Canopy Architecture of Soybean (Glycine max), Xanthium strumarium and Amaranthus retroflexus Under Different Interference Condition

In order to evaluate the effects of Xanthium strumarium or Amaranthus retroflexus competition under unsprayed and sprayed (half the recommended rate of imazethapyr) on canopy architecture of weed and soybean، field experiment was conducted at the Research Farm of University of Tehran، during the growing season of 2007 and 2008. The experimental design was factorial based on randomized complete block design with three replications. Two herbicide dose (0 and 35g a. i ha-1) and X. strumarium (at density of 0، 2، 4، 8 plants m-1 of row) or A. retroflexus (at density of 0، 4، 8، 12 plants m-1 of row) interference were studied. Different canopy architecture was found for soybean in monocultures compared to interspecific competition. In monocultures، with exception for lower layer (0-30 cm)، soybean plant had similar pattern for LA distribution within the canopy. By increasing density in untreated plot، soybean plants developed a large proportion of their leaf area in the upper portion of the canopy. soybean plants loss their lower (0-60 cm) leaves in competition with the highest densities A. retroflexus. However، canopy architecture and leaf area distribution of soybeans grown with X. strumarium or A. retroflexus was similar to soybeans grown alone in the reduced rate of herbicide. At the density of 2 and 4 plants m-1 of row، X. strumarium had leaves in all layers of the canopy and was able to develop and maintain its lower canopy leaves under the shade. In contrast، A. retroflexus retained few to no leaves within the lower layers of the canopy. It seems that more competitive ability of X. strumarium compared to A. retroflexus was mainly due to the growth habit of this plant، which has its leaves distributed evenly within the canopy. Result also showed that crop yield affected more by X. strumarium compared with A. retroflexus. Our results suggested that on the basis of X. strumarium superior competitive ability، it should be given a greater priority for control than A. retroflexus in soybean. The half rate of herbicide significantly decreased the competitive ability of X. strumarium and A. retroflexus indicating that، in soybean، application of this rate is recommended in similar condition.