Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

S. Elwynn Taylor

Second Advisor

Irvin C. Anderson


Water relations, gas exchange, and growth characteristics of intercropped soybean and velvetleaf plants were evaluated during 1985 and 1986 in a field study at Ames, Iowa. Velvetleaf predawn and midday leaf water potentials were consistently lower than those of soybean. Midday stomatal conductances, midday leaf transpiration rates, and net photosynthetic rates were higher for velvetleaf during the vegetative growth stage of both species, but thereafter were higher for soybean. Despite consistently lower leaf water potentials, velvetleaf was able to maintain a seasonal range of stomatal conductances generally comparable to those of soybean. No interspecific differences were detected in the relationship between leaf photosynthetic rates and stomatal conductance. In the drier than average year, soybeans intercropped with velvetleaf had slightly lower predawn leaf water potentials in comparison to monocultured soybeans. No differences in midday leaf water potentials, stomatal conductances, or photosynthetic rates were detected between monocultured and intercropped soybean;Soybean plants initially were taller and had more leaf area and total aboveground dry matter than intercropped velvetleaf. However, by midseason there were no interspecific differences in height and velvetleaf had surpassed soybeans in amount of leaf area and total dry weight per plant. By season-end velvetleaf plants had produced 3 to 4 times more aboveground dry matter than soybean plants. Velvetleaf had a slightly longer vegetative growth period and it abscissed a greater percentage of lower leaves in comparison to soybean. Early in the season velvetleaf allocated more resources to leaf material and less to stem material in comparison to soybean, however, a reversal of this pattern was observed later in the season. Velvetleaf specific leaf weight was greater than that of soybean from midseason on. Velvetleaf relative growth rates and net assimilation rates were consistently greater than those of soybean. Velvetleaf appears to be more efficient than soybean in the production of dry matter per unit leaf area. A rapid rate of growth and efficient allocation of dry matter appear to contribute to velvetleaf success as a weed of soybean. These factors, along with tall stature and an open, branching canopy structure, appear to allow velvetleaf to successfully compete for light with the soybean crop.



Digital Repository @ Iowa State University,

Copyright Owner

Joan Zaprzalka Cheetham



Proquest ID


File Format


File Size

83 pages