The aging process of sapwood ray parenchyma cells in four woody species
Creators: name: Yang, Kung Chi
Publisher: name: University of British Columbia
Year: 2011

description: Aging of ray parenchyma cells from the young sap-wood to recently formed heartwood was studied in single stems of Pinus banksiana Lamb., Picea mariana (Mill.) B.S.P., Abies balsamea (L.) Mill, and Populus tremuloides Michx. Season, radial location of cell within sapwood, and cell location vertically within a ray at a given radius were considered as factors which might influence the aging process. A 12 mm increment core was extracted at breast height, from the north aspect of a tree of each species in May and July for moisture content determination. Another set of cores from the south aspect of the same trees was collected in May, June, July, August, October, and November or December. These cores were used to investigate the physiological and cytological properties of living sapwood ray parenchyma cells. Qualitative and quantitative observations were made of the status of ray cells both with light and transmission electron microscopy in order to draw inferences concerning the sapwood/heartwood transformation from the aging of sapwood ray parenchyma cells. The sapwood moisture content of the three conifers studied was higher than that of heartwood, whereas in Populus tremuloides it was lower than that of heartwood. The sapwood moisture content in May was consistently greater than in July. Vitality of the sapwood ray parenchyma cells expressed by a new nuclear elongation index decreased from the outer sapwood towards the heartwood. The survival rate of the cells decreased curvilinearly from the middle sapwood towards the heartwood. At a given sapwood increment, a greater percentage of dead ray parenchyma cells was found among the marginal cells than among the central cells of a ray. No statistically significant difference was found between the vitality of the marginal and central cells, nor between any two contiguous sampling periods with exceptions in Pinus banksiana and Picea mariana between two contiguous sampling periods from July to December. No typical pattern for the distribution of lipid content was found. The pattern of starch distribution displayed significant species, radial, vertical and seasonal variation and showed two general patterns across the sapwood. Pattern A described a decreasing trend from the outer sapwood towards the inner sapwood. Pattern B was characterized by a relatively low starch content both in the outer as well as the inner sapwood. The starch content in Populus tremuloides and the lipid content in Pinus banksiana and Picea mariana displayed no statistically significant difference between marginal and central ray cells. The majority of ray parenchyma cells showed a statistically significant difference between two contiguous sampling periods in starch and lipid contents. There was no inverse relationship between the starch and lipid content over the growing season studied. Young ray parenchyma cells were rich in chromatin and cytoplasm which contained numerous cell organelles. These cells were characterized by amyloplasts which possessed one or more elongated starch granules with thylakoids and osmiophilic globuli, numerous small lipid droplets and mostly rod-like mitochondria. In contrast, aged ray parenchyma cells featured an aggregated, dense nucleus and cytoplasm which contained few cell organelles. These aged cells possessed enlarged swollen starch granules, large lipid droplets or lumps with two staining densities, round shaped mitochondria with inconspicuous cristae and a rough/broken plasmalemma. Some heartwood substances originated from the lipid lumps which appeared frequently in dying ray cells. Based on microscopic observations and measurements of the loss of vitality of ray parenchyma cells, a declining survival rate, the disintegration of cell organelles and the origin of heartwood substances from lipid lumps, it can be concluded that heartwood formation is largely associated with the death of sapwood ray parenchyma cells. The death of these cells is due to the passage of time.