Translocation: Plant Physiology and Phloem Water Potentials

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Transport system in a plant is concerned with the movement of materials from source to sinks. According to Lacher .W 1985, a source is a region of a plant, which is manufactures sugars during photosynthesis and supplies materials of any kind to the transport system (e.g leaves) and a sink as a region where the sugars and minerals are being removed or lost from the system to be used up or stored (for example the root and production of fruit). Transport of soluble products of photosynthesis is called translocation. Translocation of soluble products of photosynthesis occurs in the part of the vascular tissue known as the phloem. The phloem is the principle food conducting tissue associated with xylem in the vascular system. The basic components of phloem are sieve elements, companion cells and phloem parenchyma and phloem fibres. It is placed outside the cambium of the vascular bundle (www./http://infotrac.thomsonlearning.com). It is well known that soluble products of photosynthesis formed in the photosynthetic tissue, enter the sieve tubes by active transport. The movement of materials once they are in the sieve tubes is still under debate as to which and what is the driving force. Although sugars and amino acids tend to move along concentration gradients the speed at which they travel is too fast to be explained simply by diffusion. There presently four proposals of translocation mechanisms; the Munch model, streaming model, electro-osmotic model and the peristaltic tubule model (Sperry J.S; 2003). In 1930 Ernst Munch proposed the Pressure Flow hypothesis also known as the Mass- flow hypothesis which states that a flow of solution in the sieve elements is driven by an osmotic pressure gradient that is generated between source and sink. The pressure gradient is established as a consequence of the phloem loading at the source and phloem unloading at the sink (Knox et al, 2005). At the source region sugar is actively loaded into the sieve tube of the phloem. This...
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