The positional distribution of cell death of ray parenchyma in a conifer, Abies sachalinensis.
We monitored the distribution of death of secondary xylem cells in a conifer, Abies sachalinensis. The cell death of tracheids, which are tracheary elements, occurred successively and was related to the distance from cambium. Thus, it resembled programmed cell death. By contrast, the death of long-lived ray parenchyma cells had the following features: (1) ray parenchyma cells remained alive for several years or more; (2) in many cases, no successive cell death occurred even within a given radial cell line of a ray; and (3) the timing of cell death differed among upper and lower radial cell lines and other lines of cells within a ray. These results indicate that the death of long-lived ray parenchyma cells involves a different process from the death of tracheids. The initiation of secondary wall formation and the lignification of ray parenchyma cells in the current year's annual ring were delayed in the upper and lower radial cell lines of a ray. In addition, the density of distribution and orientation of cortical microtubules in such cells were different from those in cells in other radial lines. Ray parenchyma cells in the previous year's annual ring within the upper and lower radial cell lines of a ray contained many starch grains. Our results indicate that positional information is an important factor in the control of the pattern of differentiation and, thus, of the functions of ray parenchyma cells that are derived from the same cambial ray cells.