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A number of upland rice varieties were grown in the highlands of northern Thailand at altitudes of 800, 1200, and 1450 m in 1978 and 1979. Growth rate declined and growth duration was extended as altitude increased. At the 1200 and 1450 m sites yield was severely restricted by spikelet sterility, probably induced by cool temperatures. Some varieties were completely sterile at the highest altitude, where yields ranged up to 2.6 t ha" 1 . At 800 m several varieties yielded more than 5 t ha~' in 1978, but yields at this lower site were reduced by about 20% in 1979. Translocation to the grain of assimilates stored in the vegetative parts appeared to have been restricted. Lodging was a problem at all sites.Upland rice, defined as that grown under rain-fed, unbunded conditions, accounts for one sixth of the total world area sown to rice, but yields are much lower than those of paddy crops (de Datta, 1975). In the highlands of northern Thailand upland rice is the basic subsistence crop grown by the hilltribe people, shifting cultivators who have migrated from China, Burma and Laos. Yields are commonly less than 1 t ha" 1 , often due largely to poor agronomic management but probably also to the use of poorly adapted varieties, because few varieties yield successfully at altitudes above 1000 m. Temperature decreases with increasing altitude and the level of solar radiation in these mountainous areas is low during the wet season as a result of the high degree of cloud cover.Considerable research has been carried out on the growth and yield development of lowland rice under a range of environmental conditions, but upland rice has been largely neglected. However, the effects of variations in temperature and solar radiation, for example, are likely to be similar for both upland and lowland varieties provided that other factors such as water and nutrient supply are not severely limiting growth.The effects of temperature on growth and development of rice have been reviewed previously (e.g. Owen, 1971;Munakata, 1976;Nishiyama, 1976;Yoshida, 1977). Cool temperatures reduce crop growth rate and extend the duration of all phases of growth; panicle initiation is completely inhibited if temperatures are cold enough. Within a range of 22° to 31°C spikelet number per plant has been shown to increase as temperature drops (Yoshida, 1977). The effect of temperature on tillering depends also on the effects of solar t Present address:

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