The ultrastructure of quiescent buds of Tilia europaea

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Abstract
Shoot apices from quiescent axillary buds of the European linden tree were collected at three periods during the winter and prepared for electron microscopy by freeze-fracturing without pretreatment (i.e., no chemical fixation or glycerol infiltration). Cell structures were well preserved and not damaged by ice crystal formation. Tissue fine structure was comparable with other winter-quiescent tissues (particularly cambia) and showed the following characteristics: plentiful oil droplets, smaller quantities of other storage materials (protein, starch), inactive dictyosomes, and unusual arrangements of endoplasmic reticula (particularly a peripheral reticulum). Oil droplets were not bounded by a membrane. The quantity of oil (by cytoplasmic volume) declined steadily through the winter, presumably fuelling metabolic activity. The relatively flaccid condition of plasma membranes indicated that osmotic pressure was similar inside and outside the cells. Membrane-particle partition coefficients (Kp) between plasma membrane fracture moieties was unusual (< 1), and the significance of this observation in relation to cold acclimatisation is discussed. The appearance of the mouths (neck constrictions) of plasmodesmata changed between midwinter (closed) and early spring (open) and the observations are interpreted to indicate dynamic changes in a sphincter controlling intercellular exchange.