Dormancy and Hardiness in the Plum

In the plum there are widely differing degrees of dormancy among the different species and varieties. There appears to be a definite relation between dormancy and hardiness. Assiniboine is extremely hardy and its dormancy is very profound, in contrast with Tonka and Stella, two semihardy varieties in Minnesota, which appear to enter real dormancy for only a very short period in early winter, if they do so at all. 2. During the period of dormancy the moisture content of the semihardy varieties fluctuates with the temperature. Periods of low temperatures are accompanied by a loss of moisture from the leaf and fruit buds, and higher winter temperatures, which are seldom above freezing in Minnesota, by an increase in moisture content. In comparison with the semihardy varieties the moisture content of Assiniboine remains at a definite and fairly constant minimum throughout dormancy. 3. When the fluctuations in the moisture content of buds were found to occur under orchard conditions, this phase of the problem was checked under control in the laboratory by placing the twigbud-system in sealed chambers over different concentrations of sulphuric acid. By this method water movement in the tissues of Assiniboine was found to take place more slowly than in Stella and Tonka. Lenticel number per unit area was found to be correlated with the difference in moisture retaining capacity. 4. Somewhere near the mid-point of the dormant period fundamental metabolic changes occur which affect the ecological reactions of apparently dormant plum trees in a striking manner. As a result of these changes, or at least coincident with them, winter killing in the flower buds is found for the first time, the moisture retaining capacity of the fruit and leaf buds is changed, and anthesis occurs when fruit buds are subjected to favorable growth temperatures in the greenhouse. These changes are interpreted as indicating the time when the rest period is broken. 5. The dormant condition reached by the hardy forms, such as Assiniboine, appears to involve fundamental protoplasmic changes. Among these there may be a change in colloidal properties creating an increased imbibition which may account for the marked retention of water against the force of dehydration. 6. This investigation has a direct bearing upon the applied problem of selecting seedling fruits for hardiness. A study of the moisture relations in forms differing in hardiness may furnish the fruit breeder with a criterion, correlated with hardiness, which will hasten markedly the final selections.