1. Gaseous Exchange in Plants
1.1 Structure of Somatal Complex
Plants do not have an elaborate system to facilitate absorption of carbon dioxide from the environment and release oxygen, the by-product of photosynthesis. All living organisms, including flowering plants, respire aerobically. Thus they need oxygen and produces carbon dioxide as a metabolic waste. Most of the photosynthetic or living cells are in close proximity to the atmosphere.
The surfaces of leaves and herbaceous stems contain many pores for entry of gases known as the stoma opening. The cells in the leaves are loosely arranged to form a continuous network of inter-cellular spaces to facilitate the diffusion of gases. In the woody stems, there are also similar openings, lenticels where corky cells are loosely packed for the same purpose.
The leaves and stems of flowering plants are covered with a cuticularized epidermis which reduces water loss but at the same time prevents any significant exchange of gases. In herbaceous flowering plants, gases may enter or exit plants through tiny pores, stoma on the epidermal surface.
The stomatal pore is bordered by a pair of modified epidermal cells called guard cells which can draw apart or close together like sliding doors. The 2 guard cells are bean shaped and firmly joined at both ends but are able to separate in the mid region of their length. There is a large sap-filled vacuole in the guard cells. Unlike other epidermal cells, the guard cells possess chloroplasts, radial microfibrils and an unevenly thickened cellulose cell wall. The inner wall lining the stoma is much thicker and less elastic than the outer wall which is away from the stoma. The surrounding epidermal cells flanking the guard cells are called subsidiary cells.
Stomatal complex is the combination of guard cells and the stoma. Stomata opening and closing depends on changes in turgor of the guard cells. When the guard cells are turgid (gain water), the outer wall expands more than the inner wall, causing the guard cells to buckle outwards and the stomatal pore enlarges. The stomatal pore closes when the guard cells become flaccid (lose water). The guard cells respond to a complex set of environmental and internal cues to regulate the size of the stomata openings.
1.2 Mechanism of Stomata Opening and Closing
The currently favoured theory to explain stomata opening and closing is based on the accumulation of potassium ions observed in the guard cells prior to the opening of stomata. Photosynthesis starts at daybreak and its rate increases with increasing light intensity. Carbon dioxide levels in the cells within the leaf are lowered. The decrease helps trigger active transport of potassium ions into the guard cells. Blue wavelengths of light, which penetrate the atmosphere better also have the same effects. Accumulation of potassium ions causes the stomata to open. During stomatal closure, the potassium ions are actively removed from guard cells, reversing the process.
An accumulation of potassium ions in the guard cells will result in the development of a large potential difference across the surface membranes. The water potential of the cytoplasm in the guard cells becomes more negative than the surrounding epidermal cells. Endosmosis of water into the guard cell from the surrounding increases turgor in the guard cells and causes stomata to open,
Likewise, when potassium ions are actively pumped out of the guard cells, the water potential in the guard cells becomes less negative. Exosmosis causes the guard cells to become flaccid and closes the stomata.
Where is the source of ATP for active transport of potassium ions? The chloroplasts within the guard cells generate ATP via photosystem I of the light dependant system.
Anions are required to maintain electricalneutrality in the guard cells. In onions, electricalneutrality is maintained by the active uptake of chloride ions along with the potassium ions. In other plants, malate ions accumulate in the cytoplasm of guard cells during stomatal opening. Malate is also an anion and may be produced from phosphoenolpyruvate in the C4 pathway.
There are 2 feedback loops, one for carbon dioxide and one for water that control stomatal action. Light promotes photosynthesis, which lowers carbon dioxide levels in the leaf; the leaf’s response is to cause more potassium to move into guard cells and water follows osmotically, causing stomata to open. There is also the direct effect of blue light on guard cells, which causes stomatal opening independent of carbon dioxide levels.
For water, when more water exits in transpiration than can enter through the roots, abscisic acid (ABA) is released or produced from mesophyll cells which lead to the movement of potassium ions out of the guard cells; water follows osmotically, so stomata closes. If the rate of drying is extremely rapid, water is lost from guard cells directly, bypassing the ABA step, leading to closure.
1.3 Transpiration
The presence of stomatal pores on the epidermal surfaces of plants allows gaseous exchange; mainly entry of carbon dioxide and exit of oxygen gas during photosynthesis. However, it also results in significant amount of water loss from the plant. It accounts for 99% of the total water loss from plants, mainly the leaves. Water can also be lost from the lenticels and cuticle.
Transpiration refers to the evaporation of water, diffusion or water vapour from mesophyll cells in the leaves, stem and other plant parts through (mainly) the stomata into the atmosphere.
Transpiration is an inevitable consequence of gaseous exchange in plants. Only 1-2% of water absorbed by plants is used in photosynthesis. Foe plants to continue surviving, there is a need to strike a compromise between photosynthesis and transpiration. Different species of plants have special structural adaptations to control water loss at the stoma. The guard cells also respond to a complex set of environmental and internal cues to regulate the size of the stomatal openings.
1.4 Methods to measure Transpiration Rate
1.4.1 Using a Potometer
A potometer contains a leafy shoot, the stem of which was cut and fixed into the potometer underwater to stop any air entering its xylem vessels. All the joints are water tight. Assuming that the shoot does not use water for any metabolic process, the rate at which it takes up water from the potometer is the same as the rate of transpiration. This rate can be measured by following the movement of an air bubble along the graduated scale of the potometer over a measured time interval. The actual volume of water taken up can be estimated if the volume of the graduated tube corresponding to each division is determined.
1.4.2 Using Humidity and Temperature sensors and a Data-logger
The humidity and temperature sensors are connected to the data-logger, which is in turn plugged into the PC. One branch of the plant, the humidity and temperature sensor are placed into the polythene bag and the end tied up with a string. PicoLog is set to record at one sample per second, with a maximum of 1200 samples. The graph is set to show humidity. The spreadsheet is set to show humidity and temperature.
1.5 Factors affecting Transpiration Rate
1.5.1 Environmental Factors
1.5.1.1 Temperature
High temperatures increase the rate of transpiration in 2 ways. Firstly, heat energy provides the latent heat of vaporization of water, causing an increase in the rate of evaporation of water from the cell walls of mesophyll cells. Secondly, heat energy also increases the random thermal movement of molecules in the water vapour inside air spaces in the mesophyll.
1.5.1.2 Light Intensity
Plants transpire much more rapidly when exposed to light than in the dark. This is mainly because light stimulates the opening of stomata and warms the leaves. Thus light greatly increases the rate of transpiration.
1.5.1.3 Relative Humidity (RH)
The RH in the sub-stomatal spaces is very high. The extent to which the atmosphere is saturated with water vapour determines the steepness of the water vapour gradient between the sub-stomatal space and the atmosphere. The steeper the water vapour gradient, the faster will the water vapour escape through the stomata.
1.5.1.4 Wind Speed
Water vapour tends to build up on the surface of the leaf as it diffuses out of the stoma, most highly saturated immediately outside each stoma and becomes progressively less saturated as water vapour diffuses away. The diffusion paths of the water vapour molecules forms a hemisphere around the stoma called a diffusion shell/boundary layer as water vapour are deflected by the perimeter of the stoma.
Air movements blow away these diffusion shells or boundary layer thereby increasing the rate of evaporation from the leaf due to the presence of a steep diffusion gradient between the leaf and the atmosphere.
1.5.1.5 Availability of Water
Transpiration depends on the walls of the mesophyll cells being thoroughly wet. For this to be so, the plant must have an adequate water supply from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss or turgor occurs and the stomata close. This immediately reduces the rate of transpiration. Often, the loss of turgor may extend to other plant parts and wilting occurs.
1.5.1.6 Atmospheric Pressure
The lower the atmospheric pressure, the greater the rate of evaporation. Alpine plants which live at higher altitudes where the atmospheric pressure is lower than at sea level are liable to have a high rate of transpiration.
1.5.2 Internal Factors
1.5.2.1 Concentration of Carbon Dioxide
During photosynthesis, carbon dioxide is fixed and the reduction of internal concentration of carbon dioxide opens the stomata and thus increasing transpiration.
1.5.2.2 Concentration of Abscisic Acid
The negative feedback mechanism controls the opening and closing of the stomata. Water stress is known to cause the concentration of ABA to rise quite quickly in the leaves and this result in the closure of stomata.
1.5.2.3 Stomata
1.5.2.3.1 Abundance and Density
Transpiration rate depends on the total number of stomata present. The rate of diffusion of water vapour through small pores is proportional to the perimeter of the pore, not its area. The more stomata present, the greater the rate of transpiration.
The more closely spaced the stomatal pores, the thicker the boundary layer of water vapour on the surface and the slower the water loss from the leaf.
1.5.2.3.2 Distribution
Stomata may be found on the upper, lower or both surfaces of the leaves. The more stomata found on the upper epidermis, the higher the rate of transportation due to incident light on the upper epidermis of leaves.
1.6 Adaptations to reduce Transpiration Rate
1.6.1 Thick Cuticle
Even mildly water-stressed plants will rapidly wilt and die without their cuticle. Epidermal cells secrete this translucent, water-impermeable layer which coats their cell wall regions exposed to air. At the cuticle surface are deposits of waxes, cutin: water-insoluble lipids with long fatty acid tails. Plants living in the arid areas are susceptible to desiccation so they have thick cuticle to reduce evaporative loss.
1.6.2 Epidermal Hairs / Trichomes
Epidermal hairs trap a layer of still air with high relative humidity immediately outside the stoma opening. This provides a less steep water vapour gradient between the sub-stomatal space and the atmosphere, hence reducing the rate of evaporative water loss.
1.6.3 Sunken Stomata and Hypostomy
Some leaves have stomata that are only found in crypts / depressions. The crypts like the epidermal hairs serve to trap moisture in the region immediately outside the stoma. By having fewer stomata and several located in the same region also help to thicken the boundary layer which reduces evaporative loss.
Stomata on the lower epidermis avoid incident light, which causes stomata to open, thus lowering transpiration rate.
1.6.4 Bulliform Cells
These cells are specialized epidermal cells usually found on specific locations on the upper epidermis. They are large and contain mainly water such that the cells will shrink when water evaporates, acting as hinges which allows the leaves to roll and also traps moisture within the leaves.
1.7 Significance of Transpiration
Transpiration often results in water deficits and desiccation injury, especially when high temperature and humidity favour transpiration but the soil is deficient of water. Thus, could there be any positive advantage in transpiration to be gained by the plant?
Transpiration does speed up the movement of xylem sap. It increases the rate and quantity of water moved. This may be a more significant requirement in tall trees.
Because mineral nutrients move largely in the xylem sap, transpiration may benefit nutrient distribution. Minerals in the xylem sap will be carried along with a rapidly moving transpiration stream, but the rate-limiting step is more likely to be the rate at which nutrients are absorbed by the roots and delivered to the xylem. Moreover, salts continue to be distributed even when plants are not transpiring.
Evaporation of water from the mesophyll cells that accompanies transpiration requires energy and therefore results in cooling of the leaves. Even if the leaves were not cooled by evaporative water loss, other processes such as convection might remove sufficient energy to prevent the leaf from reaching lethal temperatures.
Transpiration may have a positive influence on the growth of some plants. These plants cease to grow under conditions of high humidity. However, many plants are able to complete their life cycle without apparent harm in high humidity where transpiration is minimal.
2. Why Flowering Plants need a Transport System?
A plant cannot carry out photosynthesis without water, minerals and solar energy. It secures water and minerals by sending a system of roots into the soil. It secures light by displaying leaves in the air. The greater its success in displaying the leaves above those of competing plants, the further it has removed them from their supply of water in the soil. A vascular system is therefore needed to bridge the gap and transport water and minerals efficiently from roots to the leaves. The vascular system is also needed to transport organic materials formed in the leaves, mainly sugars, amino acids to the living but non-photosynthetic cells of the plant. The transport of materials in plants is called translocation. The vascular system consists of a series of conducting tubes that extend through all the organs of the plant.
Water and minerals are transported from the roots to the leaves. The plants have specialized vascular tissues known as xylem, for efficient uptake of water and minerals from the soil by the roots and allow the ascent of these substances to the aerial parts of the plant
Some plants are especially tall, measuring more than 50 meters from the ground, making this process extremely challenging to transport water and minerals against gravity. Other plants grow in water scarce environment such as arid places in the desert or the mangrove. As such, these plants have other adaptations to allow them to survive under such hostile environment.
The products of photosynthesis such as sucrose, amino acids are transported away from the leaves to the parts of the plant where they are needed. All the cells that are unable to photosynthesize need a share of these materials especially the apices of the roots, shoots of the stem and branches, fruits where rapid cell division and growth are occurring.
In certain plants, food is transported for storage in perennating organs such as tubers, bulbs and corns until the following growth season. When the next season arrives, the stored food is transported in soluble form to the growing portions of the new plant.
Flowering plants have phloem, specialized vascular tissues for efficient distribution of organic compounds to various parts of the plant according to demand.
2.1 Histology of Xylem Tissue
2.1.1 Conducting Cells
2.1.1.1 Tracheids
These are individual cells tapered at each end so that the tapered end of one cell overlaps that of the adjacent cell. They have thick lignified walls and are dead at maturity. Their walls are perforated by pits so that water can flow from one tracheid to the next. The xylem of ferns and gymnosperms (conifers) consists of tracheids only. They are present in angiosperms but not important in conducting water. They are imperforated cells. Regions where perforations are located are known as perforation plates.
2.1.1.2 Xylem Vessels
Xylem vessels (dead) are formed by differentiation of cells called vessel elements (living). Vessel elements grow end to end to form a continuous tube when the end walls between individual elements disintegrate at maturity. Cytoplasmic contents also disintegrate when the cells are fully developed leaving a hollow or empty tube.
Protoxylem is first formed xylem with cell walls thickened by rings and spirals of lignin which allow the plant to stretch as it grows. Such vessels are only found in short-lived structures, such as the petiole of a leaf or in the growing tip of a root or shoot. In more permanent plant structures, these vessels eventually collapse and are replaced by the pitted and reticulated vessels of the metaxylem. Pitted vessels are uniformly lignified except at the small pores which are seen as pits. Reticulated vessels are thickened by interconnected bars of lignin.
2.1.2 Packing Cells
These are non-specialized cells for packing and are found interspersed with the conducting cells. They can be either non-lignified or lignified parenchymal cells. They are noted for storage reserves in the form of starch and fat. In mature state, they are able to differentiate into other cell types under special condition. This can repair and replace an organ after damage to a plant.
2.2 Histology of Phloem Tissue
2.2.1 Sieve Tube cells / Sieve Tube Elements / Sieve Tube Members (Living)
Sieve tube cells consist of narrow, elongated cells about 150-1000 microns and 10-15 microns in diameter. They lack nuclei, endoplasmic reticulum, mitochondria, plastids or other organelles which disintegrate during development. A few such organelles persists immediately adjacent to the cellulose cell wall, but elsewhere, they are absent. The cells are joined end to end to form long tubes running parallel to the long axis of the plant. It is the sieve tubes that form the channels for translocation. Sieve tubes are only found in angiosperms; conifers have less specialized sieve cells which are not aligned in a vertical manner.
Each sieve tube cell has perforated end walls known as sieve plates. The perforations between one cell and the next are perfectly matched and allow the passage of materials from one to another. The sieve tubes with their perforated plates seem to be well suited for transport. However, the perforations are often blocked with plasma protein (p-protein) and, during dormancy, with carbohydrate. Sealing off the sieve pores gives the plant some protection from leakage of the phloem contents during grazing by herbivores. Large amounts of callose also appear to be deposited on the sieve plates of older, non-functional sieve elements. The function of callose seems to be one of plugging off sieve elements that have been injured or are no longer functional, thus preserving the integrity of the translocating system.
2.2.2 Companion Cells (living)
These cells are small with thin cellulose cell walls. They are closely associated adjacent to each sieve tube cell. They possess a nucleus, dense ER, ribosomes and numerous mitochondria. Plasmodesmata connect each sieve tube cell with its adjacent companion cell or cells. The companion cells are sites of intense metabolic activity which control the activity of the sieve tube cells. On a purely structural ground, we might predict that the translocation of food materials occurs along the sieve tubes, with the adjacent companion cells providing the necessary energy for some part of the process.
2.2.3 Parenchyma Cells (living)
These are non-specialized cells with the usual thin cellulose cell walls for packing and are found at the periphery of the phloem.
2.2.4 Phloem Fibers and Sclereids (dead)
Phloem fibers are found in dicots but not in monocots. They are exactly similar to the sclerenchyma fibers. They provide mechanical support. Fibers occur occasionally in primary phloem, but more frequently in the secondary phloem of dicots. Since the secondary phloem is subjected to stretching as growth continues, fibers probably help to resist this pressure.
Sclereids occur especially in older phloem. They may be found alone or in combination with fibers.
2.3 Roots – Dicot and Monocot
At its tip is a meristem that produces the cells from which the first (primary) root structures will develop. Mitosis in this meristem increases the length of the root. The meristem is protected from abrasion and damage in the soil by the root cap.
As soon as new cells are produced by the meristem, they undergo a period of elongation and followed by differentiation. Cells at the epidermis become epidermis. When first formed, epidermal cells have root hairs, which greatly increase the surface area of the root and are the main entrance point for water and minerals.
A band of parenchymal cells in the regions of the cortex, develop beneath the epidermis. It serves to store food. Its inner surface is bound by a single layer of cells known as the endodermis. The tissues within the endodermis make up the stele. The other boundary of the stele is the pericycle, from which branches of the root i.e. lateral roots arise. The stele contains xylem and phloem. The xylem is arranged in bundles radially and the phloem alternates with the xylem. In monocot, the xylem is polyarch and in a dicot, the xylem usually has 3 to 6 arches.
In older dicot roots, another meristem, the cambium, develops between the xylem and phloem. Mitosis in the cambium produces new xylem to the inside and new phloem to the outside.
2.4 Translocation of Water
2.4.1 Pathways of Water Movement
Water leaving a plant leaf by transpiration is replaced by water absorbed through the roots. In tall trees, it means having to move water up to 100 meters against gravity. Water moves from the soil into the root, up the stem and to the leaves where it evaporates and diffuses into the atmosphere. Short distance movement of water from cell to cell in leaves (from xylem to stoma) and across the root cortex (from soil to xylem) follows 3 possible routes
2.4.1.1 Apoplast Pathway
It avoids contents of living cells. Then cellulose is very porous with air spaces a.k.a. apoplast, allowing water to diffuse freely and rapidly from cell to cell. It is the most favoured pathway for water movement as it provides the least resistance. These free spaces include water-filled spaces of dead cells, hollow tubes of xylem vessels and intermolecular spaces in cell walls.
2.4.1.2 Symplast Pathway
Plasmodesmata connect the cytoplasm of one cell to another adjacent cell, forming a continuous network of cytoplasm with no intervening cell surface membranes. Cytoplasm presents a considerable resistance to the flow of water through the cell, due to various organelles and membranes.
2.4.1.3 Vacuolar Pathway
The least taken pathway as water molecules need to pass through several barriers which slows its movement. Water moves by osmosis through the tonoplast (vacuolar membrane), vacuolar sap, cytoplasm and plasma membrane,
Long distance ascent of water from the roots to the leaves via mainly the xylem vessels in the stem.
2.4.2 Mechanisms of Water Movement
2.4.2.1 Osmosis and Water Potential Gradient
Measurements have showed that there is a gradient of water potential through the whole plant, least negative in the soil surrounding the roots and most negative in the atmosphere surrounding the leaves. The evaporation of water from the cells in the leaves maintains the existing water potential gradient in the plant, facilitating the flow of water by osmosis through the plant from root to leaves.
2.4.2.2 Capillary Force
If one end of a glass capillary tube is placed in water, the water rises in some way up the tube by capillarity. Xylem vessels have an internal diameter ranging from 20-400 microns and as they are stacked end to end, they form a capillary tube from root to leaf. Consequently, water will rise up the xylem from the root by capillarity as water molecules can adhere to the hydrophilic surface of xylem vessels. However, even the finest vessels, it would only rise about 1 meter which is not far enough to reach of even a small tree. Capillary action of water depends on adhesion and cohesion forces os water molecules.
2.4.2.3 Root pressure
If a stem of a well watered plant is severed, the cut end will exude copious quantities of water for a considerable time, suggesting that there is a force pushing water up the stem from the roots. This force is known as root pressure.
Root pressure can be measured by attaching a suitable mercury manometer to the cut end of the stem. Its value depends on the species of plant, the growing conditions and the time of the year, but root pressure of up to 200 kPa have been measured. Metabolic inhibitors, low temperatures and shortage of oxygen cause a reduction of root pressure which suggests it is an active process involving the expenditure of energy. Root pressure does nor account for the rise of water to the tops of tall trees.
The stele of the root behaves like an osmometer. At night, when transpiration is very low or zero, the root cells are still expanding energy to pump mineral ions into the xylem. The endodermis surrounding the stele of the root helps prevent the leakage of these ions back out of the stele. Accumulation of minerals in the stele lowers water potential; water flows in, generating a positive pressure that forces fluid up the xylem. This upward push is the root pressure.
This is not a major mechanism because it is only able to push water for only a few meters. Some plants cannot generate root pressure at all. Root pressure cannot keep pace with transpiration after sunrise.
2.4.2.4 Transpiration Pull
Experimental results suggest that water is pulled from above. Evaporation of water at the porous clay cup creates a tension which is transmitted to the water and mercury in a glass tube. When great care is taken to ensure that no dissolved air is present in the water, it is possible to draw the mercury column as high as 226 cm. This is equivalent to raising a column of water almost 30 meters (100) feet) into the air and is 3 times greater than the height that atmospheric pressure working against a perfect vacuum could attain.
If water in all the xylem is under tension, there should be a resulting inward pull (because of adhesion) on the walls of the vessels. The inward pull on all the xylem vessels in an actively transpiring tree should result in a decrease in the diameter of the trunk of a tree. Although the reduction in diameter of a single vessel is very small, the reduction in diameter of a tree trunk is large and can be measured using a dendrograph. The minimum diameter is reached just after mid day, at the time of greatest transpiration.
The problem faced by plants is not only to hold up the column of water but also to prevent it from breaking in the middle. Cohesive forces between water molecules hold a continuous column of water together, and when water transpires from the leaf, the whole of the water column move up the xylem.
In dry weather, when there is a shortage of water in the soil, the tension gets so high that the water columns break. This is called cavitation and it results in a bubble of gas forming in the vessel. The boarded pit act as valves ensuring only a few vessels is affected, thus safeguarding the system as a whole. The pits also allow the transpiration stream to find a way round an air bubble by moving into an adjacent vessel, past the bubble and then back into the original vessel chain. Also, the new xylem vessels which are produced each year in secondary growth help to compensate for the vessel blocked by air bubbles.
2.4.3 From Roots to Leaves
The water leaving a plant leaf by transpiration is replaced by water absorbed through the roots. Water moves unidirectionally from roots to leaves. This process is best explained by the cohesion-adhesion-tension theory.
2.4.3.1 From Leaves to Atmosphere
Water evaporates from the spongy mesophyll cells lining the sub-stomata spaces. The water vapour diffuse through the stomata into the atmosphere since the water potential in the atmosphere is more negative than that in the sub-stomatal space.
The water lost from the mesophyll cells is replaced by water drawn from the xylem in the veins of the leaves. The water potential in the mesophyll cells is more negative than the cells adjacent to them. Water diffuses by osmosis from the xylem vessels where the water potential is least negative, down the water potential gradient, to the mesophyll cells which are more negative. Water moves rapidly via the apoplast, symplast and the vacuolar pathway.
2.4.3.2 Through the Stem
Transpiration, which occurs in the leaves, provides the pulling or tensile force to draw water up from the roots via the xylem in the stem. This force is the surface tension (negative pressure) in the walls of the mesophyll cells in the leaves due to evaporation of water from the leaves to the atmosphere. This results in a transpiration stream – a continuous column of water that hangs from the top of the plant.
The strong cohesive or attractive forces between water molecules, due to intermolecular hydrogen bonding, prevent the disruption of this column of water in the xylem.
Water molecules also form hydrogen bonds with the hydrophilic surface of xylem walls. The adhesion of water molecules to xylem walls facilitates the ascent of water up the stem. The narrow xylem vessels provide a large surface area for adhesion of water molecules sufficient to support considerable mass of water.
The bulk flow of water to the top of a tree is solar powered (ATP not required), because it is the absorption of sunlight by the leaves that causes the evaporation responsible for transpiration pull.
2.4.3.3 From Soil to Roots
Transport of water across most of the root occurs in the same way as transport through the leaf i.e. by apoplast, symplast and vacuolar routes. Root hair cells are separated from the xylem vessels in the central stele by several layers of parenchyma cells forming the cortex, a single layer of cells in the endodermis and several layers of cells forming the pericycle.
Plasmodesmata connect the cytoplasm of one cell with that of another, forming a symplast in the root similar to that in the leaf. Water diffuses by osmosis, down the water potential gradient, from soil with the least negative water potential, across the root cortex and finally to the pericycle where the water potential is more negative.
Alternatively, water can freely diffuse through the spaces between cortical root cells and apoplast spaces within their cellulose cell walls. Water will diffuse down the water potential gradient until it reaches the cells of the endodermis. Unlike the cells in the rest of the root, the walls of endodermal cells have a casparian strip, which partially or completely encircle each cell (depending on the developmental stage). These strips contain suberin, a waxy substance which is not permeable to water.
To travel beyond the endodermis, water must pass from the apoplast into the cytoplasm of the endodermal cells. This allows active control of the passage of water and any dissolved inorganic ions it contains into the xylem and is also essential for the development of root pressure. From the cytoplasm of the endodermal cells, water is drawn to the pericycle and be pulled through the apoplast by the transpiration stream.
2.5 Translocation of Minerals
2.5.1 Mechanism of Uptake of Minerals
Plants need a number of minerals / inorganic ions for their normal metabolism. Typically, inorganic ions are taken up from the soil water via the roots. Dissolved in soil water, minerals pass into the walls of the root hair cells. Active uptake of minerals into cells also occurs.
Once in the root, some minerals are retained within the cytoplasm and vacuole or absorbed into the cellulose walls of the root cells. Most rare moved to the xylem, either through the symplast or apoplast route.
To enter the symplast route, ions must be actively pumped across the surface membranes of the epidermal cells of the root. From the cytoplasm of these cells, they may pass along the symplast during cytoplasmic streaming through the plasmodesmata.
Passage of ions through the apoplast route is passive, partly by diffusion but mainly by being carried along in the water which is pulled through the root by transpiration. When they reach the Casparian strip in the wall of the endodermal cells, ions in the apoplast must cross the cell surface membrane into the cytoplasm of an endodermal cell, this is achieved by active transport. From endodermal cells, ions are actively transported into the xylem with ions that followed the symplast route. Provided xylem and phloem are in contact, ions may also pass laterally form xylem vessels into sieve tube cells, where they are carried in the cell sap both up and down the stem.
There are 3 roles of the Casparian strip. Firstly is to regulate the types and amount of ions absorbed by the plant form the environment since it forces the ions to move through transport proteins in the plasma membrane. Secondly, it is to prevent the back flow of ions to the environment since it is impermeable to water and solutes. Thirdly, it is to maintain a very negative water potential in the pericycle for continuous absorption of water form the environment, even in a salt water environment.
Translocation is the long distance transport of products of photosynthesis (assimilates) by phloem to the rest of the plant. The source is the organ where sugar is produced by photosynthesis of by the breakdown of starch. The sink is the organ that consumes or stores the sugar.
2.5.2 Evidence for Selectivity and active transport
Concentration of certain ions may be many times higher in the cells than in the environment. This indicates that they enter the plant against a concentration gradient. Moreover, certain ions are more concentrated than the others. These observations suggest that ions are selectively absorbed by active transport, involving the expenditure of energy. It has beem found that uptake of minerals is increased by raising the temperature and decreased by oxygen deprivation or treatment with a metabolic poison. All are string indicators that active transport is involved.
2.6 Translocation of Food
Organic compounds are made in the leaves (source) of a plant during photosynthesis. Most of these molecules are transported in aqueous solution form the leaves to areas called sinks where they are utilized or stored. These sinks include growing regions, such as new leaves, flowers and fruits and stores such as seeds, tap roots, stem tubers, corns and bulbs. Thus organic molecules may be transported in phloem both upwards and downwards. This contrasts with transport in the xylem, which occurs upwards form roots to leaves only. Although amino acids and plant growth regulators are carried by phloem, up to 90% of the photosynthate is carried as sucrose. Sucrose is an ideal transport sugar. It is highly soluble yet being relatively un-reactive plays little part in cell metabolism and is not used by phloem tissues during its transit.
The mechanism of transport within the phloem is poorly understood. Although evidence that sugars are transported in the phloem stems from 1930s, there is no agreement as to how this movement occurs. Plant biologists are in agreement of one thing: the rate of movement of sugars is much too fast to be the result of passive diffusion. A few theories have been postulated to explain translocation in the phloem. Many of them are not mutually exclusive and it is possible that translocation occurs differently in different plants. The difficulty in making measurements in translocating phloem tissue without disrupting the process itself has led to a lack of convincing evidence for, or against, the various theories
2.6.1 Mass Flow (Pressure Flow) hypothesis
Please refer to a published text for further explanation.
2.6.1.1 Evidence in support of pressure flow hypothesis
The contents of the phloem sieve tubes are under marked pressure and sieve tube sap exudes when the phloem tissue is cut or damaged. Appropriate gradients in concentration of sucrose between the sink and source regions have been found in numerous plants.
2.6.1.2 Criticisms of pressure flow hypothesis
Sugars and amino acids have been observed to move at different speeds and in different directions in the same vascular bundles. In these cases phloem translocation may not occur in the direction of the “deepest” sink, but in different directions according to the metabolites being transported. The possible explanation is that it is possible for fluids to be travelling in opposite directions in two different, nearby sieve tubes in the same vascular bundle, or through 2 separate vascular bundles.
It offers no explanation for the existence of sieve plates which acts as a series of barriers impeding mass flow. The possible explanation is that one suggested function of the sieve plates is a means of sealing off damaged sieve tube elements by deposition of callose across the pores. Also, the presence of sieve plates may greatly increase the resistance along the pathway and results in the maintenance of a substantial pressure gradient between source and sink.
Phloem tissue has been found to have a high turnover of ATP along its length. Phloem is thought to play an active rather than a passive role in translocation.
2.6.2 Current widely accepted Model
2.6.2.1 Short distance phloem loading at the sugar source
Sugar produced in the mesophyll cells of a leaf must be loaded into sieve tube members before it can be exported to sugar sinks. Sugars may diffuse passively all the way from mesophyll cells to sieve tube members via the symplast pathway, through plasmodesmata linking the cells.
In many species, sieve tube members accumulate sucrose to concentrations 2 to 3 times higher than concentrations in mesophyll cells and thus phloem loading requires active transport. Much of the sucrose moves out of the cells into the apoplast in the vicinity of the sieve tube members and companion cells. This sucrose is accumulated from the apoplast by the sieve tube members and their companion cells. In some plants, companion cells have numerous ingrowths of their cell walls and plasma membrane, an adaptation that increases the cells’ surface area to accommodate transport proteins and enhances the transfer of solutes between apoplast and symplast. Such cells are called transfer cells.
Proton pumps in the plasma membrane of companion / transfer cells and sieve tube members do the work that enables the cells to accumulate sucrose in the sieve tubes. Companion cells have numerous mitochondria which can supply ATP to drive proton pumps which move hydrogen ions out of the companion cells and sieve tube cells. The proton gradient across the plasma membrane stores potential / electrochemical energy. Another membrane protein, sucrose-hydrogen ion symport, then uses this potential energy to co-transport sucrose into the cell along with hydrogen ions returning into the cytoplasm against sucrose concentration gradient.
2.6.2.2 Short Distance Phloem unloading at the Sugar Sink
Downstream, at the sink end of a sieve tube, phloem unloads its sucrose. In cells of some sink organs, the metabolism of sugar or the storage of sugar as starch maintains a low sugar concentration, which favours the continuing exit of sugar from sieve tubes. Pathways and mechanisms of sugar movement vary, depending on species. Both symplastic and apoplastic pathways may be involved. In some plants, sucrose may be unloaded form the phloem by active transport. In other species, diffusion is sufficient to move sucrose from phloem to the surrounding cells of the sink organ.
2.6.2.3 Long distance bulk flow / pressure flow between source and sink
Phloem loading results in a high solute concentration at the source end of a sieve tube, which causes the water potential to become more negative and influx of water into the sieve tube by osmosis from adjacent xylem vessels. Hydrostatic / turgor pressure develops within the sieve tube at the source end.
At the sink end, the pressure is relieved by the loss of water accompanied by the exodus of sugars leading to water potential becoming less negative in the sieve tube relative to adjacent tissues. Water that leaves the sieve tubes is recycled back from the sink to the source by xylem vessels
The pressure difference at the 2 opposite ends provides the driving force which cause phloem sap to flow rapidly from source to sink, down a gradient of hydrostatic pressure, carrying water and dissolved sugars along.
2.6.3 Electro-osmosis
This is the passage of water across a charged membrane. It is postulated that an electrical potential might be maintained across the sieve plate, the lower side being negative relative to the upper side. Such a potential might be maintained by an active pumping (by companion cells) of positive ions i.e. potassium ions, in an upward direction. Polar water molecules would then be swept along with the stream of ions drawn through the pores by the potential difference. This stream would carry the solutes i.e. sucrose present in the sieve tubes with it. Although this hypothesis is theoretically possible, there is little experimental evidence that it actually happens, though in certain plants potential difference of the right order of magnitude are claimed to have been detected.
2.6.4 Cytoplasmic Streaming Hypothesis
There are fine protein filaments which span the sieve tube cells from end to end. These filaments are continuous from one sieve tube cell to the next via the pores in the sieve plates. High magnification electron micrographs suggest that in the vicinity of the sieve plate, the protein filament take the form of microtubules of approximately 20 nm in diameter, but as they traverse the sieve tube cells, they break up into finer strands. In 1962, it was suggested that solutes such as sucrose might be transported by streaming along these protein filaments, the necessary energy coming from the sieve tubes themselves or the companion cells. It is envisaged that some strands convey solutes downwards and some upwards, thus accounting for the bi-directional flow of materials that is known to occur in the sieve tubes. How this occurs is unknown, but one speculation is the protein is contractile, rather like that found in muscle, and material is swept along by some kind of wave like movement of the filaments.
2.7 Experimental Evidence on Plant Transport
2.7.1 Bark Ringing Experiment
The phloem of a woody stem is located just underneath the bark. The xylem is located in the wood. Hence, removal of a complete strip of bark around a woody stem, known as ringing, removes phloem but leaves the xylem intact. When the ring was removed in summer, the plant did not wilt. The sugars from the leaves (destined for the roots) accumulated in the outer stem above the ring. There is no further increase in mass of roots. Phloem tissue cannot regenerate across a wide gap.
2.7.2 Analysis of sugar concentration in the green plant
Measurements of concentration of sugar in leaves, bark and woody tissues of cotton plants throughout a 24 hour period was carried out. The data were obtained by analysis of batches of plants harvested at intervals. Large samples of cotton plants were analyzed in order to overcome variations between individual plants.
2.7.3 Using aphid stylets as micropipette
The use of feeding aphids enables an analysis of contents of phloem sieve tube cells. Aphids have hollow, needle-like mouthparts which they insert into sieve tubes of herbaceous stem to feed on the sap. When feeding aphids are anaesthetized by a stream of carbon dioxide gas and carefully severed from their stylets, leaving the stylet in situ. The sections through the plant show that stylets always pierce an individual phloem sieve tube cell.
For several days after a feeding aphid has been cut from its stylet, liquid droplets form at the severed end of the stylet. These can be collected using a fine capillary tube and analyzed qualitatively by chromatography and quantitatively by chemical analysis. The fluid was shown to be an alkaline watery solution containing a mixture of organic compounds and inorganic ions. Up to 90% of the organic solute is sugar and up to 12% are amino acids, ATP, proteins including enzymes, hormones, alkaloids, vitamins and herbicides may also be found in lower concentrations.
2.7.4 Using radioactive carbon as a Tracer
Carbon dioxide labeled with radioactive carbon 14, fed to green leaves in the light is turned into radioactively labeled sugars and the sugars are transported about the plant. The movements of the labeled sugars can be followed by sectioning tissues and locating the radioactivity by autoradiography.
In autoradiography, the dried tissues or the entire leaf is pressed firmly against photographic film in the dark for an extended period. Then when the film is developed, the presence of radioactivity in parts of the leaves of the tissues shows up as fogging of negatives.
2.7.5 Using radioactively labeled ions
If plant roots are treated with solutions containing the radioactive isotopes of inorganic ions, subsequent autoradiograph show radioactivity in both phloem and xylem. If xylem tissue and phloem tissue in the lower stem are separated by paraffined paper and a solution containing radioactive inorganic ions is applied to the roots, radioactivity is found in the lower stem only in the xylem. Higher up in the stem, where the xylem and phloem have been left in contact, radioactivity is found in both tissues.
Applications
of solutions containing radioactive inorganic ions to leaves result in
radioactivity within the stem. However, radioactivity is now found in the
phloem but not the xylem.
