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

Transport in Plants – Water Potential

Roots xylem stomata

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?

What’s in dirt?!

Mineral Acquisition

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

Epiphytes, Parasitic, and Carnivorous PlantsStaghorn fern,

an epiphyteEPIPHYTESAnchored on another plant, self-nourished

PARASITIC PLANTSAbsorb sugar/minerals

from host plant

Mistletoe, a photosynthetic parasite

Pitcher plantscavity filled with digestive fluid

Venus flytrap

To gain extra nitrogen