Storage, mobilization and interrelations of starch, sugars, protein and fat in the ray storage tissue of poplar trees

Abstract

The seasonal pattern in starch, various sugars, protein, and fat, and their interrelationship, has been followed in 3-year-old branch wood of poplar trees (Populus x canadensis Moench ‘robusta’) under natural site conditions. The deposition of starch, protein and fat proceeds at different times. Starch accumulates from May until October, fat mainly during the summer months, and protein when the leaves are yellowing in September and October. The maximum concentrations in the branch wood were 15–18 μg starch, 6–9 μg protein, 4–8 μg fat, 10–15 μg sucrose, and up to 30 μg total sugars per milligram dry weight (DW). During starch deposition periods no increased sucrose level is found in the tissue. The maximum daily starch deposition rate was 0.2–0.4 μg starch/day/mg DW of wood. During starch hydrolysis in late autumn and winter, a dramatic increase in sucrose and its galactosides is measured (up to 15–27 μg/mg DW in total). In early spring, before budbreak, the concentrations of these sugars diminishes sharply. In contrast to this clear-cut starch-to-sugar conversion in autumn no significant starch-to-fat conversion is detected. An elevated content of free glycerol, however, is found in winter. In spring, starch and storage protein are mobilized completely, or almost completely, in poplar twig wood. A noteworthy pool of maltose is found transiently during autumn (up to 8 μg/mg DW) and again in spring. The results demonstrate that the individual storage materials, e.g. starch, protein, and fat, are accumulated fairly independently in the wood storage parenchyma. Tissue sugar levels, in contrast, appear to be closely related to the seasonal variations in starch content, on the one hand, and to the acclimation and deacclimation of the cells, on the other. The interrelations of the storage materials and sugars are discussed.