I.Water potential II.Transpiration III.Active transport & bulk flow IV.Stomatal control V.Mineral...
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Transcript of I.Water potential II.Transpiration III.Active transport & bulk flow IV.Stomatal control V.Mineral...
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
Lecture 10 Outline (Ch. 36, 37)
2
Physical forces drive the transport of materials in plants over a range of distances
Transport occurs on three scales
1. Within a cell – cellular level2. Short-distance cell to cell –tissue level3. Long-distance in xylem & phloem -
whole plant level
Transport in Plants
Transport occurs by 3 mechanisms:A. Osmosis & DiffusionB. Active TransportC. Bulk Flow
To survive– 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 Potential
Water molecules are attracted to:• Each other (cohesion)• Solid surfaces (adhesion)
• Free water flows from regions of high water potential to regions of low water potential
Water Potential
• Adding solutes
• Adding pressure
Water potential = Potential energy of water = Energy per volume of water in megapascals (MPa)
ψTotal = ψsolute + ψpressure
Ψ changes with:
0.1 Msolution
H2O
Purewater
P = 0
S = 0.23
= 0.23 MPa = 0 MPa
(a)
• Solutes added decreases ψ
(water less likely to cross membrane)
Water Potential
(in an open area, no pressure, so ψp = 0)
• Application of physical pressure increases ψ
(water more likely to cross membrane)
H2OP
= 0.23S
= 0.23
= 0 MPa = 0 MPa
(b)
H2O
P = 0.30
S = 0.23
= 0.07 MPa = 0 MPa
(c)
Water Potential
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
ψcell = – 0.7 MPa + 0.5 MPa = – 0.2 MPa
ψ = ψs + ψp
ψsolution = –0.3 MPa (solution has no pressure potential)
Water Potential
Which direction will water move?
• 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:
Initial flaccid cell:
Plasmolyzed cellat osmotic equilibriumwith its surroundings
P = 0
S = 0.7
P = 0
S = 0.9
P = 0
S = 0.9
= 0.9 MPa
= 0.7 MPa
= 0.9 MPa
Water Potential
If the same flaccid cell is placed in a solution with a lower solute concentration
Distilled water:Initial flaccid cell:
Cell at osmotic equilibrium with its surroundings
P = 0
S = 0.7
P =
S =
P =
S =
= MPa
= MPa
= MPa
Uses of turgor pressure:
• Inexpensive cell growth
• Hydrostatic skeleton
• Phloem transport
Water Potential
For the situation below, what will be the final solute potential inside the plant cell?
Ψs = -0.3Ψp = 0.4
Ψ =Ψs = -0.5Ψp =
Ψ =
Ψs =Ψp = 0
Ψ =
Final Cell
Solution
Initial Cell
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
Symporters (cotransporters) contribute to the gradient that determines the directional flow of water.
SoilH2O
Mineralions
Symporter
Water
Soil
Cytosol
H+
Water Route
Water enters plants via the roots – why?
How do water and minerals get from the soil to vascular tissue?
Here, pumps in H+ and mineral ions
16
Minerals & ions pumped into root cells, then moved past endodermis
What happens to ψ between soil and endodermis?
Where is osmosis occurring?
Water Potential
Once water & minerals cross the endodermis, they are transported through the xylem to upper parts of the plant.
Water Potential
Casparian Strip – waxy belt of suberin that blocks water and dissolved minerals – must go through the cell membrane.
1818
Water exits plantthrough stomata.
Smoothsurface
Rippledsurface
Water film that coats mesophyll cell walls evaporates.
Water moves up plant through xylem.
Adhesion to xylem cells
Cohesion between watermolecules
H2O
Xylem
19
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
Transpiration = loss of water from the shoot system to the surrounding environment.
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?
22
What happens if rate of transpiration nears zero?
• Guttation
Xylem
i.e. – at night, water pressure builds up in the roots
Stomata ControlH+ pumped out
K+ flow in
H2O flow in
stomata open
Why?
Why?
K+ channels, aquaporins and radially oriented cellulose fibers play important roles.
Cues for opening stomata:
Light
Depleted CO2
Internal cell “clocks”
Phloem tissue
• Direction is source to sink• Near source to near sink• Phloem under positive
pressure
Phloem
Are tubers and bulbs sources or sinks?
Phloem sap composition:
• Sugar (mainly sucrose)• amino acids• hormones• minerals• enzymes
Aphid
Vessel(xylem)
H2O
H2O
Sieve tube(phloem)
Source cell(leaf)
Sucrose
H2O
Sink cell(storageroot)
1
Sucrose
2
43
1
2
3
4
Tra
nsp
irat
ion
str
eam
Pre
ssu
re f
low
PhloemPressure Flow Hypothesis
Where are sugars made?
Sugars actively transported into companion cells plasmodesmata to sieve tube elements
Via H+/sucrose cotransporters
Water potential increased, turgor pressure increased, sap PUSHED through phloem
Sugars removed (actively) at sink water potential decreased, water leaves phloem
Water follows (WHY?!)
26
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?
28
H2O
Root hair
K+
Cu2+ Ca2+Mg2+
K+
K+
H+
H+
Soil particle–
– – – – – – ––
Mineral Acquisition
CO2
Steps:1. Roots acidify soil solution via respired CO2 and H+/ATPase pumps
2. H+ attracted to soil particle (-) which “releases” cations3. Roots absorb cations
Cation Exchange
• Makes cations available for uptake.
Which are more likely to be leached from soil after heavy rains/watering:
1. cations: K+, H+, Mg+, Ca++
2. anions: NO3-, PO4-, SO4-
3. Both equally likely to be leached
4. Neither – ions are strongly attracted to the soil
30
30Essential 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
31
• 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
Essential Nutrients and Deficiencies
Phosphate-deficient
Healthy
Potassium-deficient
Nitrogen-deficient
Deficiency of any one can have severe effects on plant growth
32
• Mycorrhizae• Root nodulation• Parasitic plants• Carnivorous plants
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
Relationship with other organisms
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
Soil
N2 N2
Nitrogen-fixingbacteria
Organicmaterial (humus)
NH3
(ammonia)
NH4+
(ammonium)
H+
(From soil)
NO3–
(nitrate)Nitrifyingbacteria
Denitrifyingbacteria
Root
NH4+
Soil
Atmosphere
Nitrate and nitrogenous
organiccompoundsexported in
xylem toshoot system
Ammonifyingbacteria
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
(a) Pea plant root
Nodules
Roots
• Inside the nodule– Rhizobium bacteria assume a
form called bacteroids, which are contained within vesicles formed by the root cell
(b) Bacteroids in a soybean root nodule. In this TEM, a cell froma root nodule of soybean is filledwith bacteroids in vesicles. The cells on the left are uninfected.
5 m
Bacteroidswithinvesicle