The objective of this study was to compare the emission rate and expansion of the leaves, duration of the leaf area (DLA) and the extinction coefficient (k) for the crops soybean and of the bean, and for the weeds Euphorbia heterophylla sensitive and Euphorbia heterophylla resistant to the herbicides inhibiting of the ALS enzyme, Bidens pilosa and Desmodium tortuosum. The experiment was developed in the field, in soil classified as Red-Yellow Claysoil, in the period of october of 2000 to march of 2001. Each plantspecies consisted of a treatment. The treatments were arranged in a randomized complete block design with four replications. The mensurations of the photosynthetically active radiation (PAR) were accomplished in two points of the plants: above and bellow the canopy, by means of a light ceptometer. The emission rate and the expansion of leaves was calculated at the end of the cycle of the crops. The DLA and k were calculated before and after the plant flowering. It was not observed differences in the development of the biotypes of E. heterophylla with relation to the rate of appearance of leaves, expansion rate, DLA or k. Among the cultures, the bean presented smaller leaf emission rate (0.591 / day) compared to the soybean (0.933 / day). Among the weeds, the largest leaf emission rate was with D. tortuosum (0.699 / day). The leaf expansion rate observed by the soybean was superior to all the other species (6.77 cm².dia-1). All plant species presented larger value for DLA after the flowering compared before flowering. The soybean presented larger value of k (before and after the flowering 0.52 and 0.93, respectively) compared to the other species, demonstrating high potential of interception of solar radiation.
Extinction coefficient; duration of the leaf area; Glycine max; Phaseolus vulgaris
