A novel approach to the in situ measurement of oxygen concentrations in the sapwood of woody plants

A novel technique for the physico-chemical analysis of xylem sap by underwa ter access to the sapwood of trees is described. In situ measurements of di ssolved oxygen in the sapwood are performed by combining this technique wit h a novel optical method for oxygen detection. In early spring, the oxygen concentration of the sapwood of Betula pendula was in the range of 80-230 m u mol O-2 L-1, corresponding to an oxygen deficit of 40-75% of air saturati on. Oxygen concentration maxima and minima occurred early in the morning an d in the afternoon, respectively, whereas xylem sap temperatures showed the reverse pattern. In the sapwood, hypoxia. increased from the beginning of bud break until frondescence, when a deficit of 86% of air saturation marke d the upper limit of oxygen depletion. There seemed to be no relationship b etween daily variations of oxygen concentration and xylem sap pressure. In summer, sap flow was a major determinant for the diurnal variation of disso lved oxygen concentration. Oxygen supply to the sapwood was determined by b oth radial influx into the trunk through intercellular gas spaces and trans port of dissolved oxygen via xylem sap flow. Radial influx seemed to be fav oured during night-time, when the trunk was warmer than ambient air. During daytime, the hypoxia of the sapwood rose and increased sharply in the even ing, when sap flow velocity approximated zero. High temperature in the sapw ood enhanced the respiratory oxygen consumption of the wood parenchyma whil e the supply of dissolved oxygen via the transpiration stream became ineffe ctive.