Does Leaf Position within a Canopy Affect Acclimation of Photosynthesis to Elevated CO(2)?

Previous studies of photosynthetic acclimation to elevated CO(2) have focused on the most recently expanded, sunlit leaves in the canopy. We examined acclimation in a vertical profile of leaves through a canopy of wheat (Triticum aestivum L.). The crop was grown at an elevated CO(2) partial pressure of 55 Pa within a replicated field experiment using free-air CO(2) enrichment. Gas exchange was used to estimate in vivo carboxylation capacity and the maximum rate of ribulose-1,5-bisphosphate-limited photosynthesis. Net photosynthetic CO(2) uptake was measured for leaves in situ within the canopy. Leaf contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), light-harvesting-complex (LHC) proteins, and total N were determined. Elevated CO(2) did not affect carboxylation capacity in the most recently expanded leaves but led to a decrease in lower, shaded leaves during grain development. Despite this acclimation, in situ photosynthetic CO(2) uptake remained higher under elevated CO(2). Acclimation at elevated CO(2) was accompanied by decreases in both Rubisco and total leaf N contents and an increase in LHC content. Elevated CO(2) led to a larger increase in LHC/Rubisco in lower canopy leaves than in the uppermost leaf. Acclimation of leaf photosynthesis to elevated CO(2) therefore depended on both vertical position within the canopy and the developmental stage.