SPICER, R.* and N. M. HOLBROOK. Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge MA, 02138. - Senescence of secondary xylem in angiosperms vs. conifers: the aging and death of parenchyma cells during the transition from sapwood to heartwood.
Most tree species support the metabolic activity of parenchyma cells
in the secondary xylem for a finite number of years, with the death of
these cells marking the transition from sapwood to heartwood. Although
the range in longevity of xylem parenchyma is remarkable (cells live
from just two to over 150 years), it is not known what causes their
death nor whether the program of senescence is similar across conifers
and angiosperms. We used fluorescent staining of nuclei (DAPI) to
generate profiles of ray parenchyma survival across the sapwood of
mature Populus grandidentata, Quercus rubra, Acer
saccharum, Tsuga canadensis and Pinus strobus. Live
cell staining (triphenyl-tetrazolium chloride) indicated that the
presence of nuclei served as an accurate surrogate for cell vitality.
Results suggest a distinct difference in the pattern of cell death
between conifers and angiosperms. Both T. canadensis and P.
strobus showed a gradual loss of living parenchyma across 10 to 25
annual rings of sapwood, whereas 100% of P. grandidentata rays
were alive to within one annual ring of the sapwood/heartwood
boundary. Q. rubra and A. saccharum showed the same
abrupt decline in living parenchyma as P. grandidentata,
suggesting that this pattern is independent of the number of years of
sapwood maintained. The gradual decline observed in conifers is the
result of entire rays dying, rather than individual cells within rays,
stressing the importance of symplasmic connections. TEM of parenchyma
in both angiosperms and conifers indicates that ray cells in the
innermost sapwood contain fewer mitochondria than cells in the outer
sapwood, but that organelles (e.g., mitochondria, nuclei, ER) are
otherwise normal in appearance. Future work will compare the cell
morphology of parenchyma in the innermost sapwood with that of
parenchyma subject to anoxia, one hypothesized agent of cell death in
the transition from sapwood to heartwood.
Key words: Acer saccharum, heartwood formation, Pinus strobus, Populus grandidentata, Quercus rubra, sapwood, senescence, Tsuga canadensis, xylem parenchyma