Phloem Loading and Unloading

Abstract

Since the classic discoveries of Malphighi, Hartwig and others it is known that in higher plants the photosynthetically derived assimilates are translocated, mostly in the form of sucrose, by specialized cells, the sieve tubes, to the growing tips and the storage organs. However, the principal mechanism of translocation has been constantly in dispute over the last 100 years. Although it is now generally accepted that diffusion alone cannot account for the observed transport rates over long distance, it is not clear how the acceleration of long distance transport occurs, that is, by pressure flow as MUNCH proposed (MUNCH 1930) or by catalyzed transfer along the sieve-tube cytoplasm (e.g. SCHUMACHER 1967). The unequivocal proof for or against these hypothesis is not available because of the lack of suitable experimental material, such as pure sieve tubes or pure companion cells; nevertheless in the last 10 years some decisive progress was made on certain aspects of transport by phloem so that easily understandable models can now be used as a basis for further experiments.