Freezing Injury in Plants

Two groups of biologists are chiefly interested in freezing injury and its prevention: those concerned with the use of low temperatures for the long­ term preservation of animal tissues and microorganisms, and those con­ cerned with the problem of how plants in nature survive subzero tempera­ tures. Both groups have amassed large volumes of data, but :neither has made much effort to study the results obtained by the other or to incorpo­ rate the results into more effective concepts of freezing injury. Part of the lack of communication is due to the different experimental approaches used by the two groups. Those concerned with preservation have devoted most of their effort to improving survival by the addition of protective solutes; and they have devoted some effort to manipulating the physical variables involved in freezing, especially cooling velocity. They have generally used single cells or isolated pieces of animal tissue rather than intact animals, paying little attention to the biochemical and physiolog­ ical differences underlying the wide variations in susceptibility to freezing shown by various species. On the other hand, botanists concerned with frost injury have been chiefly interested in the marked changes in sensitivity to freezing that oCcur seasonally in many plants, and in correlating these changes with biochemical and physiological alterations. But they have de­ voted little effort to uncovering the relation between survival and changes in physical factors such as cooling and warming velocity; and they usually define injury in terms of the median lethal temperature for plants subjected to single cooling and single warming velocities, velocities that are one to four orders of magnitude lower than those used by investigators concerned with preservation. Comparatively little effort has been devoted to simpler single-cell plant systems, and, as far as I know, none to plant material in tissue culture. The purpose of this review is to identify and circumscribe these differ­ ences in concepts, approaches, and results, at the same time pointing out the common features involved in the freezing of all living systems. The review will not be a synopsis of recent data on freezing injury and hardiness in plants: instead, it will begin with a discussion of the physical and chemical