Cytochemical and Subcellular Organization of the Shoot Apical Meristem of Dry and Germinating Jack Pine Embryos

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Abstract
Water added to dry seeds of jack pine evoked changes at the shoot apex which reflected the mobilization and consumption of food reserves for the synthesis of new protoplasm and for the formation of the prospective shoot. In shoot apices the distal zone contained large cells having nuclei with low affinity for stains. Flank and subapical zones had smaller cells with densely stained nuclei. These features did not alter during imbibition nor after the first wave of mitosis at 96 h when radicles emerged from seeds. Cells in the distal zone divided rarely. Mitosis, most abundant in subapices, decreased acropetally to the distal zone.In dry seeds, protein bodies and amyloplasts increased in size and frequency per cell from the distal zone in the basipetal direction towards the flanks and subapical zone. During imbibition most proteinaceous reserves, rich in arginine N, disappeared from all cells of the shoot apex. Lipid bodies in distal cells were small, numerous, and disappeared slowly as germination proceeded. Proplastids, mitochondria, and unidentified pleomorphic bodies were distributed in close proximity to the nucleus particularly in apical cells.Products of imbibed tritiated water (nonexchangeable tritium), and tritiated D-glucose, thymine, thymidine, and uracil were localized at the shoot apex and revealed assimilatory patterns consistent with cytochemical zonation. The covalent incorporation of tritium from water, while spread through most cells, was greatest in nuclei of flank and subapical cells. The radioactivity from glucose was found mainly in cell walls. Labelling by uracil, thymine, and thymidine by salvage mechanisms was especially abundant in nuclei of the flanks. These events coincided with dense RNA and DNA staining during germination. Collectively, observations indicated that at the flanks, water and seed reserves were concentrated at specific binding sites for the synthesis and dispersion of macromolecules before the visual production of leaf primordia.

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