Apical meristem formation during zygotic embryo development of white spruce

Abstract

The first notable sign of shoot and root meristem development in zygotic embryos of white spruce (Picea glauca (Moench) Voss) was the appearance of starch in the respective poles of the embryo. Starch granules gradually accumulated in the subapical cells of the shoot pole and were soon followed by vacuolation in the subapical cells. Vacuolation reached its highest degree in these cells at the early embryo stage. With the formation of the large vacuolated cells, the surface cell layer at the summit of the shoot pole enlarged and differentiated into the surface initials of the shoot meristem. These cells were large with distinct nuclei. As the embryo matured, the large vacuoles within the subapical cells were replaced by small ones with a concomitant increase in the cytoplasmic density of the subapical cells. After germination, the surface initials remained distinct; however, the subapical cells had different fates depending on their location. The subapical cells located next to the surface initials became the central mother cells of the shoot meristem and those located near the cotyledon junction divided periclinally and gave rise to the epicotyl rib meristem. The remaining subapical cells near the procambium became part of the pith. In the root pole, starch could be found in the cap region terminating underneath the developing procambium at the club-shaped stage of embryo development. Two layers of root initials appeared at the junction between the developing procambium and the root cap. The initials were distinct as they were larger in size than surrounding cells. Cell division activity could not be detected in the layer of initials immediately next to the procambium, while mitotic activity could be seen in the adjoining layer next to the root cap. As the embryo matured, cells surrounding the root initials also took on structural characteristics similar to the root meristem initials. As a result, the region of the root meristem initials expanded in size, and the bilayered configuration became obscure. After germination, mitotic figures could be found in the root initials. As seedling continued to grow, fewer mitotic figures could be found in the root meristem cells adjacent to the procambium pole. In roots of mature plants, a group of isodiametric cells could be discerned between the procambium and the root cap. Mitotic activity was not readily detected within this group of isodiametric cells but mitotic figures could be found in surrounding cells.Key words: apical meristems, Picea glauca, white spruce, zygotic embryogenesis.