The effect of black and eastern black nightshade on the amount of photosynthetically active radiation (PAR) intercepted by a processing tomato canopy was studied along with the correlation between PAR and tomato growth and yield. During 1989 and 1990, black and eastern black nightshade were established at densities of 0 to 4.8 m−2 within rows of transplanted, irrigated processing tomatoes. Increasing the density of either nightshade species decreased the number of tomato fruit; however, eastern black nightshade reduced tomato yield more than black nightshade. Eastern black nightshade was taller than the tomatoes, reducing PAR reaching the top of the tomato canopy. PAR reaching the top of the tomato canopy was positively correlated with yield and negatively correlated with eastern black nightshade density. Eastern black nightshade intraspecific competition decreased both stem and berry weight. Black nightshade was never taller than tomatoes and did not affect PAR reaching the top of the crop canopy. Increasing the density of black nightshade decreased berry dry weight but increased the weight of stems and leaves.

The competitive effects of common cocklebur (Xanthium pensylvanicum Wallr.) on soybean [Glycine max (L.) Merr. ‘Lee 74′] were studied on Peridge silt loam during 1975 and 1976. Interspecific competition reduced the dry matter (DM) and leaf area of the individual plants in comparison to the plants grown under intraspecific conditions. Twelve weeks after emergence the percentage reductions in DM and leaf area due to interspecific competition were similar, were greater in common cocklebur than soybeans, and were greater in the dry year (1976) than in the wet year (1975). The root systems of both species varied with plant age, soil depth, and seasonal moisture. In general, the density of roots of both species was greatest in the soil profile at a depth interval of 0 to 13 cm and in the dry year more roots were found deeper in the soil. Extraction of soil water began in mid-July at the 91-cm depth and in late July at the 137-cm depth. Roots of common cocklebur and of the interspecific treatment explored a greater volume of soil for water than did those of soybeans. The total amount of water used by a pure stand of soybeans and a mixed stand of soybeans and common cocklebur was greater than that used by a pure stand of common cocklebur. Approximately 25% less water was lost as evapotranspiration (ET) in the dry year than in the wet year. Values for water use efficiency (WUE) increased during the season. Little difference in WUE between species was found in the wet year and up to 10 weeks after emergence in the dry year.