Intraspecific, interspecific, and seasonal variation in acclimation of photosynthesis to rising atmospheric carbon dioxide concentration

This thesis aims to explain how species, genotype, developmental stage, and environmental factors that alter sink demand for carbohydrate influence acclimation of photosynthesis to elevated [CO₂]. Variation in acclimation was addressed using meta-analytical techniques and three open-air CO ₂ experiments: a Free Air CO₂ Enrichment (FACE) experiment with Glycine max (SoyFACE); a FACE experiment with Lolium perenne, a temperate grassland species (Swiss FACE); and an open-top chamber experiment within a clonal Florida scruboak ecosystem. In these experiments, photosynthesis was measured with an open-gas exchange system and in vivo rates of carboxylation (V_c,max) and electron transport (J_max) were derived to assess changes in photosynthetic capacity. The meta-analytic review of 111 soybean studies revealed that stimulation of leaf CO₂ assimilation with growth at elevated [CO₂] was 48%, despite a 40% decrease in stomatal conductance, and a 10% decrease in V_c,max. Stimulation of CO₂ assimilation was three times greater in nodulating soybeans than non-nodulating soybeans. This observation was supported in the SoyFACE experiment, where a nodulating variety showed no change in photosynthetic capacity at elevated [CO₂], but the isogenic line mutated to prevent nodulation showed a 29% decrease in V_c,max and an 18% decrease in J_max. In the Swiss FACE experiment, growth of L. perenne at elevated [CO₂] resulted in a 36% increase in daily leaf CO₂ uptake with no significant change over the ten years of the experiment. Marked acclimation of photosynthesis did however occur when L. perenne was grown under low nitrogen fertilization and late in regrowth, when source activity was high compared to sink demand for photosynthate. In the Florida scrub-oak community, there was significant interspecific and seasonal variation in acclimation of photosynthesis to elevated [CO₂]. Quercus myrlfolia did not show any change in photosynthetic capacity with prolonged exposure to elevated [CO₂]; the resultant increase in photosynthesis was 72%. The co-dominant, Q. geminata showed a loss of photosynthetic capacity with growth at elevated [CO₂] during most measurement periods. These results from diverse field experiments consistently support the hypothesis that acclimation of photosynthetic capacity at elevated [CO₂] under field conditions occurs when genetic and environmental factors limit sink demand for carbohydrate.