Photosynthetic acclimation to elevated CO2 in a sunflower canopy

Sunflower canopies were grown in mesocosom gas exchange chambers at ambientand elevated CO2 concentrations (360 and 700 ppm) and leaf photosyntheticcapacities measured at several depths within each canopy. Elevated [CO2]had little effect on whole-canopy photosynthetic capacity and total leafarea, but had marked effects on the distribution of photosynthetic capacityand leaf area within the canopy. Elevated [CO2] did not significantlyreduce the photosynthetic capacities per unit leaf area of young leaves atthe top of the canopy, but it did reduce the photosynthetic capacities ofolder leaves by as much as 40%. This effect was not dependent on the canopylight environment since elevated [CO2] also reduced the photosyntheticcapacities of older leaves exposed to full sun on the south edge of thecanopy. In addition to the effects on leaf photosynthetic capacity,elevated [CO2] shifted the distribution of leaf area within the canopy sothat more leaf area was concentrated near the top of the canopy. Thischange resulted in as much as a 50% reduction in photon flux density in theupper portions of the elevated [CO2] canopy relative to the ambient [CO2]canopy, even though there was no significant difference in the total canopyleaf area. This reduction in PFD appeared to account for leaf carbohydratecontents that were actually lower for many of the shaded leaves in theelevated as opposed to the ambient [CO2] canopy. Photosynthetic capacitieswere not significantly correlated with any of the individual leafcarbohydrate contents. However, there was a strong negative correlationbetween photosynthetic capacity and the ratio of hexose sugars to sucrose,consistent with the hypothesis that sucrose cycling is a component of thebiochemical signalling pathway controlling photosynthetic acclimation toelevated [CO2].