transport by phloem

Transport by phloem

Organic solutes such as glucose, sucrose produced during photosynthesis are translocated through phloem tissue.

The transport of photosynthates from the production centres (leaves) to the consumption centres (apices, roots, fruits, buds, tubers) through phloem is called translocation of organic solutes or long distance transport.

Translocation through phloem occurs in upward, downward and radial directions from the source (leaves) to the sink i.e., consumption centres.

Chemical analysis of the phloem sap revealed the presence of sugars upto 90%.

Sucrose constitutes 5-15% of the total sugars.

Other sugars like raffinose (triose), stachyose (tetrose) and verbascose (pentose) are also present in small quantities.

This analysis strongly suggests that phloem is the tissue concerned with the translocation of organic solutes.

Pure phloem sap may be collected by using sap sucking aphid

Concept Builder

Rabideau and Burr (1945) supplied C¹⁴O₂ to a leaf during photosyinthesis (Tracer technique). Sugars synthesized in this leaf got labelled with ¹⁴C (tracer). Presence of labelled sugars (radioactivity) in the phloem showed that solutes are translocated through phloem.

Theories of Translocation of Organic Solute

1. Protoplasmic streaming hypothesis: This theory was proposed by a Dutch botanist, Hugo de Vries in 1885 and was supported by C.F. Curtis (1935). According to this theory:

(a)  Protoplasm of the sieve tubes show continuous streaming from one end to the other.

(b)  Organic solutes (sugars) which enter the sieve tube are passively carried by the streaming protoplasm from one end of the sieve tube to the other.

(c)  Solutes move from one sieve tube to the next sieve tube by diffusion through the pores of the sieve plate. Thus, the streaming protoplasm acts as a conveyer belt or two-way escalator.

(d)  Different substances move in different directions at the same time in the same sieve tube.

2. Pressure flow or mass flow hypothesis: This theory was proposed by Munch (1929) and elaborated by Crafts (1938). According to this theory, organic solutes are translocated "en masse" through the sieve tubes from the supplying end or source (leaves) to the consumption end or sink (roots, fruits, tubers).

(a) Mesophyll cells synthesize sugars during photosynthesis. As these get dissolved in cell sap, the osmotic concentration and DPD of mesophyll cells increases (Ψ_w decreases).

(b) Water enters the mesophyll cells from the xylem. Hence, the turgor pressure or pressure potential (Y_p) of the mesophyll cells increases.

(c) Sugars dissolved in water move from mesophyll cells into the symplast system of sieve tubes through companion cells (Phloem loading).

(d) Solutes are carried "en masse" through the symplast to finally reach the consumption centres.

(e) At the consumption end, food materials (solutes) are either used up (as in roots) or are stored in an insoluble form (as in fruits, tubers). Hence, the osmotic concentration and, consequently, the turgor pressure in these cells will be low.

(f)Thus, a continuous turgor pressure gradient gets established across the symplast between the cells of the source and the cells of the sink.

(g) Water returns to the source (leaf) through the apoplast system

Objection to mass flow hypothesis :

(a) Bidirectional transport of organic solute in the same sieve tube needs explanation.

(b) Slime content and other fibrils of the sieve tube reduce the speed of flow of solutes even under high pressure.

(c) Mass flow is not a purely physical process as described by Munch because phloem cells utilised 0.1-0.5 percent of sucrose translocated through them. This is an evidence to show that phloem translocation (both loading and unloading) is an active process and requires metabolic energy.

Factors affecting Translocation of Solutes

Temperature:

Optimum temperature for translocation is 20°C and 30°C. The rate of translocation increases with increase in temperature. The temperature influences the root more than the shoot, since it acts as sink for the sugars.

Light:

The root/shoot dry weight ratio increases with increased light intensities. This indicates that translocation to root increases as compared to shoot when light intensity is increased.

Metabolic inhibitors:

The metabolic inhibitors can inhibit carbohydrate translocation. These include dinitrophenol (DNP), arsenite, azide, fluoride and hydrogen cyanide.

Mineral deficiencies:

The absorption and translocation of sucrose by a leaf is facilitated by boron. It helps sucrose to move easily through the cell membranes in the form of boron-sucrose complex.

Hormones:

Sucrose is much more efficiently translocated when growth regulators are applied such as kinetin, 1AA and gibberellic acid.

GUTTATION (Term by Burgerstein)

Plants growing under humid conditions in a moist warm soil often exhibit droplets of water along the margins of their leaves.

Phenomenon is commonly seen in Oat, Tomato, Cucumber, Garden Nasturtium and Saxifraga etc.

The loss of water in the form of liquid is called guttation.

In moist and humid conditions, the rate of absorption of water greatly exceeds transpiration.

The root pressure is built up which pushes the water up in the xylem ducts, from where it comes out on the leaf surface through special structures called hydathodes.

Hydathodes are present at the tips of veins in leaves.

A hydathode consists of a pore in the epidermis followed by large intercellular spaces and loosely arranged parenchyma called epithem and blindly ending xylem elements.

Guttated water contains inorganic and organic salts and is not pure.

Concept Builder

1.Osmotic pressure of 1 molar solution of a non-electrolyte would be 22.4 atmospheres at 0ºC.

2.Equimolar concentrations of two solutions of non-ionising substances will have same osmatic pressure.

3.The value of osmotic potential of an electrolyte will be greater by the degree of its dissociation into ions at a given temperature.

4.Plasmolysis can be demonstrated in epidermal peel of Rhoeo discolor leaf.

5.The auxin treated cells show increase in their metabolism. Respiration in these cells increases and more of energy is provided for the absorption of water.

6.At low temperature, water in the intercellular spaces freezes into ice, thus having higher OP. It causes exosmosis of water from cells causing desiccation.

7.If a fresh water plant is transferred to marine water, it dies due to exosmosis.

8.Root pressure is measured by manometer.

9.Stocking (1956) considered root pressure as an active process responsible for guttation and bleeding in plants.

10.Maximum root pressure recorded in plants is 2 bars which is sufficient to raise the water column to a height of 20 meter.

11.Root pressure is absent in gymnosperms.

12.Potometers are used for measuring/comparing the rates of transpiration.

13.Cobalt chloride paper method is also used to compare the rates of transpiration. Moisture coming out of stomata turns blue cobalt chloride paper to pink.

14.Porometers are used for assessing the total pore area (stoma).

15.Generally, stomata are photoactive (open during the day and close at night). But in succulents like Bryophyllum, Opuntia and Cacti, stomata close during the day and open at night (scotoactive).

16.Trarispiration in old stems and fruits occurs through lenticels.

17.Fresh weight of a plant or leaf would be maximum in the morning and minimum in the afternoon.

18.If half of the total number of stomata on a leaf close down, the rate of transpiration is not reduced by half.

19.Cytokinins helps in opening of stomata while ABA (abscisic Acid) and low O₂ close stomata.

20. Plants growing at high altitudes show xeromorphy (adaptation to minimise transpiration).

21. Transpiration ratio: the amount of water lost per unit of dry matter produced during the growing season of a plant.

22.Plants growing at high altitudes show xeromorphy (adaptation to minimise transpiration)

21.Transpiration ratio: the amount of water lost per unit of dry matter produced during the growing season of a plant.

22.In Saxifraga, the rate of guttation is high during flowering.

23. Mechanical shock causes stomatal closure.

24.Stomatal index =

25.Transpiration index =

26.Psychrometer is used to measure relative humidity and rate of transpiration.

27.Diameter of tree decreases during the day. It is due to narrowing of tracheary elements due to development of negative pressure. It is measured by dendrograph.

28.Matric potential Y_m: It is used for surfaces which bind water. It is also negative, e.g. soil particles, cell wall, cytoplasm etc.

29.Gravity potential Y_g : It denotes the effect of gravity on Y_w. It depends on the height (h) of water above the reference state of water, the density at water and acceleration due to gravity. Value of Y_g is negligible upto a height of 5m from the reference level and also value of Y_m is ignored.

Ψw=Ψs+Ψp