PLANT PHYSIOLOGY , Vol 106, Issue 2 689-695, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Partitioning of the Leaf CO2 Exchange into Components Using CO2 Exchange and Fluorescence Measurements
A. Laisk and A. Sumberg
Institute of Molecular and Cell Biology, Tartu University, 181 Riia Street, Tartu, Estonia EE2400
Photorespiration was calculated from chlorophyll fluorescence and
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetics and
compared with CO2 evolution rate in the light, measured by three
gas-exchange methods in mature sunflower (Helianthus annuus L.) leaves. The
gas-exchange methods were (a) postillumination CO2 burst at unchanged CO2
concentration, (b) postillumination CO2 burst with simultaneous transfer
into CO2-free air, and (c) extrapolation of the CO2 uptake to zero CO2
concentration at Rubisco active sites. The steady-state CO2 compensation
point was proportional to O2 concentration, revealing the Rubisco
specificity coefficient (Ksp) of 86. Electron transport rate (ETR) was
calculated from fluorescence, and photorespiration rate was calculated from
ETR using CO2 and O2 concentrations, Ksp, and diffusion resistances. The
values of the best-fit mesophyll diffusion resistance for CO2 ranged
between 0.3 and 0.8 s cm-1. Comparison of the gas-exchange and fluorescence
data showed that only ribulose-1,5-bisphosphate (RuBP) carboxylation and
photorespiratory CO2 evolution were present at limiting CO2 concentrations.
Carboxylation of a substrate other than RuBP, in addition to RuBP
carboxylation, was detected at high CO2 concentrations. A simultaneous
decarboxylation process not related to RuBP oxygenation was also detected
at high CO2 concentrations in the light. We propose that these processes
reflect carboxylation of phosphoenolpyruvate, formed from phosphoglyceric
acid and the subsequent decarboxylation of malate.
