TRANSPORT OF PHOTOASSIMILATES IN YOUNG TREES OF FRAXINUS AND SORBUS -MEASUREMENT OF TRANSLOCATION IN-VIVO

Translocation of photoassimilates was studied on 2-year-old trees of F raxinus excelsior and Sorbus aucuparia using the short-lived isotope C -11. Leaflets of different leaves on the same plants were radiolabelle d showing that both carbon distribution and speeds of transport may va ry with leaf position. Within 2 h after pulse feeding with (CO2)-C-11, mainly the lower leaves distributed radiolabel to the roots in Fraxin us, whereas in Sorbus, the upper leaves were also involved. By repeate d pulse applications to selected leaves, temporal profiles of C-11 tra nsport were followed on individual plants from April to October. Early in the season, within 2 h after pulse labelling, 30 - 40 % of the fix ed radiolabel was exported from leaves in Fraxinus and about 20% in So rbus. Thereafter export started to decline, particularly in Fraxinus, and the distribution of radiolabel between stem and roots could alter depending on the position of the feed leaf. Speeds of translocation ob tained along the rachis and stem showed high variability, but they did not necessarily slow down before the end of the season. The speeds mo nitored at the rachis of Fraxinus leaves (30 - 75 cm h(-1)) were gener ally lower than those found on Sorbus (50 - 130 cm h(-1)). As reported in the literature, the two tree species translocate different carbohy drates and show remarkable differences in the ultrastructure of their vascular systems. In that context it is interesting that the temporal profiles of C-11 radioactivity obtained from F. excelsior and S. aucup aria could clearly be distinguished by their characteristic shapes. Th e results are discussed in terms of anatomical characteristics of the conducting tissues and possible differences in phloem loading.