12. Characteristics of weed rice strains

1)Institute of Food Crops, Jiangsu Academy of Agricultural Sciences, Nanjing, China

2) National Institute of Genetics, Misima, 411 Japan

A weed is defined as an unwelcome plant which has adapted to habitats disturbed by man. Many crops are accompanied by their related weed types, in addition to their wild progenitors. In rice, the existence of weed types in and around arable lands (mainly directly seeded fields) was reported in many countries (Hara 1942; Chou 1948, 1981; Oka and Chang 1959; Arashi 1974; Eastin 1979). Studies on the origin of weed rices and their genetic nature might provide important information for understanding the evolution of rice and also for practical rice breeding.

Twenty-four weed rice strains obtained from seven countries were examined regarding various morphological as well as physiological characters and ten isozyme loci. Each strain was quantitatively evaluated for "wildness" and for Indica vs. Japonica variation using discriminant scores.

1) Weed rice strains possessed intermediate characteristics between wild and cultivated types showing a wide variation (Table 1). Most strains were similar to wild types in that they had a high degree of seed shedding, seed dormancy, black hulls and light seeds, although they looked like cultivated rice during the vegetative growth. But, some were similar to the cultivated type not only in outward appearance but in life-history traits also, being distinguished only by their red pericarp. Such mimics of cultivars are harvested and sown together with cultivated rice unconsciously.

2) Judging from two systems of Indica-Japonica discriminant scores, one based on three characters (phenol reaction, KC1O\3\ susceptibility and apiculus hair length) and the other on 10 isozyme loci (Amp-2, Amp-3, Acp-1, Cat-1, Est-2, Est-9, Pgd-1, Pgi-1, Pgi-2, and Pox-2), weed rice strains appeared to be differentiated into the Indica and Japonica types to some extent (Fig. 1). Strains collected from China where Japonica cultivars are grown were of the Japonica type. Indica-like weed strains were found among the collection from India and Thailand where Indica cultivars are distributed. A population sample obtained from Korea was a typical Japonica in isozymes but showed a wide variation in characters. Interestingly, those collected from the hill area of Nepal, where Japonica-like and Indica-like plants were found in mixture in the same field, were Indica-Japonica intermediates.

Weed rices found in humid tropics where the common wild rice grows have originated most probably from natural hybridization between wild and cultivated types followed by selection for weediness. However, the origin of weed rices found in the area where wild rice is not present remains unknown. They might be either 1) a transitional form changing from wild to cultivated types, or 2) a relic or primitive cultivar, or 3) an offtype derived from the present cultivars.

Table 1. Distribution of index values for "wildness", which is the average of standardized values of seed shedding rate, seed dormancy and minus seed weight

Fig. 1. Weed rice strains scattered by the scores for Indica-Japonica classification

X\1\=P+1.313K-0.28A

P: Phenol reaction

K: KClO\3\ susceptibility

A: Apiculus hair length

X\2\=1/10 (sum) F\1\/(F\1\+F\J\)

F\1\ and F\J\ are allelic frequencies in the Indica and Japonica groups, respectively, at each of 10 isozyme loci. Strains connected by broken line were collected from the same field.

References

Arashi, K., 1974. Consideration on Red Rice in Japan, 295pp. Yusankaku, Tokyo, (in Japanese)

Chou, S.L., 1948. China is the place of origin of rice. J. Rice Soc., China 7(5):53-54. (in Chinese)

Chou, S.L. 1981. Science and Technology of Rice Culture, 354pp. Agric. Press, Beijing, (in Chinese)

Eastin, E.F., 1979. Selected bibliography of red rice an other wild rices (Oryza spp.). Texas Agric. Exp. Station, 59pp.

Hara, S., 1942. Genetical studies on the wild rice in Formosa. Jpn. J. Genet. 18: 183-184. (in Japanese)

Oka, H.I. and W.T. Chang, 1959. The impact of cultivation on populations of wild rice, Oryza sativa f. spontanea. Phyton 13: 105-117.