PLANT PHYSIOLOGY , Vol 114, Issue 1 337-344, Copyright © 1997 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Does Long-Term Elevation of CO2 Concentration Increase Photosynthesis in Forest Floor Vegetation? (Indiana Strawberry in a Maryland Forest)
C. P. Osborne, B. G. Drake, J. LaRoche and S. P. Long
John Tabor Laboratories, Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, United Kingdom (C.P.O., S.P.L.)
As the partial pressure of CO2 (pCO2) in the atmosphere rises,
photorespiratory loss of carbon in C3 photosynthesis will diminish and the
net efficiency of light-limited photosynthetic carbon uptake should rise.
We tested this expectation for Indiana strawberry (Duchesnea indica)
growing on a Maryland forest floor. Open-top chambers were used to elevate
the pCO2 of a forest floor habitat to 67 Pa and were paired with control
chambers providing an ambient pCO2 of 38 Pa. After 3.5 years, D. indica
leaves grown and measured in the elevated pCO2 showed a significantly
greater maximum quantum efficiency of net photosynthesis (by 22%) and a
lower light compensation point (by 42%) than leaves grown and measured in
the control chambers. The quantum efficiency to minimize photorespiration,
measured in 1% O2, was the same for controls and plants grown at elevated
pCO2. This showed that the maximum efficiency of light-energy transduction
into assimilated carbon was not altered by acclimation and that the
increase in light-limited photosynthesis at elevated pCO2 was simply a
function of the decrease in photorespiration. Acclimation did decrease the
ribulose-1,5-bisphosphate carboxylase/oxygenase and light-harvesting
chlorophyll protein content of the leaf by more than 30%. These changes
were associated with a decreased capacity for light-saturated, but not
light-limited, photosynthesis. Even so, leaves of D. indica grown and
measured at elevated pCO2 showed greater light-saturated photosynthetic
rates than leaves grown and measured at the current atmospheric pCO2. In
situ measurements under natural forest floor lighting showed large
increases in leaf photosynthesis at elevated pCO2, relative to controls, in
both summer and fall. The increase in efficiency of light-limited
photosynthesis with elevated pCO2 allowed positive net photosynthetic
carbon uptake on days and at locations on the forest floor that light
fluxes were insufficient for positive net photosynthesis in the current
atmospheric pCO2.
