Lecture 10 Outline (Ch. 36, 37)
I.
Water potential
II.
Transpiration
III. Active transport & bulk flow
IV. Stomatal control
V.
Mineral acquisition
VI. Essential nutrients
VII. Symbioses & other modes
of nutrition
VIII. Summary
Transport in Plants
Physical forces drive the transport of
materials in plants over a range of distances
Transport occurs on three scales
1.
2.
3.
Within a cell – cellular level
Short-distance cell to cell –tissue level
Long-distance in xylem & phloem whole plant level
Transport occurs by 3 mechanisms:
A. Osmosis & Diffusion
B. Active Transport
C. Bulk Flow
2
Transport in Plants – Water Potential
Roots  xylem  stomata
To survive
Water Potential
– Plants must balance water uptake and loss
• What is Osmosis? What is diffusion?
• Water potential : predicts water movement due to
solute concentration & pressure
– designated as psi (ψ)
Water molecules are
attracted to:
• Each other (cohesion)
• Solid surfaces (adhesion)
Water Potential
• Free water flows from regions of high water
potential to regions of low water potential
Ψ changes with:
• Adding solutes
• Adding pressure
Water potential = Potential energy of water =
Energy per volume of water in megapascals (MPa)
ψTotal = ψsolute + ψpressure
Water Potential
(a)
• Solutes added
 decreases ψ
0.1 M
solution
(water less likely to cross
membrane)
Pure
water
(in an open area, no
pressure, so ψp = 0)
H2O
 = 0 MPa
P = 0
S = 0.23
 = 0.23 MPa
Water Potential
• Application of physical pressure
 increases ψ
(water more likely to cross membrane)
(b)
(c)
H2O
P = 0.23
S = 0.23
 = 0 MPa  = 0 MPa  = 0 MPa
H2O
P = 0.30
S = 0.23
 = 0.07 MPa
Let’s say Ψ inside a plant cell is -0.5 Mpa and
outside the cell the solution Ψ is 0.2 Mpa.
What will happen in
terms of movement of
water and to the cell?
Water Potential
Water Potential
ψ = ψs + ψp
Which
direction
will water
move?
ψcell = – 0.7 MPa + 0.5 MPa = – 0.2 MPa
ψsolution = –0.3 MPa (solution has no pressure potential)
Water Potential
• Water potential
– Affects uptake and loss of water by plant cells
• If a flaccid cell is placed in an environment with a higher
solute concentration
– The cell will lose water and become plasmolyzed
0.4 M sucrose
solution:
Plasmolyzed cell
at osmotic
equilibrium
with its
surroundings
P = 0
S = 0.9
 = 0.9 MPa
P = 0
S = 0.9
 = 0.9 MPa
Initial flaccid cell:
P = 0
S = 0.7
 = 0.7 MPa
If the same flaccid cell is placed in a
solution with a lower solute concentration
Initial flaccid cell:
P = 0
S = 0.7
 =
MPa
Distilled water:
P =
S =
 =
MPa
Cell at osmotic
equilibrium with
its surroundings
P =
S =
 = MPa
Water Potential
Uses of turgor
pressure:
•
•
•
Inexpensive cell
growth
Hydrostatic
skeleton
Phloem
transport
For the situation below, what will be the final
solute potential inside the plant cell?
Solution
Ψs = -0.5
Ψp =
Ψs = -0.3
Ψp = 0.4
Ψ=
Ψ=
Final Cell
Ψs =
Ψp = 0
Ψ=
Initial Cell
Water Route
Most plant tissues
- cell walls and cytosol are continuous cell to cell (via?)
- cytoplasmic continuum called the symplast
apoplast = continuum of cell walls plus extracellular spaces
Water Route
How do water and minerals get from the soil to vascular tissue?
Symporters
(cotransporters)
contribute to the gradient
that determines the
directional flow of water.
H2O
Soil
Soil
Cytosol
Here, pumps in H+ and mineral ions
Water enters plants
via the roots – why?
Symporter
H+
Mineral
ions
Water
Water Potential
Minerals & ions pumped
into root cells, then moved
past endodermis
What happens to ψ between soil
and endodermis?
Where is osmosis occurring?
16
Water Potential
Once water & minerals cross the endodermis, they are
transported through the xylem to upper parts of the plant.
Casparian Strip – waxy belt of suberin that blocks water and
dissolved minerals – must go through the cell membrane.
Xylem
Water exits plant
through stomata.
Smooth
surface
Rippled
surface
H2O
Water moves up
plant through xylem.
Water film that coats
mesophyll cell walls
evaporates.
Adhesion to xylem cells
Cohesion
between water
molecules
18
18
Transpiration = loss of water from the shoot
system to the surrounding environment.
Bulk Flow = movement of fluid due to pressure gradient
•
Transpiration drives bulk flow of xylem sap.
•
Water is PULLED up a plant – against gravity
•
Ring/spiral wall thickening protects against vessel collapse
19
Xylem Ascent by Bulk Flow
• The movement of xylem sap is against gravity
– maintained by the transpiration-cohesion-tension
• Stomata help regulate the rate of transpiration
• Leaves generally have broad surface areas
• These characteristics
– Increase photosynthesis
– Increase water loss through stomata
20
µm
We know that water moves from areas of
higher (more positive) water potential to regions
of lower (more negative) water potential.
A. How does ψ of the root compare to that in the
soil outside the root?
B. How does ψ in the air compare to that in the
leaf of a plant undergoing transpiration?
Xylem
What happens if rate of transpiration nears zero?
i.e. – at night, water pressure
builds up in the roots
•
Guttation
22
H+ pumped out
Stomata Control
K+ flow in
H2O flow in
Why?
stomata open
Why?
K+ channels, aquaporins and
radially oriented cellulose
fibers play important roles.
Cues for opening stomata:
Light
Depleted CO2
Internal cell “clocks”
Phloem
•
•
•
Direction is source to sink
Near source to near sink
Phloem under positive
pressure
Phloem sap composition:
Phloem tissue
•
•
•
•
•
Sugar (mainly sucrose)
amino acids
hormones
minerals
enzymes
Are tubers and bulbs sources or sinks?
Aphid
Phloem
Pressure Flow Hypothesis
Vessel
(xylem)
Sieve tube Source cell
(phloem) (leaf)
H2O
Sucrose
H2O
1
Where are sugars made?
2
Pressure flow
Water potential increased, turgor pressure
increased, sap PUSHED through phloem
Sugars removed (actively) at sink
 water potential decreased,
water leaves phloem
2
3
Transpiration stream
Sugars actively transported into
companion cells  plasmodesmata
to sieve tube elements
Via H+/sucrose
Water follows (WHY?!) cotransporters
1
4
Sink cell
(storage
root)
3
H2O
Sucrose
4
Overview: A Nutritional Network
• Every organism
– Continually exchanges energy and materials with its
environment
• The branching root and shoot system provides high SA:V to
collect resources
– Plants’ resources are diffuse (scattered, at low
concentration)
What are these diffuse resources?
26
Mineral Acquisition
What’s in dirt?!
Mineral Acquisition
Soil particle
Cation Exchange
• Makes cations
available for
uptake.
K+ –
––
–
Cu2+ K+
– –
Mg2+
– +
K
– –
Ca2+
H+
CO2
H+
Root hair
H2O
Steps:
1. Roots acidify soil solution via respired CO2 and
H+/ATPase pumps
2. H+ attracted to soil particle (-) which “releases” cations
3. Roots absorb cations
28
Which are more likely to be leached
from soil after heavy rains/watering:
1.
2.
3.
4.
cations: K+, H+, Mg+, Ca++
anions: NO3-, PO4-, SO4Both equally likely to be leached
Neither – ions are strongly attracted to the soil
30
Essential Nutrients and Deficiencies
• Plants require certain chemicals to thrive
• Plants derive most organic mass from the CO2 of air
– Also depend on soil nutrients like water and minerals
Essential elements:
Required for a plant to
complete its life cycle
30
Essential Nutrients and Deficiencies
• Photosynthesis = major source of plant nutrition
• Overall need
– Macronutrients – used in larger amounts
• Nine = C, O, H, N, K, Ca, Mg, P, and S
– Micronutrients – used in minute amounts
• Seven = Cl, Fe, Mn, Zn, B, Cu, and Mo
Healthy
Deficiency of any
one can have
severe effects on
plant growth
Phosphate-deficient
Potassium-deficient
Nitrogen-deficient
31
Relationship with other organisms
•
•
•
•
Mycorrhizae
Root nodulation
Parasitic plants
Carnivorous plants
32
Relationship with other organisms
• Symbiotic associations with mycorrhizal fungi are found in
about 90% of vascular plants
– Substantially expand the surface area available for
nutrient uptake
– Enhance uptake of phosphorus and micronutrients
The fungus gets: sugars from plant
Agriculturally, farmers and foresters
…Often inoculate seeds with
spores of mycorrhizae to promote
mycorrhizal relationships.
Nitrogen, Soil Bacteria and Nitrogen Availability
• Plants need ammonia (NH3) or nitrate (NO3–) for: Proteins, nucleic
acids, chlorophyll…
• Nitrogen-fixing soil bacteria convert atmospheric N2 to
nitrogenous minerals that plants can absorb
N2
N2
Atmosphere
Soil
N2
Nitrogen-fixing
bacteria
Denitrifying
bacteria
H+
(From soil)
Soil
+
NH4
NH3
(ammonia)
–
+
NH4
(ammonium)
Organic
material (humus)
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
Nitrifying
bacteria
NO3
(nitrate)
Ammonifying
bacteria
Root
Symbiotic relationships form between nitrogen-fixing bacteria
and certain plants - Mainly legume family (e.g. peas, beans)
• Nodules: Swellings of plant cells
“infected” by Rhizobium bacteria
Nodules
Bacteroids
within
vesicle
5 m
Roots
(a) Pea plant root
(b) Bacteroids in a soybean root
nodule. In this TEM, a cell from
a root nodule of soybean is filled
with bacteroids in vesicles. The
cells on the left are uninfected.
• Inside the nodule
– Rhizobium bacteria assume a
form called bacteroids, which
are contained within vesicles
formed by the root cell
Epiphytes, Parasitic, and Carnivorous Plants
EPIPHYTES
Anchored on another
plant, self-nourished
PARASITIC PLANTS
Absorb sugar/minerals
from host plant
Staghorn fern,
an epiphyte
Pitcher plants
cavity filled with
digestive fluid
Venus flytrap
Mistletoe, a
photosynthetic parasite
To gain extra
nitrogen