In this article we will discuss about the translocation of organic substances in phloem.
An organised transport system is essential for the efficient functioning of different organs in a multicellular plant. Nutrients were absorbed from the soil through the roots and are utilized in rest of the organs of the plant. Similarly, metabolites are synthesized in the leaves and are used up in other tissues and organs.
The transport of nutrients and metabolites may occur to short or long distances. The role of xylem in water movement seems certain but the role of phloem in the movement of organic substances is still debated.
Phloem structure and function have attracted the attention of basic and applied botanists because of its importance in growth, mobility of solutes and pesticides, etc. In the early phases simple experiments like ringing and girdling (Fig. 26-1) were carried out with the chief motive to disconnect the normal flow of material through the phloem.
It was observed that growth below the girdling decreased. As early as 1837, structure of phloem was known and also exudation of sugars was demonstrated. However, the presence of sieves at places in the sieve tubes did not answer questions regarding the solute transport satisfactorily.
It was in early 1930s that studies in cotton revealed that phloem contributed towards assimilate transport. Such inferences were later confirmed by radioactive tracers. For instance, through the usage of 14C during photosynthesis, labelled metabolites were spotted out in the phloem cells. To begin with phloem is separated from the xylem of the stem (Fig. 26-2).
This is usually done by making an incision in the bark and inserting a piece of wax paper to form a barrier around the stem between the xylem and the phloem. In this way lateral translocation between the two tissues is prevented. The tracer or radioactive substances (14CO2) are supplied either to the leaves above or below the girdle. Any of the two tissues, either phloem or xylem be interrupted separately.
We may also supply radioactive organic compounds or a solution of radioactive ions. After a specific interval of time the plant is segmented and the label distribution because of translocation is determined. The determinations are done in xylem and phloem above and below the girdled portion. A comparison of the data from the tissues will indicate the nature of translocation.
Other techniques like autoradiography and usage of radioactive phosphorus have also proved quite rewarding. It has been concluded that salts and inorganic substances move upward in xylem while they move downward through phloem. On the other hand, organic substances move up and downward through phloem.
However, organic nitrogen may move up in the xylem, or phloem in trees. Lateral translocation of solutes occurs through active transport or osmosis. In recent years simple but elegent techniques e.g., phloem-feeding aphids have been employed.
The aphids insert their stylet into the sieve tubes. Once the stylet is removed, phloem exudate flows out and can be safely collected for biochemical analysis. Now we know that assimilates from the leaves move through sieve tubes and pass to the rest of the organs.
Several of the plant species exhibit high level of exudation through phloem. These are Ricinu-communis, Cucurbita species, palm species, Yucca, etc. Thus with proper excision it is possible to obtain enough quantities of phloem exudate for chemical analysis. In general it is characterized by high dry matter content and sucrose is the chief metabolite in it.
Also by and large reducing sugars are also absent. About 90% of the exudate matter is made up of sugars. In addition amino acids, some enzymes, ATP, vitamins, growth substances are also present. In Ricinus besides several inorganic compounds, auxin, gibberellin, cytokinin are also found.
In some instances even ABA has been spotted out. Clearly their presence was likely to affect the sugar transport and its utility. Elsewhere we also stated that the flowering stimulus also is transmitted through the phloem.
The movement of metabolites through the phloem indicates a distinct relationship between the source and the sink. Thus, a source is the tissue where there is storage or synthesis of carbohydrate. In other words, such tissues possess more of the metabolites than they are utilized. The sink, on the other hand, is a tissue or an organ which receives these metabolites.
Phloem provides a link between source and the sink. Figure 26-3 shows movement of carbohydrates from source to the sink and indicates place of its origin and destination. It may be mentioned that the over-all growth and development of the plant reflects the partition of assimilates from source to sink in space and time. The size and metabolic activities in the sources and sinks contribute significantly in plant growth and productivity.
Under maximized conditions of source and sink the productivity also goes up. Thus with maximum photosynthesis, efficient transport of assimilates and the yield components also increase. Thus under the constraints of source and sink-limitations, the yield does not increase. These is a reason to assume that compared to the source, the sink is highly important for the increase in yield.
If the leaf is a source, the entry of assimilate into the minor vein and then the phloem is very important. The sucrose is synthesised in the cytoplasm and we do not have sufficient information regarding its transfer to the leaf phloem.
Recent studies have provided evidence that sucrose accumulates in companion cells and then moves into the sieve tubes. In summary sucrose is synthesised in the mesophyll cells and enters the free space of the leaf and moves to the phloem along a concentration gradient.
Thus accumulation of sucrose from the free space is an energy-dependent process. Such an accumulation prior to translocation is called ‘vein-loading’. The latter process seems to provide the driving force for the translocation of building the requisite pressure. The size and age of the leaf plays significant role in the amount of assimilate exported. Also the size and activity of the sink has a great influence in this regard. In C4 plants, high photosynthetic rate is accompanied by high rate of assimilate translocation. Anatomical specialisation plays an important role in this direction. Moreover, in recent years it has also been reported that the speed of translocation is very high in C4 compared in C3 plants.
By and large it is observed that leaves situated lower down the plant transport assimilated to the roots whereas those situated above transport metabolites to apex. The middle situated leaves do so in both the directions. This is not a rule except that one may say that arrangement of source-sink on the plant determines the pattern of distribution of assimilates.
There is hardly any doubt that actively growing organs receive maximal assimilates e.g., elongating internodes, expanding leaves, developing fruits and seeds, etc. The directional movement of metabolites as studied through radioactive substances indicates their bidirectional movement transport.
Thus to begin with in a seedling the young leaf acts as a sink but in mature state it becomes the source. The pattern of leaf development plays an important role in determining the shift from sink to source.
With the onset of reproductive phase now sinks come into being and with the formation of young fruits and seeds the size of sinks elaborate. As is a common experience the roots and also the leaves formation decreases. Similarly in the plants where tuber, corm or rhizomes are produced there is initiation of sinks and elaboration of their size in due course. Again in such species new leaves’ formation declines.
With the shift from the vegetative to the reproductive phase there is also development of anatomical features. The anatomy links the source and the sink. Besides these internal features, the environmental effects like temperature, water, mineral distribution also affect the source sink relationships. Such alterations are caused by the effects of these factors on growth and development of different organs.
Recent studies have also provided evidences that it is possible to manipulate the rate of supply of assimilates by a source and also rate of utilization by the sink. One of the possible explanations offered is that the carbohydrate accumulation along the path from source to sink could be contributory to the accumulation of sugars in the leaves.
The role of hormones in bringing about the coordination between source-sink is also becoming evident in recent years. For instance, there is high accumulation of metabolites at sites where there is high accumulation of growth substances. One possibility could be stimulatory effect of hormones on sink.
In addition to environmental factors, hormones, parasites also disturb the transport of assimilates.
Besides carbohydrates, phloem also contributes towards mineral nutrients distribution. Lateral movement of ions is occurring through phloem. This is especially true of phosphate and potassium ions. On the contrary, calcium ions move through xylem.
Leaves sprayed with phosphate ions transport it through phloem. Such a differential response of the two tissues to the mobility of the different ions is perhaps a method to enforce economy of mineral metabolism within the plant. At any rate the pattern of distribution and transport of minerals in the phloem is similar to that of the sugars.
The recent investigations have also provided evidences regarding the mobility of herbicides and pesticides, etc. through the phloem. Even the synthetic growth regulators are also translocated through this tissue.
Large number of papers have also appeared where quantitative aspects of assimilate transport have been worked out. Such studies have used radiotracers, aphids as the basic techniques to measure the velocity of transport in the phloem. For details a reference be made to the review of Marshall and Sagar (1976).
Besides these facets, mechanisms of phloem transport have received much attention.
In the following a brief account on the structure and function of phloem will be given:
Figure 26-5 is a stereogram portion of sieve tube and companion cell (Fig. 26-4) with their plasmodial connections. Notice abundant mitochondria, ribosomes, Golgi bodies and plastids in the sieve tube whereas companion cell has elongated but large nucleus and ER.
Several divergent views exist on the functioning of the sieve tubes and such a diversity of thinking is partly due to the different views held on its structure and the methods employed in studying it. However, there is unanimity of opinion regarding their non-lignification of walls and delicate structure.
Also all the sieve tubes have sieved pores on the horizontally situated sieve plates. Thus the communication with the adjacent tubes is through sieve pores and is not comparable to the xylem elements.
Figure 26-6 shows diagrammatic representation of possible configurations of sieve elements contents:
a. It shows obstruction of sieve pores by the cytoplasm,
b. It will be observed that the empty lumen of the adjacent sieve tubes is connected through the sieve pores in an unobstructed way,
c. The material in the sieve elements is filamentous and is continuous between the two adjacent sieve tubes,
d. The material in the sieve elements is in the form of filamentous strands which are interconnected through the sieve pores.
These strands or ‘rope’ of filaments are contiguous between the two sieve tubes, e. yet another interpretation is that the contents in the sieve tube are bound by membrane and these minor ‘capillaries’ connect the two adjacent sieve tubes through the sieve pores.
These is a general agreement on one aspect that sieve tubes abound in P-proteins (phloem proteins) to which some of the organelles may be attached. The attachment of the organelles has one advantage that these may not be swayed away and block the sieve pores.
Yet another controversy centres around the nature and structure of the sieve pores. Here again the agreement is atleast on one point that in its functional state the pores are open to allow and facilitate movement of the metabolites. Recent studies employing sophisticated methods have shown clearly that the sieve pores are not always completely plugged or even completely open.
Firstly, sieve tube seems to be a living system which is highly sensitive to the metabolic inhibitors and need energy for their proper functioning. One of the metabolites used in the energy is sucrose. Besides carbohydrates it also transmits large number of diverse substances.
Secondly, the movement velocity in the sieve elements is very high and at specific times it carries large amounts of metabolites.
Similarly the TP of the sieve tube is very high and the metabolites may move in a bidirectional way as well. One of the interesting questions which needs specific answer is whether the elements in the sieve tube move up and down simultaneously.