Scanning electron microscopic study on freezing behaviour of tissue cells in dormant bud of mulberry ( Morus sp.)

摘要

The freezing behaviour studies of dormant buds, were examined, employing scanning electronmicroscopy (SEM) and light microscopy. The differences and effect of freezing behaviour on dormant budswere observed. The dormant bud primordia of several woody plant species avoid freezing injury by deepsupercooling. By slow cooling (5oC/day) of dormant buds to –30oC, all living cells in bud tissues exhibiteddistinct shrinkage without intracellular ice formation detectable by SEM. However, the recrystallizationexperiment of these slowly cooled tissue cells, which was done by further freezing of slowly cooled budswith liquid nitrogen (LN) and then rewarming to –10oC, confirmed that some of the cells in the apicalmeristem, area in which cells had thin walls and in which no extracellular ice accumulated, lost freezablewater with slow cooling to –30oC, indicating adaptation of these cells by deep super cooling. Water in planttissues will not supercool unless heterogeneous ice nucleating substances are absent and the spread of icefrom adjacent tissue can be prevented. Deep supercooling could not occur in dormant bud primordia if xylemvessels formed a continuous conduit connecting the dormant bud primordia with the remainder of the plant. Ifxylem continuity was established, ice could propagate via the vascular system and nucleate the water withinthe primordia. It is concluded that no extracellular ice crystals accumulated in such tissues containing deepsupercooling cells with thin cell walls.