Translocation is the process plants use to move soluble organic nutrients throughout their structure within the phloem tissue. This plant-wide distribution system is necessary for delivering the energy produced in photosynthetic areas to all other cells. The primary function of translocation is to distribute the carbon skeletons and energy required for growth, metabolism, and storage across the entire plant body. Unlike the one-way transport of water and minerals that occurs in the xylem, the movement of substances in the phloem is dynamic and can be directed as needed.
The Phloem’s Cargo: Principal Substances Moved
The main substance transported through the phloem is the carbohydrate sucrose, the primary form of energy distributed to non-photosynthetic parts of the plant. Sucrose is a disaccharide made of a glucose and a fructose molecule. Plants prefer sucrose over glucose for long-distance transport because it is less chemically reactive, preventing interference with other metabolic processes during its journey.
The phloem sap is a water-based solution rich in these sugars, often containing a concentration between 10 to 30 percent near production sites. Beyond sucrose, the phloem transports secondary substances crucial for plant function, including amino acids (protein building blocks) and mineral ions.
The phloem also serves as a complex signaling network, carrying regulatory molecules like plant hormones (phytohormones) such as auxins and gibberellins, which regulate growth and development. Messenger RNA (mRNA) and mobile proteins are also found in the phloem sap, traveling long distances to influence gene expression in distant cells.
Understanding Directionality: Why Movement Can Be Upward
The direction of movement in the phloem is not fixed like the flow in the xylem, but depends on the relationship between a “source” and a “sink.” A source is any part of the plant that produces or releases more sugar than it consumes, such as a mature leaf performing photosynthesis. A sink is any part that consumes or stores more sugar than it produces, including roots, developing fruits, or growing shoots.
Translocation always moves substances from a source to a sink, meaning the direction can change depending on the plant’s metabolic needs and stage of development. For instance, sugar produced in a lower, mature leaf may be translocated upward to supply energy to a newly forming flower bud or an actively growing apical meristem at the top of the plant.
The source-sink relationship is dynamic and can reverse. For example, a root or tuber storing starch during the dormant season can become a source in the spring, breaking down its stored starch into sucrose and sending it upward to support the rapid growth of new leaves. This bidirectional, needs-based transport contrasts sharply with the unidirectional flow of water through the xylem.
The Pressure Flow Mechanism
The movement of phloem sap from a source to a sink is explained by the Pressure-Flow Hypothesis. This process relies on a gradient of hydrostatic pressure, created by the active loading and passive unloading of sucrose into the phloem. At the source, sucrose is actively transported into the sieve-tube elements via companion cells, requiring metabolic energy.
This influx of dissolved sugar drastically lowers the water potential inside the sieve-tube elements. Water moves rapidly from the adjacent xylem tissue into the phloem by osmosis, drawn by the high solute concentration. This continuous entry of water builds up substantial turgor pressure, which drives the sap forward toward the sink, where the sugar concentration is lower.
At the sink, sucrose is unloaded and either consumed for energy or converted into an insoluble storage form like starch. The removal of sugar increases the water potential within the sieve-tube elements. Water moves back out of the phloem by osmosis, often returning to the xylem, which relieves the pressure at the sink. This continuous cycle of pressure generation and release results in the bulk flow of phloem sap.