In this article we will discuss about the Flow of Source and Sink in Phloem Translocation.
It is the long distance movement of organic substances from the source or supply end (region of manufacture or storage) to the region of utilization or sink. But the source and sink may be reversed depending on the season or need of the plants.
Sugar stored in roots may be mobilised to become a source of food in the early spring when the buds of trees act as sink and require energy for their growth and development. Since the source-sink relationship is variable, the direction of movement of organic solutes in phloem can be upwards or downwards i.e., bidirectional (c.f. unidirectional upwards in xylem).
Directions of Translocation of Organic Solutes:
Translocation of organic solutes can occur in the following directions:
1. Downward Translocation:
It is the most common mode of translocation. The leaves manufacture food in excess of their own requirement. The excess food comes out of leaves and is trans-located in the downward direction to stem (for storage, metabolism, maintenance of its cells and secondary growth, if any) and root system (for storage, growth, metabolism and maintenance).
2. Upward Translocation:
In deciduous plants renewal of growth and development of new foliage are dependent upon upward transport of food from reserves present in the roots and stems. Growth of the stem apices, formation of flowers, fruits, etc. require the movement of assimilates from leaves in an upward direction.
3. Lateral Translocation:
It is little except when source and sink lie on the opposite sides.
4. Bidirectional Translocation:
Rabideau and Burr (1945) found that labelled carbohydrates moved out of the leaves in both upward and downward directions. The two types of translocation are believed by many workers to occur in different sieve tubes.
Differences between Diffusion and Translocation
Pathway of Translocation:
The most common organic nutrient trans-located in plants is sucrose. The channels of transport are sieve tubes (in flowering plants) and sieve cells (in non flowering vascular plants) of phloem. It was proved for the first time by Czapek (1897).
The evidences are as follows:
1. There are only two paths for long distance translocation, tracheary elements and sieve tubes. The former are dead while the latter are living. Translocation of organic solutes seems to be through sieve tubes because it is inhibited by steam girdling which kills living cells.
2. In girdling or ringing experiments , a ring of bark is cut from the stem. It also removes phloem. Nutrients collect above the ring where the bark also swells up and may give rise to adventitious roots (Fig. 11.40). Growth is also vigorous above the ring.
The tissues below the ring not only show stoppage of growth but also begin to shrivel (Roots can be starved and killed if the ring is not healed after some time. Killing of roots shall kill the whole plant) clearly showing that bark or phloem is involved in the movement of organic solutes which occurs in one direction, i.e., towards root.
Girdling experiments are performed in fruit trees to make more food available to fruits. However, the rings are kept narrow and cambium is not touched so that the incision heals up after some time. (Girdling experiments cannot be carried out in monocots and dicots with bi-collateral bundles because of the absence of a single strip of phloem).
3. Mason and Maskell (1928) inserted a wax paper between phloem and xylem. Parts of the bark were also removed except for a narrow strip. They found evidence that the organic solutes passed through the narrow strip of bark containing the phloem.
4. By means of aphid stylets, Weatherley (1959) found that sieve tubes contained a concentrated solution of organic substances under a pressure.
5. Radio-autographs show that assimilates with incorporated radioactive elements pass out of the leaves and travel towards the sink ends through phloem.
6. Sieve tubes contain a high organic solute content— 5-10% soluble carbohydrates (mostly surcose), about 1% nitrogenous compounds (mostly amino acids), organic acids besides traces of hormones and other organic solutes. Sucrose is most suitable form of carbohydrate translocation as it is non-reducing and chemically stable. It does not react with other substances during translocation.
7. Tonoplast is absent in sieve tube cells so that cytoplasm is in direct contact with vacuolar contents.
8. Sieve tube cytoplasm can tolerate high concentration of solutes without being plasmolysed.
9. Cytoplasm of one sieve tube cell is continuous with that of the adjacent sieve tube cells through sieve plates so as to form continuous filaments. The centre of sieve tube cells is empty with cytoplasmic strands being peripheral.
10. Sieve tube cells possess granules and filaments of P-protein with ATPase activity.
11. Relatively large amounts of organic solutes are trans-located. The rate of translocation of organic nutrients is such that a sieve tube must be refilled 3-10 times per second. Crafts and Lorenz (1944) found that a pumpkin fruit receives 5500 gm of the organic solution in 33 days with a rate of 0.61 gm. of dry weight or translocation of 110 cm per hour.
12. Lateral movement from phloem to living cells or from source to phloem occurs through transfer cells and symplasm.
Mechanism of Phloem Translocation:
Several theories have been put forward to explain the mechanism of translocation of organic nutrients through the phloem e.g., diffusion, activated diffusion, protoplasmic streaming, interfacial flow, elect osmosis, trans cellular strands, contractile proteins, mass flow. Mass flow hypothesis is the most accepted one.
Mass Flow or Pressure Flow Hypothesis:
It was put forward by Munch (1927, 1930). According to this hypothesis, organic substances move from the region of high osmotic pressure to the region of low osmotic pressure in a mass flow due to the development of a gradient of turgor pressure (Fig. 11.41).
This can be proved by taking two interconnected osmometers, one with high solute concentration and the other with little osmotic concentration. The two osmometers of the apparatus are placed in water (Fig. 11.42). More water enters the osmometer having high solute concentration as compared to the other.
It will, therefore, come to have high turgor pressure which forces the solution to pass into the second osmometer by a mass flow. If the solutes are replenished in the donor osmometer and immobilised in the recipient osmometer, the mass flow can be maintained indefinitely.
Sieve tube system is fully adapted to mass flow of solutes. Here the vacuoles are fully permeable because of the absence of tonoplast. A continuous high osmotic concentration is present in the source or supply region, e.g., mesophyll cells (due to photosynthesis).
The organic substances present in them are passed into the sieve tubes through their companion cells by an active process. A high osmoticconcentration, therefore, develops in the sieve tubes of the source. The sieve tubes absorb water from the surrounding xylem and develop a high turgor pressure (Fig. 11.43).
It causes the flow of organic solution towards the area of low turgor pressure. A low turgor pressure is maintained in the sink region by converting soluble organic substances into insoluble form. Water passes back into xylem.
(i) Sieve tubes contain organic solutes under a pressure because an injury causes exudation of solution rich in organic solutes,
(ii) Direction of flow of organic solutes is always towards concentration gradient. A fall of 20% concentration was observed by Zimmermann (1957) over a distance of eight metres,
(iii) Defoliation of shoots causes disappearance of concentration gradient in its phloem,
(iv) Bennet (1937) observed viruses to move in phloem in a mass flow in the direction of movement of organic solutes at a rate of about 60 cm/hr.
(v) All the substances dissolved in sieve tubes are found to move with the same velocity with minor differences,
(vi) The hypothesis can be simulated experimentally.
(i) Vacuoles of the adjacent sieve tube cells are not continuous. The cytoplasm present near the sieve plates exerts resistance to the mass flow,
(ii) Catalado (1972) have observed that the rate of flow of water (72 cm/hr.) and solutes (35 cm/hr.) to be different in the same sieve tube,
(iii) Phloem transport is not influenced by water deficit,
(iv) The cells at the source end of mass flow should be turgid but they are often found to be flaccid in case of germinating tubers, corms, etc..
Some comments and discussion in a recent post about stratigraphy motivated me to finish this post about the future of sedimentary geology, which I started a few months ago but never finished.
In January 2008, Nature (#451) included a supplement highlighting International Year of Planet Earth, (IYPE) which is a joint initiative by the United Nations Educational, Scientific, and Cultural Organization (UNESCO) and the International Union of Geological Sciences (IUGS).The supplement contains about 15 essays spanning such topics as deep Earth composition, history of geology, and several about climate-related topics. One of the essays (the only one I’ve actually read), “From landscapes into geologic history” by Philip Allen, covered a broad range of Earth surface-related topics with few words and, in my opinion, did it very well.
The fact that I recently completed a graduate degree combined with the timing of this essay inspired me to think about the future of my discipline – sedimentary geology. The best way to start this post is with the opening statement of Allen’s essay:
Erosional and depositional landscapes are linked by the sediment-routing system. Observations over a wide range of timescales might show how these landscapes are translated into the narrative of geological history.
The future of many scientific disciplines, some say, is integration with other disciplines. How do different components of a system interact? Allen is advocating a more comprehensive “systems” view that has gained increasing attention, at least since I’ve been paying attention (~10 years now).*
A classic paper I’ve blogged about by Harry Wheeler (1964) touched upon this notion when he stated:
…stratigraphers must concern themselves with the interpretation of degradational as well as aggradational patterns. Conversely, the geomorphologist who ignores depositional phenomena is equally delinquent.
Wheeler mentions a couple specialties by name — stratigraphy and geomorphology. Almost 45 years later we still have these sub-fields, as we should, but I think we are appreciating the overlap and interdependence of systems more and more. That appreciation and interest is motivating researchers to investigate and unravel the interactions of the systems.
This is essentially what Allen’s essay is about … and he brings in more scientific specialties in addition to stratigraphy and geomorphology:
The growing field of study of Earth surface processes is uniting the normally disparate disciplines of solid Earth geology, geomorphology and atmospheric and oceanographic sciences.
Earth surface processes is indeed a growing field. One might argue that it’s just a different name for what we’ve already been doing. Perhaps it is … but the formalization of it (as evidenced by a journal with that name) is relatively new.
Having the discussion of what is and what is not a new discipline and how to define that is not what I’m interested in with this post. I am more interested in the bigger picture. We are we headed?
Part of this growing appreciation and interest in the entire Earth surface system, especially with regards to erosion, transport, and deposition of sediment, is an approach that has been termed source-to-sink. I’m not sure where/when that term was coined … it doesn’t really matter. Researchers have certainly been investigating and discussing source-to-sink (or less alliteratively, source-to-basin) aspects of modern and ancient sedimentary systems for a long time. But, again, it seems to me that it is gaining appreciation as an approach in and of itself more in recent years.
What do we mean by source-to-sink? One way to visualize this is to think about a single grain of sand. Let’s say you got your typical quartz sand grain that is weathered out of an aging granitic outcrop. What happens along the path from liberation to ultimate deposition?
Geomorphologists primarily look at the net-erosional parts of the system (the ‘source’) and try to get clues from the landscape about tectonics. What is the rate of erosion? How does that relate to rates of uplift? Can we study the long-term evolution of a uplifting/eroding landscape and deduce the local as well as far-field tectonic history? How does the climate affect patterns of denudation? Where, when, and why does that sand grain come loose and start making it’s way down system?
Sedimentary geologists are primarily concerned with the net-depositional parts of the system, or the ‘sink’. How many times was that sand grain deposited along the way? How long did the whole journey take? How long did it remain in an intermediary location? Where is the final site of deposition before it’s buried and put into the stratigraphic record – in a river? a delta? the deep sea? Why that location for that system? What does that tell us about the system as a whole? And so on, and so forth.
Allen expresses these ideas, more eloquently than I can, when he states:
The mass fluxes associated with the physical, biological and chemical processes acting across the landscape involve the transport of particulate sediment and solutes. Sediment is moved from source to sink — from the erosional engine of mountainous regions to its eventual deposition — by the sediment-routing system. The selective long-term preservation of elements of the sediment-routing system to produce the narrative of the geological record is dictated by processes operating in Earth’s lithosphere.
What’s compelling about this as an approach is the potential for improving our understanding with respect to controls on sedimentation. Sedimentary geologists have for a long time discussed how external forcings such as climate, sea level, and tectonics (to name a few) control patterns of sedimentation. But, I would argue that we still lack a true understanding of the interactions of various external forcings. [I have a paper in press right now from some work I did on a Holocene system that attempts to address this … I’ll post about it when it’s out].
Allen also touches on factors that are intrinsic to the sedimentary system itself:
…the sediment flux signal from the contributing upland river catchment is likely to be transformed, phase-shifted and lagged by the internal dynamics of the routing system. If this is the case, how can we possibly decipher the forcing mechanisms for a particular record in deposited sediment without knowing how it has been transformed by the internal dynamics of the sediment-routing system?
In other words, if the “noise” related to the transport and deposition of our single sand grain overpowers the signal from the external forcing (e.g., climatic fluctuation) can we even detect it? This is the question at this point. I saw a talk by University of Minnesota geologist, Chris Paola, at a conference in April that concluded that, at least in some cases (and based on scaled-down experiments), the internal dynamics will indeed completely destroy the original signal.
So, where do we go from here? If we put our efforts towards characterizing and unraveling the interactions of modern and geologically-recent (e.g., late Pleistocene-Holocene) source-to-sink systems – where we have a relatively good handle on timing of paleoenvironmental changes – can we apply what we will learn to the ancient? Allen articulates this question quite well when he states:
Time transforms sediment-routing systems into geology, and like history, selectively samples from the events that actually happened to create a narrative of what is recorded. Progress in understanding modern sediment-routing systems now leaves us poised to answer the important question: how do we simultaneously use the modern to generate the time-integrated ancient, and ‘invert’ the ancient to reveal the forcing mechanisms for change in the past?
This is, of course, the fundamental goal of looking at the sedimentary record in the first place (and has been for centuries). Will a source-to-sink approach and integration with other Earth surface processes lead us down a research path worth following? I think so … but, then again, this is what I do, so perhaps I’m biased in my thinking. The real challenge, which is something almost all science will face in this century, is how to effectively integrate and synthesize complex systems while not losing the rich details.
In my opinion, this is very challenging … but it is also very exciting.
* In terms of a “systems” view, I’m speaking within the context of research activities … that is, conducting new science. A somewhat separate discussion would be implementation of a systems view within the context of geoscience education. This is another can of worms that perhaps my geoblogger colleagues who are educators (Kim, Ron, Callan, etc.) might want to start a thread about. Personally, I’m a strong advocate of appreciating a systems view in scientific investigation … when it comes to education, however, I think the core disciplines of geology (e.g., mineralogy, petrology, structural geology, sedimentology, etc.) are absolutely, positively necessary. In my opinion, an undergraduate requires solid training and experience in the nitty-gritty before integration and interdependence of systems can truly be appreciated. But … like I said, let’s save that for another time … or I’ll tag a willing geoblogger with that. Anybody? UPDATE: Kim over at All My Faults… started a thread about teaching Earth systems here.
Allen, P. (2008). From landscapes into geological history Nature, 451 (7176), 274-276 DOI: 10.1038/nature06586