Plants require a method to distribute the energy they create to every cell. They possess an intricate transport network, analogous to an animal’s circulatory system, which ensures the delivery of necessary materials. This movement of food, primarily sugars, from where it is made to where it is needed or stored is called translocation. This transport is fundamental to a plant’s growth and survival, allowing non-photosynthetic parts like roots and developing fruits to receive nourishment.
The Role of Phloem in Transport
Translocation is the long-distance transport of organic compounds, known as photosynthates, through the plant’s vascular system. The specific tissue responsible is the phloem, one of the two main conducting tissues in vascular plants. The phloem forms a network of continuous tubes that extends from the leaves to the deepest roots, acting as the main highway for nutrient distribution.
Phloem tissue is composed of specialized living cells, primarily sieve-tube elements and their associated companion cells. Sieve-tube elements are long cells arranged end-to-end, forming the transport channel. They feature perforated end walls called sieve plates that facilitate fluid movement, and they lack a nucleus and most organelles at maturity to minimize obstruction to the flow of phloem sap.
Metabolic functions for the sieve-tube elements are carried out by the adjacent companion cells, which are rich in mitochondria and ribosomes. Companion cells play a direct role in the loading and unloading of sugars into the sieve tubes, often through active transport. Translocation contrasts sharply with the function of the other vascular tissue, the xylem, which conducts water and minerals unidirectionally from the roots to the leaves.
Source and Sink Dynamics
Translocation involves the movement of substances from a “source” to a “sink” within the plant. A source is any part that produces or releases sugars in excess of its own needs, such as a mature leaf performing photosynthesis. The primary sugar translocated in the phloem is the disaccharide sucrose, which is ideal for transport.
A sink is any part of the plant that consumes or stores these organic compounds, requiring a net import of sugars. Typical sink tissues include growing points, developing flowers, fruits, and young, non-photosynthetic leaves. Storage organs, such as potato tubers, can act as a sink when accumulating sugar and later reverse roles to become a source when the plant mobilizes reserves for new growth.
The direction of translocation is not fixed and can be multi-directional, moving both up and down the stem depending on the locations of the source and the active sink. Besides sucrose, the phloem also transports amino acids, hormones, and various signaling molecules. The rate of movement is influenced by the sink strength, which is the sink’s ability to draw sugars from the phloem based on its metabolic demand.
The Pressure Flow Mechanism
The mechanism explaining this mass movement is known as the Pressure Flow Hypothesis, or Mass Flow Hypothesis. This theory is driven by a difference in hydrostatic pressure, or turgor pressure, created between the source and the sink ends of the phloem. The process begins at the source, where sucrose is actively loaded into the sieve-tube elements, often with the help of companion cells.
The high concentration of solutes, mainly sucrose, inside the phloem at the source drastically lowers the water potential within the sieve tubes. This causes water to move by osmosis from the adjacent xylem vessels into the phloem, increasing the fluid volume. The influx of water generates a high turgor pressure at the source end of the sieve tube.
This high pressure forces the phloem sap, a water-based solution rich in sugars, to flow by bulk movement toward the sink, where the pressure is lower. At the sink, sucrose is actively unloaded from the sieve-tube elements for growth, metabolism, or storage. The removal of the sugar increases the water potential within the phloem at the sink.
The resulting higher water potential causes the water to diffuse back out of the phloem and into the nearby xylem vessels. This water movement reduces the turgor pressure at the sink, maintaining the pressure gradient that drives the continuous flow of sap. The continuous loading and unloading of sucrose, coupled with the osmotic movement of water, ensures a steady flow of nourishment through the sieve tube.