Age-dependent bark photosynthesis of aspen twigs

The photosynthetic performance of trembling aspen (Populus tremula L.) twig s and leaves was studied in relation to selected structural features of asp en bark. PFD transmittance of intact periderm was reduced by about 90% in c urrent-year twigs through peridermal thickening. However, because of drasti c changes within the bark microstructure, PFD transmittance increased in 1- year-old twig segments up to 26% of the incident PFD. On a unit surface are a basis, the chlorophyll content of young twigs (425 mg Chl m(-2)) almost r eached that of leaves (460 mg Chl m-2). The chlorophyll content of aspen ba rk chlorenchyma was clearly age-dependent, even increasing in current-year twigs with advancing internodal age. The low bark chlorophyll alb ratios (a bout 2.6 compared with 3.9 in leaves) indicate that bark chloroplasts are s hade-adapted. Positive net photosynthesis was not found in aspen twigs, but apparent respiration was distinctly reduced in the light due to light-driv en carbon refixation (bark photosynthesis) within the chlorenchymal tissues . Under constant microclimatic conditions, dark respiration rates were stro ngly correlated with stem-internal CO2 refixation. In accordance with incre asing dark respiration rates, the efficiency of this carbon recycling was g enerally greater in the metabolically more active, younger twig segments th an in older segments; carbon refixation rates reached up to 80% of dark res piration values. At least in young twigs and branches and thus in the light -exposed outer parts of tree crowns, respiratory CO2 losses by the tree ske leton could efficiently be reduced. Refixation of carbon dioxide may be of great importance for carbon budgets in the environmentally controlled or pa thogen-induced leafless states of deciduous aspen trees.