Cytochemical and subcellular organization of root apical meristems of dry and germinating jack pine embryos

Abstract

By cytochemical and electron microscopic methods, the emerging radicle of Pinusbanksiana Lamb. was shown to contain an abundance of food reserves largely proteins, lipids, and starch grains. After 4 days of imbibition, the food reserves were dispersed to the daughter cells that tended to form through a sequence of divisions, linear arrays of adhering cells. The main cytochemical changes were associated with the later stages of imbibition and the consumption of carbohydrates and nitrogenous compounds for the synthesis of newly dispersed macromolecules. Peroxidase (EC 1.11.1.7) not found to any significant extent in the dry embryo, appeared by day 4 in the root cap and epidermal cells. As for DNA synthesis, cells of the quiescent zone were shown by autoradiography to incorporate very little [methyl-3H]thymidine. By contrast, most cells in the radicle incorporated thymidine in advance of the first wave of cell division between 3 to 4 days.The initial tight packing of subcellular organelles and the high lipid content of dry cells made the evaluation of subcellular changes difficult. In the nucleus and cytoplasm, the increase of ribosomes and polysomes was supported by the increased cytoplasmic staining for total RNA and acidic proteins. In emerging radicles, the overall redistribution of organic reserves and the associated subcellular reorganization related to the tactical displacement of cells and to new channels for the initial uptake of water and nutrients from the soil.