Freezing injury in onion bulb cells: I. Evaluation of the conductivity method and analysis of ion and sugar efflux from injured cells.
Onion (Allium cepa L.) bulbs were frozen to -4 and -11 C and kept frozen for up to 12 days. After slow thawing, a 2.5-cm square from a bulb scale was transferred to 25 ml deionized H(2)O. After shaking for standard times, measurements were made on the effusate and on the effused cells. The results obtained were as follows.Even when the scale tissue was completely infiltrated, and when up to 85% of the ions had diffused out, all of the cells were still alive, as revealed by cytoplasmic streaming and ability to plasmolyze. The osmotic concentration of the cell sap, as measured plasmolytically, decreased in parallel to the rise in conductivity of the effusate. The K(+) content of the effusate, plus its assumed counterion, accounted for only 20% of the total solutes, but for 100% of the conductivity. A large part of the nonelectrolytes in the remaining 80% of the solutes was sugars.The increased cell injury and infiltration in the -11 C treatment, relative to the -4 C and control (unfrozen) treatments, were paralleled by increases in conductivity, K(+) content, sugar content, and pH of the effusate. In spite of the 100% infiltration of the tissue and the large increase in conductivity of the effusate following freezing, no increase in permeability of the cells to water could be detected.The above observations may indicate that freezing or thawing involves a disruption of the active transport system before the cells reveal any injury microscopically.
Department of Horticultural Science and Landscape Architecture, University of Minnesota, St. Paul, Minnesota 55108.
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