Ice and a slice
For those of us who live in regions prone to frost in winter, one of the most fascinating phenomena is the ability of certain plants to survive exposure to temperatures below freezing, often in a repeated diurnal cycle of freeze–thaw which may last for several days or even weeks. How do they do it? In some frost-tolerant species, frosted plants look badly wilted, and indeed they are. In this issue, Margaret McCully and colleagues (Canberra, Australia, pp. 665–674) describe how cells of these plants lose water under freezing conditions and regain it during a thaw. The authors used plants of Trifolium repens (white clover) and Escholschzia californica (California poppy) frosted in situ where they grew. Petioles were taken at different times in the freeze–thaw cycle and examined by cryo-microscopy and by conventional light microscopy. It is well known that many frost-tolerant organisms are able to prevent intracellular ice formation and these plant species are no exception. Ice formation starts in the intercellular spaces and, as freezing proceeds, water is drawn from the cells, thus leading to wilting. What is so interesting about the species examined here is that significant ice formation (‘blocks of extracellular icicles’) form in a ‘fault zone’ between the epidermal layers and the parenchyma. However, the outer cell layers do not separate completely from the parenchyma; there are anchorage points opposite the vascular bundles that hold the layers together. Thus, the petiole does not fall to pieces despite the extensive ice formation. On thawing, the process is reversed: the ice melts, water is taken up by the cells, which regain their size and turgor, and the sub-epidermal space shrinks. Overall, this is a clear mechanism for avoidance of intracellular ice formation. However, in our understanding of the process, many questions remain, as the authors themselves indicate, including those concerning the developmental regulation of the fault zone and of the anchorage points.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
