Ultrastructural Changes Related to Frost Hardiness in the Cortical Parenchyma Cells from Mulberry Twigs

Ultrastructural changes in the cortical parenchyma cells ofmulberry (Morus bombyciz Koidz. cv. ‘Gorogi’) twigswere related closely to the seasonal cycle of frost hardiness.Changes in hardiness from summer to winter and vice versa tookplace particularly in the plasmamembrane. During stages that result in a rapid increase in hardiness,the sequence of cytological changes for the replacement of theplasmamembrane was marked invaginations of the plasmamembrane;pinching off into vesicles in the peripheral cytoplasm; engulfmentof these vesicles into vacuoles; generation of numerous microvesiclesby the Golgi apparatus; the appearance of these microvesicleswith the plasmamembrane. Similar cytological changes that areassociated with increased hardiness occur during artificialhardening at 0°C for 5 to 20 days in both the autumn andspring. In the midwinter cells, however, the plasmamembranehas a fairly smooth structure. Thus, the highly folded structureof the plasmamembrane is not always characteristic of extremelyhardy cells. Ultrastructural changes associated with hardinessare discussed in terms of membrane renewal. ¹Contribution No. 2267 from the Institute of Low TemperatureScience. (Received October 11, 1980; Accepted December 5, 1980)