ABSTRACT
This chapter presents an account of phloem transport which explains much of the behavior of the photoassimilate transport system in responding to endogenous and environmental factors. Most physiologists agree that long-distance phloem transport occurs by means of a mechanism which is a generalization of E. Munch's proposal. Delivery of photosynthate to the sieve tubes from photosynthetic cells of the mesophyll has been reviewed recently by A. J. E. van Bel and by M. A. Grusak. After chilling the pathway, transport recovers in plants at the low temperature, and so it is not unexpected that with sufficiently slow cooling there is no effect on transport. The long-distance phloem transport pathway between source and sink regions is through the sieve elements of the phloem tissue. An apoplastic path of unloading is found where sinks accumulate significant concentrations of osmotically active solutes and is obligatory where there is symplastic discontinuity, as between the generations in seeds.
