PARENT SESSION
Oral Session # 24: Photosynthesis and Respiration.
Presiding: A Leakey
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 203.

Photosynthetic acclimation of scrub-oaks to long-term elevated CO₂: A summary of seven years.

LI, J. -H.^*,1, LINKE, R. B.², MADRITSCH, A. ², MEISTER, M. H. ², HINKLE, C. R. ³, DRAKE, B. G. ¹, ¹ Smithsonian Environmental Research Center, Edgewater, MD 21037, USA² University of Vienna, A-1090, Vienna, Austria³ Dynamac Corp., Kennedy Space Center, FL 32899, USA

ABSTRACT- The Florida scrub-oak ecosystem has been exposed in situ to elevated atmospheric carbon dioxide concentration (C_a) (350 mol mol^-1 above ambient) in large open-top chambers since May 1996. The photosynthetic responses of Quercus myrtifolia willd. and Quercus geminata Small, the dominant and codominant species, have been studied at physiological and biochemical levels on detached branches in the lab and on intact leaves in the field during the last seven years. Photosynthetic acclimation has been species and season specific, occurring in Q. myrtifolia only initially, then later in Q. geminata only, and not occurring in young leaves for both species and during senescence for Q. myrtifolia. Discrepancies on acclimation have also occurred comparing measurements made on predawn excised branches in the lab with intact leaves in the field. Maximum rate of carboxylation (V_cmax) and maximum rate of potential electron transport (J_max) measured indoor were 21% lower than that measured outdoor in August and September 2002. For Q. myrtifolia, significant acclimation was indicated from outdoor measurements but not from indoor measurements at the later stage. Photosynthetic acclimation in Q. myrtifolia may be caused by a limitation of carbon sink through feed-back inhibition since elevated C_a significantly increased sugars and starch contents but did not affect leaf nitrogen, total soluble protein and Rubisco contents per unit leaf area. Large variations arise between species, exposure period, season and excised vs. intact measurements highlight the importance of long-term elevated C_a studies with native ecosystem as well as measurement approaches and methods.

Key words: Photosynthetic acclimation, Quercus geminata, Elevated CO₂ , Quercus myrtifolia