Abstract in Englisch:

Reassimilation of internal CO₂ via corticular photosynthesis (PScₒrt) has an important effect on the carbon economy of trees. However, little is known about its role as a source of O₂ supply to the stem parenchyma and its implications in consumption and movement of O₂ within trees. PScₒrt of young Populus nigra (black poplar) trees was investigated by combining optical micro-optode measurements with monitoring of stem chlorophyll fluorescence. During times of zero sap flow in spring, stem oxygen concentrations (cO₂) exhibited large temporal changes. In the sapwood, over 80% of diurnal changes in cO₂ could be explained by respiration rates (Rd₍mₒd₎). In the cortex, photosynthetic oxygen release during the day altered this relationship. With daytime illumination, oxygen levels in the cortex steadily increased from subambient and even exhibited a diel period of superoxia of up to 110% (% air sat.). By contrast, in the sapwood, cO₂ never reached ambient levels; the diurnal oxygen deficit was up to 25% of air saturation. Our results confirm that PScₒrt is not only a CO₂-recycling mechanism, it is also a mechanism to actively raise the cortical O₂ concentration and counteract temporal/spatial hypoxia inside plant stems.