Seasonal Ultrastructural Changes in Xylem Parenchyma Cells of Apple Twigs in Relation to Cold Hardiness
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Ultrastructural changes in xylem parenchyma cells of apple twigs were studied in relation to seasonal fluctuation in cold hardiness. During active growth from mid-June to mid-July, the most striking changes were cell wall thickening and the formation of pits. In the cytoplasm, the appearance of rough endoplasmic reticulum (ER), polysomes, dictyosomes, and vesicles indicate that these organelles participate in cell wall thickening and the formation of pits. In early August immediately after cessation of shoot elongation, starch granules in the plastids began to increase in size and number; protein-lipid bodies appeared. During cold acclimation from late September to mid-October, the vacuoles shrank in size, and the cell became full of plastids containing large starch granules. In early November, unidentified organelles that were similar in size to plastoglobuli appeared near the thylakoid membranes in plastids. In mid-January when cold hardiness reached a maximal level, small vacuoles were dispersed throughout the cytoplasm. In late February, the formation of plastid initials occurred, while plasmodesmata formed in the pits. During deacclimation from late March to mid-April, the vacuoles enlarged, starch granules in the plastids decreased in size, and unidentified organelles reappeared in the plastids. At the onset of flowering in mid-May, starch granules in the plastids increased in size again. Moreover, during a one-year cycle, the rough ER was prominent along the cell wall, while mitochondria appeared to be adjacent or attached to the surface of the plastids. Based on these results, the relationship between ultrastructures in xylem parenchyma cells of the apple twigs and cold hardiness, especially the deep supercooling characteristic is discussed.
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園芸学会 | 論文
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