Light-modulation of corticular CO2-refixation in young birch stems (Betula pendula Roth.)

Photosynthetic bark reduces the flux of CO2 from woody tissues to the atmosphere by recycling endogenous CO2 derived from mitochondrial respiration. In young birch trees (Betula pendula Roth.) this process is modulated by the light intensity regime at the growing site. Although positive net photosynthesis was not yet found in intact birch twigs, CO2 efflux was distinctly reduced upon illumination by 65% in high-light grown birches (HL: 100% sunlight) and by up to 82% in low-light grown trees (LL: 20% of full sunlight). Bark chlorenchymes were found to be optimized for light harvesting and photosynthetic performance under LL-conditions, due to the shading effect of the outer peridermal (or rhytidomal) layers. Compared on a unit area basis the inner bark tissue contained up to 47%-49% of the chlorophyll of the concomitant leaves under both HL and LL conditions, respectively. Peridermal light transmittance was changed by the light intensity regime during growth, due to changes within the microstructure of the outer bark. The inner bark morphology, quantified as specific bark area (SBA), was found to be strongly correlated with twig respiration (r(2) = 0.83-0.96) and the CO2-refixation rate (r(2) = 0.81-0.95) under LL and HL conditions and thus can be used to predict the efflux of CO2 from the stein organs. Besides leaf dark respiration (R-d) also twig R-d was clearly related with the light environment. Under shading the investigated twigs showed a lower R-d but a higher relative CO2-refixation (expressed as a percentage of dark respiration); up to 100% of the respired carbon was refixed in LL twigs. Furthermore, twig R-d was the physiological parameter that correlated most strongly with CO2-refixation rate.