Electron Microscopic Observations of the Apical Meristem of Growing Roots in Trifoliate Orange Grafted with Satsuma Mandarin

Abstract

The apical initials and derived cells for central cylinder, cortex, epidermis, and rootcap of the growing roots of 3-year-old satsuma mandarin (Citrus unshiu Marc. cv. Okitsu Wase) trees grafted on trifoliate orange rootstocks were studied using light phase and transmission electron microscopes. Initial cells, plerome, periblem and dermatogen/calyptrogen could be recognized under a light phase microscope, but the details of the meristematic cells and their differentiation patterns in the initial and derived layers were not clearly observed. Transmission electron microscopy revealed distinctly the root apex showing the organization of 3 rows of initial cells for plerome, periblem and dermatogen/calyptrogen. All the initial rows were composed of a group of cells with a transverse direction. The uppermost row was the plerome initial followed by the periblem initial and finally the dermatogen/calyptrogen initial toward the root apex. The cells in the initial rows showed many oval to tubular proplastids but starch granules were completely absent, whereas the plerome cells contained spherical and smaller starch granules in the tubular proplastids. In contrast, the proplastids in the periblemic cells formed larger starch granules. The periblem also appeared to develop intercellular spaces radiating from the root apex, while the plerome cells still appeared to be dense. In addition, numerous osmiophilic lipid-like globules were observed in the plerome and periblemic cells. The calyptrogen cells contained numerous well-developed amyloplasts filled with compound starch granules each of which consisted of 5-15 granules.