Responses of ray parenchyma cells to wounding differ between earlywood and latewood in the sapwood of Cryptomeria japonica

Abstract

Changes in cellular contents of ray parenchyma cells during the formation of reaction zone differ between earlywood and latewood in the sapwood of Cryptomeria japonica. Changes over time in the cellular contents of xylem parenchyma cells provide important clues to the mechanism of the early events in the wound reaction of trees. In this study, we monitored the events that occur during the death of ray parenchyma cells after wounding. We examined nuclei, starch grains, and colored substances in ray parenchyma cells by light microscopy and the autofluorescence of cell walls of tracheids by confocal laser-scanning microscopy in Cryptomeria japonica after artificial wounding. In addition, we compared cytological changes in ray parenchyma cells in the longitudinal and radial directions. Finally, we analyzed the differences between earlywood and latewood in terms of the responses of ray parenchyma cells to wounding. Behind the wound, changes in cellular contents were visible first in latewood regions in the second annual ring behind the wound. The progression of changes in cellular contents of ray parenchyma cells stopped near the growth-ring boundary. These results indicate that the growth-ring boundary might prevent the spread of some factor(s) that induces cytological changes in ray parenchyma cells. Above the wound, most colored substances were localized in ray parenchyma cells that were located near wounds in latewood regions. Thus, even at an equal distance from the wound, the amount of secondary metabolites in ray parenchyma cells differed between earlywood and latewood. Our observations suggest that differences in the anatomical features of neighboring tracheids between earlywood and latewood might influence changes in cellular contents of ray parenchyma cells during reactions to wounding in Cryptomeria japonica.