leaves per unit area, varieties that are capable of tolerating extreme conditions such as cold injury, diseases and pests.

Secondly, on response of social and commercial needs for the mulberry fruits, we are screening and breeding varieties suitable for the fruit production. The screened strains and artificial tetraploid strains will be registered as recommended varieties for cultivation through local adaptability test for four years at more than three places.

There are 18 recommended mulberry cultivars that meets a lot of requirements in Korea. All of recommended mulberry cultivars originate from the 3 main species: 11 varieties of M. alba L., 5 of M. bombycis Koidz. and 2 of M. lhu Koidz.

The following main methods are employed in mulberry breeding in Korea.

2.3.1 Artificial hybridization and selection

The first step of plant breeding program is to identify that exhibit variation for the trait or traits of interest to the breeder. Desirable traits or combinations of traits should be sought among plants in existing populations, such as recommended cultivars and breeding lines.

When the desired traits cannot be found in any of the existing germplasm sources, the breeder creates variation by hybridization or induced mutation. Artificial hybridization is the most widely using method for the breeding of mulberry. Female and male parents are selected for hybridization. Parent selection is important because it determines the range and nature of the variability in the F2 generation and sets the potential limit for successful selection in the segregating generations.

After selection of desired seedlings they are further propagated by the vegetation method. The main characters for mulberry selection are as follows:

• High nutritive value for the silkworms and high leaf yield per tree and unit area.
• Resistance to the local mulberry diseases
• Response to indigenous cultural practices
• Shorter internodal distance, allowing rearing silkworms with mulberry shoots.
• High percentage of leaves from the whole vegetation yield
• To have more branches per tree.

In Korea, 7 out of 18 recommended varieties are by selection and 11 cultivars were by hybridization. 14 cultivars out of 18 are diploids, whereas 3 are triploid created by crossing tetraploid with diploid. 100 strains generated by artificial hybridization are preserved after selection by their characteristics

By crossing artificial tetraploids with diploids at Department of Sericulture and Entomology, NIAST, RDA, we bred 3 recommended triploid varieties for the commercial cultivation, namely Shingwangppong having high rooting ability, high yielding Milsungppong and Suilppong.

By this method at Department of Sericulture and Entomology, NIAST, RDA were evolved the high yielding triploid varieties of Shingwangppong, Milsungppong and Suilppong, which are crossed tetraploids with diploids and 7 diploid varieties of Subongppong, Sugyeppong, Shinilppong, Susungppong, Chungunppong, Sangilppong, and Hansungppong.

2.3.2 Polyploidy breeding

Polyploidy in mulberry, either natural or artificial has a great economic value in evolution of superior varieties so that are possibly used in breeding. Tetraploid varieities might have several beneficial variations in characteristics, as compared with their original diploids.

Generally, comparison of the diploid and tetraploid cultivars shows several specific characters such as thicker in leaves, higher vegetative growth, slower evaporation from leaves, larger in leaf size, and larger in fruits (Tojyo, 1966). Higher yielding and bearing bigger in fruits are important character in rearing silkworm and fruit production.

For the take advantage of characteristics of tetraploids, we have been tried to produce tetraploid genotypes by colchicine treatment on seedligns and sprouting winter buds of hardwood cuttings since 1995.

40 tertaploid mulberry accessions created are as follows;

Mulberry is cultivating for the fruit production in many European and Middle Eastern countries. As mulberry fruit has been increasingly evaluated as desirable for use in fresh and processed food and drink such as jam and mulberry wine, opportunities for using the mulberry trees as fruit source have grown. 3 accessions out of 40 tetraploids are under local adoptability test for registration as recommended variety for production of mulberry fruits. We expect these tertaploid mulberry accessions adaptable to practical use. It is well known that tetraploids bears bigger fruits and sorosis compared with original diploid varieties (Tojyo, 1966).

By using these characteristics, therefore, inducing artificial tetraploids are used for breeding mulberry varieties suitable for the production of fruits. And furthermore, induced artificial polyploids have been being used as the useful stocks for breeding superior triploids by intercrossing tetraploids with diploids.

Triploid plants have been found to be superior in their nutritive quality of leaf and vegetative growth (Seki and Oshikane, 1959; Park et al, 1991; Sung et al, 2001)

2.3.3 Mutation breeding

Inducing mutations artificially might also be beneficial to eliminate certain undesirable characters. Such beneficial mutant characters can successfully be perpetuated through vegetative propagation, without disturbing the genetic balance of the original variety.

New cultivars would be produced more quickly and more economically than by conventional methods involving hybridization and selection. Therefore, we tried to induce mutations using irradiation, we failed to breed commercial cultivars. Mutation breeding was not applied for mulberry breeding since 1988 in Korea.