Nuclear Magnetic Resonance of Water in Cold Acclimating Red Osier Dogwood Stem

Abstract

The pulsed and continuous-wave nuclear magnetic resonance of water in cold-acclimating red osier dogwood (Cornus stolonilera Michx) stem showed reduced relaxation times and increased line width. The reduction of relaxation times suggests an over-all restriction in the motional characteristics of the water. The increased line width is not related to a molecular property of the water, but is useful in estimating the initiation of cold acclimation. Biphasic relaxation characteristics may be related to partitioning of the water at the cellular level. The liquid water content of the stem was a weak function of temperature between -25 and -55 C, corresponding to approximately 0.15 gram of water per gram of dry stem. The quantity of unfrozen water at subfreezing temperatures was not strongly dependent on the degree of cold acclimation. It is concluded that the ability of dogwood to survive low temperatures depends on its ability to tolerate diminished quantities of liquid water.Low temperature injury of plants is thought to result from intracellular ice formation or from freezing of extracellular water which may cause severe dehydration of the cells. Red osier dogwood (Cornus stolonifera Michx) has been studied previously and many details of the physiological alterations associated with acclimation to freezing stress and environmental factors which induce acclimation have been reported (10, 24). For example, changes in the quantities of water, lipids, carbohydrates, proteins, and nucleic acids usually occur (22,34). Although water has not been studied extensively as part of this problem, the status of water almost certainly is a vital factor in determining the resistance of living organisms to freezing stress. Nuclear magnetic resonance provides a particularly convenient tool for the study of water in plants because it is nondestructive and rich ' Scientific Journal Series Paper 8558 of the Minnesota Agricultural Experiment Station. in structural as well as dynamical information. NMR2 is used in two ways in the present study: to study the state of the unfrozen water in dogwood stems at low temperatures and to determine the amount of unfrozen water at low temperatures.NMR has been extensively used to study water in animal tissues such as muscle. In these samples, the NMR line is significantly broader than it is in pure water and the spin-spin relaxation time (T,2) is significantly shorter (2 to 6, 12, 13). These results are often interpreted to mean that cellular water consists of several populations of water molecule, including a bound fraction which experiences restricted motion. Some controversy has developed concerning the extent of the region of restricted water mobility and models range from a simplified two-phase model with a small fraction bound (2, 4, 5) to one consisting of all bound water (3,6,13

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

“…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).…”

“…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.…”

“…The free induction decay after the second and each succeeding pulse was monitored less than 6 ,usec after the pulse center. The liquid water content, LT, was calculated using equation 1 (2). Tr is the free induction decay of the partly frozen sample.…”

Water permeability of chloroplast envelope membranes

“…Water is generally present in plant tissue in two or more compartments that are isolated (on an NMR time scale) by slow exchange rates. The compartments in bark and stems have been identified [7,8] as extracellular and intracellular water. Different relaxation times from vacuolar and cytoplasmic water can be distinguished in maize roots [5].…”

“…Proton spin relaxation has been used to detect water compartments in wheat leaves [3] and * To whom correspondence should be addressed crowns [4], maize roots [S] and leaves [6], dogwood stems [7], ivy bark [8], Elodea [9], and Nitella [lo]. Water is generally present in plant tissue in two or more compartments that are isolated (on an NMR time scale) by slow exchange rates.…”