9 - The Low Profile Directors of Carbon and Nitrogen Economy in Plants: Parenchyma Cells Associated with Translocation Channels

Abstract

This chapter explores the underlying cellular mechanisms that govern uptake from and release into the channels. The physiology of the channel-associated cells is crucial to the M/inch concept of mass flow. With progressive study on the sieve element-companion cell (SE/CC) complexes, the views on phloem loading, transport, and unloading have changed dramatically with consequent adaptations of the mass flow concept. For optimal functioning of the release/retrieval systems, a certain degree of symplasmic isolation of the SE/CC complexes in the transport phloem seems appropriate. Different mechanisms of phloem loading have been discovered. Originally, the concept of multiprogrammed phloem loading was purely hypothetical and was based on the differences in ultra structure and architecture between the minor veins in various plant families. Linking the data on the membrane potentials of the cell elements in transport phloem with the multiprogrammed concept of phloem loading reveals a remarkable correlation. One may take this chapter as a plea to evaluate experimentally the importance of the channel-associated cells for the distribution of C and N in plants. The channel-associated cells play a major role in the syndromes of phloem physiology that may be associated with the climate and/or the growth strategy. Despite its presumptive syndrome-bound behavior, the phloem remains a dynamic system with numerous control points for the distribution of organic matter.