11th Bio Botany · Part

(CoCl . 2H O or CoCl . 4H O) due to water vapours coming out through stomata. The rate of transpiration is more on the lower surface than in the upper surface of the dorsiventral leaf.

. . Significance of transpiration Transpiration leads to loss of water, as stated earlier in this lesson % of absorbed water is lost in transpiration. It seems to be an evil process to plants.

However, number of process like absorption of water, ascent of sap and mineral absorption directly rely on the transpiration. Moreover plants withstand against scorching sunlight due to transpiration. Hence the transpiration is a “ necessary evil ” as stated by Curtis . .

Translocation of ­Organic Solutes Leaves synthesize food material through photosynthesis and store in the form of starch grains. When required the starch is converted into simple sugars. They must be transported to various parts of the plant system for further utilization. However, the site of food production (leaves) and site of utilization are separated far apart.

Hence, the organic food has to be transported to these areas. The phenomenon of food transportation from the site of synthesis to the site of utilization is known as translocation of organic solutes . The term solute denotes food material that moves in a solution. .

. Path of Translocation It has now been well established that phloem is the path of translocation of solutes. Ringing or girdling experiment will clearly demonstrate the translocation of solute by phloem. - - .

. PM . . Source and Sink Source is defined as any organ in plants which are capable of exporting food materials to the areas of metabolism or to the areas of storage.

Examples: Mature leaves, germinating seeds. Sink is defined as any organ in plants which receives food from source.Example: Roots, tubers, developing fruits and immature leaves (Figure . ). Figure .

: Source and Sink . . Phloem Loading The movement of photosynthates (products of photosynthesis) from mesophyll cells to phloem sieve elements of mature leaves is known as phloem loading . It consists of three steps.

i. Sieve tube conducts sucrose only. But the photosynthate in chloroplast mostly in the form of starch or ­ triose-phosphate which has to be transported to the cytoplasm where it will be converted into sucrose for further translocation. ii.

Sucrose moves from mesophyll to nearby sieve elements by short distance transport. iii. From sieve tube to sink by ­long-distance transport. .

. Ringing or girdling experiment Ring of bark removed Xylem Swollen tissue Water f bark oved Swo tiss Ring re Figure . : Ringing experiment The experiment involves the removal of all the tissue outside to vascular cambium (bark, cortex, and phloem) in woody stems except xylem. Xylem is the only remaining tissue in the girdled area which connects upper and lower part of the plant.

This setup is placed in a beaker of water. After some time, it is observed that a swelling on the upper part of the ring appears as a result of the accumulation of food material (Figure . ). If the experiment continues within days, the roots die first.

It is because, the supply of food material to the root is cut down by the removal of phloem. The roots cannot synthesize their food and so they die first. As the roots gradually die the upper part (stem), which depends on root for the ascent of sap, will ultimately die. .

. Direction of Translocation Phloem translocates the products of photosynthesis from leaves to the area of growth and storage, in the following directions, Downward direction : From leaves to stem and roots. Upward direction : From leaves to developing buds, flowers, fruits for consumption and storage. Germination of seeds is also a good example of upward translocation.

Radial direction : From cells of pith to cortex and epidermis, the food materials are radially translocated. - - . . PM However, the theory was rejected because the speed of translocation is much higher than simple diffusion and translocation is a biological process which any poison can halt.

. Activated diffusion theory This theory was first proposed by Mason and Maskell ( ). According to this theory, the diffusion in sieve tube is accelerated either by activating the diffusing molecules or by reducing the protoplasmic resistance to their diffusion. .

Electro-Osmotic theory The theory of electro osmosis was proposed by Fenson ( ) and Spanner ( ). According to this, an electric-potential across the sieve plate causes the movement of water along with solutes. This theory fails to explain several problems concerning translocation. .

Munch Mass Flow hypothesis Mass flow theory was first proposed by Munch ( ) and elaborated by Crafts ( ). According to this hypothesis, organic substances or solutes move from the region of high osmotic pressure (from mesophyll) to the region of low osmotic pressure along the turgor pressure gradient. The principle involved in this hypothesis can be explained by a simple physical system as shown in figure . .

Figure . : A model demonstrating the Mass flow hyphothesis Two chambers “A” and “B” made up of semipermeable membranes are connected by tube “T” immersed in a reservoir of water. Chamber “A” contains highly concentrated sugar solution while chamber “B” contains dilute sugar solution. The following changes were observed in the system, i.

The high concentration sugar solution of chamber “A” is in a hypertonic state which Why plants transport sugars as sucrose and not as starch or glucose or fructose? Glucose and Fructose are simple monosaccharides, whereas, Sucrose is a disaccharide composed of glucose and fructose. Starch is a polysaccharide of glucose. Sucrose and starch are more efficient in energy storage when compared to glucose and fructose, but starch is insoluble in water.

So it cannot be transported via phloem and the next choice is sucrose, being water soluble and energy efficient, sucrose is chosen as the carrier of energy from leaves to different parts of the plant. Sucrose has low viscosity even at high concentrations and has no reducing ends which makes it inert than glucose or fructose.During photosynthesis, starch is synthesized and stored in the chloroplast stroma and sucrose is synthesized in the leaf cytosol from which it diffuses to the rest of the plant. . .

Phloem Unloading From sieve elements sucrose is translocated into sink organs such as roots, tubers, flowers and fruits and this process is termed as phloem unloading . It consists of three steps: . Sieve element unloading : Sucrose leave from sieve elements. .

Short distance transport : Movement of sucrose to sink cells. . Storage and metabolism : The final step when sugars are stored or metabolized in sink cells. .

. Mechanism of Translocation Several hypotheses have been proposed to explain the mechanism of translocation. Some of them are given below: . Diffusion hypothesis As in diffusion process, this theory states the translocation of food from higher concentration (from the place of synthesis) to lower concentration (to the place of utilization) by the simple physical process.

- - . . PM draws water from the reservoir by endosmosis. ii.

Due to the continuous entry of water into chamber “A”, turgor pressure is increased. iii. Increase in turgor pressure in chamber “A” force, the mass flow of sugar solution to chamber “B” through the tube “T” along turgor pressure gradient. iv.

The movement of solute will continue till the solution in both the chambers attains the state of isotonic condition and the system becomes inactive. v. However, if new sugar solution is added in chamber “A”, the system will start to run again. A similar analogous system as given in the experiment exists in plants: Chamber “A” is analogous to mesophyll cells of the leaves which contain a higher concentration of food material in soluble form.

In short “A” is the production point called “ source ”. Chamber “B” is analogous to cells of stem and roots where the food material is utilized. In short “B” is consumption end called “ sink ”. Tube “T” is analogous to the sieve tube of phloem.

Mesophyll cells draw water from the xylem (reservoir of the experiment) of the leaf by endosmosis leading to increase in the turgor pressure of mesophyll cell. The turgor pressure in the cells of stem and the roots are comparatively low and hence, the soluble organic solutes begin to flow en masse from mesophyll through the phloem to the cells of stem and roots along the gradient turgor pressure. In the cells of stem and roots, the organic solutes are either consumed or converted into insoluble form and the excess water is released into xylem (by turgor pressure gradient) through cambium. Merits: i.

When a woody or herbaceous plant is girdled, the sap contains high sugar containing exudates from cut end. ii. Positive concentration gradient disappears when plants are defoliated. Objections: i.

This hypothesis explains the unidirectional movement of solute only. However, bidirectional movement of solute is commonly observed in plants. ii. Osmotic pressure of mesophyll cells and that of root hair do not confirm the requirements.

iii. This theory gives passive role to sieve tube and protoplasm, while some workers demonstrated the involvement of ATP. . Mineral Absorption Minerals in soil exist in two forms, either dissolved in soil solution or adsorbed by colloidal clay particle.

Previously, it was mistakenly assumed that absorption of mineral salts from soil took place along with absorption of water. But absorption of minerals and ascent of sap are identified as two independent processes. Minerals are absorbed not only by root hairs but also by the cells of epiblema. Plasma membrane of root cells are not permeable to all ions and also all ions of same salt are not absorbed in equal rate.

Penetration and accumulation of ions into living cells or tissues from surrounding medium by crossing membrane is called mineral absorption . Movement of ions into and out of cells or tissues is termed as transport or flux . Entry of the ion into cell is called influx and exit is called efflux . Various theories have been put forward to explain this mechanism.

They are categorized under passive mechanisms (without the involvement of metabolic energy) and active mechanisms (involvement of metabolic energy). . . Passive Absorption .

Ion-Exchange: Ions of external soil solution were exchanged with same charged (anion for anion or cation for cation) ions of the root cells. There are two theories explaining this process of ion exchange namely: - - . . PM i.

Contact exchange and ii. Carbonic acid exchange. i. Contact Exchange Theory: Figure .

: Contact Exchange theory According to this theory, the ions adsorbed on the surface of root cells and clay particles (or clay micelles) are not held tightly but oscillate within a small volume of space called oscillation volume . Due to small space, both ions overlap each other’s oscillation volume and exchange takes place (Figure . ). ii.

Carbonic Acid Exchange Theory: According to this theory, soil solution plays an important role by acting as a medium for ion exchange. The CO released during respiration of root cells combines with water to form carbonic acid (H CO ). Carbonic acid dissociates into H + and HCO – in the soil solution. These H + ions exchange with cations adsorbed on clay particles and the cations from micelles get released into soil solution and gets adsorbed on root cells (Figure .

). Figure . : Carbonic Acid Exchange theory . .

Active Absorption Absorption of ions against the concentration gradient with the expenditure of metabolic energy is called active absorption . In plants, the vacuolar sap shows accumulation of anions and cations against the concentration gradient which cannot be explained by theories of passive absorption. Mechanism of active absorption of salts can be explained through carrier concept. Carrier Concept: This concept was proposed by Van den Honert in .

The cell membrane is largely impermeable to free ions. However, the presence of carrier molecules in the membrane acts as a vehicle to pick up or bind with ions to form carrier-ion-complex, which moves across the membrane. On the inner surface of the membrane, this complex breaks apart releasing ions into cell while carrier goes back to the outer surface to pick up fresh ions (Figure . ).

Figure . : Carrier Concept The concept can be explained using two theories: . Lundegardh’s Cytochrome Pump Theory: Lundegardh and Burstrom ( ) observed a correlation between respiration and anion absorption. When a plant is transferred from water to a salt solution the rate of respiration increases which is called as anion respiration or salt respiration .

Based on this observation Lundegardh ( and ) proposed cytochrome pump theory which is based on the following assumptions: i. The mechanism of anion and cation absorption are different. ii. Anions are absorbed through cytochrome chain by an active process, cations are absorbed passively.

iii. An oxygen gradient responsible for oxidation at the outer surface of the membrane and reduction at the inner surface. According to this theory, the enzyme dehydrogenase on inner surface is responsible for the formation of protons (H + ) and electrons (e – ). As electrons pass outward through electron transport chain there is a corresponding inward passage of anions.

Anions are picked up by oxidized cytochrome - - . . PM . .

Donnan equilibrium Within the cell, some of the ions never diffuse out through the membrane. They are trapped within the cell and are called fixed ions. But they must be balanced by the ions of opposite charge. Assuming that a concentration of fixed anions is present inside the membrane, more cations would be absorbed in addition to the normal exchange to maintain the equilibrium.

Therefore, the cation concentration would be greater in the internal than in the external solution. This electrical balance or equilibrium controlled by electrical as well as diffusion phenomenon is known as the Donnan equilibrium . Summary There are two types of transports namely short and long distance in plants to translocate sap and solutes. Based on energy requirement, the transport may either be passive or active.

The process of diffusion, facilitated diffusion, imbibition and osmosis are driven by concentration gradient like a ball rolling down to a slope and hence, no energy is needed. The water absorbed (either active or passive) from the soil by root hairs must reach the xylem for further transportation. There are three possible routes to reach the xylem from root hairs. They are i) apoplast ii) symplast and/or iii) transmembrane.

Various theories explain the path of sap in the xylem and Dixon’s Cohesion-tension theory is the most accepted one. Transpiration is mostly carried out by stomata, which has guard cells. The general mechanism of stomatal movement is based on entry and exit of water molecules in guard cells. Many theories are there to explain how water enters and exits from guard cells.

The theory of potassium transport enumerates two different reactions separately run for opening and closing of stomata. Contrary to ascent of sap by xylem in an upward direction, the path of solute which consists of the photosynthetic products is always in phloem and translocate multidirectional. The point of origin of translocation is photosynthetic leaves which oxidase and are transferred to other members of chain as they transfer the electron to the next component (Figure . ).

Figure . : Cytochrome Pump theory H - H ½ H O ¼ O Fe + Fe + Fe + Fe + Fe + Fe + e - e - A - A - A - A - A - A - A - A - A - A - A - Dehydrogenase Reactions c + c + OUTSIDE INSIDE The theory assumes that cations (C + ) move passively along the electrical gradient created by the accumulation of anions (A – ) at the inner surface of the membrane. Main defects of the above theory are: (i) Cations also induce respiration. (ii) Fails to explain the selective uptake of ions.

(iii) It explains absorption of anions only. . Bennet-Clark’s Protein-Lecithin Theory: In , Bennet-Clark proposed that the carrier could be a protein associated with phosphatide called as lecithin . The carrier is amphoteric (the ability to act either as an acid or a base) and hence both cations and anions combine with it to form Lecithin- ion complex in the membrane.

Inside the membrane, Lecithin-ion complex is broken down into phosphatidic acid and choline along with the liberation of ions. Lecithin again gets regenerated from phosphatidic acid and choline in the presence of the enzyme choline acetylase and choline esterase (Figure . ). ATP is required for regeneration of lecithin.

Lecithin Lecithinase Phosphatidic Acid Choline Choline Esterase Acetyl Choline ATP Choline Acetylase OUTSIDE INSIDE C + C + C + A _ A _ A _ Figure . : Protein-Lecithin theory - - . . PM .

Munch hypothesis is based on a. Translocation of food due to TP ­gradient and imbibition force b. Translocation of food due to TP c. Translocation of food due to imbibition force d.

None of the above . If the concentration of salt in the soil is too high and the plants may wilt even if the field is thoroughly irrigated. Explain . How phosphorylase enzyme open the stomata in starch sugar interconversion theory?

. List out the non-photosynthetic parts of a plant that need a supply of sucrose? . What are the parameters which control water potential?

. An artificial cell made of selectively permeable membrane immersed in a beaker (in the figure). Read the values and answer the following questions? a.