Relationship between biomass of an individual hill and root lodging tolerance in direct seeded rice [Oryza sativa]

The relationship between biomass of an individual hill and pushing resistance was investigated using lodging tolerant US. rice cultivar 'Lemont' and suscestible Japanese rice cultivar 'Nipponbare'. The plants were grown in the field at the same density (44.4 hills per square m) in 1990 and at different densities (22.2, 44.4, 88.8 hills per square m) in 1993 with or without laying a porous membrane between the top-soil and sub-soil layer. At the heading stage, panicle number (E), plant height (H), pushing resistance (R) at 10cm height (h), root and shoot dry weight (W,U), and diameter of culm base originating nodal roots (D) were measured for each individual hill. Loding index [I = (H * U) / (h * R) ], indicating a level of tolerance to root lodging, was higher in the hills with less E. This tendency was not significantly affected by porous membrane laying treatment which prevented roots from elongating to sub-soil layer. The ratio of R to W was lower in the plant with less E or the plant grown under highly dense condition. Above results indicate that factors other than root mass and penetration to sub-soil layer are relating to the decrease of lodging tolerance in rice plant with less E. The possible influence of D on root lodging tolerance was discussed based on the balance of physical moments working on the culm base.