34.16:
Phloem and Sugar Transport
Photosynthesis generates sugar in plant leaves. However, the sugar is often needed in distant parts of the plant. Translocation is the process that distributes the products of photosynthesis to other plant tissues.
Leaves are typically called sugar sources, which are sites that produce more sugar than they consume. In contrast, the roots, stems, and fruits are typically sugar consumers, called sugar sinks.
Transport of sugar between source and sink occurs in plant tissue called phloem. In Angiosperms, phloem is composed of specialized cells called sieve-tube elements, arranged end to end to create long tubes. Companion cells are located alongside each sieve-tube element.
Sugar can reach the phloem by several routes. It may travel through the extracellular space and cell walls or through pores that directly connect the cytoplasm of neighboring cells.
A watery solution, called Phloem sap, composed of sugar, amino acids, hormones, and minerals, flows through the sieve tube elements.
According to the pressure-flow hypothesis, water follows sugar into the phloem by osmosis, increasing the pressure within the phloem and thus driving phloem sap movement.
Phloem sap then flows to the nearest sink tissue, which has a low sugar concentration. The sugar either diffuses or is actively transported out of the phloem. As the sugar is unloaded, water follows by osmosis and relieves pressure within the phloem.
Using translocation through the phloem, plants can distribute resources to the tissues in which they are needed at a particular time or season.
34.16:
Phloem and Sugar Transport
Like many living organisms, plants have tissues that specialize in specific plant functions. For example, shoots are well adapted to rapid growth, while roots are structured to acquire resources efficiently. However, sugar production is primarily restricted to the photosynthetic cells that reside in the leaves of angiosperm plants. Sugar and other resources are transported from photosynthetic tissues to other specialized tissues by a process called translocation.
Within a plant, tissues that produce more sugar than they consume are sugar sources – leaves are the primary example of this. Roots, shoots, flowers, and fruits are usually considered to be sugar sinks, as they require more sugar than they can make. Translocation distributes sugar, hormones, amino acids, and some signaling molecules from sugar sources to sugar sinks through a tube-like structure of vascular plants called phloem. Flow can be bidirectional in the phloem, which is composed of cells joined end-to-end by plasmodesmata to form the sieve-tube elements. These cells have thickened cell walls, giving them mechanical support, and are accompanied by neighboring companion cells that facilitate phloem health and loading of solutions into the phloem from surrounding tissues.
Phloem loading can occur via the apoplastic or symplastic routes and may be either passive or active. These pathways to phloem may operate at the same time or sequentially, and there is some evidence that plants can switch between loading modes depending on plant water and energy demands. In many instances, the sucrose/H+ symporter couples the loading of sucrose into the phloem with transport of a hydrogen ion.
According to the pressure-flow hypothesis, the sugar concentration gradient promotes the flow of water into the phloem, resulting in the generation of pressure. As a result, the phloem sap moves towards areas of lower pressure, at the nearest sugar sink. At the sugar sink, sucrose is transported to the area of lower sugar concentration, driving movement out of the phloem. Water follows the sucrose, relieving pressure in the phloem.