Seasonal changes of sucrose-phosphate synthase and sucrose synthase activities in poplar wood (Populus x canadensis Moench <robusta>) and their possible role in carbohydrate metabolism

Two important enzymes of sucrose metabolism, sucrose-phosphate synthase (SPS, EC 2.4.1.14) and sucrose synthase (SuSy, EC 2.4.1.13), were investigated in the ray parenchyma cells of the trunk wood of Populus x canadensis Moench robusta throughout the year. The activity of SPS increases dramatically in autumn in parallel with leaf fall, reaches a maximum level in winter at the time of the starch-to-sugar conversion and declines in spring during starch resynthesis and mobilisation. In summer, the activity of SPS remains at a very low level. These seasonal changes in SPS activity were identical both under V-max- and under V-lim- assay conditions. In temperature-controlled storage experiments with twig sections, the activation state of SPS, termed as V-lim/V-max x 100, was substantially higher after storage at -5 degreesC in contrast to storage at +10 degreesC. A Western blot analysis, using a polyclonal antibody, revealed a molecular weight of about 130 kD for the SPS-polypeptide in poplar wood with highest levels of SPS enzyme protein in winter and lowest levels in summer. SPS of other tree species (Acer, Fagus, Salix) exhibited a molecular weight in a similar range. The activity of SuSy started to increase in late autumn, was high in winter and declined in spring. In contrast to SPS, SuSy shows a remarkably high activity in the outer wood area in summer while it remained low in the middle and inner area of the trunk wood. This high SuSy activity correlates with the differentiation of the xylem cells rather than with starch deposition. The significance of the SPS in autumn and winter for the starch-to-sugar conversion during cold adaptation of xylem parenchyma cells and of the SuSy for wood formation processes is discussed.