A model for extracellular freezing based on observations on Equisetum hyemale.

In frost hardy plants, the lethal intracellular formation of ice crystals has to be prevented during frost periods. Besides the ability for supercooling and pre-frost dehydration of tissues, extracellular ice formation is another strategy to control ice development in tissues. During extracellular ice formation, partially large ice bodies accumulate in intercellular spaces, often at preferred sites which can also be expandable. In this contribution, the physico-chemical processes underlying the water movements towards the sites of extracellular ice formation are studied theoretically, based on observations on the frost hardy horsetail species Equisetum hyemale, with the overall aim to obtain a better understanding of the physical processes involved in extracellular ice formation. In E. hyemale, ice accumulates in the extensive internal canal system. The study focuses on the processes which are triggered in the cellular osmotic-mechanic system by falling, and especially subzero temperatures. It can be shown that when the temperature falls, (1) water flow out of cells is actuated and (2) "stiff-walled" cells lose less water than "soft-walled" cells. Furthermore, (3) cell water loss increases with increasing (= less negative) turgor loss point. These processes are not related to any specific activities of the cell but are solely a consequence of the structure of the cellular osmotic system. On this basis, a directed water flow can be initiated triggered by subzero temperatures. The suggested mechanism may be quite common in frost hardy species with extracellular ice formation.