Carbon isotope discrimination in photosynthetic bark

We developed and tested a theoretical model describing carbon isotope discr imination during photosynthesis in tree bark. Bark photosynthesis reduces l osses of respired CO2 from the underlying stem. As a consequence, the isoto pic composition of source CO2 and the CO2 concentration around the chloropl asts are quite different from those of photosynthesizing leaves. We found t hree lines of evidence that bark photosynthesis discriminates against C-13. First, in bark of Populus tremuloides, the delta C-13 of CO2 efflux increa sed from -24.2 parts per thousand in darkness to -15.8 parts per thousand i n the light. In Pinus monticola, the delta C-13 of CO2 efflux increased fro m -27.7 parts per thousand in darkness to -10.2 parts per thousand in the l ight. Observed increases in delta C-13 were generally in good agreement wit h predictions from the theoretical model. Second, we found that delta C-13 Of darkrespired CO2 decreased following 2-3 h of illumination (P <0.01 for Populus tremuloides, P <0.001 for Pinus monticola). These decreases suggest that refixed photosynthate rapidly mixes into the respiratory substrate po ol. Third, a field experiment demonstrated that bark photosynthesis influen ced whole-tissue delta C-13. Long-term light exclusion caused a localized i ncrease in the delta C-13 Of whole bark and current-year wood in branches o f P. monticola (P <0.001 and P <0.0001, respectively). Thus bark photosynth esis was shown to discriminate against C-13 and create a pool of photosynth ate isotopically lighter than the dark respiratory pool in all three experi ments. Failure to account for discrimination during bark photosynthesis cou ld interfere with interpretation of the delta C-13 in woody tissues or in w oody-tissue respiration.