A laboratory method for testing frost hardiness of apple trees is described. Results demonstrated that diffused electrolytes from pre-hardened twigs provided a reliable means of testing their hardiness. There was a definite association between increased diffusion of electrolytes from hardened tissues and increased injury of twigs. This association was found useful in establishing both the relative degrees of hardiness of three named varieties of apples, Antonovka, McIntosh and Northern Spy, and the absolute values for the ultimate frost resistance of their tissues. Thus, although the total conductance of electrolytes of the three varieties was about the same before the artificial freezing treatments, ranging from 900 to 1000 micromhos, the average survival and the conductance of diffused electrolytes were as follows: hardy Antonovka 71 per cent and 296 micromhos; less hardy McIntosh 53 per cent and 445 micromhos; and least hardy Northern Spy 23 per cent and 611 micromhos. Results also established quantitative values for the ultimate frost hardiness of apple trees, namely: no appreciable injury when the conductance from hardened tissues of twigs was within 200 to 250 micromhos or less. Conductance of electrolytes above 350 to 450 micromhos generally signified total killing; whereas intermediary readings indicated partial injury to twigs.The method used also permitted a study of the effects of several factors considered of vital importance in: (a) developing frost resistance of overwintering plants such as seasonal maturity, cold hardening and dehardening of tissues, and (b) causing excessive frost injuries such as increasing intensities and rates of freezing temperatures. The validity of the used laboratory methods is discussed.
Assessment of frost resistance of varieties and forms of apple trees