Let us make an in-depth study of the translocation of organic solutes in higher plants. After reading this article you will learn about 1. Directions of Translocation 2. Path of the Translocation of Organic Solutes 3. Mechanism of Translocation through Phloem and 4. Other Theories of Mechanism of Translocation.
The movement of organic food materials or the solutes in soluble from one place to another in higher plants is called as translocation of organic solutes.
Translocation of organic solutes is essential in higher plants because:-
(i) In higher plants, only the green parts can manufacture food and it must be supplied to other non-green parts for consumption and also for storage.
(ii) During the germination of the seeds, the insoluble reserve food material of the seed is converted into soluble form and is supplied to the growing regions of young seedling till it has developed its own photosynthetic system i.e., leaves.
Translocation of organic solutes always takes place from the region of higher concentration of soluble form i.e., the supply end (source) to the region of lower concentration of its soluble form i.e., the consumption end (sink).
Directions of Translocation:
Translocation of organic solutes may take place in the following directions:
1. Downward Translocation:
Mostly, the organic food material is manufactured by leaves and is trans located downward to stem and the roots for consumption and storage.
2. Upward Translocation:
It takes place mainly during the germination of seeds, tubers etc. when stored food after being converted into soluble form is supplied to the upper growing parts of the young seeding till it has developed green leaves.
Upward translocation of solutes also takes place through stem:
(i) To buds which resume growth in the spring
(ii) To developing leaves situated closer to its apex
(iii) To opening flowers and developing fruits which are situated near the ends of the branches.
3. Radial Translocation:
Radial translocation of organic solutes also takes place in plants from the cells of the pith to cortex.
Path of the Translocation of Organic Solutes:
1. Path of Downward Translocation:
Downward translocation of the organic solutes takes place through phloem.
This view is supported by the following evidences:
(i) Tissues other than phloem cannot account for downward translocation:
Ascent of sap takes place through xylem, so naturally organic solutes are not trans located through it. The cells of the ground tissue are structurally neither suitable for translocation nor they contain soluble organic solutes which could be trans located.
These cells usually have organic solutes in insoluble form. Thus, only phloem is left which can account for translocation of the organic solutes. The end to end arrangement of the sieve tubes in phloem whose cross walls are perforated by sieve pores form continuous channels and is best suited for it (Fig. 15.1). Further, in Cucurbits where the leaves are usually larger, the stem contains bicollateral vascular bundles to cope with the rapid translocation of food materials through it.
(ii) Blocking of phloem:
Translocation of food materials stops when sieve pores are plugged due to the deposition of a chemical compound, the callose.
(iii) Chemical analysis of phloem sap:
Cells of phloem contain large quantities of organic solutes mainly sugars such as sucrose in soluble form.
(iv) Isotopic studies:
It has been observed that if a leaf of the plant is allowed to photosynthesize in presence of labelled 13CO2 the translocation of carbohydrates labelled with 13C isotope takes place through the stem. But, if some segments of the stem including phloem were killed by hot wax, no movement of carbohydrates could be detected.
(v) Ringing experiment:
If a ring of bark including phloem is removed from the stem of a plant, the downward translocation of food material stops and food material accumulates just above the ring. As a result after some time, the tissue above the ring swells and may even develop adv. roots (Fig. 15.2) while the lower parts of the plant below the ringed portion gradually dry up.
2. Path of upward translocation:
There has been controversy regarding the path of upward translocation of organic solutes in plants. Although translocation of organic solutes takes place through phloem, but under certain conditions it may take place through xylem.
3. Path of Radial Translocation:
Radial translocation of organic solutes from pith to cortex takes place through medullary rays.
Mechanism of Translocation through Phloem:
Various theories have been put forward to explain the mechanism of phloem conduction but they are not fully satisfactory. Among them Munch’s (1930) hypothesis is most convincing.
Munch’s Mass Flow or Pressure Flow Hypothesis:
According to this hypothesis put forward by Munch (1930) and elaborated by Craft (1938) and others, the translocation of organic solutes takes place en mass through phloem along a gradient of turgor pressure from the region of higher conc. of soluble solutes i.e., supply end to the region of lower conc. i.e., consumption end. The principle involved in this hypothesis can be explained by a simple physical system as shown in the Fig. 15.3.
Two membranes X and Y permeable only to water and dipping in water are connected by a tube T to form a closed system. Membrane X contains more concentrated sugar solution than in membrane Y. Due to higher osmotic pressure of the concentrated sugar solution in membrane X, water enters into it so that its turgor pressure is increased. The increase in the turgor pressure results in mass flow of sugar solution to membrane Y through the tube T till the concentration of sugar solution in both the membranes is equal.
If in the above system it could be possible to maintain continuous supply of sugars in membrane X and its utilization or conversion into insoluble form in membrane Y, the flow of sugar solution from X to Y will continue indefinitely.
According to Munch’s hypothesis, a similar analogous system for the translocation of organic solutes exists in plants. As a result of photosynthesis, the mesophyll cells in the leaves contain higher concentration of organic food material in them in soluble form and correspond to membrane X or supply end. The cells of stem and roots where the food material is utilized or converted into insoluble form correspond to membrane Y or consumption end. While the sieve tubes in phloem which are placed end to end correspond to the tube T.
Mesophyll cells draw water from the xylem of the leaf due to higher osmotic pressure and suction pressure of their sap so that their turgor pressure is increased. The turgor pressure in the cells of stem and the roots is comparatively low and hence, the soluble organic solutes begin to flow en mass from mesophyll through phloem down to the cells of stem and the roots under the gradient of turgor pressure. In the cells of stem and the roots the organic solutes are either consumed or converted into insoluble form and the excess water is released into xylem through cambium (Fig. 15.4).
Demerits of Munch’s Hypothesis:
(1) This hypothesis accounts for the translocation in only one direction at a time, although there may be simultaneous upward and downward translocation of solutes.
(2) There is considerable doubt regarding the magnitude of the turgor pressure at the supply end which may not be sufficient enough to overcome the resistance offered by the sieve plates in the translocation of solutes through sieve tubes.
(3) Turgor pressure may not always be higher at the supply end.
(4) This hypothesis is based on purely physical assumptions and does not take into account the fact that whole of the translocation process is dependent upon the plant’s metabolism and the metabolic energy.
Other Theories of Mechanism of Translocation:
(1) Protoplasmic Streaming Theory:
According to this hypothesis first proposed by De Vries (1885) and later supported by Curtis (1935) protoplasmic streaming occurs in sieve tube elements of phloem and the solute molecules caught up in the circulating cytoplasm are carried from one end to the other end of sieve tube from where they diffuse to the next sieve tube elements through the cytoplasmic strands in the sieve plates.
This theory was supported because:
(i) It accounted for simultaneous movement of solutes in both upward and downward directions in the same sieve tube and
(ii) That the factors like low temperature and oxygen deficiency which retard protoplasmic streaming also checked the translocation of solutes.
But, the strongest objection against this theory is that the protoplasmic streaming has not been observed in mature sieve tube elements.
Protoplasmic theory has recently been re-emphasized by Cany (1952) and Thaine (1962, 64) who observed the ‘trans cellular strands’ (cytoplasmic strands) traversing the sieve tube elements in petiolar tissue.
They also observed:
(i) The movement of solute particles from one sieve tube element to another and
(ii) Particles moving in opposite directions in adjacent trans cellular strands in the same sieve tube element.
(2) Interfacial Flow Hypothesis:
According to this hypothesis proposed by Van den Honert (1932) the solute particles could move along the interfaces such as between the vacuole and the protoplast. But this theory did not find support, the main objection against this theory being (i) the lack of evidences in support of such a mechanism in plants and (ii) that the plant membranes are not static but constantly changing.
(3) Activated Diffusion Hypothesis:
According to this hypothesis put forward by Mason and Phillis (1936) the protoplasm of sieve tube elements in some way hastens the diffusion of the solutes probably (i) by activating the diffusing molecules or (ii) by decreasing the resistance of the protoplasm to their diffusion. Although they could think of the participation of the respiratory energy during this process but were unable to give details of such a mechanism, and hence, this theory also has not been accepted.
(4) Electro-Osmotic Theory:
According to this theory put forward by Fensom (1957) and Spanner (1958) the translocation of solutes through sieve tubes takes place probably due to an electric potential across the sieve plates. The electric-potential could be maintained by the circulation of K+ at the sieve plates. But due to lack of evidences this theory could not be elaborated further.