Translocation is the movement of materials from leaves to other tissues throughout the plant. Plants produce carbohydrates (sugars) in their leaves by photosynthesis, but nonphotosynthetic parts of the plant also require carbohydrates and other organic and nonorganic materials. For this reason, nutrients are translocated from sources (regions of excess carbohydrates, primarily mature leaves) to sinks (regions where the carbohydrate is needed). Some important sinks are roots, flowers, fruits, stems, and developing leaves. Leaves are particularly interesting in this regard because they are sinks when they are young and become sources later, when they are about half grown.
Phloem Structure and Function
The tissue in which nutrients move is the phloem . The phloem is arranged in long, continuous strands called vascular bundles that extend through the roots and stem and reach into the leaves as veins. Vascular bundles also contain the xylem , the tissue that carries water and dissolved minerals from the roots to the shoots. When plants increase in diameter (secondary growth) they do so by divisions of a layer of cells just under the bark; this cell layer makes new xylem to the inside (forming the wood of the tree trunk) and a thin, continuous cylinder of new phloem to the outside.
The contents of the phloem can be analyzed by cutting off the stylets (mouth parts) of phloem-feeding insects such as aphids and collecting the drops of sap that exude. Phloem sap is composed largely of sugar dissolved in water. All plants translocate sucrose (table sugar) and some also transport other sugars such as stachyose, or sugar alcohols such as sorbitol. Many other organic compounds are found, including amino acids , proteins , and hormones . Glucose , the sugar found in the circulatory system of animals, is not translocated.
In order to accommodate the flow of sap, the internal structure of the conducting cells of the phloem, the sieve elements, is drastically altered. As the sieve elements mature, they lose many of the organelles commonly found in living cells and they modify others. The nucleus disappears, as do the vacuoles, microfilaments, microtubules, ribosomes , and Golgi bodies. Therefore, the inside (lumen) of the cell is left essentially open. The sieve elements are greatly elongated in the direction of transport and are connected to one another to form long sieve tubes. Large pores perforate the end walls of the sieve elements to facilitate flow through the tube. The connecting walls thus look like a sieve, giving the cell type its name.
Some sieve elements can live for a long time, as many as one hundred years in palm trees, even though they have no nucleus or any of the machinery needed for protein synthesis. Cells closely associated with them, called companion cells, apparently keep them alive. The association of sieve elements and companion cells is one of the most intimate and complex in nature, and one of the least understood. It now appears that both small and large molecules can move from companion cells to sieve elements through the plasmodesmata that connect them. Plasmodesmata are minute pores that traverse the common walls between plant cells. They have an intricate internal structure. Interest in plasmodesmata is high because viruses move through them to cause infections. If a virus enters the phloem this way it will travel with the sap, spread widely around the plant, and infect sink organs. Since viruses are much larger than plasmodesmata, they must be disassembled in one cell and reassembled when they get to their destination.