Extracellular freezing in leaves of freezing-sensitive species

Low-temperature scanning-electron microscopy was used to study the freezing of leaves of five species that have no resistance to freezing: bean (Phaseolus vulgaris L.), tobacco (Nicotiana tabacum L.), tomato (Lycopersicon esculentum L.), cucumber (Cucumis sativus L.), and corn (Zea mays L.). In the leaves of the four dicotyledonous species, ice was extracellular and the cells of all tissues were collapsed. In contrast, in maize leaves ice was extracellular in the mesophyll, and these cells were collapsed, but the epidermal and bundle-sheath cells apparently retained their original shapes and volume. It is concluded that the leaves of the freezing-sensitive dicotyledonous species tested were killed by cellular dehydration induced by extracellular freezing, and not by intracellular freezing. Freezing injury in maize leaves apparently resulted from a combination of freezing-induced cellular dehydration of some cells and intracellular ice formation in epidermal and bundle-sheath cells.