Document: ERI-3-30-3

The relationship between mesophyll conductance and photosynthetic acclimation to elevated CO₂.

SINGSAAS, E.L.* ¹, D.R.ORT ^1,2 and E.H.DELUCIA ¹

University of Illinois Urbana-Champaign, Urbana, IL 61801 USA ¹
U.S.D.A., Agricultural Research Service, Urbana, IL 61801 USA ²

Abstract:
Photosynthetic acclimation is often defined by parameters calculated from photosynthetic CO₂ response curves. These calculations depend on knowledge of the CO₂ concentration inside the chloroplast, C_c, but in absence of that information the leaf airspace CO₂ concentration, C_i, is used. Mesophyll conductance, g_m, is needed to calculate C_c from C_i. Our objective was to determine whether elevated CO₂ affects g_m in trees and to determine how potential changes affect estimates of photosynthetic acclimation. Using gas-exchange and chlorophyll fluorescence, we measured g_m on Populus tremuloides and Liquidambar styraciflua trees growing at 360 and 560 L L^-1 CO₂ at two FACE sites. Mesophyll conductance increased in elevated CO₂ by 11% in P. tremuloides and 26% in L. styraciflua. It was positively correlated with light-saturated photosynthesis rates. Using the g_m values determined for these trees, we analyzed the photosynthetic CO₂ response of trees at each FACE site and determined that estimates of acclimation changed by as much as 30%. We conclude that g_m changes proportionally with photosynthesis and should be considered when determining photosynthetic acclimation to CO₂ treatments.

Keywords: gas exchange, mseophyll, acclimation, photosynthesis

This abstract is being presented at: 10:30 AM in session:
Oral Session #40: Elevated CO₂ In Forest Systems.