Translocation of Organic Substances in Trees. III. The Removal of Sugars from the Sieve Tubes in the White Ash (Fraxinus Americana L.).

Abstract

The sieve tubes remain turgescent after artificial defoliation and the natural leaf-fall in autumn. It is therefore concluded that they are not "leaking". Two metabolic mechanisms are described, both of which may be responsible for the removal of sugars from the sieve tubes. First there is a rapid intercon-version of stachyose into sucrose after artificial and natural defoliation. It is suggested that an [alpha]-D-galactosidase on the side wall cytoplasm of the sieve tubes removes D-galactose units from the oligosaccharides, thus producing raffinose from stachyose and sucrose from raffinose. Second sucrose itself is removed from the sieve tubes. This process seems reversible and controlled by a hormonal factor that comes from the leaves (sucrose exit in the presence, sucrose re-entry in the absence of the hormone). The sucrose removal is very rapid immediately after defoliation (hormone still available), but slows down and stops after 5 days, after which sucrose increases again in the sieve tubes (hormone exhausted). From the breakdown rate of stachyose immediately after defoliation, and the measured stachyose gradient along the trunk before defoliation, it was possible to calculate the rate of stachyose translocation as 62, 73, and 54 cm per hour for the 3 defoliation experiments. The total molar gradient of the 3 sugars and D-mannitol, which constitute about 90% of the sieve tube substances, is positive in the downward direction of the trunk during the whole summer. After defoliation this gradient disappears and at the same time some of the gradients of the individual substances become negative. This suggests that the turgor gradient is the driving force of translocation through the sieve tubes.