• branch chambers;
  • carbon isotopes;
  • day respiration;
  • photosynthetic discrimination


We report diurnal variations in 13C discrimination (13Δ) of Picea sitchensis (Bong.) Carr. branches measured in the field using a branch chamber technique. The observations were compared to predicted 13Δ based on concurrent measurements of branch gas exchange. Observed 13Δ values were described well by the classical model of 13Δ 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 13Δ did not capture the observed diurnal variability. At dawn and dusk, we measured very high 13Δ values that were not predicted by either of the said models. Exploring the sensitivity of 13Δ to possible respiratory isotope effects, we conclude that isotopic disequilibria between the gross fluxes of photosynthesis and day respiration can explain the high observed 13Δ 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 13Δ values.