Plant structure and growth mechanisms

1. Objectives: Students will be able to
• Describe resource acquisition and transport in vascular plants
• Demonstrate that soil is a living, finite resource and that plants require essential elements to complete their life
cycle
• Explain that plant nutrition often involves relationships with other organisms
Physical forces drive the transport of materials in plants over a range of distances
Q1 Transport in vascular plants occurs on three scales: with the help of Fig 1 write a short account of the 3 different
scales
Figure 1 An overview of transport in a vascular plant.
Revise your understanding so far…….
What tissue transports the water and minerals?
What tissue transports the sugar solution and other substances
made by the plant?
And what cells do the tissues contain?
The Central Role of Proton Pumps
The most important active transport protein in
the plasma membranes of plant cells is the
proton pump, which uses energy from ATP to
pump hydrogen ions (H+
) out of the cell. This
results in a proton gradient with a higher H+
concentration outside the cell than inside
(Figure 36.3). The gradient is a form of
potential (stored) energy because the
hydrogen ions tend to diffuse “downhill” back
into the cell, and this “flow” of H+
can be
harnessed to do work. And because the proton
pump moves positive charge, in the form of H+
, out of the cell, the pump also contributes to a voltage known as a
membrane potential, a separation of opposite charges across a membrane. Proton pumping makes the inside of a plant
cell negative in charge relative to the outside. This voltage is called a membrane potential because the charge separation
is a form of potential energy that can be harnessed to perform cellular work.
Figure 2 Proton pumps provide energy for solute transport. By pumping H+
out of the cell, proton pumps produce an H+
gradient and a charge separation called a membrane potential. These two forms of potential energy can be used to drive
the transport of solutes.
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BIO 304 Plant Structure and Physiology: Part 2 Resource Acquisition and Transport

2. Q2 Describe and explain 3 examples of where the membrane potential is used to transport substances in plants (see Fig
3)
Figure 3 Solute transport in plant cells.
Effects of Differences in Water Potential - why water can move by osmosis
To survive, plants must balance water uptake and loss. The net uptake or loss of water by a cell occurs by osmosis, the
passive transport of water across a partially permeable membrane.
Q3 Can you remember the definition of osmosis?
How can we predict the direction of osmosis when a cell is surrounded by a particular solution?
In the case of an animal cell, if the plasma membrane is impermeable to the solutes, it is enough to know whether the
extracellular solution has a lower or higher solute concentration relative to the cell. Water will move by osmosis from the
solution with the lower solute concentration to the solution with the higher solute concentration.
Q4 What will happen when red blood cells are placed in a) distilled water and b) concentrated salt solution? Explain.
But a plant cell has a cell wall, which adds another factor affecting osmosis: physical pressure. The combined effects of
solute concentration and physical pressure are incorporated into a measurement that is called water potential,
abbreviated by the Greek letter psi (ψ). The cell wall will stop the plant cell from bursting.
Q5 Explain what has happened to the plant cell (Fig 4) in these conditions: A is in distilled water, B is in a solution which
has the same concentration. C is in concentrated salt solution.
A B C
Fig 4
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3. Water potential determines the direction of movement of water. The most important thing to remember is that free
water, water that is not bound to solutes or surfaces, moves from regions of higher water potential to regions of lower
water potential if there is no barrier to its free flow. For example, if a plant cell is immersed in a solution having a higher
water potential than the cell, uptake of water will cause the cell to swell.
By moving, water can perform work, such as expanding a cell. The “potential” in water potential is water′s potential
energy—water′s capacity to perform work when it moves from a region of higher ψ to a region of lower ψ.
Q6 Explain what is meant by the terms ‘turgid’ and ‘flaccid’
Plant biologists measure ψ in units of pressure called megapascals ( MPa).(or KPa) Physicists have assigned the value of
zero to the water potential of pure water in a container open to the atmosphere (ψ = 0 MPa) under standard conditions
(at sea level and at room temperature). One MPa is equal to about 10 atmospheres of pressure. (An atmosphere is the
pressure exerted at sea level by an imaginary column of air extending through the entire height of the atmosphere—
about 1 kg of pressure per square centimeter.) A few examples will give you an idea of the magnitude of a megapascal:
Your lungs exert less than 0.1 MPa. A car tire is usually inflated to about 0.2 MPa. Water pressure in home plumbing is
about 0.25 MPa. In contrast, most plant cells exist at approximately 1 MPa.
How Solutes and Pressure Affect Water Potential
The water potential equation takes account of both pressure potential and solute potential:
Q7 Write down the equation for water potential (where ψ is the water potential, ψS is the solute potential (osmotic
potential), and ψP is the pressure potential.
The solute potential (ψS) of a solution is proportional to the number of dissolved solute molecules. Solute potential can
also be called osmotic potential because solutes affect the direction of osmosis. Solutes may be any type of dissolved
chemical.
By definition, the ψS of pure water is 0.
Q8 What happens when solutes are added to this pure water?
Pressure potential(ψP) is the physical pressure on a solution. Unlike ψS, ψP can be positive or negative. For example, the
water in the dead xylem cells of a transpiring plant is often under a negative pressure (tension) of less than −2 MPa.
Conversely, much like the air in a balloon, the water in living cells is usually under positive pressure. The cell contents
press the plasma membrane against the cell wall, producing what is called turgor pressure.
Quantitative Analysis of Water Potential
Now that we have introduced the water potential equation and its components, let′s put it to use. First we will look at
water movement in an artificial system. Then we will apply the equation to a living plant cell.
The artificial model represented in Figure 5 shows the movement of water within a U–shaped tube that has a membrane
separating the two arms of the tube. The membrane is permeable to water but not to solutes. What happens if we fill
the right arm of the tube with a 0.1 M solution (ψS = −0.23 MPa) and fill the left arm with pure water (ψS = 0)?
When there is no physical pressure (that is, when ψP = 0), the water potential ψ will be equal to ψS. So the ψ of the left
arm of the tube (pure water) will be 0, whereas the ψ of the right arm will be −0.23 MPa. Because water always moves
from regions of higher water potential to regions of lower water potential, the net water movement in this case will be
from the left arm of the tube to the right arm (Figure 5a) The water level in the right arm will rise.
Q9 Explain what will happen in each of these 3 cases.
Keep in mind the key point: Water moves in the direction of higher to lower water potential
(Figure 5b) If we now apply a physical pressure of +0.23 MPa to the solution in the right arm, we raise its water potential
from a negative value to 0 MPa (ψ = −0.23 + 0.23).
(Figure 5c) We increase ψP to +0.30 MPa, then the solution has a water potential of +0.07 MPa (ψ = −0.23 + 0.30)
(Figure 5d) Finally, imagine using a plunger to pull upward on the pure water instead of pushing downward on the
solution .
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4. Figure 5 Water potential and water
movement: an artificial model. In this U–
shaped apparatus, a selectively permeable
membrane separates pure water from a
0.1 M solution containing a particular
solute that cannot pass freely across the
membrane. Water moves across a
selectively permeable membrane from
where water potential is higher to where it
is lower. The water potential (ψ) of pure
water at atmospheric pressure is 0 MPa. If
we know the values of pressure potential
(ψP) and solute potential (ψS), we can
calculate water potential: ψ = ψP + ψS. The
values for ψ and ψS in the left and right
arms of the U–tube are given for initial
conditions, before any net movement of
water. (a) The addition of solutes reduces
water potential (to a negative value). (b, c)
Application of physical pressure increases
water potential. (d) A negative pressure
(tension) decreases water potential.
Uptake and loss of water by plant cells.
First, imagine a flaccid (limp) cell that has
a ψP of 0. Suppose this flaccid cell is bathed
in a solution of higher solute concentration
(more negative solute potential) than the
cell itself (Figure 6a) .
Now let′s place the same flaccid cell in pure
water (ψ = 0) (Figure 6b)
Q10 Explain what happens in each case
Aquaporin Proteins and Water Transport
Water potential is the force that moves water across the membranes of plant cells, but how do the water molecules
4
Figure 6 Water relations in plant cells. In these experiments, identical cells, initially flaccid, are placed in two different
environments. (The protoplasts of flaccid cells are in contact with their walls but lack turgor pressure.) The blue arrows
indicate the initial direction of net water movement.

5. actually cross the membranes? Because water molecules are so small, they move relatively freely across the lipid bilayer,
even though the middle zone is hydrophobic (see ppt). Water transport across biological membranes, however, is too
specific and too rapid to be explained entirely by diffusion through the lipid bilayer. Indeed, water typically crosses
vacuolar and plasma membranes through transport proteins called aquaporins (see ppt). These selective channels do not
affect the water potential gradient or the direction of water flow, but rather the rate at which water diffuses down its
water potential gradient. Evidence is accumulating that the rate of water movement through these proteins is regulated
by phosphorylation of the aquaporin proteins induced by changes in second messengers such as calcium ions (Ca2+
).
Three Major Compartments of Vacuolated Plant Cells
Transport is also regulated by the compartmental structure of plant cells. Outside the protoplast is a cell wall that helps
maintain the cell′s shape. However, it is the selectively permeable plasma membrane that directly controls the traffic of
molecules into and out of the protoplast. The plasma membrane is a barrier between two major compartments: the cell
wall and the cytosol (the part of the cytoplasm contained within the plasma membrane but outside the intracellular
organelles). Most mature plant cells have a third major compartment, the vacuole, a large organelle that can occupy as
much as 90% or more of the protoplast′s volume (Figure 4a). The vacuolar membrane, or tonoplast, regulates molecular
traffic between the cytosol and the vacuolar contents, called cell sap. Its proton pumps expel H+
from the cytosol into the
vacuole. The resulting pH gradient is used to move other ions across the vacuolar membrane by chemiosmosis.
Figure 7 Cell compartments and routes for short–distance transport.
Q 11 Figure 7 shows the cell compartments and routes for short distance transport. Summarize the important factors
you can learn from this Figure. Use the terms: plasmodesma (pl plasmodesmata,) symplast, apoplast, vacuole
transmembrane route
Roots absorb water and minerals from the soil
Fig 8 summarizes the way water and mineral salts are absorbed from the soil. Use Fig 8 points 1-5 to help you answer
the next question.
Q12 Describe the mechanisms for short distance transport between the soil and xylem.
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6. Figure 8 Lateral transport of minerals and water in roots.
The Roles of Root Hairs, Mycorrhizae, and Cortical Cells
Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and
where root hairs are located. Root hairs, which are extensions of epidermal cells, account for much of the surface area of
roots (see ppt 1). Soil particles, usually coated with water and dissolved minerals, adhere tightly to the root hairs. The soil
solution flows into the hydrophilic walls of epidermal cells and passes freely along the apoplast into the root cortex. This
exposes the symplast of all the cells of the cortex to the soil solution, providing a much greater surface area of
membrane than the surface area of the epidermis alone.
As the soil solution moves along the apoplast into the roots, cells of the epidermis and cortex take up water and certain
solutes into the symplast. Although the soil solution is usually very dilute, active transport enables roots to accumulate
essential minerals, such as K+
, to concentrations hundreds of times higher than in the soil.
Q13 Why are the cortical cells so important in the uptake of water and solutes?
Most plants form mutually beneficial relationships with fungi, which facilitate the absorption of water and minerals from
the soil. Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hyphae
(filaments) (see ppt) The hyphae absorb water and selected minerals, transferring much of these resources to the host
plant.
Q14 Why are the fungal hyphae so helpful?
Water and minerals ascend from roots to shoots through the xylem
Here we will focus on the long–distance transport of xylem sap. The sap flows upward from roots throughout the shoot
system to veins that branch throughout each leaf. Leaves depend on this efficient delivery system for their supply of
water. Plants lose an astonishing amount of water by transpiration, the loss of water vapor from leaves and other aerial
parts of the plant. Consider the example of maize (commonly called corn in the U.S.). A single plant transpires 125 L of
water during a growing season. A maize crop growing at a typical density of 75,000 plants per hectare transpires almost
10 million L (10 million kg) of water per hectare every growing season (equivalent to about 1.25 million gallons of water
per acre per growing season). Unless the transpired water is replaced by water transported up from the roots, the leaves
will wilt and the plants will eventually die. The upward flow of xylem sap also brings mineral nutrients to the shoot
system.
Factors Affecting the Ascent of Xylem Sap
Xylem sap rises to heights of more than 100 m in the tallest trees. Is the sap pushed upward from the roots, or is it pulled
upward by the leaves? Let′s evaluate the relative contributions of these two possible mechanisms.
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7. Pushing Xylem Sap: Root Pressure
At night, when transpiration is very low or zero, root cells continue pumping mineral ions into the xylem of the vascular
cylinder. Meanwhile, the endodermis helps prevent the ions from leaking out. The resulting accumulation of minerals
lowers the water potential within the vascular cylinder. Water flows in from the root cortex, generating root pressure, an
upward push of xylem sap. The root pressure sometimes causes more water to enter the leaves than is transpired,
resulting in guttation , the exudation of water droplets that can be seen in the morning on tips of grass blades or the leaf
margins of some small, herbaceous eudicots (Figure on ppt) . Remember: Guttation fluid is different from dew, which is
condensed moisture produced during transpiration.
Q15 Why is root pressure is a minor mechanism driving the ascent of xylem sap in most plants?
For the most part, xylem sap is not pushed from below by root pressure but pulled upward by the leaves themselves.
Pulling Xylem Sap: The Transpiration–Cohesion–Tension Mechanism
To move material upward, we can apply positive pressure from below or negative pressure from above (as when sucking
liquid through a straw). Here we will focus on the process by which water is pulled upward by negative pressure in the
xylem. As we investigate this mechanism of
transport, we will see that transpiration
provides the pull, and the cohesion of water
due to hydrogen bonding transmits the
upward pull along the entire length of the
xylem to the roots.
Fig. 9
Transpirational Pull. Stomata, the
microscopic pores on the surface of a leaf,
lead to airspaces that expose the mesophyll
cells to the carbon dioxide they need for
photosynthesis. The air in these spaces is
saturated with water vapour because it is in
contact with the moist walls of the cells. On
most days, the air outside the leaf is drier;
that is, it has a lower water potential than the
air inside the leaf. Therefore, water vapour in
the airspaces of a leaf diffuses down its water
potential gradient and exits the leaf via the stomata. It is this loss of water vapor from the leaf by diffusion
and evaporation that we call transpiration.
Q16 With the help of Fig 9, points 1-5, describe how the transpirational pull is produced in a leaf.
But how does loss of water vapor from the leaf translate into a pulling force for upward movement of water through a
plant?
The leading hypothesis is that negative pressure that causes water to move up through the xylem develops at the air–
water interface in mesophyll cell walls. Water is brought to the leaves via the xylem in leaf veins and then is drawn into
the mesophyll cells and into their cell walls. This movement depends on adhesion of water to cellulose microfibrils and
other hydrophilic components in plant cell walls. At first, water evaporates from a thin water film lining the airspaces
surrounding mesophyll cells. As more water is lost to the air, the air–water interface retreats deeper into the cell wall
and becomes more curved (Figure 9 ) . As even more molecules evaporate, the degree of curvature and the surface
tension of the water molecules increase, and the pressure at the air–water interface becomes increasingly negative.
Water molecules from the more hydrated parts of the leaf are then pulled toward this area, reducing the tension. These
pulling forces are transferred to the xylem because each water molecule is cohesively bound to the next by hydrogen
bonds.
Transpirational pull depends on some of the special properties of water: adhesion, cohesion, and surface tension.
The role of negative pressure fits with what you learned earlier about the water potential equation because negative
pressure (tension) lowers water potential. Since water moves from where its potential is higher to where it is lower, the
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1
1
2
3
4
5

8. increasingly negative pressure at the air–water interface causes xylem cells to lose water to mesophyll cells, which lose
water to the airspaces, where it diffuses out through stomata. In short, the negative water potential of leaves provides
the “pull” in transpirational pull.
Q17 How does Cohesion and Adhesion result in the Ascent of Xylem Sap? Use Fig 10 to help.
Figure 10 Ascent of xylem sap. Hydrogen bonding forms an unbroken chain of water molecules extending from leaves
all the way to the soil. The force that drives the ascent of xylem sap is a gradient of water potential (ψ). For the bulk flow
over long distance, the ψ gradient is due mainly to a gradient of the pressure potential ( ψP). Transpiration results in the
ψP at the leaf end of the xylem being lower than the ψP at the root end. The ψ values shown at the left are a “snapshot.”
During daylight, these specific values may vary, but the direction of the water potential gradient remains the same.
The upward pull on the sap creates tension within the xylem. Tension will pull the walls of the pipe inward. You can
actually measure a decrease in the diameter of a tree trunk on a warm day.
Q18 Suggest what specialized feature of xylem stops the walls from collapsing inwards.
Stomata help regulate the rate of transpiration
Leaves generally have broad surface areas and high surface area–to–volume ratios.
The broad surface area is a morphological adaptation that enhances the absorption of light needed to drive
photosynthesis. The high surface area–to–volume ratio aids in the uptake of carbon dioxide during photosynthesis as
well as in the release of oxygen. Upon diffusing through the stomata, CO2 enters a maze of air spaces formed by the
spongy parenchyma cells. These cells cause the internal surface area of the leaf to be 10 to 30 times greater than the
external surface area.
8

9. Although broad surface areas and high surface area–to–volume ratios increase photosynthesis, they also have the
serious drawback of increasing water loss by way of the stomata. Thus, a plant′s tremendous requirement for water is
part of the cost of making food by photosynthesis. By opening and closing the stomata, guard cells help balance the plant
′s requirement to conserve water with its requirement for photosynthesis (Figure 11) .
Figure 11 Open stomata (left) and closed stomata (colorized SEM).
Effects of Transpiration on Wilting and Leaf
Temperature
Q19 Describe why transpiration can cause wilting and
affect leaf temperature.
Figure 12 The mechanism of stomatal opening and
closing.
When guard cells take in water from neighbouring cells
by osmosis, they become more turgid and curved. This
bending increases the size of the pore between the guard
cells. When the cells lose water and become flaccid, they
become less curved and close the pore.
Q20 Explain how the changes in turgor pressure happen.
in the swelling and shrinking of guard cells by varying the
permeability of the membranes to water.
Suggest how proton pumps may be involved
A second stimulus causing stomata to open is depletion
of CO2
Movement from Sugar Sources to Sugar Sinks: Translocation
Sieve tube elements in phloem are the specialized cells that transport sucrose solution. They are arranged end to end to
form long sieve tubes. Between them are perforated sieve plates, structures that allow the flow of sap along the sieve
tube.
Phloem sap is an aqueous solution that is made up of disaccharide sucrose, as well as minerals, amino acids, and
hormones.
The direction that phloem sap travels is variable, may be up or down the plant. The movement is always from a sugar
9

10. source to a sugar sink.
For each sieve tube, the direction of transport depends on the locations of the source and sink connected by that tube.
So adjacent tubes may carry sap in opposite directions. Direction of flow varies by season or developmental stage of the
plant.
Q21 Explain what is meant by a sugar source and a sugar sink. Give examples.
Figure 13 Loading of sucrose into phloem.
Sugar must be loaded into sieve–tube members before being exported to sinks.
Q22 Use Fig 13 and your knowledge of proton pumps and co-transport to explain how this is done.
Pressure Flow: The Mechanism of Translocation in Angiosperms
Phloem sap flows from source to sink at fast rates. The sap moves through a sieve
tube by bulk flow driven by positive pressure.
The building of pressure at the source end and reduction of that pressure at the sink
end cause water to flow from source to sink, carrying the sugar along. Xylem recycles
the water from sink to source.
Figure 14 Pressure flow in a sieve tube.
The pressure flow hypothesis explains why phloem sap always flows from source to
sink.
Q23 Use the figure to explain the pressure flow hypothesis.
How transport tissues are adapted to their function
XYLEM VESSELS
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11. Structure Reason it helps
No end walls or perforation plates Makes continuous column, water flows easily
No cell contents/cytoplasm (dead) Water flows easily, there is little resistance to
flow
Cell walls have pits Allows lateral movement of water out into
other vessels or into other tissues
Cell walls thickened with lignin Prevents inwards collapse of walls due to
water column being under tension
PHLOEM
Structure Reason it helps
End walls have sieve plates with
sieve pores
Phloem sap flows easily, little resistance
Peripheral cytoplasm Easier flow of fluid
No nucleus in sieve tube elements Allows more space for sucrose; metabolic
processes carried out by companion cells
Companion cells have many
mitochondria
Generates ATP for active transport
Sieve tube elements are elongated
and stacked end to end
Enables transport of sap from source to sink
Plasmodesmata between comp cell
and sieve tube cells
Sucrose solution moves into sieve tube
element
11

12. Soil and
Plant Nutrition: Plants require essential elements to complete their life cycle
Soil, water, and air all contribute to plant growth. Plants extract mineral nutrients, essential chemical elements, from the
soil in the form of inorganic ions. Plants acquire nitrogen, for example, in the form of nitrate ions (NO3
−
). However,
mineral nutrients add little to the plant′s overall mass. 80–90% of a plant is water, and plants grow mainly by
accumulating water in the central vacuoles of their cells. Water is also a nutrient that supplies most of the hydrogen
atoms and some of the oxygen atoms incorporated into organic compounds by photosynthesis. Only a small fraction of
the water that enters a plant contributes atoms to organic molecules. It has been estimated that more than 90% of the
water absorbed by maize plants is lost by transpiration.
Q24 Apart from the water lost in transpiration, what happens to the water absorbed by the plant?
Macronutrients and Micronutrients
More than 50 chemical elements have been identified among the inorganic substances in plants, but not all of these
elements are essential. A chemical element is considered an essential element if it is required for a plant to complete a
life cycle and produce another generation. In studying the chemical composition of plants, we must distinguish elements
that are essential from those that are merely present in the plant. To some extent, the chemical elements in a plant
reflect the soil composition. Plants growing on mine tailings, for instance, may contain gold or silver, but these minerals
have no nutritional function.
To determine which chemical elements are essential elements, researchers use hydroponic culture, in which plants are
grown without soil by using mineral solutions. Such studies have helped identify 17 essential elements that are needed
by all plants. ( Table 37.1, on the next page).
Macronutrients: 9 elements which plants require in relatively large amounts.
Micronutrients: 8 elements which plants need in very small amounts.
Q25 Make a list of the essential macro- and micronutrients
Micronutrients function in plants mainly as cofactors, non–protein helpers in enzymatic reactions. Iron, for example, is a
metallic component of cytochromes, the proteins in the electron transport chains of chloroplasts and mitochondria. It is
because micronutrients generally play catalytic roles that plants need only minute quantities.
The symptoms of a mineral deficiency depend on the function and mobility of the nutrient.
Function: For example, magnesium is a component of chlorophyll. Deficiency of magnesium causes yellowing of the
leaves, chlorosis. In some cases, the relationship between a mineral deficiency and its symptoms is less direct. For
instance, iron deficiency can cause chlorosis even though chlorophyll contains no iron, because iron ions are required as
a cofactor in one of the steps of chlorophyll synthesis.
Mobility within the plant: If a nutrient moves about freely, symptoms will show up first in older organs because young,
growing tissues have more “drawing power” for nutrients in short supply.
For example: A plant starved of magnesium will show signs of chlorosis first in its older leaves. Magnesium is fairly mobile
and is moved first to young leaves.
If a mineral is less mobile, for example iron, then the deficiency shows up first in the young leaves; the older ones have
an adequate supply of the mineral and keep it..
The symptoms of a mineral deficiency are often distinctive enough for a plant physiologist or farmer to diagnose its cause
Deficiencies of nitrogen, phosphorus, and potassium are most common. Shortages of micronutrients are less common
and tend to occur in certain geographic regions because of differences in soil composition. One way to confirm a
diagnosis is to analyze the mineral content of the plant and soil. The amount of a micronutrient needed to correct a
deficiency is usually quite small. For example, a zinc deficiency in fruit trees can usually be cured by hammering a few
zinc nails into each tree trunk. Moderation is important because overdoses of many nutrients can be toxic to plants.
Hydroponic culture can ensure optimal mineral nutrition by using nutrient solutions that can be precisely regulated.
However, this method is not used widely in agriculture because it is relatively expensive compared with growing crops in
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BIO 304 Plant Function and Physiology: Part 2 Soil and Plant Nutrition Lecturer: Susan Tyzack

13. soil. (see PPT)
Q 26 Suggest what difficulties might arise with hydroponic farming – why is it so expensive?
Soil quality is a major factor in plant distribution and growth
The major factors determining which plants can grow well in a certain location are the texture and composition of soil.
Texture = the general structure of soil- amounts of various sizes of soil particles.
Composition = the organic and inorganic chemical components of soil.
Plants that grow naturally in a certain type of soil are adapted to its texture and composition and can absorb water and
extract essential mineral nutrients. In turn, plants affect the soil. The soil–plant interface is a critical part of the chemical
cycles that sustain terrestrial ecosystems.
Texture and Composition of Soils
Soil has its origin in the weathering of solid rock. The freezing of water that has seeped into crevices can mechanically
fracture rocks. Acids dissolved in the water can also help break rocks down chemically. Organisms like bacteria, mosses,
lichens are able to invade the rock; they accelerate breakdown by chemical and mechanical means. Some organisms, for
example, secrete acids that dissolve the rock. Roots that grow in fissures (cracks) lead to mechanical fracturing.
The result of this activity is topsoil, a mixture of particles derived from rock, living organisms, and humus, the remains of
partially decayed organic material. The topsoil and other distinct soil layers, or horizons, are often visible in vertical
profile where there is a road cut or deep hole. The topsoil, also known as the A horizon, is the richest in organic material
and is therefore most important for plant growth.
The texture of topsoil depends on the sizes of its particles, which are classified in a range from coarse sand to
microscopic clay particles. The most fertile soils are often loams made up of roughly equal amounts of sand, silt and clay.
The fertility of loam soil is because a) particles are fine enough to provide a large surface area for retaining
minerals and water, which adhere to them b) particles are coarse enough to provide air spaces containing oxygen
that can be used by roots for cellular respiration.
Q27 What do you think will happen if there is poor drainage in the soil?
Soil composition includes organic components as well as minerals. A teaspoon of topsoil has about 5 billion bacteria that
cohabit with various fungi, algae and other protists, insects, earthworms, nematodes, and roots of plants. The activities
of all these organisms affect the soil′s physical and chemical properties. Earthworms, for instance, turn over and aerate
the soil by their burrowing and add mucus that holds fine soil particles together. Meanwhile, the metabolism of bacteria
alters the mineral composition of the soil. Plant roots can also affect soil composition and texture. For instance, they can
affect soil pH by releasing organic acids, and they reinforce the soil against erosion.
Humus consists of decomposing organic material formed by the action of bacteria and fungi on dead organisms, faeces,
fallen leaves, and other organic refuse. Humus prevents clay from packing together and builds a crumbly soil that retains
water but is still porous enough for the adequate aeration of roots. Humus is also a reservoir of mineral nutrients that
are returned gradually to the soil as microorganisms decompose the organic matter.
Q 28 Use Fig 15 and the PPT to explain the way water is contained around soil particles and the uptake of cations by
cation exchange
The film of water bound less tightly to the particles is the water generally available to plants. It is not pure water but a
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Fig.15

14. soil solution containing dissolved minerals in the form of ions. Roots absorb this soil solution. To be available to roots,
mineral ions must be released from the soil particles into the soil solution.
Q29 What do you think would be the effect of acid rain on the availability of minerals to plants?
Soil Conservation and Sustainable Agriculture
It may take centuries for a soil to become fertile through the breakdown of rock and the accumulation of organic
material; human mismanagement can destroy that fertility within a few years. Soil mismanagement has been a recurring
problem in human history. For example, the Dust Bowl was an ecological and human disaster that took place in the
southwestern Great Plains region of the United States in the 1930s. Before farming, the region was covered by hardy
grasses that held the soil in place in spite of the long recurrent droughts and torrential rains characteristic of the region.
However, in the late 1800s and early 1900s, many homesteaders settled in the region, planting wheat and raising cattle.
These land uses left the topsoil exposed to erosion by winds that often sweep over the area. Bad luck in the form of a
few years of drought made the problem worse. Much of the topsoil was blown away, making millions of hectares of
farmland useless. Hundreds of thousands of people abandoned homes and land, a desperate situation written about by
John Steinbeck in ‘The Grapes of Wrath.’
To understand soil conservation, we must remember that agriculture is not natural. In forests, grasslands, and other
natural ecosystems, mineral nutrients are usually recycled by the decomposition of dead organic material in the soil. In
contrast, when we harvest a crop, essential elements are removed and not replaced. In general, agriculture depletes the
mineral content of the soil. To grow 1,000 kg of wheat grain, the soil gives up 20 kg of nitrogen, 4 kg of phosphorus, and
4.5 kg of potassium. Each year, the soil fertility diminishes unless fertilizers are applied to replace lost minerals. Many
crops also use far more water than the natural vegetation, forcing farmers to irrigate.
The three most important factors in soil conservation are: careful fertilization, thoughtful irrigation and
prevention of erosion.
Q30 Give reasons why more than 30% of the world’s farmland suffers from low productivity
Table 1. Global land degradation
(data from UN Environment Programme)
Q31 Sketch a graph to illustrate the data on the table. Comment on the
data in Table 1. Suggest reasons for the figures for land degradation in
Europe and Africa.
Discuss reasons for the figures for grain production
Fertilizers
Prehistoric farmers may have started fertilizing their fields after noticing that grass grew faster and greener where
animals had defecated. ‘Artificial’ fertilizers: In developed nations today, most farmers use
commercially produced fertilizers containing minerals that are mined or prepared by industrial processes. These
fertilizers are usually enriched in nitrogen, phosphorus, and potassium, the macronutrients most commonly deficient in
farm and garden soils. Fertilizers are labeled with a three–number code called the N–P–K ratio, indicating the content of
these minerals. A fertilizer marked “15–10–5,” for instance, is 15% nitrogen (as ammonium or nitrate), 10% phosphorus
(as phosphoric acid), and 5% potassium (as the mineral potash).
Organic fertilisers: Manure, fishmeal, and compost are called “organic” fertilizers because they are of biological origin
and contain decomposing organic material. This must be converted into inorganic nutrients that roots can absorb.
Minerals a plant takes up from the soil are in the same form whether from organic fertilizer or a chemical factory.
Continent % of land
degraded
% of world
grain
production
Africa 16.2 5
Asia 15.1 50
Europe 22.3 16
N.America 7.0 23
S.America 13.0 6
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15. Q 32 Describe some advantages and disadvantages of both types of fertiliser
Soil pH is an important factor in fertilisation because it affects cation exchange and the chemical form of minerals. Plants
may be starved of an element if it is bound too tightly to clay particles or is in a chemical form the plant cannot absorb.
Changes in H+
concentration may make one mineral more available but make another less available. At pH 8, for
instance, the plant can absorb calcium, but iron is almost completely unavailable.
Soil pH should be matched to a crop′s mineral needs. If the soil is too alkaline, adding sulphate will lower the pH. Soil that
is too acidic can be adjusted by adding lime (calcium carbonate or calcium hydroxide).
A major problem with acidic soils, particularly in tropical areas, is that aluminum dissolves at low pH and becomes toxic
to roots. Some plants cope with high aluminum levels by secreting organic anions that bind the aluminum and render it
harmless.
Irrigation
Water is most often the limiting factor in plant growth. Irrigation can transform a desert into a garden, but farming in
arid regions is a huge drain on water resources. Problems caused by irrigation include: drying up of rivers and salt
accumulation due to evaporation of irrigation water.
Q33 Explain the effect of salinization on plants.
Erosion
Topsoil from thousands of acres of farmland is lost to water and wind erosion each year. Certain precautions, such as:
planting rows of trees as windbreaks, terracing hillside crops, and cultivating in a contour pattern, can prevent loss of
topsoil. Crops such as alfalfa and wheat provide good ground cover and protect the soil better than maize and other
crops that are usually planted in more widely spaced rows.
If managed properly, soil is a renewable resource in which farmers can grow food for many generations. The goal of soil
management is sustainable agriculture, using a variety of farming methods that are conservation–minded,
environmentally safe, and profitable.
Soil Reclamation
Some areas are unfit for agriculture because of contamination of soil or groundwater with toxic heavy metals or organic
pollutants. Non–biological technologies, such as removal and storage of contaminated soil in landfills are expensive and
often disrupt the landscape. Phytoremediation (Phyto-= plant, remediation=making better) is a biological, non-
destructive technology that seeks to reclaim contaminated areas cheaply by using the ability of some plants to extract
soil pollutants and concentrate them in portions of the plant that can be easily removed for safe disposal. For example,
alpine pennycress (Thlaspi caerulescens) can accumulate zinc in its shoots at concentrations 300 times higher than most
plants can tolerate. Such plants show promise for cleaning up areas contaminated by smelters, mining operations, or
nuclear testing. Phytoremediation is part of the more general technology of bioremediation, which includes the use of
prokaryotes to detoxify polluted sites.
Check your understanding with these questions:
a). What are the general characteristics of good soil?
b). Explain how the phrase “too much of a good thing” can apply to watering and fertilizing plants
Bacteria and plant nutrition: the special case of nitrogen
Of all the mineral nutrients, nitrogen contributes most to plant growth and crop yields. Plants require nitrogen as a
component of proteins, nucleic acids and other important organic molecules.
The atmosphere is nearly 80% nitrogen so why then should plants suffer nitrogen deficiencies?
This atmospheric nitrogen is gaseous N2 which plants cannot use. For plants to absorb nitrogen, it must first be converted
to ammonium (NH4
+
) or nitrate (NO3
−
).
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16. Q34 With the help of the PPT label the stages shown in Fig 16.
All life on Earth depends on nitrogen fixation, a process performed only by some bacterial species. Several of these
species live freely in the soil, while others live in plant roots in symbiotic relationships.
Some plant families have mutualistic relationships, for example symbiotic nitrogen fixation, involving roots and bacteria.
The Role of Bacteria in Symbiotic Nitrogen Fixation
Symbiotic relationships with nitrogen–fixing bacteria provide some plant species with a built–in source of fixed nitrogen
for assimilation into organic compounds. From an agricultural perspective, the most important and efficient symbioses
between nitrogen–fixing bacteria and plants occur in the legume family (leguminous plants), including peas, beans,
soybeans, peanuts, alfalfa, and clover.
A legume′s roots have swellings called nodules consisting of plant cells that have been “infected” by nitrogen–fixing
Rhizobium (“root living”) bacteria. Inside the nodule, Rhizobium bacteria take a form called bacteroids, which are
contained in vesicles formed by the root cell. Rhizobium bacteria fix atmospheric N2and supply it as ammonium, a form
readily used by the plant. Legume–Rhizobium symbioses generate more useful nitrogen for plants than all industrial
fertilizers, and the symbiosis provides the right amounts of nitrogen at the right time at virtually no cost to the farmer. In
addition to supplying the legume with nitrogen, symbiotic nitrogen fixation significantly reduces spending on fertilizers
for subsequent crops.
Q35 Label the diagram (Fig.17) and outline the 4 important stages of root nodule development.
Fig. 17 Development of a soybean root nodule
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Fig.16 The role of soil bacteria in nitrogen nutrition of plants

17. Nitrogen fixation requires an anaerobic environment, so the location of the bacteroids inside living, nonphotosynthetic
cells is ideal. Lignified external layers may also limit gas exchange. Some root nodules are reddish in colour, owing to a
molecule called leghemoglobin (leg– for “legume”), an iron–containing protein that, like haemoglobin, binds reversibly
to oxygen. This protein is an oxygen “buffer,” keeping the concentration of free O2 low and regulating the oxygen supply
for the intense respiration that the bacteria require to produce ATP for nitrogen fixation.
Each legume is associated with a particular strain of Rhizobium. The symbiotic relationship between a legume and
nitrogen–fixing bacteria is mutualistic because the bacteria supply the plant with fixed nitrogen while the plant provides
the bacteria with carbohydrates and other organic compounds. Most of the ammonium produced is used by nodules to
make amino acids, which are then transported to the shoot via the xylem.
Symbiotic Nitrogen Fixation and Agriculture
The agricultural practice of crop rotation depends on symbiotic nitrogen fixation. One year a non–legume such as maize
is planted, and the following year alfalfa or some other legume is planted; this restores the concentration of fixed
nitrogen in the soil. Instead of being harvested, the legume crop may be ploughed in so that it will decompose as “green
manure,” reducing the need for manufactured fertilizers. To ensure that the legume encounters its specific Rhizobium,
the seeds are soaked in a culture of the bacteria or dusted with bacterial spores before sowing.
Many plant families besides legumes include species that benefit from symbiotic nitrogen fixation. For example, alder
trees and certain tropical grasses host bacteria. Rice crops benefit indirectly from symbiotic nitrogen fixation. Rice
farmers culture a free–floating aquatic fern called Azolla, which has symbiotic cyanobacteria that fix nitrogen and
increase the fertility of the rice paddy. The growing rice eventually shades and kills the Azolla, and decomposition of this
organic material adds more nitrogenous compounds to the paddy.
Case Study: read the information and answer the questions.
In a study carried out in Kwalei village, 18 km east of Lushoto, investigators looked at the effect of declining soil fertility
on crop yields. Farmers did not use mineral fertilisers because they were too expensive, and they had not got enough
animals to supply manure. In addition, a lot of fertiliser was lost because of sloping fields. A few farmers reported that
spreading leaves from various local shrubs over their fields and ploughing them into the soil(green manure) seemed to
improve fertility. This practice was made the focus of the investigation.
Fig 1. Shows the change in soil inorganic nitrogen content over time, after material from 4 different shrubs had been
ploughed into the soil.
a) Describe the
changes in
nitrogen content
over time for the
different species
of shrub.
b) Suggest what
advice you could
give to a farmer
about which
shrub to use and
when to plant the
food crop.
c) Apart from the use of green manure, what other advice could you give the farmer in order to improve soil
fertility?
d) Use Table 2 to state which shrubs are best at releasing each of the main nutrients.
e) What evidence on Table 2 would suggest that the farmers are right to use these shrubs as green manure?
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18. Fungi and Plant Nutrition
Mycorrhizae (“fungus roots”) are modified roots consisting of mutualistic associations of fungi and roots. The
fungus benefits from a steady supply of sugar donated by the host plant.
Q36 Describe the benefits to the plant
Mycorrhizae are formed by most plant species, and might have been one of the evolutionary adaptations that enabled
plants to colonize land in the first place.
Q37 Give 2 pieces of evidence to support this. Suggest why mycorrhizae would be an advantage in developing land
plants.
The Two Main Types of Mycorrhizae: ectomycorrhizae and endomycorrhizae.
Ectomycorrhizae: mycelium (mass of branching hyphae) covers the surface of the root in a dense ‘mantle’. Fungal
hyphae extend from the mantle into the soil, greatly increasing the surface area for water and mineral absorption.
Hyphae also grow into the cortex of the root, forming a network in the apoplast. Compared with “uninfected” roots,
ectomycorrhizae are generally thicker, shorter, and more branched. They do not form root hairs, which would be
unnecessary given the extensive surface area of the fungal mycelium. About 10% of plant families have species that form
ectomycorrhizae, and the most are woody, including members of the pine, spruce, oak, walnut, birch, willow, and
eucalyptus families.
Endomycorrhizae. Fine fungal hyphae extend from the soil into the root, no dense mantle. Hyphae extend into the root
cells by digesting small patches of the root cell walls. However, a fungal hypha does not actually pierce the plasma
membrane and enter the cytoplasm; it grows into a tube formed by invagination of the root cell′s membrane. The action
is analogous to poking a finger gently into a balloon; your finger is like the fungal hypha, and the balloon skin is like the
root cell′s membrane. After the fungal hyphae have penetrated in this way, some form densely branched structures
called arbuscules (“little trees”), which are important sites of nutrient transfer between the fungus and the plant. Hyphae
may also form oval vesicles, which possibly store food for the fungus. Endomycorrhizae look like “normal” roots with root
hairs, but a microscope reveals a symbiotic relationship of enormous importance to plant nutrition. Endomycorrhizae are
much more common than ectomycorrhizae and are found in over 85% of plant species, including important crop plants
such as maize, wheat, and legumes.
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19. Q 38 Label the figures of mycorrhizae below
Check your understanding: make a table to compare and contrast root nodules and mycorrhizae.
19