Integration of photosynthetic acclimation to CO2 at the whole-plant level

Primary events in photosynthetic (PS) acclimation to elevated CO₂ concentration (CO₂]) occur at the molecular level in leaf mesophyll cells, but final growth response to CO₂] involves acclimation responses associated with photosynthate partitioning among plant organs in relation to resources limiting growth. Source–sink interactions, particularly with regard to carbon (C) and nitrogen (N), are key determinants of PS acclimation to elevated CO₂] at the whole-plant level. In the long term, PS and growth response to CO₂] are dependent on genotypic and environmental factors affecting the plant's ability to develop new sinks for C, and acquire adequate N and other resources to support an enhanced growth potential. Growth at elevated CO₂] usually increases N use efficiency because PS rates can be maintained at levels comparable to those observed at ambient CO₂] with less N investment in PS enzymes. A frequent acclimation response, particularly under N-limited conditions, is for the accumulation of leaf carbohydrates at elevated CO₂] to lead to repression of genes associated with the production of PS enzymes. The hypothesis that this is an adaptive response, leading to a diversion of N to plant organs where it is of greatest benefit in terms of competitive ability and reproductive fitness, needs to be more rigorously tested. The biological control mechanisms which plants have evolved to acclimate to shifts in source–sink balance caused by elevated CO₂] are complex, and will only be fully elucidated by probing at all scales along the hierarchy from molecular to ecosystem. Use of environmental manipulations and genotypic comparisons will facilitate the testing of specific hypotheses. Improving our ability to predict PS acclimation to CO₂] will require the integration of results from laboratory studies using simple model systems with results from whole-plant studies that include measurements of processes operating at several scales. Abbreviations: CAM, crassulacean acid metabolism; FACE, Free-Air CO₂ Enrichment; Pi, inorganic phosphate; LAR, leaf area ratio (m² g^-1); LWR, leaf weight ratio (g g^-1); NAR, net assimilation rate (g m^-2 d^{- 1}); PS, photosynthetic; RGR, relative growth rate (g g^-1 d^-1); R:S, root/shoot ratio; rubisco, ribulose bisphosphate carboxylase/oxygenase; RuBP, ribulose bisphosphate; SLA, specific leaf area (m² g^-1); SPS, sucrose phosphate synthase; WUE, water use efficiency (g biomass g H₂O^-1).