Xylem sap flow as a major pathway for oxygen supply to the sapwood of birch (Betula pubescens Ehr.)

The role of xylem sap flow as an aqueous pathway for oxygen supply to the wood parenchyma of Betula pubescens saplings was investigated. Using micro-optode sensors the oxygen status of the sapwood was quantified in relation to mass flow of xylem sap. Sap flow was gradually reduced by an increasing oxygen depletion in the root space. The effect of sap flow on radial O-2 transport between stem and atmosphere was assessed by a stoichiometrical approach between respiratory CO2 production and O-2 consumption. Restriction of sap flow set in 36.5 h after the onset of O-2 depletion, and was complete after 71 h. Interruption of sap flow drastically increased the O-2 deficit in the sapwood to 70%. Sap flow contributed about 60% to the total oxygen supply to the sapwood. Diurnal O-2 flow rates varied between 3 and 6.3 nmol O-2 m(-2) leaf area (LA) s(-1) during night- and daytime, respectively. Maximum O-2 flow rates of 20 nmol O-2 m(-2) LA s(-1) were reached at highest sap flow rates of 5.7 mmol H2O m(-2) LA s(-1). Sap flow not only affected the oxygen status of the sapwood but also had an effect on radial O-2 transport between stem and atmosphere.