Water relations and morphological aspects of soybean-velvetleaf interspecific competition

Abstract

Field and greenhouse research was conducted to evaluate the effects of interspecific competition on water relations, morphology, and yield of soybeans [Glycine max (L.) Merr. cv. Hutton] and velvetleaf (Abutilon theophrasti Medik. # ABUTH). Under greenhouse conditions, velvetleaf photosynthetic rates exceeded those of soybean at velvetleaf leaf water potentials (Ψ[w1]) > -2.5 MPa. However, as stress intensified, photosynthetic rates declined more rapidly in velvetleaf than in soybean. Stomatal conductance of soybean and velvetleaf decreased curvilinearly with reductions in Ψ[w1], but photosynthetic rates declined linearly and curvilinearly as soybean and velvetleaf stomatal conductance decreased, respectively. Interspecific competition significantly reduced soybean and velvetleaf seed yields in each year of a 2 yr field study. Neutron attenuation showed that monocultured velvetleaf and soybean roots extracted water from 1 and 1.5 m depths within the soil profile, respectively. Little or no soil water was extracted at 1.5 m depths in the intercropped plots. Leaf water potential did not differ significantly between monocultured and intercropped soybeans in 1984 or 1985. In 1985, (Ψ[w1]) values were significantly lower in intercropped than monocultured velvetleaf during anthesis. Leaf water potential values were 0.3 and 0.4 MPa lower in the youngest, fully-expanded leaves of intercropped than monocultured velvetleaf during midmorning and midafternoon hours, respectively. Rates of photosynthesis, stomatal conductance, and transpiration also differed significantly between monocultured and intercropped velvetleaf during anthesis. Interspecific competition had little or no effect on Ψ[w1], photosynthetic and transpiration rate, or stomatal conductance velvetleaf in 1984. Significant reductions in leaf area index (LAI) and total leaf number of intercropped soybean were observed 8 wks after emergence in 1985. Interspecific competition had little or no effect on soybean LAI in 1984. Leaf area index, number of main stem nodes, total number of leaves, and plant dry weight were significantly lower in intercropped than monocultured velvetleaf 4 and 6 wks after emergence in 1984 and 1985, respectively. The data suggest that soybean yield reductions in soybean-velvetleaf interspecific competition are attributable to resource limitations other than water (i.e. sunlight and nutrients) in South Central Texas.