Mechanism of translocation

Diffusion Hypothesis– according to this hypothesis, the conc. of food material is higher at their site of synthesis –the leaves and their conc. is low in roots. So, the food material diffuses to roots.

Protoplasmic streaming theory – According to Devries and Curtis soluble food materials in sieve tubes move from one end to another end due to cytoplasmic streaming.

It was purposed by Hugo de Vries (1885). Organic solutes which enter the sieve tube are possibly carried by the streaming protoplasm from one end to other of sieve tube. Organic solutes move by diffusion through the pores of sieve plate. Thus the streaming protoplasm acts as conveyer belt. Different substances move in different direction at the same time in the same sieve plate.

Objections:

• The observed rate of protoplasmic streaming is much slower than the rate of translocation.

• Protoplasmic streaming has not been observed in matured sieve plate of most plant.

Protein – Lecithin Theory:

This theory was suggested by bennet Clarke (1956). According to his theory –

• Carrier molecule composed of protein with the phospholipids called as lecithin.
• There are different phospholipid groups present in membrane correspond with the number of competitive group of cations and anions.
• Phosphate group in the phosphatide is regarded as active centers of carrier.
• The ions are liberated on the inner surface of the membrane by decomposition of the lecithin by the enzyme lecithinase.
• The regeneration of the carrier lecithin from phosphatide acid and choline takes place in presence of enzymes viz. choline acetylase and choline esterase and ATP. vi. ATP acts as the energy source for active transport.

Munch mass flow hypothesis:

Munch’s “Mass Flow” Hypothesis- Munch (1930) proposed that the soluble food material in the phloem shows mass flow. Mesophyll cells of leaves synthesize sugars due to which the OP of these cells increases resulting in absorption of water through xylem. Now, the TP of these cells also increases towards upper side and produces mass flow in the protoplasm of sieve tube towards lower side. Thus, the sugars move downwards into the roots where they are utilized during respiration and growth and their conc. lowers down. This process continues.

Objection:

• The main defect of Munch- hypothesis is that it explains only unidirectional downward flow of soluble food materials.

• Mass flow is purely physical process but phloem transport is active process and requires energy.

• Saline content and other fibrils of sieve tube reduce the speed of flow even under high pressure.

Transcellular streaming (Thaine, 1964)

The solute passes in the sieve tube in straight strands which are tubular continuous from one tube to another through sieve pores. These transcellular strands are proteinaceous in nature and acts as bidirectional translocation of solutes through rhythmic expansions. Metabolic energy is used for this purpose.

Objections:

• It failed to explain the induction of ATP in sieve tube.
• Such type of transcellular strands have not been observed under electron microscope.

Phloem loading and unloading

The process by which sugar is loaded into the sieve elements from the chloroplast of the mesophyll cells has been described by Gunning, Pate and co-workers. There are two possible parallel pathways

• The sugar may move through the symplasm, chiefly by diffusion. There are many plasmodesmata connecting the cells all the way from the mesophyll to the sieve element, so that symplastic transport of sugar is possible.

• Sugar may move from the mesophyll to the phloem by diffusing through the apoplasm or extracellular free space (cell-walls of endodermis, pericycle, xylem tracheids and vessels).

Phloem unloading and sink loading

• Whereas phloem loading at the source is an active process (requiring energy), unloading of the phloem at the sink is generally a passive process (i.e., does not require energy).
• In general, levels of sucrose in the cytosol of the sink tissue are kept low by enzymatic hydrolysis to glucose and fructose, and by the icorporation of the resulting sugars into polysaccharides (e.g., starch).
• It has been shown, however, that the uptake of assimilates by wheat endosperm cells and legume embryos is facilitated by an energy dependent membrane process similar to that responsible for phloem loading from the apoplast, although the movement is down a concentration gradient.
• In tomato, there is evidence that suggests that the rate of assimilate import is inversely related to the sucrose content of the fruit, implying that the rate of import depends on the concentration gradient.
• Differences in rate of fruit growth have been associated with sucrose hydrolysis by acid invertase. Sucrose is also converted into glucose and fructose before entering the maize endosperm.
• The rate of assimilate uptake is also influenced by sink metabolism and physical restrictions for the movement of assimilate. In wheat and maize, rate of kernel growth is related to number of endosperm cells and the number of starch granules per cell. Position of a sink relative to other competing sinks may possibly influence rate of assimilate import in terms of proximity (to assimilate supply) and resistance of the pathway (i.e., kernels and distal tip of the maize ear tend to abort first or are frequently smaller than kernels close to the base of an ear).