Topics: Phloem, Plant physiology, Sieve tube element Pages: 7 (2087 words) Published: February 1, 2013
Translocation of Organic Solutes
Plants synthesize food materials by photosynthesis and store the same in the form of starch grains in the chloroplasts found in mesophyll cells.  Then the found material is converted into simple sugars; then it is transported to regions where it is required – stem apex, young flower buds fruits and storage organs are the sites to which the organic food is translocated.  The site of synthesis and the site of need are separated by time and space.  The structures found between these sites play a significant role in translocation.


Structures responsible for translocation
Girdling experiments involving the removal of a ring of cortex including phloem cells below leaves prevent the flow of food materials towards roots which accumulate at the upper rim o the rig and shows swelling.  Radioactive tracers like 14CO2 sucrose phosphates supplied to lead are found to be translocated through sieve tube elements.  While mealy bugs pierce their stylets into sieve tubes and suck the sap, the white bugs have specialized in feeding on the sieve tube sap.  Thus various studies reveal that the food material is translocated through sieve tubes.  



Rate of translocation
Using radio active tracers at the source of supply like leaves, it is possible to measure the rate of translocation.  Such experiments have shown that, the organic food material is translocated at the rate of 100-200 cm per hour which is relatively 1000 times faster than the rate of passive diffusion.  However, not all plants exhibit the same rate of transport and it varies from plant to plant; added to this various factors also contribute to the rate of movement.  

Direction of movement:
Food material always moves from the leaves of different regions of the plant body organic solutes found in the leaves nearer to the stem apex move upwards, and leaves found at the base move down wards.  But the substances in the leaves found in between, move in both directions.  But the bi-directional movement takes place in the same sieve tube or in different sieve cells is a debatable point.  Some claim that it is unidirectional and others view it as bi-directional, that too in the same cell.

Translocating substances
Analysis of mealy bug’s stylet drops of sieve tube sap indicate that, the major amount of the sap is made up of sucrose.  Though glucose and fructose are found in traces, they are not the tube translocates but they are the degraded products of sucrose in transit.  Along with sucrose, rhamnose, and other sugars are also found to be translocated but in traces.  Amino acids, other organic acids and phytohormones are also found to be translocated though sieve tubes.  

Factors controlling the rate of translocation
1.     Temperature:  Organic solutes that move out of leaves towards their respective destinations are thermo-regulated.  Increase in the temperature of soil increases the rate of movement of materials towards roots decrease in the temperature results in the fall in the rate of movement of solutes in the leaves.  Thus the rate of metabolism of both supply end and receiving and have a direct relationship to the rate of translocation.  

2.     Light:  Light is an important factor that affects the life of the plant in many respects like growth, flowering, photosynthesis, germination, etc. Besides these effects, is has a control over the translocation of food materials.  Generally during day time, only a small amount of food materials is translocated out of the leaves.  Out of this small amount, 2/3rd moves towards stem tip and the rest towards root.  On the contrary, most of the food is translocated from the leaves during night or dark conditions. But more of food is translocated to roots than to stem at nights. The most favorable action spectrum has been found to be red light.  Metabolic state of tissue: Actively dividing tissue needs more energy for its activities hence it needs more food...
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