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39
Nutrition and Transport
in Plants
Concept Outline
39
...

Plant Nutrients
...

Soil
...


39
...

Nutritional Adaptations
...
Some plants entice
bacteria to produce organic nitrogen for them
...
About 90% of all vascular plants rely on
fungal associations to gather essential nutrients, especially
phosphorus
...
3 Water and minerals move upward through the
xylem
...
The bulk movement of water and dissolved
minerals is the result of movement between cells, across cell
membranes, and through tubes of xylem
...
Water and minerals
enter the plant through the roots
...
A combination of the
properties of water, structure of xylem, and transpiration of
water through the leaves results in the passive movement of
water to incredible heights
...
Too much water is
harmful to a plant, although many plants have adaptations
that make them tolerant of flooding
...
4 Dissolved sugars and hormones are transported
in the phloem
...
Sucrose and
hormones can move from shoot to root or root to shoot in
the phloem
...


FIGURE 39
...
Most plants absorb water and essential
nutrients from the soil, but carnivorous plants are able to obtain
some nutrients directly from small animals
...
In
this chapter, we address two major questions: (1) what inputs, besides energy from the sun, does a plant need to survive? and (2) how do all parts of the complex plant body
share the essentials of life? Plants, like animals, need various
nutrients to remain alive and healthy
...
Plants acquire these
nutrients through photosynthesis and from the soil,
although some take a more direct approach (figure 39
...

Carbohydrates produced in leaves must be carried throughout the plant, and minerals and water absorbed from the
ground must be transported up to the leaves and other parts
of the plant
...

777

39
...


Plant Nutrients
The major source of plant nutrition is the fixation of atmospheric CO2 into simple sugar using the energy of the
sun
...
O2 is a product of
photosynthesis and atmospheric component that also
moves through the stomata
...

However, CO2 and light energy are not sufficient for the
synthesis of all the molecules a plant needs
...
1)
...
There are nine macronutrients: carbon, hydrogen, and oxygen—the three elements
found in all organic compounds—as well as nitrogen

Table 39
...
Each of these nutrients approaches or, as in the
case with carbon, may greatly exceed 1% of the dry weight
of a healthy plant
...
2)
...

Nutritional requirements are assessed in hydroponic
cultures; the plants roots are suspended in aerated water
containing nutrients
...
The plants are then

Essential Nutrients in Plants

Examples of Important Functions

MACRONUTRIENTS

Carbon
Oxygen
Hydrogen
Nitrogen

(CO2 )
(O2, H2O)
(H2O)
(NO3–, NH4+)

44
44
6
1–4

Potassium
Calcium

(K+)
(Ca++)

0
...
2–3
...
1–0
...
1–0
...
05–1

Major component of organic molecules
Major component of organic molecules
Major component of organic molecules
Component of amino acids, proteins, nucleotides, nucleic acids,
chlorophyll, coenzymes, enzymes
Protein synthesis, operation of stomata
Component of cell walls, maintenance of membrane structure and
permeability, activates some enzymes
Component of chlorophyll molecule, activates many enzymes
Component of ADP and ATP, nucleic acids, phospholipids, several
coenzymes
Components of some amino acids and proteins, coenzyme A

MICRONUTRIENTS (CONCENTRATIONS IN PPM)

Chlorine
Iron
Manganese
Zinc
Boron
Copper
Molybdenum

778

(Cl–)
(Fe++, Fe+++)
(Mn++)
(Zn++)
(BO3– or B4O7=)
(Cu++)
(MoO4=)

Part X Plant Form and Function

100–10,000
25–300
15–800
15–100
5–75
4–30
0
...
2
Mineral deficiencies in
plants
...
(b) Chlorinedeficient plant with
necrotic leaves (leaves
with patches of dead
tissue)
...

(d) Zinc-deficient plant
with small, necrotic
leaves
...
The
agricultural implications
of deficiencies such as
these are obvious; a
trained observer can
determine the nutrient
deficiencies that are
affecting a plant simply
by inspecting it
...
3)
...
To give an idea of how
small the quantities of micronutrients may be, the standard
dose of molybdenum added to seriously deficient soils in
Australia amounts to about 34 grams (about one handful)
per hectare, once every 10 years! Most plants grow satisfactorily in hydroponic culture, and the method, although
expensive, is occasionally practical for commercial purposes
...

One application has been the investigation of elevated levels of CO2 (a result of global warming) on plant growth
...
This decreases the nutritional value of the
leaves to herbivores
...


Suspected
nutrient is
not essential

splant
Tran
Monitor
growth

Normal
growth
Complete
nutrient
solution

Solution lacking
one suspected
essential nutrient
Abnormal
growth

Suspected
nutrient is
essential

FIGURE 39
...
A seedling is
first grown in a complete nutrient solution
...

The growth of the seedling is then studied for the presence of
abnormal symptoms, such as discolored leaves and stunted growth
...


Chapter 39 Nutrition and Transport in Plants

779

Soil
Plant growth is affected by soil composition
...
It is composed of a mixture of ingredients, which may include sand,
rocks of various sizes, clay, silt, humus, and various other
forms of mineral and organic matter; pore spaces containing
water and air occur between the particles
...

The crust includes about 92 naturally occurring elements;
table 2
...

Most elements are combined as inorganic compounds called
minerals; most rocks consist of several different minerals
...
About 5 metric tons of carbon is tied
up in the organisms that are present in the soil under a
hectare (0
...
4), which is a
mixture of mineral particles of varying size (most less than
2 mm thick), living organisms, and humus
...
When topsoil is lost
because of erosion or poor landscaping, both the waterholding capacity and the nutrient relationships of the soil
are adversely affected
...
Some of the soil water, because of
its properties described below, is unavailable to plants
...
Another fraction of the water
is held in small soil pores, which are generally less than
about 50 micrometers in diameter
...
When it is depleted through evaporation or root uptake, the plant will wilt and eventually die
unless more water is added to the soil
...
When these
communities are replaced by cultivated crops, the situation
changes drastically: the soil is much more exposed to erosion and the loss of nutrients
...

One solution to this is crop rotation
...
Both crops remove nutrients from the soil, but the
plants have different nutritional requirements, and therefore
the soil does not lose the same nutrients two years in a row
...
Sometimes farmers allow a field to lie fallow—that is,

780

Part X Plant Form and Function

FIGURE 39
...
The uppermost
layer in soil is called
topsoil, and it contains
organic matter, such as
roots, small animals, and
humus, and mineral
particles of various sizes
...
Beneath
the subsoil are layers of
bedrock, the raw
material from which soil
is formed over time and
weathering
...
This allows natural processes to rebuild the field’s store of nutrients
...
You can
do the same thing in a lawn or garden by leaving grass clippings and dead leaves
...

Fertilizers are also used to replace nutrients lost in cultivated fields
...
All of these elements are needed in large
quantities (see table 39
...
Both chemical and organic fertilizers are
often added in large quantities and can be significant sources
of pollution in certain situations (see chapter 30)
...
Substances such as manure or the remains of
dead animals have traditionally been applied to crops, and
plants are often plowed under to increase the soil’s fertility
...
However, organic fertilizers build up the
humus content of the soil, which often enhances its waterand nutrient-retaining properties
...

Soils contain organic matter and various minerals and
nutrients
...


39
...


Nutritional
Adaptations

Nitrogen-Fixing Bacteria

Carnivorous Plants
Some plants are able to obtain nitrogen directly from other organisms,
just as animals do
...
By
capturing and digesting small animals
directly, such plants obtain adequate
nitrogen supplies and thus are able to
grow in these seemingly unfavorable
environments
...
5)
...

The Venus flytrap (Dionaea muscipula), which grows in the bogs of
coastal North and South Carolina,
has three sensitive hairs on each side
of each leaf, which, when touched,
trigger the two halves of the leaf to
snap together (see figure 39
...
Once
the Venus flytrap enfolds a prey item
within a leaf, enzymes secreted from
the leaf surfaces digest the prey
...
They have a
limited number of times they can
open and close as a result
...

Unlike Venus flytraps they do not
close rapidly and it is possible that
the two share a common ancestor
...

Once inside the pitchers, insects slide
down into the cavity of the leaf,
which is filled with water and digestive enzymes
...
They sweep small animals
into their bladderlike leaves by the
rapid action of a springlike trapdoor,
and then they digest these animals
...
5
A carnivorous plant
...
Insects enter the
pitchers and are trapped and digested
...


Plants need ammonia (NH3) to build
amino acids, but most of the nitrogen
is in the atmosphere in the form of
N 2
...

Some of these bacteria live in close
association with the roots of plants
...
6)
...
Hosting these bacteria
costs the plant in terms of energy, but
is well worth it when there is little
ammonia in the soil
...


Mycorrhizae
While symbiotic relationships with
nitrogen-fixing bacteria are rare, symbiotic associations with mycorrhizal
fungi are found in about 90% of the
vascular plants
...

In terms of plant nutrition, it is important to recognize the significant
role these organisms play in enhancing phosphorus transfer to the plant
...
Functionally,
the mycorrhizae extend the surface
area of nutrient uptake substantially

FIGURE 39
...
A root hair of
alfalfa is invaded by Rhizobium, a bacterium
(yellow structures) that fixes nitrogen
...


Carnivorous plants obtain
nutrients, especially nitrogen,
directly by capturing and
digesting insects and other
organisms
...

Fungi help plants obtain
phosphorus and other nutrients
from the soil
...
3

Water and minerals move upward through the xylem
...
It
is not unusual for a large tree to have leaves more than 10
stories off the ground (figure 39
...
Did you ever wonder
how water gets from the roots to the top of a tree that
high? Water moves through the spaces between the protoplasts of cells, through plasmodesmata (membrane connections between cells), through cell membranes and through
the continuous tubing system in the xylem
...
We also know that water
first enters the roots and then moves to the xylem
...

While most of our focus will be on the mechanics of
water transport through xylem, the movement of water at
the cellular level plays a significant role in bulk water
transport in the plant as well, although over much shorter
distances
...
In the case of
parenchyma cells it turns out that most water also moves
across membranes rather than in the intercellular spaces
...

We now know that osmosis is enhanced by water channels
called aquaporins
...
In plants they exist in vacuole and
plasma membranes
...
Some aquaporins only appear or open during drought stress
...
They are important not only in maintaining
water balance within a cell, but in getting water between
many plant cells and the xylem
...

Once water enters the xylem, it can move upward
100 m in the redwoods
...
However,
most of the force is “pulling” caused by water evaporating
(transpiration) through the stomata on the leaves and
other plant surfaces
...

The result is an unusually stable column of liquid reaching great heights
...
7
How does water get to the top of this tree? We would expect
gravity to make such a tall column of water too heavy to be
maintained by capillary action
...


Xylem

Stoma
H2O
Carbohydrates

Water exits the plant
through stomata in leaves
...


H2O
Water and carbohydrates
travel to all parts of the plant
...

The water potential of
soil is high
...
8
Water movement through a plant
...


Water Potential
Plant biologists often discuss the forces that act on water
within a plant in terms of potentials
...

Water coming through a garden hose is an example of
physical pressure
...
By
applying pressure (on the side that has the greater concentration of solute), it is possible to prevent osmosis
from taking place
...
Water will enter a
cell osmotically until it is stopped by the pressure potential caused by the cell wall
...
If two adjacent cells
have different water potentials, water will move from the
cell with the higher water potential to the cell with the
lower water potential
...

Water potential in a plant regulates movement of
water
...
On the surface of
leaves and other organs, water loss called transpiration
creates a negative pressure
...
8)
...

Aquaporins enhance water transport at the cellular
level, which ultimately affects bulk water transport
...
This
works because of the strong cohesive forces between
molecules of water that allow them to stay “stuck”
together in a liquid column and adhesion to walls of
tracheids and vessels
...
8)
...
Because the mineral ion concentration in
the soil water is usually much lower than it is in the plant,
an expenditure of energy (supplied by ATP) is required
for the accumulation of such ions in root cells
...
Once in the roots, the ions,
which are plant nutrients, are transported via the xylem
throughout the plant
...
9)
...
Eventually, on their journey inward, they reach the endodermis and any further
passage through the cell walls is blocked by the Casparian

strips
...
However, transport through the cells of the endodermis is selective
...

Transpiration from the leaves (figure 39
...

However, at night, when the relative humidity may approach 100%, there may be no transpiration
...

Active transport of ions into the roots still continues to
take place under these circumstances
...

In terms of water potential, we say that active transport increases the solute potential of the roots
...
This phenomenon is called root pressure, which in reality is an osmotic phenomenon
...
9
The pathways of mineral transport in roots
...
In passing
through the cortex, they must either follow the cell walls and the spaces between them or go directly through the plasma membranes and
the protoplasts of the cells, passing from one cell to the next by way of the plasmodesmata
...


784

Part X Plant Form and Function

Upper epidermis
Palisade mesophyll
Vascular bundle
Water exits
plant through
stomata

Spongy mesophyll
Intercellular space (100% humidity)
Stoma

Epidermis

Water moves
up plant through
xylem

Water molecule
Water enters
plant through
roots

FIGURE 39
...
Water evaporating from the leaves through the stomata causes the movement of water upward in the xylem and the
entrance of water through the roots
...
When root pressure is very high, it can force
water up to the leaves, where it may be lost in a liquid
form through a process known as guttation (figure 39
...

Guttation does not take place through the stomata, but instead occurs through special groups of cells located near
the ends of small veins that function only in this process
...

Water enters the plant by osmosis
...
Root
pressure, which often occurs at night, is caused by the
continued, active accumulation of ions in the roots at
times when transpiration from the leaves is very low or
absent
...
11
Guttation
...

Chapter 39 Nutrition and Transport in Plants

785

Water and Mineral Movement

Transpiration of Water from Leaves

Water and Mineral Movement through the Xylem

More than 90% of the water taken in by the roots of a
plant is ultimately lost to the atmosphere through transpiration from the leaves
...
As you saw in chapter 38, these intercellular
spaces are in contact with the air outside of the leaf by way
of the stomata
...
This water
is continuously replenished from the tips of the veinlets in
the leaves
...

Photosynthesis requires a supply of CO 2 entering the
stomata from the atmosphere
...
Structural features such as stomata and the cuticle have evolved in response to one or both of these requirements
...
After the sun sets,
transpiration from the leaves decreases
...

The water potential that is responsible for water movement is largely the product of negative pressure generated
by transpiration, which is driven by the warming effects of
sunlight
...
So, what
does work? Otto Renner proposed the solution in Germany in 1911
...
” Evaporation from the leaves produces a tension on the entire water column that extends
all the way down to the roots
...
The tensile strength of a column of water varies
inversely with the diameter of the column; that is, the
smaller the diameter of the column, the greater the tensile
strength
...

The water molecules also adhere to the sides of the tracheid or xylem vessels, further stabilizing the long column
of water
...
Anatomical adaptations decrease the probability of this
...
Air
bubbles are generally larger than the openings, so they
cannot pass through them
...
Deformed cells or freezing
can cause small bubbles of air to form within xylem cells
...
This is more likely to occur with
seasonal temperature changes
...

Most minerals the plant needs enter the root through
active transport
...
Phosphorus, potassium, nitrogen, and sometimes iron may be abundant in the
xylem during certain seasons
...

Keep in mind that minerals that are relocated via the
xylem must move with the generally upward flow
through the xylem
...
Calcium, an essential nutrient, cannot be
transported elsewhere once it has been deposited in plant
parts
...
On a shortterm basis, closing the stomata can control water loss
...
However, the stomata must be open at least
part of the time so that CO2 can enter
...
The gas dissolves mainly in water on the
walls of the intercellular spaces below the stomata
...
A plant must respond
both to the need to conserve water and to the need to
admit CO2
...
The sausage- or dumbbellshaped guard cells stand out from other epidermal cells not
only because of their shape, but also because they are the
only epidermal cells containing chloroplasts
...
You can make a model of this
for yourself by taking two elongated balloons, tying the
closed ends together, and inflating both balloons slightly
...
Now place

Chloroplasts

Guard cell

Chloroplasts

H2O

Epidermal cell

H2O

H2O

Nucleus

H2O

H2O

H2O

H2O

H2O

Thickened inner
cell wall (rigid)

H2O

H2O
Stoma open
Solute potential is high;
water moves into guard
cells

(a)

H2O

H2O
Stoma closed
Solute potential is low;
water moves out of
guard cells

(b)

FIGURE 39
...
(a) When potassium ions from surrounding cells are pumped into guard cells, the guard cell turgor
pressure increases as water enters by osmosis
...
(b) When the potassium ions leave the guard cells and their
solute potential becomes low, they lose water and turgor, and the stoma closes
...
Hold the open ends together
...
Real guard cells rely on
the influx and efflux of water, rather than air, to open and
shut
...
This creates a solute potential within the guard cells that causes water to enter
osmotically
...
12a)
...
When stomata close, sucrose,
rather than K+, leaves the cell through sucrose transporters
...
12b)
...
Experimental evidence is consistent
with several pathways regulating stomatal opening and
closing
...
In some species, Cl– accompanies the K+ in and out of the guard cells, thus maintaining electrical neutrality
...

When a whole plant wilts because there is insufficient
water available, the guard cells may also lose turgor, and as
a result, the stomata may close
...
When they are turgid,
the stomata open, and CO2 enters freely; when they are
flaccid, CO2 is largely excluded, but water loss is also retarded
...
This hormone is released from chloroplasts and
produced in leaves
...
Plants likely control the
duration of stomatal opening through the integration of
several stimuli, including blue light
...


Chapter 39 Nutrition and Transport in Plants

787

Other Factors Regulating Transpiration
...
When CO2 concentrations are high,
guard cells of many plant species lose turgor, and their
stomata close
...
The
stomata also close when the temperature exceeds 30° to
34°C when transpiration would increase substantially
...

In chapter 10, we mentioned CAM photosynthesis, which
occurs in some succulent like cacti
...
CAM photosynthesis conserves water in dry environments where succulent
plants grow
...
One involves dormancy during dry times
of the year; another involves loss of leaves
...
Plants are often deciduous in regions with
severe winters, when water is locked up in ice and snow and
thus unavailable to them
...

Thick, hard leaves, often with relatively few stomata—
and frequently with stomata only on the lower side of the
leaf—lose water far more slowly than large, pliable leaves
with abundant stomata
...
These trichomes also increase humidity at the leaf
surface
...
Within these
depressions the water vapor content of the air may be high,
reducing the rate of water loss
...
” Flooding rapidly depletes available oxygen in the
soil and interferes with the transport of minerals and carbohydrates in the roots
...
Hormone levels change in flooded plants—ethylene (the only
hormone that is a gas) increases, while gibberellins and cytokinins usually decrease
...

Oxygen-deprivation is among the most significant problems
...
Generally, standing water flooding is more harmful
to a plant (riptides excluded)
...

Physical changes that occur in the roots as a result of
oxygen deprivation may halt the flow of water through the
plant
...
One
adaptive solution is that stomata of flooded plants often
close to maintain leaf turgor
...
13
Adaptation to flooded conditions
...


Adapting to Life in Fresh Water
...
This involved a major change in controlling salt balance
...
13)
...
14)
...
Oxygen may be
transported from the parts of the plant above water to
those below by way of passages in the aerenchyma
...

Some plants normally form aerenchyma, whereas others,
subject to periodic flooding, can form it when necessary
...

Plants also respond to flooded conditions by forming larger
lenticels (which facilitate gas exchange) and additional adventitious roots
...
14
Aerenchyma tissue
...
(a) Water lilies float on the surface of ponds where oxygen is collected and
transported to submerged portions of the plant
...
The specialized parenchyma tissue that
forms these open spaces is called aerenchyma
...


Adapting to Life in Salt Water
...
The salt must be excluded, actively secreted, or diluted as it enters
...
These roots,
called pneumatophores (see chapter 38), have large
lenticels on their above-water portions through which oxygen enters; it is then transported to the submerged roots
(figure 39
...
In addition, the succulent leaves of mangroves contain large quantities of water, which dilute the
salt that reaches them
...

Transpiration from leaves pulls water and minerals up
the xylem
...
Stomata open when their guard cells
become turgid
...
Biochemical, anatomical, and
morphological adaptations have evolved to reduce water
loss through transpiration
...
However, plants can survive flooded conditions,
and even thrive in them, if they can deliver oxygen to
their submerged parts
...
15
How mangroves get oxygen to their submerged part
...
However, modified
roots called pneumatophores supply the submerged portions of
the plant with oxygen because these roots emerge above the water
and have large lenticels
...


Chapter 39 Nutrition and Transport in Plants

789

39
...


Phloem Transport Is Bidirectional
Most carbohydrates manufactured in leaves and other
green parts are distributed through the phloem to the rest
of the plant
...
Carbohydrates concentrated in storage
organs such as tubers, often in the form of starch, are also
converted into transportable molecules, such as sucrose,
and moved through the phloem
...
Radioactive carbon dioxide (14CO2) gets incorporated into glucose as a
result of photosynthesis
...
Such studies
have shown that sucrose moves both up and down in the
phloem
...

Aphids thrust their stylets (piercing mouthparts) into
phloem cells of leaves and stems to obtain abundant sugars
there
...
16)
...
Using aphids to obtain the critical samples and
radioactive tracers to mark them, it has been demonstrated
that movement of substances in phloem can be remarkably
fast; rates of 50 to 100 centimeters per hour have been
measured
...
As we will explore in the
next chapter, environmental signals can result in the rapid
translocation of hormones in the plant
...
Experimental evidence supports
much of this model
...

Carbohydrate sources include photosynthetic tissues, such
as the mesophyll of leaves, and food-storage tissues, such as
the cortex of roots
...


790

Part X Plant Form and Function

(a)

(a)

FIGURE 39
...
(a) Aphids, like this individual of
Macrosiphon rosae shown here on the edge of a rose leaf, feed on
the food-rich contents of the phloem, which they extract through
their piercing mouthparts (b), called stylets
...


In a process known as phloem loading, carbohydrates
(mostly sucrose) enter the sieve tubes in the smallest
veinlets at the source
...
Companion cells and
parenchyma cells adjacent to the sieve tubes provide the
ATP energy to drive this transport
...
17
Diagram of mass flow
...
Moving from the
mesophyll cells of a leaf or another part of the plant into the conducting cells of the phloem, the sucrose molecules are then transported to
other parts of the plant by mass flow and unloaded where they are required
...
Turgor pressure in the sieve tubes increases
...
At the sink,
carbohydrates are actively removed
...
17)
...

Transport of sucrose and other carbohydrates through
sieve tubes does not require energy
...


Chapter 39 Nutrition and Transport in Plants

791

Chapter 39
Summary

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com/raven6e

www
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com

Questions

Media Resources

39
...

• Plants require a few macronutrients in large amounts
and several micronutrients in trace amounts
...

• Plant growth is significantly influenced by the nature
of the soil
...


1
...


• Nutrients
• Soil

39
...

• Some plants entice bacteria to produce organic
nitrogen for them
...

• About 90% of all vascular plants rely on fungal
associations to gather essential nutrients
...
The atmosphere is full of
nitrogen yet it is inaccessible to
most plants
...
3 Water and minerals move upward through the xylem
...
Energy is required for active transport
...
Aquaporins are
water channels that enhance osmosis
...
The ultimate energy source for pulling water
through xylem vessels and tracheids is the sun
...
Stomata open when their guard cells
are turgid and bulge, causing the thickened inner
walls of these cells to bow away from the opening
...


3
...
What proportion of water
that enters a plant leaves it via
transpiration?
5
...
Under what environmental
condition is water transport
through the xylem reduced to
near zero? How much
transpiration occurs under these
circumstances?
7
...


39
...

• Sucrose and hormones can move up and down in the
phloem between sources and sinks
...

Sucrose is loaded into the phloem near sites of synthesis, or sources, using energy supplied by the companion cells or other nearby parenchyma cells
...
What is translocation? What
is the driving force behind
translocation?
9
...
Is this
process active or passive?

• Activity: Water
Movement
• Uptake by Roots
• Water Movement
• Student Research:
Heavy Metal Uptake



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