Transport in Plants II (cont.)Water Balance of Plants

It is wise to bring some water, when one goes out to look for water.

Arab Proverb

Big Picture

Xylem venation brings water close to every cell in the leaf

(and other living cells).

Water Relationsat 10 m

Cell

Xylem

S = P

-0.9 =

-0.9 =

-0.8 +

0.2 +

(-0.1)

(-1.1)

Hint: Think about the water relations of mesophyll cells, and don’t forget their function.

Water is pulled from the xylem into the cell walls of the mesophyll cells, and evaporates.Xylem venation brings water close to every cell in the leaf.

Water Use Water Loss

-m =r

-

T: surface tension of H2O,(7.28 x 10-8 MPa m-1)

r: radius

Transpiration

Matric potential...

P = S m

See Fig. 36.11

Diffusion

tc = 1/2 = 0.042s

L (distance) = ~1 mm

Ds of water in air = 2.4 x 10-5 m2 s-1

tc = 1/2 = L2

Ds

• across the plasma membrane and cell wall of the mesophyll cells produces tension that draws water through the xylem,

• is driven by the cwv (regulated by stomata),

• Note: m is driven by the cwv.

H2O versus CO2 Flux

…determined by pathway and diffusion coefficients,

CO2 pathway is longer,

• through cytoplasm, through two chloroplast membranes,

Conductance for H2O is ~8-10x that for CO2.

…diffusion (concentration gradients),

H2O representative concentrations:

Inside: 1.27 mole m-3,Outside: 0.47 mole m-3, = 0.8 mole m-3,

CO2 representative concentrations:

Inside: ~ 0.0 mole m-3 (ideal),Outside: ~0.014 mole m-3, = 0.014 mole m-3,

H2O gradient ~ 57x greater than CO2.

Flux = Conductance x Driving Force

~ 450 - ~600 H2O per every CO2

Guard Cell StructureFeatures

• plasmodesmata between guard cells, but not between guard cells and other cells,

• Chloroplasts,

– typically the only epidermal cells with chloroplasts,

• Highly flexible walls,

– radially reinforced with cellulose microfibrils,

• Pore that opens and closes.

Guard Cell Function

One method of regulation…[sucrose].

Stomatal Function

Guard Cell Control

• Light,

– blue light signal transduction,– ATP synthesis,– carbohydrates,

• CO2 concentration,

• Circadian rhythms,

• Hormones.

Big Picture

Cavitation...you can hear plants “cry”

• H2O in the transpiration stream is in a physically metastable state,

– experimental values for required to break a pure water column in a capillary tube exceed -30 MPa,

– -3 MPa exceeds physiolgical requirements of even the tallest trees,

– As tension increases in the water, a higher probability of gas leaking into the system occurs (air seeding),

• Gases do not resist tensile forces, thus the gas bubble expands (cavitation).

• Gases also have reduced solubility in ice, thus freezing of the xylem sap also causes cavitation.

Cavitation“cures”

• Bubbles do not spread far because the they do not spread through the pores in the pit membranes,

• Reduction of tensionin times of limited transport might allow the bubble to go back into solution,

• Root pressure increases can cause a reduction in tension

as well,

• Secondary cambium produces new xylem cells.

Xylem Cells

Bordered Pits

Pits: microscopic regions where the secondary wall of a xylem cell is

absent, and the primary wall is thin and porous.

Transport in Plants IVPhloem

There is no sugar cane that is sweet at both ends.

…Chinese proverb

Transport…molecular and ionic movement from one

location to another,

– H2O,

– Sugars and other organics,

– Ions,

– Gases,

– Proteins, RNA, Hormones, etc.

Proteins/RNA/Hormones etc.+ Everything

Phloem Transportoverview

• Long distance, bi-directional flow of sugars,

– Sugar alcohols,– Organic acids,– Amino acids, – Hormones,

• Source (phloem loading),

• Sink (phloem unloading),

• Pressure is manipulated at source and sink in order to create bulk flow in phloem conducting cells.

Phloem Cellsreview

Sieve Tube Elements

• Living cells,

• Plasma membrane,

• No nucleus,

• No tonoplast, vacuole,

• Some cytoplasm,

• Sieve plate,

– no membrane between sieve tube members, i.e. does not divide!

• P-proteins, slime bodies, callous.

P Protein and CalloseSlime and wound repair

In Cucurbita…

Phloem Protein 1 (PP1):Filament protein,

Phloem Protein 2 (PP2):Lectin (plant defense proteins).

Wound Callose (-1,3 Glucan) is synthesized on the plasma membrane for long term solution.

Slime bodies

Phloem Locationreview

Root Stele Stem Vascular Bundle Leaf Midrib

Phloem is always in close proximity to xylem.

Pressure- Flow-HypothesisMunch Hypothesis

Source• High concentration of sucrose, via photosynthesis,

– [sucrose] drives diffusion,

• Active H+-ATPase,

– electrochemical gradient drives symporters,

• - s builds, water enters the cell, + p builds.

Sink• Low concentration of sucrose,

– [sucrose] drives diffusion,

• Active H+-ATPase,

– electrochemical gradient drives antiporters,

– - s drops, water exits the cell, p drops.

Pressure-Flow-Hypothesis

p

• Notice that the s at the source is more

negative than at the sink!

• Why don’t we expect water to flow toward the source?

Water, along with solutes moves down the pressure gradient, not the water

potential gradient.

Water Cycling

Control of Transportassimilation allocation and partitioning

• Long distance, bi-directional flow of sugars,

– source and sink relationships are reversible, and under environmental and developmental control,

• Source:

– sucrose synthesis balanced with starch synthesis,

• Sink:

– Thought to be controlled by sink strength (sugar demand),

• Mechanisms for monitoring and switching unknown.

Phloem Loadingsymplastic and apoplastic

symplastic: via diffusion

apoplastic: via secondary active transport

Pressure- Flow-Hypothesispassive vs. active

Driving Force

• Primarily Diffusion,

– very low p required, and phloem transport can occur at low temperatures,

– and in the presence of H+-ATPase inhibitors.

Phloem Transport

Pressure-Flow-Hypothesis

Sugar LoadingSource

Sugar UnloadingSink

creates creates

High P Low P

P drives bulk flow

Function

Long Distance Transport

Sugars,Amino AcidsOrganics,Hormones, etc.

Requires Special Anatomy

Pores,P protein,Callose

Companion Cells

Metabolic and genetic drivers of sieve tubes.

Sieve Tubes

Membrane lined pipes

Sieve Plates

Concept Map