Temporally and spatially controlled death of parenchyma cells is involved in heartwood formation in pith regions of branches of Robinia pseudoacacia var. inermis

Abstract

Focussing on four types of parenchyma cell around pith regions of branches of Robinia pseudoacacia L. var. inermis, we examined the timing and role of cell death during heartwood formation. Large parenchyma cells that were located in the inner part of the pith died within a year. By contrast, other parenchyma cells died within 4 years, with the timing of cell death depending on the type of cell. Axial parenchyma cells of the xylem close to the pith died first. Then, small parenchyma cells died in the perimedullary zone in the outer part of the pith. Finally, ray parenchyma cells in the xylem close to the pith died. Variations in the autofluorescence of cell walls, which might have been due to deposition of heartwood substances, were observed first in xylem ray parenchyma cells and small parenchyma cells in the perimedullary zone. Our results indicate that the initiation of heartwood formation occurs within 4 years in pith regions of branches in Robinia pseudoacacia L. var. inermis. Moreover, it appears that not only xylem ray parenchyma cells but also small parenchyma cells in the perimedullary zone might be involved in the synthesis of heartwood substances.