A Method of Studying Stresses Occurring in Plant Tissue During Freezing 1

Extracellular water is considered to be the continuous liquid system in plant tissue through which substances in solution can diffuse freely and includes the film of moisture surrounding cells as well as water less closely associated with living protoplasts such as in xylem vessels. Electrolytes do not diffuse freely into and out of living protoplasts, their water content being isolated from the principal path of migration. The fact that cellular organization of living .tissue constitutes an effective diffusion ~barrier is demonstrated by the high resistance of normal tissue to a weak direct electric current (about 5 >( ~ ohms pe r centimeter along the longitudinal axis of leaves of cereal seedlings) and by the decrease in resistance to less than 5% of this value after the tissue has been killed. The diffusion barrier is not broken by low voltage (in the range of 0 to 3 volts per centimeter), since within this range the resistance of the tissue is independent of the voltage applied. When slightly higher voltages are used, the resistance decreases reversibly with increasing voltage, indicating a weakening of the barrier. The diffusion barrier can be irreversibly destroyed by applying a sufficiently high voltage. Within the low voltage range, the tissue is unaffected by the amount of current up to a total of about 0.8 coulombs (the equivalent of 10 microampere days) (6). The method of obtaining freezing patterns was developed from previous studies of electrophoresis of foreign ions and a stem rust toxin through the extracellular water of wheat leaves (7). The effect of environmental factors on the resistance of the electrophoretlc unit used for studying migration provided ,the basis for evaluating changes in the liquid extracellular moisture content of plant tissue during freezing. The same type of unit was used for both problems and consisted of two electrical contacts and the plant tissue extending between them. Other units, in which a film of various solutions on paper or glass replaced the plant tissue, were used in developing the method of studying stress caused by freezing. The contact was made through a paste of finely divided charcoal moistened with an isotonic solution which is applied to about two square centimeters of the surface of the tissue. Dry graphite or platinum provides the best connection between the paste and the wires of the electrical circuit. The resistance of the contact is much less than that of the tissue and does not change appreciably with slight changes in the moisture con~tent of the paste. However, the resistance will be high if the paste is very