Agronomic and growth characteristics of upland and lowland rice varieties.
Based on a study of 25 cultivars, the plant characteristics and growth features of the so-called lowland and upland rice cultivars vary continuously. One or more cultivars in 1 group fall in the range of the other group for 1 or more major characteristics associated with their performance in upland culture. Low tillering potential and constant leaf area appear to be distinctive features of many upland cultivars. Under severe water stress, most upland cultivars are less damaged by drought and have lower panicle sterility than lowland types, but certain lowland types, such as Dular and IR5, tolerate drought as well as the upland cultivars. Drought resistance is associated with a high proportion of thick roots, a dense root system, a high proportion of long roots, and a high root:shoot ratio. There is a genotype-environment interaction among upland and lowland cultivars with respect to root development. Many upland cultivars are more responsive to water stress and produce more long and thick roots under dry growing conditions. Leaf characters such as moderate droopiness and the ability to fold when water stress occurs may also be associated with drought resistance under field conditions.