Interactions between soybean and weeds in a replacement series system, considering the effects of water stress

Due to the increase of water deficiency in many farm regions and its meaning on weed interference, competitive interactions between soybean and three weeds were evaluated under water stress (20 to 40 days after transplanting) and no stress conditions. Three independent experiments were carried out in a growth chamber, being each one composed by the weeds Alternanthera tenella, Tridax procumbens or Digitaria ciliaris, along with the crop, in which soil water condition and plant composition effects were evaluated while in competition. A replacement series system was used, including both monoculture of each species and a mixture with a ratio of 50% between weed and soybean. A completely randomized design was used in factorial arrangement, with treatments distributed in three levels for plant composition factor (soybean and weeds monocultures, in addition to the soybean + weed mixture) and two levels for the water factor (with or without stress), amounting six treatments in each experiment. Soybean dry mass was higher than weed dry mass, when growing without water stress. However, under water stress conditions, the dry mass of soy was reduced in all experiments, mainly in the D. ciliaris comparative experiment. Water restriction was also significant in the plants' photosynthesis reduction in most of the experiments, reducing leaf area duration and efficiency of water use. Analysing all variables shows greater weed tolerance than soybean when submitted to water deficit and with distinct changes of their interactions and mechanism of competition, in each experiment. Devido ao aumento da deficiência hídrica em muitas regiões agrícolas e sua importância na interferência de plantas daninhas, avaliaram-se as interações competitivas de soja com três espécies de plantas, sob condições normais de crescimento ou com deficiência hídrica temporária (20-40 dias após transplante). Os três experimentos foram realizados em câmara de crescimento, com as espécies Alternanthera tenella, Tridax procumbens ou Digitaria ciliaris, juntamente com a cultura, avaliando-se os efeitos do conteúdo de água no solo e da composição de plantas, na capacidade competitiva de cada espécie. Utilizou-se o sistema de séries substitutivas, abrangendo o monocultivo e a mistura na proporção de 50% entre cada planta daninha e a soja. Utilizou-se o delineamento inteiramente casualizado, em esquema fatorial, com tratamentos distribuídos em três níveis para o fator composição de plantas (monocultivos de s