Chapter 11: Translocation in the Phloem

The pressure flow hypothesis serves as the central model explaining long-distance transport, where hydrostatic pressure gradients drive bulk flow of phloem sap from source tissues with high sugar concentrations to sink tissues with lower concentrations. Source-sink relationships are examined in detail, covering how photosynthetic leaves function as primary sources while developing fruits, roots, and meristems act as metabolic sinks. The chapter delves into the biochemical processes of phloem loading, contrasting apoplastic and symplastic pathways and highlighting the role of sucrose transporters and proton pumps in active accumulation. Phloem unloading mechanisms are similarly explored, showing how sink tissues extract and utilize transported nutrients through various cellular pathways. Additional topics include the regulation of translocation rates by environmental factors, the transport of signaling molecules and proteins alongside carbohydrates, and the integration of phloem transport with whole-plant carbon allocation strategies. The material also addresses how phloem function responds to developmental changes, stress conditions, and seasonal variations, emphasizing the dynamic nature of this transport system in coordinating plant growth and metabolism across different organs and tissues.