EFFECT OF TIMING AND SEVERITY OF WATER-DEFICIT ON 4 DIVERSE RICE CULTIVARS .1. ROOTING PATTERN AND SOIL-WATER EXTRACTION

The ability of a plant to extract soil water during water deficit affe cts crop growth and may affect plant recovery from water deficit, and hence grain yield. Rooting pattern and soil water extraction of four r ice (Oryza sativa L.) cultivars were investigated during a period of w ater deficit imposed during either the vegetative or reproductive stag e of growth. The four rice cultivars chosen, CPIC8, Lemont, Rikuto-Nor in 12 (RN), and Todoroki-Wase (TW), were known to differ in response t o water deficit under upland conditions. Root growth ceased in all cul tivars when water deficit was imposed at either the vegetative or repr oductive stage. The cultivars differed in their inherent rooting patte rn, with total root length ranging from 1.3 to 33.7 km m(-2), but show ed 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 extr action were similarly ranked among cultivars (CPIC8 > Lemont > RN > TW ). Selecting for deeper roots and large root length density would assi st 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 de velops around or after panicle initiation, selection for root characte ristics does not require drought conditions.