Limiting Factors of Distribution of the genus Oryza and Preventing Factors of Their Collection

片山忠夫（鹿児島大学農学部）The germplasm of rice is rich in genetic diversity, which have been stemmed largely from its wide geographical dispersal and eco-genetic diversification. Because they can grow in vast uncropped or marginal areas, for example, drought, salinity, acidity, Fe toxicity, deep water. On the distribution of wild rice species in the world, some reports have been already published. Although some conclusions on the distribution will be drawn after the experiments made with the use of the materials collected from the several viewpoints, considerations on the distribution of wild rice species has mainly described in the present report. The following items are of fundamental sense as the background. It is important to know where genetic diversity exits or where rice variations have performed well in spite of several problems as endemic diseases or insects, problem soil, cool temperatures, salinity, or deep water. To solve these items, factors affecting the distribution of the wild rice species, belonging to the genus Oryza, were briefly discussed in the present paper from the two large calegories, i.e., factors, limiting distribution and preventing collection of the wild rice species. Basing on the experiences of the field survey, the limiting factors of natural distribution of the wild rice species were considered in the main seven viewpoints, i.e., i) dry and wet seasons, ii) degree of water depth, iii) population size, iv) topographical constitution, v) relation to old ruins and villages, vi) link with human race and life, vii) air or water pollution. The preventing factors of collection during the scientific tour were discussed in the main nine viewpoints, i.e., i) route of tour, ii) road situation, iii) flood damage, iv) changing natural habitat by inanimate factors, v) changing natural habitat by animate factors, vi) disturbance by men, vii) time lag found in seasons of tour, viii) soil condition, ix) political status