Seasonal changes of activity of a starch granule bound endoamylase and of a starch phosphorylase in poplar wood (Populus x canadensis Moench <robusta>) and their possible regulation by temperature and phytohormones

The activity of a starch granule bound endoamylase (EC 3.2.1.1) and of a starch phosphorylase (EC 2.4.1.1) was followed in the twig wood of 10-year-old poplar trees (Populus x canadensis Moench robusta) throughout the year. The activities of both enzymes increased in fall from very low levels in late summer to a maximum in winter. In late winter, the activities of both enzymes decreased continuously until the period of starch resynthesis in early spring and then fell rapidly parallel to bud break. The starch content monitored in the wood decreased in fall parallel to the increase of these enzyme activities. During the winter period, twigs stored at different temperatures revealed a remarkable influence of temperature on endoamylase activity and an almost linear relationship between storage temperature and starch content. Lowering the temperature resulted in higher endoamylase activities and a decrease in starch content while higher temperatures induced the opposite changes. Starch phosphorylase activity also responded to the lowering of temperature but showed a much slower reaction. By means of cycloheximide treatment of twigs, evidence was found that at least a part of the temperature induced increase in endoamylase activity was derived from de novo synthesis of the enzyme protein. Supplying the twigs with ABA or IAA via the vessels also induced changes in endoamylase activity. While ABA led to increased endoamylase activities in fall, IAA triggered a marked decrease of endoamylase activity in spring. The significance of the starch granule bound endoamylase in the regulation of the starch-sugar-level in parenchyma cells of the wood during winter is discussed.