The transport of products of photosynthesis (also called assimilates or photosynthates) from Source to Sink in plants is called translocation. The Source is a net exporter of assimilates (e.g., leaves), while the Sink is a net importer of assimilates (e.g., tubers, fruits, roots, stems). A storage tissue can become a Source when stored material is mobilized and exported.
3.1.1 Translocation Mechanism
Translocation occurs in phloem tissue via sieve elements (with associated companion cells) and metabolic energy is required for this process. Rates of movement in the phloem can sometimes exceed 1 m per hour and
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substances can move in different directions at the same time. Movement by diffusion is much too slow to account for such rapid rates of movement observed in the phloem. Some movement in individual sieve elements may be explained by cytoplasmic streaming—this is a rotational movement of the cytoplasm around the periphery of many cells due to the action of microfilaments. Cytoplasmic streaming is readily observed in young but not in mature sieve elements, and observed rates of movement are still too slow to explain the rapid rates of movement that can occur in phloem tissue.
3.1.2 Munch’s Pressure-Flow Theory
The most widely accepted explanation of the translocation mechanism is given by Munch’s Pressure-Flow theory. This theory suggests that movement in the phloem is due to mass flow along a turgor (hydrostatic) pressure gradient.
Assimilates enter the sieve tubes of the phloem by active transport (phloem loading) at the Source (e.g., leaf). The osmotic potential falls as solutes accumulate in the sieve elements at the Source. Water is then dragged in by osmosis from surrounding tissue and ultimately from the xylem. The pressure increases as water enters the sieve tube leading to the mass flow of water and dissolved substances along the sieve tube under a hydrostatic pressure gradient.
Assimilates are removed from the sieve tubes (unloading) at a Sink (where assimilates are utilized). The water potential of the solution in the sieve tube increases as dissolved substances move out and the solution becomes more dilute. Water moves out when the water potential of the solution in the sieve tube becomes higher than that of the surrounding cells. This leads to a fall in the hydrostatic pressure at that location in the sieve tube, which serves to bring more phloem sap toward the active Sink.
Water flowing out of the sieve tubes at the Sink will ultimately return to the xylem. Translocation is therefore linked to water flow in xylem (Fig.
6.4).
There are two mechanisms that can prevent uncontrolled loss of phloem sap in cases where the sieve tube is damaged:
1. Formation of P-protein (phloem protein) plugs. P-protein filaments form a fine network next to the plasma membrane of sieve elements. If the sieve tube becomes damaged, the P-protein (along with other contents of the phloem) surges toward the cut end due to the internal hydrostatic pressure. The tangled mass of protein filaments and protein bodies form a “P-protein” plug, which helps to seal the cut end of the sieve tube. However, not all flowering plants have P-proteins.
2. Proliferation of callose. This is a carbohydrate polymer that is synthesized by the plasma membrane especially under stress conditions. Callose is deposited into the tangled mass in the sieve pores of damaged sieve tubes, which serves to seal off the damaged sieve elements. Callose proliferates when there is a pressure drop, which helps to seal the sieve pores.
Figure 6.4: Diagram illustrating the Pressure-Flow theory
SELF-ASSESSTMENT EXERCISE (SAE)
1. Explain the mechanism of translocation
2. State the two mechanisms that can prevent uncontrolled loss of phloem sap in cases where the sieve tube is damaged
4.0 CONCLUSION
In plants, food is prepared by the leaves by the process of photosynthesis.
The food prepared by the leaves is in the form of simple sugars (glucose).
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No other part of the plant can prepare food. So, all the parts of a plant require food for getting energy, maintenance and growth. That is why; the food prepared by the leaves is transported to all the other parts of a plant through phloem. The transportation of food from the leaves to other parts of the plant is called translocation. The food made by the leaves of the plant is necessary to be translocated to all the other parts of the plant so that every part of the plant can utilize the food for obtaining energy as well as for growth and repair.
5.0 SUMMARY
In summary, translocation is the process within plants that functions to deliver nutrients and other molecules over long distances throughout the organism. Translocation occurs within a series of cells known as the phloem pathway, or phloem transport system, with phloem being the principal food-conducting tissue in vascular plants. Nutrients are translocated in the phloem as solutes in a solution called phloem sap. The predominant nutrients translocated are sugars, amino acids, and minerals, with sugar being the most concentrated solute in the phloem sap. Various cell types utilize these nutrients to support their requirements for life or store them for future use. Because translocation is responsible for the delivery of nutrients to developing seeds and fruits, this process is critical to the achievement of optimal crop yield.
6.0 TUTOR-MARKED ASSIGNMENT