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 O 2 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 O 2 at different depths within stems of Acer rubrum , Fraxinus americana , Tsuga canadensis , and Quercus rubra . In summer during active transpiration, O 2 declined from the cambium toward the heartwood boundary in F. americana , T. canadensis and Q. rubra , but remained constant in A. rubrum . Average sapwood O 2 was about 10%, with the lowest values observed in the innermost sapwood around 3–5%. Before spring leaf flush, O 2 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% O 2 , but reduced by about 65% at 1% O 2 . In F . americana , sapwood respiration was constant above 10% O 2 but reduced by 25% at 5% O 2, and by 75% at 1% O 2 , the most extreme inhibition observed. However, when prolonged (72 h) exposure to 1%, 5% and 10% O 2 was followed by re‐equilibration to 10% O 2 , no inhibition was found. Given the minor (and reversible) effect of low O 2 on parenchyma metabolism at levels common in the inner sapwood, it is unlikely that O 2 content severely limits parenchyma respiration or leads to parenchyma cell death during sapwood senescence. Within‐stem O 2 levels may instead be most relevant to metabolism in the cambial zone and phloem, for which sapwood could serve as a significant source of O 2 .