Effect of timing and severity of water deficit on four diverse rice cultivars I. Rooting pattern and soil water extraction

The ability of a plant to extract soil water during water deficit affects crop growth and may affect plant recovery from water deficit, and hence grain yield. Rooting pattern and soil water extraction of four rice (Oryza sativa L.) cultivars were investigated during a period of water deficit imposed during either the vegetative or reproductive stage of growth. The four rice cultivars chosen, CPIC8, Lemont, Rikuto-Norin 12 (RN), and Todoroki-Wase (TW), were known to differ in response to water deficit under upland conditions. Root growth ceased in all cultivars when water deficit was imposed at either the vegetative or reproductive stage. The cultivars differed in their inherent rooting pattern, with total root length ranging from 1.3 to 33.7 km m-2, but showed no difference in ability to alter root growth during stress. These inherent differences in rooting pattern are of vital importance to the cultivars' ability to extract water during water deficit. Root length density was large in the surface soil layers and declined with depth. Extractable soil water and water extraction rate were related to root length density. Total root length, root length density and water extraction were similarly ranked among cultivars (CPIC8 Lemont RN TW). Selecting for deeper roots and large root length density would assist in developing cultivars which extract more soil water and therefore are more drought resistant in upland conditions. Because the inherent rooting pattern of a cultivar is not altered by water deficit that develops around or after panicle initiation, selection for root characteristics does not require drought conditions