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The following points highlight the seven main mechanisms of phloem transport in plants. The mechanisms are: 1. Pressure Flow 2. Diffusion 3. Cyclosis 4. Electro-Osmosis 5. Transcellular Streaming 6. Microfibrillar Model 7. Correlation of Structure and Function.
Mechanism # 1. Pressure Flow:
According to this hypothesis the metabolites transport is a physical phenomenon (Fig. 26-7). The bulb A has strong whereas B has weak sucrose solution.
The flow of the solution would take place from A to B and consequently water will be forced out of B. This is lucidly indicated by the arrows. This hypothesis is chiefly based on the assumption that the sieve pores are open and free from any resistance.
Mechanism # 2. Diffusion:
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The process similar to the diffusion occurs in the sieve tube system. However, the hypothesis is not accepted even though the proponders suggest a kind of active diffusion.
Mechanism # 3. Cyclosis:
On the basis that sieve tubes are living system it has been assumed by some workers that cyclosis operates in the sieve tubes. However, the presence of cyclosis even in the undeveloped sieve tubes and also the occurrence of the sieve does not fully-support this hypothesis.
Mechanism # 4. Electro-Osmosis:
Fig. 26-8 shows electro-osmotic flow of water and solutes through sieve pores. As will be made out from the diagram, electro-osmosis seems to occur across each sieve plate leaving pressure flow to operate along each sieve tube.
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Spanner, who originally proposed this hypothesis, suggested that ions were maintained in continuous circulation through the sieve pores and back through companion cell or even walls of the sieve tubes. In other words, a potential gradient appears to be set up across the sieve plate. The hypothesis have been turned down on the basis of amount of heat generated during the proposed process.
Mechanism # 5. Transcellular Streaming:
As shown in Fig. 26-9 in this model it is assumed that transcellular strands extend through the sieve tubes and pass through the sieves pores. Further, these strands are bathed in a solution. While the aphids traverse the sieve tubes, it is this fluid which is sucked by them.
The metabolites move in these strands in different directions and the movement requires energy which is made available from the sucrose in these sieve tubes. The rate of calculated movement of metabolites in the strands is far below than actually observed.
Mechanism # 6. Microfibrillar Model:
The diagrammatic assumption of this model is illustrated in Fig. 26-10. It is assumed that the phloem material is a well-organized network of fibrillar material which constituted the structure of the sieve tubes. Possibly wire-like connections occur in the material of the phloem. Near the sieve plates/pores the contraction is more and the material is passed on from sieve tube to the next.
Mechanism # 7. Correlation of Structure and Function:
Of all the hypothesis discussed above, much of the arguments favour pressure flow hypothesis. In summary it is the concentration gradient of osmotically active substances which decrease in the direction of the transport.
All the hypothesis discussed so far take into account the non- availability of resistance from the sieve plate- pores. According to Zimmerman (1976) if sieve plates were providing some resistance they must have disappeared in the course of evolution. He argues that these structures develop in response to injury and are a reaction product.
Several of the phloem biologists have integrated the mass flow hypothesis with the physiological transfer across the plates. If true, it brings the functional attribute of companion cells in focus and also explains the presence of ATP in the sieve tube sap and presence of P-proteins at the sieve pores may represent a sort of physiological mechanism driving solutions across the plates.
On the whole the mass flow hypothesis is also compatible with the bidirectional transport in the sieve tube system. This implies that the metabolites from the leaves can be transported from the leaves to the roots and also to the shoot apex. This experiment of Eschrich utilizing aphid technique in Vicia faba, employed fluorescent dye.
The said author applied this dye dissolved in lanolin in one of the basal leaves and 14C—urea in lanolin to the upper leaf. The aphids were introduced between the two leaves. After some time the honey dew from these aphids was examined. In some aphids honey dew both the fluorescent dye and the 11C—urea, were observed.
These observations strongly support the bidirectional flow of the metabolites in the same sieve tube element keeping in view that one aphid only pierces one sieve tube at a time. Eschrich’s explanation was based on mass flow hypothesis (Fig. 26-11), except that dye flowed in one sieve tube and the urea in another and the two sieve tubes were interconnected at places. Once they intermix they are transported through mass flow.
All the hypothesis taken one by one lead on to believe that each one of them has its good and weak points. It may be borne in mind that the mechanism of transport in the phloem is intriguing challenge and the scientists who ultimately succeed ‘in convincing all interested parties will deserve a Nobel prize for peace.
