roots, tubers, flowers and fruits and this process is termed as phloem unloading . It consists of three steps: . Sieve element unloading : Sucrose leave from sieve elements. .
Short distance transport : Movement of sucrose to sink cells. . Storage and metabolism : The final step when sugars are stored or metabolized in sink cells. .
. Mechanism of Translocation Several hypotheses have been proposed to explain the mechanism of translocation. Some of them are given below: . Diffusion hypothesis As in diffusion process, this theory states the translocation of food from higher concentration (from the place of synthesis) to lower concentration (to the place of utilization) by the simple physical process.
- - . . PM draws water from the reservoir by endosmosis. ii.
Due to the continuous entry of water into chamber “A”, turgor pressure is increased. iii. Increase in turgor pressure in chamber “A” force, the mass flow of sugar solution to chamber “B” through the tube “T” along turgor pressure gradient. iv.
The movement of solute will continue till the solution in both the chambers attains the state of isotonic condition and the system becomes inactive. v. However, if new sugar solution is added in chamber “A”, the system will start to run again. A similar analogous system as given in the experiment exists in plants: Chamber “A” is analogous to mesophyll cells of the leaves which contain a higher concentration of food material in soluble form.
In short “A” is the production point called “ source ”. Chamber “B” is analogous to cells of stem and roots where the food material is utilized. In short “B” is consumption end called “ sink ”. Tube “T” is analogous to the sieve tube of phloem.
Mesophyll cells draw water from the xylem (reservoir of the experiment) of the leaf by endosmosis leading to increase in the turgor pressure of mesophyll cell. The turgor pressure in the cells of stem and the roots are comparatively low and hence, the soluble organic solutes begin to flow en masse from mesophyll through the phloem to the cells of