Elucidation of the mechanisms involved in long-distance water transport in trees requires knowledge of the water distribution within the sapwood and heartwood of the stem as well as of the earlywood and latewood of an annual ring. X-ray computed tomography is a powerful tool for measuring density distributions and water contents in the xylem with high spatial resolution. Ten-to 20-year-old spruce (Picea abies L. KARST.) and oak (Quercus robur) trees grown in the field were used throughout the experiments. Stem and branch discs were collected from different tree heights, immediately deep frozen, and used for the tomographic determinations of spatial water distributions. Results are presented for single-tree individuals, demonstrating heartwood and sapwood distribution throughout their entire length as well as the water relations in single annual rings of both types of wood. Tree rings of the sapwood show steep water gradients from latewood to earlywood, whereas those of the heartwood reflect water deficiency in both species. Although only the latest two annual rings of the ringporous species are generally assumed to transport water, we found similar amounts of water and no tyloses in all rings of the oak sapwood, which indicates that at least water storage is important in the whole sapwood.The aim of this study was to test high-resolution computed tomography (CT) for water relations in spruce (Picea abies L. KARST.) and oak (Quercus robur) stems rather then to analyze the general features of tree conductive systems that are already well documented (Hartig, 1855; Mac Dougal, 1925; C ermak et al., 1992).The CT method was developed by Cormack and Hounsfield and today has become a standard examination method in the medical field and material sciences (Hounsfield, 1980). In plants, CT has been shown to be capable of measuring the inner properties of wood. For instance, studies of CT scanning of wood were described by Taylor et al. (1984), Funt and Bryan (1987), and Lindgren (1991), showing that the method can be used for accurate and nondestructive measurements of wood density. In addition, the use of a mobile CT apparatus made it possible to detect the water distribution in stems of living trees (Habermehl et al., 1986(Habermehl et al., , 1990. The main advantage of this mobile method is that changes in the water distribution within a single tree may be monitored over periods of days, months, or years. However, the spatial resolution is low and does not effectively allow measurements of the water relations in individual annual rings.In this study, a new technique is presented that permits the mapping of water concentrations in a whole stem as well as in individual annual rings with a spatial resolution of 0.1225 mm 3 . This has become possible because the same disc is scanned under field-fresh and dry conditions, respectively, so that the density differences can be calculated from image subtraction with data values reflecting the absorption coefficients of water without any cell material. Thus, our method leads to...
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