How does elevated CO₂ or ozone affect the leaf-area index of soybean when applied independently?

• Changes in leaf-area index (LAI) may alter ecosystem productivity in elevated [CO₂] or [O₃]. By increasing the apparent quantum yield of photosynthesis (φ_c,max), elevated [CO ₂] may increase maximum LAI. However, [O₃] when elevated independently accelerates senescence and may reduce LAI. • Large plots (20 m diameter) of soybean (Glycine max) were exposed to ambient (approx. 370 μmol mol^-1) or elevated (approx. 550 μmol mol^-1) CO₂ or 1.2 times ambient [O₃] using soybean free-air concentration enrichment (SoyFACE). • In 2001 elevated CO₂ had no detectable effect on maximum LAI, but in 2002 maximum LAI increased by 10% relative to ambient air. Elevated [CO₂] also increased the φ_c,max of shade leaves in both years. Elevated [CO₂] delayed LAI loss to senescence by approx. 54% and also increased leaf-area duration. Elevated [O₃] accelerated senescence, reducing LAI by 40% near the end of the growing season. No effect of elevated [O₃] on photosynthesis was detected. • Elevated [CO₂] or [O₃] affected LAI primarily by altering the rate of senescence; knowledge of this may aid in optimizing future soybean productivity.