Interacting Effects of CO2and Nutrient Concentration

Abstract

Soybean [Glycine max(L.) Merr. ‘Tracy′] and two associated weeds, sicklepod (Cassia obtusifoliaL.) and showy crotalaria (Crotalaria spectabilisRoth), were grown in controlled-environment chambers with day/night temperatures of 29/23 C, photosynthetic photon flux density (PPFD) of 600 μE·m−2·s-1, CO2concentrations of 350 or 675 ppm, and one-eighth or one-half strength Hoagland's nutrient solution applied three times daily. Leaf areas and dry weights of plant parts were determined at 1, 3, and 5 weeks. Stomatal resistances, transpiration rates, leaf water potentials, and leaf chlorophyll contents were measured, and net assimilation rates (NAR) and leaf area durations (LAD) were calculated. In all species, growth in 675 ppm CO2enhanced dry-matter production through increases in both NAR and LAD. The increased dry-matter production with one-half strength compared to one-eighth strength Hoagland's solution was, however, caused by increased LAD. Stomatal conductances and transpiration rates decreased in 675 ppm CO2, but were not affected by nutrient level. High CO2concentration or low nutrient level generally decreased leaf chlorophyll content per unit area. Growth enhancement by high CO2was greater in one-half strength than in one-eighth strength Hoagland's solution.