Document: HAF-3-34-8

Photosynthetic and stomatal acclimation to sub-ambient and elevated CO₂ concentrations in a C₃-C₄ grassland.

MAHERALI, H.* ¹, L.J.ANDERSON ², H.B.JOHNSON ³, H.W.POLLEY ³ and R.B.JACKSON ¹

Duke University, Durham, NC 27708 USA ¹
Pennsylvania State University, University Park, PA 16802 USA ²
USDA-ARS, Temple, TX 76502 USA ³

Abstract:
The possibility that plants may be adapted to the low atmospheric carbon dioxide concentrations that prevailed during the late Pleistocene has implications for the physiological responses of terrestrial vegetation to rising [CO₂]. We measured leaf gas-exchange and photosynthetic and stomatal acclimation to [CO₂] in three dominant species of an intact C₃-C₄ grassland exposed to an experimental gradient of atmospheric CO₂ ranging from sub ambient (215 ppm) to elevated (550 ppm) levels. In the third treatment year (1999), increases in light saturated photosynthetic rates with atmospheric [CO₂] were linear and greatest in Bromus japonicus (C₃ grass), followed by Solanum dimidiatum, (C₃ forb) and Bothriocloa ischamaum (C₄ grass). Net photosynthesis (A) versus intercellular CO₂ (A-Ci) response curves revealed that only Bothriocloa and Solanum up-regulated carboxylation efficiency at sub-ambient [CO₂]. There was no evidence of photosynthetic down-regulation at elevated [CO₂] in any species. Decreases in light saturated stomatal conductance (g) with treatment [CO₂] were greatest in Solanum, followed by Bromus and Bothriocloa. The largest decline in g in all species occurred from sub-ambient to ambient [CO₂]. Preliminary analysis of g-Ci curves, however, revealed no evidence of stomatal acclimation to treatment [CO₂]. These results provide partial evidence of the responsiveness of photosynthetic biochemistry to sub-ambient [CO₂]. They suggest that the stimulation of carbon fixation by elevated CO₂ in the dominant species of a grassland are not constrained by potential adaptations to sub-ambient [CO₂].

Keywords: Carbon dioxide, Global climate change, Grasslands, Photosynthesis, Stomatal conductance

This abstract is being presented at: 2:00 PM in session:
Oral Session #30: Effects of Elevated Carbon Dioxide.