The transportation of solutes in the phloem from the source to the sink is known as the translocation. The phloem sap contains sugar, amino acids, water, other metabolites, hormones and mineral elements. It is a concentrated solution of dissolved solute; this solution is known as phloem sap. Translocation in the phloem occurs in both the directions simultaneously. The source is the part of the plant where the photosynthate is synthesized (leaf) and sink is the part which require or stores the food. The source and sink relation is variable that is it get reversed depending on the season. The stored food in the roots will be mobilized and used up in the spring season when the bud becomes the sink, since it requires energy for growth and development.
Translocation in the phloem tissue occurs by the pressure flow or mass flow mechanism. According to this hypothesis the translocation of sugars occurs from the source to sink occurs by a positive pressure. The glucose is converted to sucrose and is moved from the mesophyll tissue to the companion cells and then to the phloem sieve tube by the mechanism of active transport. This process is known as the loading, this creates a hypertonic condition in the phloem. As a result of osmosis water enters the phloem from the xylem tissue. This water uptake creates a pressure which moves the sap down to the sieve tube. Then the unloading of the sugar takes place and water is lost at the sink which is recycled back again to the source.