Penetration of hardpans by rice lines in the rainfed lowlands

Rice roots are commonly shallow in rainfed lowland conditions. Mechanical impedance is one factor that may restrict access of roots to deeper soil layers, thereby reducing the capacity of the root system to extract water from depth during late-season drought. The capacity of rice roots to penetrate hardpans was examined in experiments at Rajshahi, Bangladesh, in the 1994 wet season. Eight lines (CT9993, IR52561, IR58821, IR62266, KDML105, Mahsuri, Namsagui19, and IR20) were grown in three experiments: irrigated early, rainfed early, and rainfed late. As drought intensified from heading to dough stage, soil penetration resistance increased to 3.0 MPa at 15'25 cm depth in both rainfed experiments. A high proportion of the total root length was found in the surface layer, particularly in IR20. CT9993 and IR58821 had thicker roots than other lines. Root length density (RLD) increased in deeper soil layers in rainfed with time, but lines differed in their capacity to penetrate the compacted layer as drought intensified after heading. Only IR58821 and Mahsuri were able to increase RLD below 15 cm depth after heading to values greater than 1.6 cm cm'3, and only in the rainfed early experiment. In rainfed late, soil penetration resistance tended to increase after heading in IR20, IR52561 and IR62266, implying these lines were able to extract water below 15 cm depth, but without the concomitant increase in RLD. The greater penetration ability of Mahsuri and IR58821 was expressed in both rainfed environments at high soil penetration resistance.