Different pathways of the oxygen supply in the sapwood of young Olea europaea trees

Gaseous transport through lenticels is widely accepted to be the main pathway for oxygen supply to the parenchymatous tissues of the wood. Circumstantial evidence exists that the oxygen required for respiration by these living cells can be obtained from the transpiration stream. However, there has been no functional confirmation of this role. To address this problem and to quantify the contribution of the different pathways to the oxygen supply of the sapwood, we have developed a three-electrode miniaturized oxygen-selective sensor to be implanted into the sapwood for long-term determination of the oxygen concentration. In spring, during the active growing season, the oxygen concentration of the sapwood of young olive (Olea europaea L.) trees changed from 80—90 μmol O2 l-1 around midday to 20—30 μmol O2 l-1 in the night. These concentrations correspond to a deficit of oxygen for the sapwood between 65—70% and 88—90% of an aqueous solution saturated with air. In the daylight hours, almost all the oxygen present in the sapwood was delivered by the transpiration stream, driven by the soil—plant—atmosphere water-potential gradient. During the night the diffusion of oxygen via the sap-filled lumina of the tracheids and vessels (xylary diffusion in the aqueous phase) accounted for about 87% of all the oxygen present, whereas only the remaining 13% was assessed as supplied by radial diffusion in the aqueous or gaseous phase.