- • Changes in leaf-area index (LAI) may alter ecosystem productivity in elevated [CO2] or [O3]. By increasing the apparent quantum yield of photosynthesis (φc,max), elevated [CO2] 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 φc,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.