On the fine structure of sieve tubes and the physiology of assimilate transport in Alaria marginata

Abstract

Alaria marginata Postels and Ruprecht has a sieve tube system which extends through the lamina, especially the midrib, and through the stipe. The sieve elements originate from the innermost cortex cells and are nucleate, highly vacuolated cells that contain the usual cell organelles and membrane systems. The plastids and mitochondria show some special features in their morphology and fine structure. P protein is absent. Sieve pores, 0.11–0.3 μm in diameter, occur in cross walls between two sieve elements. They are lined by plasmalemma, and the cytoplasms of the two cells are interconnected through them. Long-distance transport of photo-assimilate follows the source–sink relationship; but its normal basipetal direction can be reversed by creating "artificial" sinks. Translocation velocity is in the range of 25 to 40 cm/h. The translocate consists mainly of mannitol and free amino acids, which were analyzed qualitatively and quantitatively. Double-labeling experiments with 32P and 14C indicate that inorganic ions are not translocated together with the 14C-labeled photoassimilates and probably move only by diffusion.