Bark photosynthesis

It is meanwhile widely accepted that aside from green leaves nearly all other plant organs are able to a reductive carbon assimilation. Flowers, fruit, and even stems and roots very often possess chloroplast containing cells. In trees, bushes, shrubs and sub-shrubs photosynthetic activity has not only been found in the living bark of young twigs, branches or even main stems, but photosynthetic activity was also found within living cells of wood and sometimes even in the pith. There seems to be a common rule: when light can penetrate a plant organ, (at least some) photosynthesis will be found.

Many different termini are used to describe the “non-foliar” CO₂ fixation within twigs, branches and stems. Bark photosynthesis, corticular photosynthesis, chlorenchymal CO₂-reduction, stem-internal CO₂-fixation, chlorenchymal CO₂-refixation or even stem photosynthesis.

Corticular photosynthesis is thought to be driven by stem-internal CO₂ derived from mitochondrial respiration and probably in part also by gaseous xylem efflux. The inorganic nutrient supply is normally ensured via the transpirational xylem stream. Light penetrates the periderm to a certain extent allowing a strongly shade adapted photosynthesis. Principally, corticular photosynthesis occurs through all seasons. Yet, winter reduces the activity to a large extent. How corticular photosynthesis is regulated is still an open question. And many more features are not well understood: Is there a systematic difference between leaf-shedding, deciduous trees and evergreen conifers? What are realistic O₂ and CO₂ concentrations in pith, wood and living bark of different tree species? Extremely high CO₂ concentrations (maximum published values were around 26%) would inhibit carbon fixation via acidification of alkaline compartments. Where does stem-internal CO₂ originate? How high is the contribution of CO₂ loss from the xylem as compared to mitochondrial CO₂ release? Can bark photosynthesis exclusively feed wound callus formation in leafless stem parts? Can bark photosynthesis help in bridging the energy gap between total defoliation (e.g. insect attack or fungal disease) and re-foliation? How do fixation rates change from young to mature trees? Knowledge of the re-fixation capacity of different age classes of twigs and branches will help to estimate the specific contribution of age classes to the total corticular photosynthesis. How big is the contribution of corticular photosynthesis as compared to the whole carbon budget of single trees? How different is species specificity? Does corticular refixation increase the competitiveness of woody plants? Do we have to integrate corticular photosynthesis in global CO₂-fixation models? Respirational loss of living limbs is thought to be as high as 26% of the annual gross primary production in forest ecosystems? How much can be compensated by stem-internal re-fixation?

The papers presented in this volume intend to fill some of the existing gaps in our knowledge on where, when and how corticular carbon fixation works. The paper of Berveiller and Damesin (carbon assimilation of tree stems: potential involvement of phosphoenolpyruvate carboxylase) discusses the idea of corticular photosynthesis being an intermediate between the classical C3 and C4 pathways. Levizou and Manetas (maximum and effective PSII yields in the cortex of the main stem of young Prunus cerasus trees: effects of season and exposure) pay attention to primary reactions of chlorenchymal electron transport. “Non-temperature related variations in CO₂ efflux rates from young tree stems in the dormant season” are reported by Saveyn, Steppe and Lemeur. The antitranspirant functions of stem periderms and their influence on corticular photosynthesis under drought stress has been examined by Wittmann and Pfanz. Peter Hietz reports on the question whether oxygen is involved in beech (Fagus sylvatica) red heartwood formation? Finally, the paper by Lüttge points to the fact that corticular photosynthesis is not solely “invented” by woody plants but also exists in CAM-using succulents, although there are clear differences as to carbon uptake via stomata and the absence of a light-reducing/blocking rhytidome.

May this series of publications on corticular photosynthesis and its functions and effects also help in stimulating further research in this new and long overlooked topic.

Hardy Pfanz