Abstract
This report reviews the distribution, origin, evolution, dissemination, and diversification of wild rice in and around tropical Asia. Some characteristics of wild rice species are described from the viewpoint of their morphological and ecological characters.
The genus Oryza is constituted by about 20 valid species, including two cultivated and many wild species. As in other cereals, the evolutionary pathway of cultivated rice may be conceptualized as: wild perennial species→wild annual species→cultivars. Wild relatives of O. sativa are widely distributing in tropical and subtropical Asia and Oceanea. Record of them has been found in Thailand, India, and South China dating back as far as 3,500 B. C. They are rich in genetic diversity, much of which remains to be tapped, and which stems largely from their wide geographical dispersal and eco-genetic diversification. From the 1960s, abundant research into and collection of wild rice in Asia has been reported.
The characteristics of wild rice species in tropical Asia have been discussed in comparison with the wild rice distributed in other regions in terms of grain morphology, anatomical and histological characters of glumes and leaves, germination behaviour, seed longevity, affinity between embryo and endosperm, flowering order and time of grains in a panicle, panicle structure, floating habit, and specificity of O. officinalis.
Several factors account for the different photoperiodic sensitivities in the genus Oryza. It can be concluded that the adaptation to natural photoperiod has played an essential role in the existence of cultivated and wild species. Some characteristics become clear from the physiological and evolutionary relationships and phylogenetic differentiation of the genus Oryza.
