韧皮部与糖运输

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.