592 FACTORS AFFECTING GROWTH 



are native to tropical or sub-tropical regions, are killed or seriously injured by 

 relatively low temperatures above their freezing point. This type of low 

 temperature injury is often called chilling injury. For example, Sellschop and 

 Salmon (1928) found that exposure to a temperature of 0.5 to 5.0° C. for 

 24 to 36 hours was fatal or markedly injurious to rice, velvet beans, cotton, 

 peanuts, and Sudan grass. Species which were only slightly injured included 

 maize, sorghums, watermelons, and pumpkins, while soy beans, buckwheat, 

 tomatoes, and flax showed no evidence of injury from such a chilling. The 

 cause of such pronounced effects of low but not freezing temperatures un- 

 doubtedly lies in disturbances which are induced in the metabolic activities and 

 physiological conditions within the cells, but little or no precise information is 

 available regarding the exact nature of such disturbances. In general the 

 longer the exposure of such plants to chilling temperatures, the greater the 

 resulting injury. 



(4) Freezing Injury. — Many plant tissues are killed or irreparably in- 

 jured when they are exposed to temperatures which are low enough to cause 

 ice formation to take place within them. This is the most frequent and 

 fundamental type of low temperature injury in temperate climates and the 

 remainder of the discussion will be devoted to it. Many plant organs, how- 

 ever, can endure subjection to such conditions. Plants or plant organs of 

 which this is true are said to be cold resistant, frost resistant, or hardy. 



II. Alechanism of Ice Formation in Plant Tissues. — When water is grad- 

 ually cooled, freezing usually does not begin at 0° C. but only after its tem- 

 perature has dropped from a fraction to several degrees below its freezing 

 point. In other words the water is first undercooled before freezing begins. 

 With the initiation of ice crystal formation heat of crystallization is released, 

 resulting in a rise of the temperature of the water to its freezing point. Simi- 

 larly the water in plant tissues usually does not freeze until after the tissues 

 have been undercooled. Some plant tissues can be undercooled as much as 

 15° C. below their true freezing point before crystallization of water begins, 

 but for most plant tissues the undercooling is only a few degrees. The actual 

 freezing points of plant tissues, however, are seldom below ~5° C. 



Because of their capacity for undercooling some plants normally susceptible 

 to frost injury can survive short exposures to freezing temperatures without 

 injury. This is true, for example, of certain species of cacti, which often can 

 be undercooled 10-15° C. without any formation of ice crystals. Once such 

 tissues actually freeze, however, they are killed or seriously injured. 



When plant tissues freeze, ice formation often takes place in the intercellu- 

 lar spaces. The crystals enlarge at the expense of water which diffuses from 

 the abutting cells. There are a number of conditions, however, under which 



