Photosynthetic Response of Soybean Canopies to Full-Season Carbon Dioxide Enrichment

Global atmospheric CO₂ concentration ([CO₂]) is increasing as a result of the burning of fossil fuels. At present there is little information about how agronomic crops will respond to future high [CO₂]. To investigate the basic process that will be most affected, soybean [Glycine max (L.) Merr.] canopies were continuously exposed to various [CO₂] and photosynthetic rates were measured throughout the growing season. Soybean was grown to physiological maturity in sunlit controlled-environment chambers in CO₂ concentrations of 330, 450, 600, and 800 µL L⁻¹. Carbon dioxide fluxes were measured on the canopies at 15-min intervals every day and used to calculate photosynthetic and respiration rates. Gross photosynthetic rate increased with each increment in [CO₂] regardless of stage of development, but there was considerable day-to-day and seasonal variation. Seasonal changes in photosynthetic rate were associated with developmental changes in the crop. Photosynthetic rates were low during early vegetative development, even after the canopy had closed, but increased threefold just before flowering to reach a peak during flowering at stage R2. They then decreased by 30% or more until just before the start of pod expansion (R3) when a 45% increase occurred. Thereafter, photosynthetic rates decreased slowly and continuously to final harvest. The daily curves of photosynthetic rate vs. photosynthetic photon flux density were further analyzed to determine canopy light utilization efficiency (α) and canopy conductance to CO₂ transfer (r). Plants grown in 800 µL L⁻¹ [CO₂] had a value of α that averaged about 40% higher than that for plants grown in 330 µL L⁻¹ and a value of r that averaged about 24% lower for the season. Differences in α between these treatments were significant throughout the season, while initial differences in r between treatments became less obvious after late vegetative growth stage V11.