Phloem: the long and the short of it.
The structure of phloem sieve-element-companion-cell complexes reflects a duality of function: to conduct photoassimilates throughout the plant, and to exchange solutes between the phloem and surrounding tissues. The conceptual integration of these long- and short-distance functions requires the abandonment of a long-cherished concept in phloem physicochemistry, that source-sink turgor differentials control flow. The manifest inability of decentralized organisms such as plants to control phloem translocation centrally disqualifies such differentials as control variables; besides, the phloem is maximally efficient if the pressure differentials are small. Testing this hypothesis and whether turgor differentials are small will require a significant recommitment to studying the quantitative anatomy of phloem.
Thompson
Matthew V
MV
Department of Cell Biology and Molecular Genetics, H.J. Patterson Hall, University of Maryland, College Park, MD 20742, USA. mvthom@umd.edu
D001692:Biological Transport; D018547:Plant Stems / Q000033:anatomy & histology / Q000378:metabolism; D010944:Plants / Q000033:anatomy & histology* / Q000378:metabolism*; D011312:Pressure