Studies on the Chemistry of the Living Bark of the Black Locust in Relation to Its Frost Hardiness. V. Seasonal Transformations and Variations in the Carbohydrates: Starch-Sucrose Interconversions

Abstract

Sucrose and starch are the principal storage carbohydrates in the living bark of the black locust. Both carbohydrates are depleted in late spring by the flush of new growth. Sucrose begins to accumulate again shortly afterwards and reaches a max. in late summer. Starch does not appear in the bark as a storage product until late in the summer and reaches a max. in early fall but the max. content is not as high, in normal trees, as is that of sucrose. Starch disappears with the coming of cold weather in late fall but sucrose is retained in high concn. throughout the winter. In the instance of one intact tree where sucrose accumulation was abnormally low, it was found that the disappearance of starch was accompanied by a concomitant rise in sucrose concn. With the advent of warm weather in early spring and prior to initiation of growth sucrose appears to be quantitatively converted into starch. This conversion occurs only when the daily temp. has a min. above 0[degree]C. Artificial application of sustained low temp. (3[degree]C) to bark tissues at this time causes the starch to be converted quantitatively back to sucrose. Reducing sugars are present in low amts. throughout the yr., reaching a max. in late winter. Fructose is absent in late spring during the period of most active growth. Trees ringed in summer accumulate much more carbohydrate in the bark above rings than do normal trees at this time and, in contrast to normal trees, this excess carbohydrate is present largely in the form of starch rather than sucrose. With incidence of low temp. in autumn a readily observable conversion of the starch to sucrose occurs in the later ringed sections of these trees. Sections above rings of trees ringed in winter show no signs of radial or terminal growth in spring. They do not become hydrated as do normal bark tissues at this time and their carbohydrates are not so radically depleted. The carbohydrates are retained in the form of sucrose and little or no conversion of this sucrose to starch is observed even at the higher temps. prevailing at this time. These observations on seasonal and unseasonal variations in the carbohydrates of the black locust suggest that a temperature-sensitive and reversible enzymatic process is operating in the bark of the tissue of this tree which determines the direct conversion of sucrose to starch and perhaps of starch to sucrose as well. It is also indicated that this process may be dependent on the state of hydration of the cells of this tissue.