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Effects of within-stem gas concentrations on sapwood physiology: Implications for sapwood aging and senescence.
Spicer, Rachel*,1, Holbrook, Noel1, 1 Harvard University, Cambridge, MA
ABSTRACT- The gas composition within woody stems is significantly lower in O2 and higher in CO2 than the ambient atmosphere, but it is not known how these native concentrations affect respiration of living xylem tissue (i.e., parenchyma cells), or might contribute to the process of sapwood senescence. We designed a series of experiments to determine (a) the pattern of O2 contents at different radial depths within woody stems, (b) rates of cellular respiration under the reduced O2 found in stems, and (c) the potential for elevated CO2 within stems to inhibit respiration. Species studied included two conifers (Tsuga canadensis, Pinus strobus), two ring-porous angiosperms (Quercus rubra, Fraxinus americana) and one diffuse-porous angiosperm (Acer rubrum). Oxygen concentrations were not consistent with a hypothesis of parenchyma death by prolonged anoxia: in most species, O2 contents in the innermost sapwood (adjacent to the heartwood, in which all parenchyma are dead) were typically between 10% and 12% (mole fraction), and rarely as low as 5%. When equilibrated to an atmosphere of 10% O2, rates of sapwood respiration were similar across species, with the innermost sapwood often, but not always, respiring at a significantly lower rate than the outer sapwood. In contrast, when respiration was expressed per live tissue (i.e., parenchyma) volume, conifers had higher rates of respiration than angiosperms, and showed no difference between inner and outer sapwood. Tissue age was not a good predictor of sapwood respiration, with tissue in the innermost sapwood respiring at the same rate across several species despite differences in age of more than 30 years. In contrast, a build-up of CO2 in the sapwood may inhibit respiration: in Pinus strobus, equilibration to an atmosphere of 10% CO2 caused a 25% reduction in respiration. Future work will focus on seasonal changes in both O2 and CO2 concentrations within stems.
Key words: carbon dioxide, respiration, oxygen, sapwood