Within-stem oxygen concentration and sap flow in four temperate tree species: does long-lived xylem parenchyma experience hypoxia?

Oxygen levels as low as 1–5% (gaseous mole fraction) occur in secondary xylem, but it is not known if there is a consistent pattern of decline in O₂ from the cambium toward the pith, or whether parenchyma cells experience hypoxic conditions deep within the stem. We developed a system for repeated in situ measurement of O₂ at different depths within stems of Acer rubrum, Fraxinus americana, Tsuga canadensis, and Quercus rubra. In summer during active transpiration, O₂ declined from the cambium toward the heartwood boundary in F. americana, T. canadensis and Q. rubra, but remained constant in A. rubrum. Average sapwood O₂ was about 10%, with the lowest values observed in the innermost sapwood around 3–5%. Before spring leaf flush, O₂ content in the outer sapwood was reduced in Q. rubra and T. canadensis relative to summer, and was occasionally lower than in the inner sapwood. Sapwood respiration in T. canadensis was constant above 5% O₂, but reduced by about 65% at 1% O₂. In F. americana, sapwood respiration was constant above 10% O₂ but reduced by 25% at 5% O_2, and by 75% at 1% O₂, the most extreme inhibition observed. However, when prolonged (72 h) exposure to 1%, 5% and 10% O₂ was followed by re-equilibration to 10% O₂, no inhibition was found. Given the minor (and reversible) effect of low O₂ on parenchyma metabolism at levels common in the inner sapwood, it is unlikely that O₂ content severely limits parenchyma respiration or leads to parenchyma cell death during sapwood senescence. Within-stem O₂ levels may instead be most relevant to metabolism in the cambial zone and phloem, for which sapwood could serve as a significant source of O₂.