References
(1) National Institute of Genetics, Mishima, 411 Japan
(2) Rice Research Center, Phatum Thani, Thailand
The upland rice cultivars distributed in Thailand are partly of the Japonica
type, while all lowland cultivars are Indicas (Oka and Chang 1963; Sato et al.
1986). It may be asked why Japonicas are confined to the upland field in
Thailand.
In general, Japonica varieties are grown as both lowland and upland rices in temperate countries and at high altitudes in tropical countries also. Tropical lowland cultivars at low altitudes are exclusively Indicas. However, some of upland cultivars at a low altitude in Thailand were found to be Japonicas, which were early-maturing. It would be necessary for upland rices to reach maturity during the rainy season.
We obtained a random sample of 50 upland cultivars from the Germplasm Center of Thailand (GS lines) and classified them into the Indica and Japonica types by Oka's (1958) method. Then, their heading behavior was investigated under artificial shortday conditions, simulating the natural daylength at 24 degrees N; the special "astrodial" was set at September 22 on June 22, so that the dates in July corresponded to that in October at 24 degrees N. The heading dates of the cultivars tested were clearly divisible into early- and late- maturity groups (Table 1. Variation in heading date of Thailand upland cultivars). All Japonica cultivars, except for one from Chiangmai, belonged to the early-maturing group. Probably the early-maturing upland Japonicas are insensitive or weakly sensitive to photoperiod.
Table 1. Variation in heading date of Thailand upland cultivars
References
Oka, H.I., 1958. Intervarietal variation and classification of cultivated rice. Ind. J. Genet. Pl. Breed. 18: 79-89.
Oka, H.I. and W.T. Chang, 1963. A note on rice varieties of Japonica type found in northern Thailand. Bot. Bull. Acad. Sinica 4: 163-168.
Sato, Y.I., S. Chitrakon and H. Morishima, 1986. The Indica-Japonica differentiation of native rice cultivars in Thailand and its neighboring countries. In New Frontiers in Breeding Researches (Proc. 5th Intern. Congr. SABRAO, Kasetsart Univ., Thailand), p. 185-193.