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Branch chambers
Carbon isotopes
Day respiration
Photosynthetic discrimination
Carbon-isotope discrimination
Gas-exchange
Mesophyll conductance
Diurnal-variation
Respired CO2
Rain-forest
In-vivo
Leaves
Respiration
Fractionation
Abstract:
We report diurnal variations in C-13 discrimination ((13)Delta) of Picea sitchensis (Bong.) Carr. branches measured in the field using a branch chamber technique. The observations were compared to predicted (13)Delta based on concurrent measurements of branch gas exchange. Observed (13)Delta values were described well by the classical model of (13)Delta including isotope effects during photorespiration, day respiration and CO2 transfer through a series of resistances to the sites of carboxylation. A simplified linear of model (13)Delta did not capture the observed diurnal variability. At dawn and dusk, we measured very high (13)Delta values that were not predicted by either of the said models. Exploring the sensitivity of (13)Delta to possible respiratory isotope effects, we conclude that isotopic disequilibria between the gross fluxes of photosynthesis and day respiration can explain the high observed (13)Delta values during net photosynthetic gas exchange. Based on the classical model, a revised formulation incorporating an isotopically distinct substrate for day respiration was able to account well for the high observed dawn and dusk (13)Delta values. [References: 69]
