A análise da competitividade foi efetuada por meio de diagramas aplicados a experimentos substitutivos e uso de índices de competitividade relativa. As variáveis estudadas foram afilhamento, estatura, área foliar e massa seca da parte aérea das plantas. O arroz-vermelho modificou negativamente o número de afilhos, a estatura e a massa seca da parte aérea das plantas dos cultivares IRGA 417 e EEA 406, demonstrando habilidade competitiva superior. Os cultivares de arroz IRGA 417 e EEA 406 não modificaram suas características morfofisiológicas quando em competição, independentemente da proporção de plantas entre ambos, demonstrando habilidades competitivas equivalentes.
Palavras-chave: características morfofisiológicas, competição, interferência, plantas daninhas.ABSTRACT -The objective of this study was to compare the relative competitive abilities between two rice cultivars and a red rice biotype. Greenhouse experiments were carried out at the Universidade Federal do Rio Grande do Sul, in the 2001/2002 growing season, in a completely randomized design, with four replications. The treatments were arranged in a replacement series assay, with five plant proportions of rice cultivars and competitor biotype being established: 100:0; 75:25; 50:50; 25:75 and 0:100, respectively. Rice was represented by the cultivars IRGA 417 and EEA 406, and the competitors by a red rice genotype as well as by EEA 406 cultivar, mimicking red rice. Competitive analysis was accomplished through diagrams usually applied to replacement series studies, plus use of relative competitive indices. The plant variables evaluated were: tillering, plant height, leaf area, and shoot dry mass. Red rice negatively modified plant tillering, plant height, leaf area, and shoot dry mass of both rice cultivars, proving superior competitive ability. The rice cultivars IRGA 417 and EEA 406 did not change their morpho-physiological plant characteristics when in association with each other, regardless of plant proportions among them, presenting equivalent competitive abilities.