Investigating early vigour in upland rice (Oryza sativa L.): Part I. Seedling growth and grain yield in competition with weeds

Rapid seedling growth of rice is reported to be associated with improved ability to tolerate weed competition, which is a major constraint in direct-seeded rice crops. To explore differences in growth and the effects of weed competition on yield, 20 diverse Asian rice cultivars were grown in the field and in greenhouse experiments. Differences in grain yield occurred between cultivars in the field and, after a single hand weeding, weed competition decreased grain yields by more than 60% compared with weed-free plots. Three of four cultivars producing the largest grain yields in weed-free plots also had the largest yields ill competition with weeds in the same season. Some cultivars, however, had relatively poor yields in competition with weeds compared with weed-free conditions. Rice biomass at 28 days was correlated with grain yield in competition with weeds, and with reduced weed biomass. Substantial differences in seedling growth were found among cultivars. Sabita and NSG 19 accumulated twice the total dry matter and leaf area per seedling by 12 days after sowing (DAS) as some of the least vigourous cultivars. Grain yields in competition with weeds in field experiments in two seasons significantly correlated with rice seedling biomass at 12 DAS in greenhouse experiments. Leaf area at 6 DAS had the greatest direct effect on seedling dry matter accumulation at 12 DAS. Leaf area at: 6 DAS, in turn, was associated with higher specific leaf area, greater dry matter partitioning to leaves at 4 DAS, larger embryos, heavier seeds, and earlier onset of photo-autotrophic growth. More than 90% of the variation in total rice seedling biomass at 12 DAS was associated with the parameters studied. Leaf area at 6 DAS and dry matter of rice seedlings are easily measurable traits that have potential to be used as an indirect tool for mass screening of rice for competitiveness against weeds. (C) 2009 Elsevier B.V. All rights reserved.