Three diversity centers were detected for isozyme variation including South Asia, China and Southeast Asia Oryza sativa is cultivated in Europe, Africa, tropical and temperate Asia, Australia, and North and South America.
ECOLOGICAL ASPECT:
Ranging from Cool Temperate Steppe to Wet through Tropical Very Dry to Wet Forest Life Zones, rice is reported to tolerate annual precipitation of 4.2 to 42.9 dm (mean of 85 cases = 15.2), annual temperature of 8.4 to 27.8°C (mean of 85 cases = 23.0), and pH of 4.3 to 8.7 (mean of 72 cases = 6.3) (Duke, 1978, 1979). According to Rutger (1981), fertile soils are desired, with pH between 5 and 7.5. Rice is a tropical, subtropical and warm temperate crop, growing best where summer temperatures of 24–25°C prevail and in full sun. Rice grows as far north in Japan as 42°N and as far south in Queensland as 23°S. On the south side of the Himalayas rice is grown to 2,400 m. Rice mostly cultured on the humid coastal lowlands and deltas of the world. Growth arrested below 10°C; rice exhibits little or no frost tolerance. Temperatures of 0.5 to 5°C are lethal after 24 hours. Aquatic rice may require a dependable supply of fresh, slowly moving water, at temperature of 21–29°C. If rainfall is less than 125 cm annually, irrigation is used to make up deficit. Rice is said to require the equivalent of 8–10 dm during its growing period. Crop is salt tolerant at some stages of growth; during germination but not seedling stages rice has even been grown to reclaim salty soils. Terrain should be level enough to permit flooding, yet sloped enough to drain readily. Soil should be of a fine texture that holds water or should have a subsoil which holds water with little seepage. Friable loam overlying heavy clay, as in many coastal and delta areas, is ideal (Reed, 1976). Although most rice cvs are shortday, there are photosensitive and long day cvs.
