Mulberry breeding, cultivation and utilization in Japan

Hiroaki Machii, Akio Koyama and Hiroaki Yamanouchi
National Institute of Sericultural and Entomological Science
Owashi, Tsukuba, Ibaraki, Japan

GENETIC RESOURCES AND BREEDING

Species and its distribution in Japan

Mulberry belongs to the genus Morus of the family Moraceae. Koidzumi (1917) classified the genus Morus into 24 species and one subspecies. Mulberry is distributed in a wide area of tropical, subtropical, temperate and sub-arctic zones. Most mulberry varieties cultivated in Japan belong to M. bombysis Koidz, M. alba and M. latifolia Poiret. Varieties belonging to M. bombysis are primarily cultivated in cold regions, such as the Tohoku district. Morus latifolia varieties are mainly cultivated in warm places, such as the Kyushu district. However, varieties of M. alba are cultivated in a wide area, from the Kyushu to Tohoku district, since they have middle traits of these two species. Besides these, M. acidosa Griff. grows naturally and is cultivated in the southwest islands, including the Okinawa islands. Morus kagayamae Koidz and M. boninensis Koidz, are indigenous to the remote islands of Hachijoujima and Ogasawara, respectively.

Ploidy of mulberry

Generally, mulberry is a diploid plant with 28 chromosomes (2n=28). However, it is rich in ploidy and many triploid varieties have been found especially among M. bombysis Koidz. It is said that M. cathayana Hemsl. has tetraploid, pentaploid and hexaploid varieties. Both M. serrata Roxb., indigenous to India, and M. tiliaefolia Makino, originally from Japan and Korea, are known to be hexaploid. M. boninensis Koidz is a tetraploid that is being endangered through cross-contamination with Morus acidosa Griff. M. nigra L. is dexaploid (2n=308), the largest number of chromosomes among phanerogams.

Inflorescence of mulberry

Mulberry has different varieties with male, female or androgynous flowers. Among the varieties with androgynous flowers, there are predominantly staminated, predominantly pistillated and even hermaphrodite types. Mulberry plants with anemophilous flowers cross each other easily and naturally.

Mulberry genetic resources

The National Institute of Sericultural and Entomological Science (NISES) in Japan has collected and maintained 1 300 accessions of both indigenous and exotic origin (Machii, Koyama and Yamanouchi, 1999). In order to utilize these genetic resources efficiently in breeding programmes, morphological traits necessary for classification and identification, agronomic traits responsible for the resistance against diseases and pests and commercial traits related to productivity and quality are investigated. Information on these traits is very useful for achieving breeding targets.

MULBERRY BREEDING METHOD

It takes many years (approximately 15-20) to develop a new variety of mulberry because it is a perennial woody plant. Breeding targets should be set with a long-term view. To date, breeding targets have been, for example, high yield, high nutritional value and resistance against diseases and pests. But today, new targets have been added to cope with changes in the sericultural system, such as large numbers of silkworm reared and adaptability to densely planted fields suitable for mechanical harvesting.

Crossing is the major breeding method adopted for the development of new mulberry varieties. The choice of parent selection plays a vital role in achieving the objective. For example, the variety "Shin-ichinose" was selected from the F1 generation of "Ichinose" x "Kokuso 21". The aim was to develop a variety with high quality, high yield and resistance against lodging from "Ichinose" (good quality and high yield), and "Kokuso 21" (vigorous growth and erect type). Two other varieties, "Tokiyutaka" and "Oyutaka" were also developed and released from this crossing. "Ichinose" (female) and "Kokuso 21" (male) have been used as parents for other varieties. They have desirable traits, and crossing is easy and simple.

Many indigenous, natural triploid varieties, such as "Ichibei", "Fukushima Oha", "Akagi" and "Tagowase" have been distributed, especially in the Tohoku area, northern Japan. Since the 1960s, polyploidy breeding has been produced artificially by colchicine, which is capable of doubling chromosome numbers.

The significance of polyploidy breeding is that the leaves of triploids are larger than diploids and the yield is higher; crossing between different ploids accumulates more genes than crossing between diploids, and is expected to have more heterotic effect; triploids show good leaf quality and resistance to coldness (Tojo, 1985). To date, five triploid varieties, "Shinkenmochi", "Aobanezumi", "Mitsushigeri", "Yukimasari" and "Yukiasahi" have been developed and released using the polyploid breeding method.

Mulberry breeding system

In Japan, mulberry breeding started at the Government Sericultural Experiment Station in 1916, and two varieties, "Kokuso 13" and "Kokuso 70" were released to farmers in 1922. After that, with the wave of economic recovery planning after the Second World War, three varieties, "Kokuso 20", "Kokuso 21" and "Kokuso 27" were released in 1949. However, these varieties were not popularized much, despite their high leaf productivity, because they were both rather sensitive to dwarf disease, which causes the most serious growth damage, and were of slightly poor quality.

In 1958, a field trial was initiated in four climatically diverse regions (cold, snowy, temperate and warm) and 13 prefecture sericulture experiment stations. In 1971, a dwarf disease resistance test was added to the programme. When a selected strain is officially recognized to have excellent characteristics, it is commercially released as a new variety. To date, 19 varieties have been released, including "Shinichinose", "Yukishinogi", "Minamisakari", "Shinkenmochi", "Hayatesakari", "Aobanezumi", "Mitsuminami" and "Senshin". Such a system is believed to be indispensable to enable Japan’s breeding network to release a new mulberry variety to farmers.

MULBERRY VARIETIES

Generally speaking, varieties belonging to M. bombysis have lobed and shallow bottom leaves. Branches are brown or grey, and the winter buds are oval and sharp-pointed. Varieties of M. alba have lobed/unlobed leaves with whitish-grey or greyish-brown branches. Morus latifolia varieties have large, unlobed, lustrous leaves and greenish-grey or whitish-grey branches. "Ichinose" and "Kairyo-nezumigaeshi", most widely cultivated in Japan, are M. alba varieties and "Kenmochi", intensively cultivated in cold areas, is a variety of M. bombysis.

As mentioned above, 19 varieties were registered and released from 1971 to 1998 by the Ministry of Agriculture, Forestry and Fisheries (MAFF). Of them, six varieties, "Minamisakari", "Hayatesakari", "Mitsuminami", "Hinosakari", "Mitsusakari" and "Senshin" are suitable for warm areas such as the Kyushu and Shikoku regions. Four varieties, "Shin-ichinose", "Tokiyutaka", "Oyutaka" and "Tachimidori" are suitable for temperate areas, such as the Kanto region. Five varieties, "Shin-kenmochi", "Aobanezumi", "Mitsushigeri", "Hachinose" and "Waseyutaka" are adaptable to cold areas, such as the Tohoku region. Four varieties, "Yukishinogi", "Yukishirazu", "Yukimasari" and "Yukiasahi" are adaptable to snowy areas, such as Niigata prefecture. There are some varieties, such as"Hayatesakari", which have been dispersed into the Tohoku from the Kyushu area, which was their place of origin.

PHYSIOLOGY AND CULTIVATION OF MULBERRY

Leaf production

The growth of mulberry is generally divided into three stages: new shoot development, growth and storage (Ohyama, 1970). New shoot development is a stage in which new shoots develop using reserves stored in the stump or root the previous year. The growth stage is when carbohydrates and other substances are produced for vegetative growth by means of photosynthesis in leaves. The storage stage is when most photosynthetic substances are stored for the following year's growth. In cultivated mulberry plants, however, photosynthetic organs are removed/harvested by pruning and leaf picking, disrupting the growth stage. The exploited mulberry plants resume growth using the remaining storage substances.

NUTRITIVE VALUE OF MULBERRY LEAVES

The silkworm eats only mulberry leaves to make its cocoon, producing silk. Mulberry leaves are rich in protein and amino acids (see Table; Machii, 1989). It is known that there is high correlation between leaf protein level and production efficiency of cocoon shell, which means the cocoon shell weight to the total amount of mulberry leaves consumed by the silkworm (see figure; Machii and Katagiri, 1991). Therefore, an increase in the protein level of mulberry leaves may lead to improvements in cocoon productivity.

Relationship between mulberry leaf protein content and the cocoon shell yield in spring and autumn

Cultivation

According to the 1996 statistical data of Japanese sericulture, the area of mulberry gardens was 14 884 ha, including 1 172 ha of densely planted fields. Regarding planting density of mulberry gardens, normal planting with 600-1 000 plants per 10 ha is common. In densely planted fields, aiming at early high yield and machinery harvesting, more than 2 500 plants/10 ha are used.

Standard application of chemical fertilizer to mulberry garden is 30 kg of N, 14 kg of phosphate and 12 kg of potassium/10 ha for alluvial soil, and 30 kg of N, 16 kg of phosphate and 20 kg of potassium/10 ha for volcanic ash soil. In either case, application of at least 1 500 kg of compost per 10 ha is recommended.

Amino acid content in mulberry leaf (mean of 119 varieties) and minimum requirement for silkworm (mg/g DM)

Amino acid	Content	(%)	SD	CV	Minimum requirement for silkworms*
Asp	20.49	(10.0)	3.63	17.72
Thr	10.52	(5.2)	1.75	16.63	7
Ser	10.12	(5.0)	1.60	15.79
Glu	23.23	(11.3)	3.96	17.03
Pro	10.93	(5.4)	3.73	34.10
Gly	12.02	(5.9)	1.95	16.22
Ala	15.75	(7.7)	2.90	18.44
Val	12.83	(6.3)	2.17	16.92	8
Cys	1.17	(0.6)	0.25	21.72
Met	2.99	(1.5)	0.61	20.48	4
Ileu	10.04	(4.9)	1.88	18.68	8
Leu	19.45	(9.5)	3.10	15.93	8
Tyear	7.40	(3.6)	1.39	18.74
Phe	12.26	(6.0)	2.06	16.78	8
GABA	2.26	(1.1)	0.69	30.70
NH3	2.89	(1.4)	0.54	18.70
Lys	12.33	(6.0)	2.58	20.91	8
His	4.61	(2.3)	0.82	17.78	5
Arg	12.96	(6.3)	2.72	20.95	8
Total	204.25	(100.0)
N(percent)	4.36		0.42	9.63

*Arai and Ito, 1967.

There are various training forms in mulberry cultivation according to the various purposes. Maintenance of stump height is one of the typical forms of training. Based on the height from the soil surface: low cut (at the height of 15-30 cm from soil surface), medium low cut (30-50 cm) and medium cut (50-100 cm) are under practice. The second is a fist shape training method: if the plant is pruned at the fold each time, a fist is formed, from where the shoots emerge. Yet, if it is pruned slightly above it, leaving a definite bud, the plant height goes up gradually and a fist is not formed. This is called a non-fist shape training method. The third is a lateral branch training method developed in the Yamanashi prefecture: branches adjacent to plants within a row are held down and tied up with a wire and the shoots emerge from the buds of the branches lying down.

Harvesting methods vary with rearing scale and frequency. Basically there are two methods: spring pruning (for the summer-autumn rearing season) and summer pruning (for both spring rearing and late autumn rearing seasons). There are also the circle harvesting method (spring pruning and summer pruning alternately every year) and alternate harvesting method (alternating spring and summer pruning to half of the same plant). These two methods are adopted to secure enough yield by sustaining the tree vigour. Meanwhile, in densely planted fields, mechanical harvesting is so essential that low pruning, at a point near the ground to prevent stump formation, is desirable.

PROPAGATION

Mulberry propagation is generally carried out by grafting and by cutting methods. Root grafting prevails because it is easy to handle and the grafted saplings have a high survivability. The cutting method can be with hard wood (using the branches grown in the previous year) and soft wood (using the spring sprouted shoots). With mulberry varieties of poor rooting ability, treatment with plant hormones is advised to stimulate rooting. Recently, tissue culture derived saplings have also been produced.

OTHER USES

Mulberry was originally cultivated in Japan and other countries for sericulture. Recently, however, mulberry has been re-evaluated because of its functional characteristics and is being utilized for various purposes, such as the following:

Fruit

Mulberry fruit changes colour from green to purple black through red with maturity. Some varieties introduced from mid-Asia have white fruit. On average, the sugar content is about 12 percent, but in some varieties it is more than 20 percent. Mulberry fruit is consumed fresh, made into jam or liquor (mulberry wine). Very recently, it was found that mulberry fruit has an anti-oxidative property.

Medicinal uses

Mulberry has been used as a medicine from ancient times. The root bark in particular has been used as a herbal medicine to reduce high blood pressure. Mulberry leaf is rich in gamma-aminobutylic acid, effective against high blood pressure, and in alanine, effective against hangovers (Machii, 1989, 1990). Moreover, it has been found that deoxynojirimycin, which is said to have an effect in lowering the blood-sugar level closely related to diabetes, is abundant in mulberry leaf. That is why, today, mulberry tea is considered to be a health food.

Paper production

Mulberry grows more quickly than other woody plants and is said to be suitable for high biomass production. Mulberry branches are being used as raw material for paper production.

Mushroom production

Mulberry stem and stem powder are found to be a good source of media for mushroom production.

Animal feed

The use of mulberry for animal production in Japan is being reported in other articles in this E-conference.

BIBLIOGRAPHY

Arai, N. & Ito, T. 1967. Nutrition of the silkworm, Bombyx mori XVI. Quantitative requirements for essential amino acids. Bull. Sericul. Exp. Sta., 21: 373-384.

Koidzumi, G. 1917. Taxonomical discussion on Morus plants. Bull. Imp. Sericult. Exp. Stat., 3: 1-62 (in Japanese)

Machii, H. 1989. Varietal differences of nitrogen and amino acid contents in mulberry leaves. Acta. Seric. Entomol., 1: 51-61. (in Japanese)

Machii, H. 1990. On gamma-aminobutylic acid contained in mulberry leaves. J. Seric. Sci. Jpn., 59: 381-382. (in Japanese)

Machii, H. & Katagiri, K. 1991. Varietal differences in nutritive values of mulberry leaves for rearing silkworms. JARQ, 25: 202-208.

Machii, H., Koyama, A. and Yamanouchi, H. 1999. A list of genetic mulberry resources maintained at National Institute of Sericultural and Entomological Science. Misc. Publ. Natl Seric. Entomol. Sci., 26: 1-77. (in Japanese)

Ohyama, K. 1970. Studies on the function of the root of mulberry plant in relation to shoot pruning at harvesting. Bull. Sericul. Exp. Sta., 24: 1-132. (in Japanese with English summary)

Tojo, I. 1985. Research of polyploidy and its application in Morus. JARQ, 18: 222-228.