Freezing of Water in Red-Osier Dogwood Stems in Relation to Cold Hardiness

Abstract

Studies of stem water in red-osier dogwood (Cornus stolonifera Michx.) using nuclear magnetic resonance spectroscopy indicated that most freezing occurs at temperatures above -30 C in cold-hardy and tender stems. Hardy and tender stems had about the same amount of unfrozen water at -40 C (0.28 gram of water per gram dry weight). When hardy stems were slowly cooled below -20 C, the temperature below which little additional freezing occurs, they survived direct immersion in liquid N2 (-196 C) The importance of water in plant injury during freezing has long been discussed. A question often asked is whether injury results directly from intracellular freezing or indirectly via extracellular freezing (13). Several studies have indicated decreases in stem water content concomitant with increasing hardiness (8,9,14,15). The stem water content of red-osier dogwood (Cornus stolonifera Michx.) decreases from about 4 g water/g dry weight in nonhardy tissue to about 1.75-0.75 g water/g dry weight in hardy plant stems (9). During cooling most of the freezing occurs above -20 C (1). By -20 C the stem water content of both tender and hardy plants is reported to be about 0.25 to 0.30 g water/g dry weight (1). Sakai (1 1, 12) has shown that several woody species normally killed by quick freezing to -196 C survived such temperatures if they were slowly prefrozen to temperatures of around -20 C (-15 to -30 C depending on species) before being subjected to -196 C. He suggested that the slow prefreezing gives the plants sufficient time for nearly all of the freezable water to be removed by exosmosis causing cell dehydration and extracellular freezing rather than intracellular freezing and death. collagen fibers, Dehl (3) found that 0.6 g water/g of collagen did not freeze at -50 C. Kuntz and co-workers (6, 7) found that 0.2 to 0.6 g of water/g protein dissolved in solution does not freeze at -35 C. Whether the amount of unfrozen water can be correlated to hardiness levels is of some controversy.Krasavtsev (5), using calorimetric methods, indicated that in a group of species of varying hardiness levels, the least hardy had 10 to 23% unfrozen water at -60 C while the most hardy species had only 8 to 9%. In earlier studies, Tumanov and Krasavtsev (14) showed that the amount of unfrozen water in shoots dropped continuously as a result of laboratory hardening. On the other hand, Burke et al. (1), using NMR5 spectroscopy, showed that there appeared to be no difference in the amounts of unfreezable water in red-osier dogwood of varying hardiness levels at subfreezing temperatures.The current studies were designed to observe stem water content in red-osier dogwood at various levels of hardiness, with particular emphasis on the amounts of unfreezable water at subfreezing temperatures. Water measurements were made using pulse NMR methods. NMR methods have been used in measuring water in biological systems (1-3, 7). Pulse NMR was used here because it is more convenient for the study of water at low temperatures than the continuous wa...

Response of Xylem Ray Parenchyma Cells of Red Osier Dogwood (Cornus sericea L.) to Freezing Stress (Microscopic Evidence of Protoplasm Contraction)

“…The reduction in cell water content is accompanied by a reduction in cell volume and collapse of the cell wall (Ashworth et al, 1988;Pearce, 1988;Malone and Ashworth, 1991;Pearce and Ashworth, 1992). This type of freezing behavior is also called extracellular freezing (Burke et al, 1976;Burke and Stushnoff, 1979). Differential thermal analysis of tissues exhibiting extracellular freezing records only one exotherm, the HTE (Malone and Ashworth, 1991).…”

“…During a freezing stress, tissue water, presumably within xylem ray parenchyma cells, remains liquid at low temperatures (supercools) by remaining isolated from extracellular ice (Burke, 1979). As a result, the parenchyma cells of supercooling tissues do not dehydrate and they retain their original shape and volume during freezing (Burke et al, 1976;Ashworth et al, 1983Ashworth et al, , 1993Malone and Ashworth, 1991). Tissues displaying freezing avoidance are moderately hardy and cannot survive temperatures below -4OOC (George et al, 1974;Burke et al, 1976;Becwar et al, 1981).…”

“…As a result, the parenchyma cells of supercooling tissues do not dehydrate and they retain their original shape and volume during freezing (Burke et al, 1976;Ashworth et al, 1983Ashworth et al, , 1993Malone and Ashworth, 1991). Tissues displaying freezing avoidance are moderately hardy and cannot survive temperatures below -4OOC (George et al, 1974;Burke et al, 1976;Becwar et al, 1981). Tissues that resist low temperatures by freezing tolerance lose their cellular water to extracellular ice (George et al, 1982).…”

“…Differential thermal analysis of tissues exhibiting extracellular freezing records only one exotherm, the HTE (Malone and Ashworth, 1991). The HTE is linked to freezing of water in xylem vessels and extracellular spaces, and this freezing event is not injurious (Quamme et al, 1972;Burke et al, 1976). Some tissues exhibiting extracellular freezing are cold hardy and can survive temperatures below -4OoC, in some cases as low as -196OC (Sakai, 1960;Guy et al, 1986).…”

“…Current concepts of freezing resistance recognize two mechanisms by which woody plant tissues survive freezing temperatures: freezing avoidance and freezing tolerance (Burke et al, 1976;George et al, 1982;Wisniewski and Ashworth, 1986). Tissues that survive low temperatures by freezing avoidance display deep supercooling characteristics.…”