Translocation of Organic Solutes

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The movement of organic food materials or the solutes in soluble from one place to another in higher plants is called as translocation of organic solutes.

In simple terms, transport of sugar in sieve tubes is called translocation.

Food synthesized in leaves is transported to shoot apices, new buds, developing fruits, etc., in upward direction and to root apices in downward direction and to the storage organs. The storage organs and the leaves are known as source and the consumption ends as sink. There is a direct relationship between a source and sink. The translocation of organic food may be downward: from leaves (sources) at higher level to stems and roots (sinks) at a lower level, upward: from a source at a lower level (leaves, storage organs, and germinating seeds) at a lower level to sink (growing, seedling, developing buds, flowers, fruits) at higher level and Lateral: occurs from cells of pith to cortex.

Sugar is actively loaded into the sieve tube element at the source. An area where sugar is made is called source e.g. green leaves and stem. Any area where sugar is stored or used is called sink e.g. young leaves, fruits, seeds and roots. According to pressure flow mechanism water containing sugar in solution flows under pressure through the phloem.

Mechanism of translocation

According to Munch (1903), transport of food material takes place through phloem along a concentration gradient. The organic food (sugars) manufactured in mesophyll cells of leaf increases their osmotic pressure. As a result water from xylem elements and neighbouring cells enters these cells raising their turgor pressure. This forces some of the dissolved food from these cells into the sieve tubes. The cells of the root and storage organs have low osmotic and turgor pressure due to low food concentration. This creates a turgor pressure gradient between the leaf and turgor pressure. As a result of this a mass flow of water containing dissolved organic food takes place from the upper end to the lower end of the plant through phloem.

Image showing mechanism of translocation according to Mucnh hypothesis.

Mechanism of Translocation According to Mucnh Hypothesis.

Image showing mechanism of translocation according to Mucnh hypothesis.

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