Growth analysis of nine diverse rice cultivars in competition

The authors measured more than 20 root and shoot growth response variables for nine diverse, mostly upland, rice cultivars in competition with barnyardgrass (Echinochloa crus-galli). The objectives were to thoroughly establish varietal differences as a basis for future comparison in the field and to test hypothesis about cultivar evaluation in breeding nurseries. The cultivars included selections from all three Isozyme groups of Oyrza sativa, indica, tropical japonica, and Aus, as well as CG-14, a traditional O. glaberrima from West Africa. In two trials in 2001, target plants were grown in pot surrounded by six barnyardgrass plants in a glasshouse at IRRI, Philippines. Height, tiller numbers, and chlorophyll content (SPAD) were measured weekly, and plants were harvested at 21 and 49 d after seeding (DAS). Measured variables included leaf, stem, and root dry mass (DM); leaf and stem area; root length; leaf and tiller angle; leaf area distribution; as well as derived variable such as specific leaf area and associated growth rates. Significant cultivar differences were found for most response variables. Two indica cultivars, an Aus cultivar and CG-14 consistently produced the most DM per plant at each harvest but did not have uniform growth traits. Thus, DM production under competition in these experiments was a function of multiple traits, and more than one path to greater DM production existed. In regressions DM was mostly a function of leaf area and other shoot-related variables; root traits were not needed to explain outcomes. In addition, the relationship between rice DM at 21 and 49 DAS was weaker than expected, perhaps indicating that greater early vegetative vigor does not automatically confer increased DM production under weed interference. Overall, results supported the authors' hypothesis that competitiveness against weeds may be increased by nondestructive screening of nursery lines for cover and some early vegetative vigor traits such as tillering and height gain rates