06 Plant Water Relation - part 11

Transport of food :

All the plant parts require continous supply of food for nutrition and developement.
In higher plants, there is a great differentiation and division of labour.
Chloroplasts are confined to green cells of leaves where food is synthesized.
The non-green parts like root and stem must received food from leaves.
The part where food is synthesized is called source and while part where it is utilized, is called sink.
Food has to travel from source to sink.
This movement of food from one part to the other part, is called translocation of food.

Path of translocation :

Food is to be translocated to longer distances in higher plants. Hence plants must have adequate channels for the transport of food.
Sieve tubes and vessels are structurally ideally suited for longitudinal (vertical) translocation.

The ringing experiment
structure and distribution of phloem
chemical analysis of phloem sap and
use of isotope 14C, clearly point out that the phloem tissue is primarily responsible for flow of food in longitudinal downward direction.

The horizontal (lateral) translocation occurs from phloem to pith or phloem to cortex via medullary rays in the stem.
Food is always translocated in the form of sucrose (soluble form) and always along the concentration gradient from source to sink.
The transport of food occurs in -

1. Vertical translocation :

In vertical (longitudinal) transport, food is translocated in downward direction from leaves (source) to stem and root (sink).
It also occurs in upward direction during germination of seed, bulbils, corm, etc.
Upward translocation also occurs from leaves to growing point of stem, to developing flowers and fruits situated near the ends of the branches of stem.

2. Lateral translocation :

It occurs in the root and stem.
When food is translocated from phloem to pith, it is called radial translocation and from phloem to cortex, it is called tangential translocation.
The transport of food through phloem is bidirectional.
Phloem sap contains mainly water and food in the form of sucrose. But sugars, amino acids and hormones are also transported through phloem.

Mechanism of sugar transport through phloem :

Several mechanisms/ theories like diffusion, activated diffusion, protoplasmic streaming, electro-osmosis, pressure-flow, etc. are put forth.
The most convincing theory is Munch's pressure flow theory or mass flow hypothesis.
Ernst Munch proposed that photosynthetic cell synthesizes glucose. Hence, its osmotic concentration increases.
Due to endo-osmosis water from surrounding cells and xylem, is absorbed.
The cell becomes turgid.
Due to increase in turgor pressure, sugar from photosynthetic cell is forced ultimately into the sieve tube of the vein. This is called loading of Vein.
At the sink end, root cell utilizes sugar and also polymerizes excess sugar into the starch. Its osmotic concentration is lowered.
Exo-osmosis occurs.
Water in the root cell is lost to surrounding cells, thereby decreasing the turgidity of cell.
Turgor pressure is lowered. Hence, a turgor pressure gradient is developed from sieve tube in the leaf to the root cell.
Consequently, food is translocated along the concentration gradient, passively. This is Vein unloading.
At the sink end sugar is used and excess water exudes into the xylem.
Main objection to this theory is that this mechanism does not explain bidirectional transport of food.
More over, according to Munch, pressure flow is purely a physical process.

Yogy's biology