Physiology and longevity of secondary xylem in trees

Trees are exceptionally long-lived organisms and continue to accumulate mass as long as they are alive. Rather than remain functional throughout the life of a tree, secondary xylem is gradually compartmentalized in the center of the stem to form a physiologically inactive core, the 'heartwood'. This process occurs in nearly every tree species described, and is vital to the maintenance of a balance between the volume of autotrophic and heterotrophic tissue. Heartwood formation is defined by the death of parenchyma cells, which have species-specific lifespans that range from two to 200 years. I designed a series of experiments to determine the cause of parenchyma cell death during heartwood formation, and considered several competing hypotheses: (a) that low oxygen levels within the stem cause anoxic cell death at a particular depth beneath the bark; (b) that the metabolic rate of parenchyma cells is greatly reduced with cell age; and (c) that parenchyma cell death during heartwood formation is a form of programmed cell death (PCD). The majority of this work was carried out in five temperate tree species with contrasting anatomies and physiologies: Acer rubrum, Fraxinus americana, Pinus strobus, Quercus rubra, and Tsuga canadensis.;First, by measuring O2 concentration in situ at different depths within the stem and the rate of sapwood respiration under different combinations of low O2 and high CO2 (both conditions known to occur in woody stems), I was able to show that respiration of xylem parenchyma is only moderately inhibited by low O2, and surprisingly robust to high CO2. Thus, within-stem gas composition is unlikely to play a role in heartwood formation. Second, by quantifying the volume fraction of living cells in woody tissue and expressing respiration on a per live cell, rather than a per tissue volume basis, I found that there is little evidence for an inherent decline in parenchyma metabolism with age, even in cells 50+ years old. Instead, I suggest that parenchyma cell death is a novel and widespread form of programmed cell death (PCD). I detected fragmentation of nuclear DNA, a hallmark of PCD in both plants and animals, in a very narrow region (< 1 mm) just outside the heartwood, where parenchyma cells die and their nuclei disappear. Past work describing ultrastructural changes in parenchyma cells in this narrow zone are also indicative of PCD. The finding that parenchyma cell death is a form of PCD suggests that heartwood formation is a program of sapwood senescence and under regulatory control. Such control would be of great benefit given the cumulative nature of plant growth and large volume of heterotrophic (i.e., respiring) tissue that would otherwise accumulate in the woody stem.