DOES LEAF POSITION WITHIN A CANOPY AFFECT ACCLIMATION OF PHOTOSYNTHESIS TO ELEVATED CO2 - ANALYSIS OF A WHEAT CROP UNDER FREE-AIR CO2 ENRICHMENT

Previous studies of photosynthetic acclimation to elevated CO2 have fo cused on the most recently expanded, sunlit leaves in the canopy. We e xamined acclimation in a vertical profile of leaves through a canopy o f wheat (Triticum aestivum L.). The crop was grown at an elevated CO2 partial pressure of 55 Pa within a replicated field experiment using f ree-air CO2 enrichment. Cas exchange was used to estimate in vivo carb oxylation capacity and the maximum rate of ribulose-1,5-bisphosphate-l imited photosynthesis. Net photosynthetic CO2 uptake was measured for leaves in situ within the canopy. Leaf contents of ribulose-1,5-bispho sphate carboxylase/oxygenase (Rubisco), light-harvesting-complex (LHC) proteins, and total N were determined. Elevated CO2 did not affect ca rboxylation capacity in the most recently expanded leaves but led to a decrease in lower, shaded leaves during grain development. Despite th is acclimation, in situ photosynthetic CO2 uptake remained higher unde r elevated CO2. Acclimation at elevated CO2 was accompanied by decreas es in both Rubisco and total leaf N contents and an increase in LHC co ntent. Elevated CO2 led to a larger increase in LHC/Rubisco in lower c anopy leaves than in the uppermost leaf. Acclimation of leaf photosynt hesis to elevated CO2 therefore depended on both vertical position wit hin the canopy and the developmental stage.