Seasonal changes in content of starch, protein and sugars in the twig wood of Salix caprea L.

The seasonal changes in content of the storage compounds (starch, sugars, and protein) were studied biochemically and electron microscopically in 2-year-old twig wood of Salix caprea L.. Sugar content of the apoplast (= xylem sap) and the symplast (= living cells) of the wood was determined separately. At its maximum, about 25g of starch, 30g of sugars, and 8 to 10g of protein are found perkg of twig wood on dry weight basis. Of the protein content about 6 g/kg can be attributed to true storage proteins while 2 to 3g/kg are "structural proteins" of the living cells. The deposition periods for starch and protein differ clearly. Starch accumulates from May until October while protein is deposited primarily during leaf senescence in fall. Protein deposition proceeds in the form of small protein storing vacuoles called protein bodies. Sugar content of the living cells of the wood is low during the vegetation period (5 to 10g/kg wood) but increases drastically in late October and in November giving rise to a winter maximum of up to 30g/kg. As starch decreases in parallel there is good evidence that this increase is due primarily to a prominent starch-to-sugar conversion. The dominating sugars in the wood during all periods of the year, except in May and June; are sucrose and its galactosides raffinose and stachyose. Xylem sap sugar content is neglectible except in late February and March when it rises to more than 3 % (w/v). Sucrose dominates by far (ca. 95 %) over the hexoses (less than 5 %). The increased sugar content of the sap is not correlated with the sugar content of the living cells suggesting a facilitated or active release during the period of catkin growth.