Soluble products of photosynthesis formed by photosynthetic tissue enter the sieve tubes by active transport. Once in the sieve tubes the speed at which they move along the concentration gradient is too fast to be explained by diffusion alone.
Theories explaining the mechanism of translocation.
The mass flow hypothesis:
Electro osmosis theory
The cytoplasmic streaming hypothesis
a) The mass flow hypothesis:
Mass flow also known as Pressure Flow refers to the bulk transport of materials from one point to another as a result of turgor pressure difference between the two points.. It was proposed by Ernst Munch, a German plant physiologist in 1930. A high concentration of organic substance inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient (osmotic gradient) that draws water into the cells. Movement occurs by bulk flow (mass flow); phloem sap moves from sugar sources to sugar sinks by means of turgor pressure, also known as hydrostatic pressure. A sugar source is any part of the plant that is producing or releasing sugar. During the plant's growth period, storage organs such as the roots are sugar sources, and the plant's many growing areas are sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward).
Copy the Illustration (Functional Approach) page 195 fig 12.17
c) Electro osmosis
This is the movement of ions in an electrical field through a fixed porous electrically charged surface. It follows observations that:-
Ions move in an electrical field to the pole with a charge opposite to their own i.e. + to and vise versa
Ions with a like charge repel each other.
Ions in aqueous solution are surrounded by a shell of water i.e. they are hydrated.
Water and its dissolved solutes e.g. sucrose which surrounds the hydrated ions is bound to the hydration shell by hydrogen bonds.
When hydrated ions move in an electric field water and dissolved solutes will follow such ions.
The sieve plates and phloem proteins are normally negatively charged thus forming the required fixed and charged porous surface.
Thus it is argued that when mass flow occurs downwards through the phloem the anions being repulsed accumulate above the sieve plate so that the cell above the sieve plate becomes negative with respect to that below. A potential difference builds up on the sieve plate and when it reaches a critical value (the threshold) protons (H+) surge from the wall of the upper cell into its cytoplasm lowering its pH and making the cytoplasm above the sieve plate positively charged. This pushes other positive ions (cations) mainly K + by electrical repulsion through the sieve plate from the upper to the lower cell ( i.e electro-osmosis occurs). This surge of hydrated potassium ions carries water molecules and dissolved solutes like sucrose across the sieve plate. Later on using ATP from the mitochondria in the companion cell, proton pumps in the surface membrane of the sieve tube cell quickly pump protons out of its cytoplasm and back into its cell wall. The cell therefore reverts to its original state and the process begins all over again.
This theory does not object Munch's mass flow hypothesis but modifies it to explain among other things:- The great velocity of translocation in the phloem, the great energy demands and the fact that presence of sieve pores does not necessarily increase resistance to flow. It therefore accounts for the need for companion cells and sieve plates in the phloem and this is compatible with Munch's hypothesis by suggesting that solutes move in the phloem by mass flow but the flow is boosted at intervals (at the sieve plates) by electro-osmosis.
Evidence that transport occurs through the xylem and phloem
i) Use of ringing experiments
The active phloem of a woody stem is located on the inside of the bark while the xylem is located in the interior woody region. Hence the removal of a complete strip of bark in a ring around a woody stem known as ringing removes phloem but leaves xylem intact. The results show that the sugar concentration increases immediately above the ring and decreases below it indicating that the downward movement of the sugar is blocked.
ii) Use of 14C labelled sucrose:
A plant is exposed to CO2 labeled with radioactive 14C which becomes incorporated into the products of photosynthesis like sucrose which are subsequently detected in the part of the stem that contains the phloem. By use of autoradiographs of sections through plant tissues it has been shown that radioactive sucrose is carried in the sieve tube cells of the phloem but not in the xylem. Such experiments have to assume that plants transport sucrose molecules containing radioactive carbon atoms (14C) in the same way as those containing only non-radioactive carbon atoms (12C). This technique could be combined with ringing experiments to ascertain the effect of removing phloem on the transport of labeled sucrose.
iii) Use of radioactively labeled ions:
If plant roots are treated with solutions containing such radio-active inorganic isotopes of organic ions and later autoradiographs prepared they show radioactivity in both the xylem and phloem. If however the xylem and phloem are separated e.g. by use of paraffined paper radioactivity is detected only in the xylem.
If plant leaves are treated with solutions containing such radio-active inorganic ions detection of radioactivity in the stem is in the phloem and not in the xylem.
These experiments suggest that ions from the roots tend to travel in the xylem while those from the leaves travel through the phloem though some communication seems to occur between the phloem and xylem.
iv) Experiments using aphid stylets:
It has been observed that when aphids feed, their stylets penetrate into the hosts phloem and sometimes into a single sieve tube from which sugar and other phloem contents are forced up the stylet by the hydrostatic pressure which exists in the sieve tubes into their alimentary canals.
Thus the use of feeding aphids enables an analysis of the contents of phloem sieve tube cells by cutting through the stylet of anaethesised aphids and collecting the exuded sap by micropipetes. Chemical analysis of this sap can then be carried out. Such analysis here shown that phloem sap is an alkaline solution containing a mixture of organic compounds and inorganic ions. Up to 90% of the organic solutes being sugar (mainly sucrose) and up to about 12% being amino acids. Other substances present in the phloem include ATP, preteins (including enzymes), hormones, alkaloids, vitamins and of course water.
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