Review: Form and Function in Evolutionary Context

1. Enormous interspecific variation in plant traits (e.g., growth rate, photosynthesis, leaf life span, leaf shape…)

2. Variation thought to represent adaptations to range of environmental conditions in diverse habitats (e.g., alpine, desert, tropical rain forest, grassland…)

3. Repeated combinations of functional characteristics in distantly related taxa across broad geographic range suggests convergent evolution--> natural selection constrains variability for suite of interrelated traits

--> patterns of universal tradeoffsReich et al. 1997

Controls over photosynthesis - CO2 supply and demand

analysisSupply controlled

bystomatal responses

to:1. humidity2. light3. internal CO2

concentration4. soil-water

availability 5. water status of the

shoot tissues

Demand (use) controlled by:

1. level of nitrogen investment in photosynthetic proteins

2. relative allocation of photosynthetic nitrogen between light-harvesting and CO2 harvesting processes,

3. inherent kinetic constraints of photosynthetic enzymes

Colonization on land required adaptations for

1. Efficient internal transport2. Desiccation resistance or avoidance 3. Reproduction without water 4. Effective dispersal 5. Competition (for multiple resources)

Chl a & b

Innovation in cell division

Water and sap conducting tissue

Indeterminant growth

Archegonium and antheridium

Niklas 1992, 1997

What selective pressures have shaped the vegetative and reproductive functions of plants

Niklas 1992, 1997 Lecture adapted from Enquist

(UA), personal communication 4/04

Flux density of water through plants (Transpiration)

On land . .

1. Plants require 138.8 moles of water (lost)per mole of CO2 that has been fixed during photosynthesis

2. BUT AVAILABLE WATER MAY BE LIMITED

Increase transpiration <-> increase photosynthesis

Decrease transpiration <-> decrease photosynthesis

Photosynthesis/transpiration compromise

Low [HLow [H22O]O]

High [H2O]

Form

A plant is a wickutilizing a natural watergradient (soil and air) to Transport nutrients . . .

. . to assimilate CO2

to fuel respiration in order to grow and reproduce.

Function

But the environmentdiffers (biotic and abiotic) -->DIVERSITY

Movement of Water

[CO2]

Pull of water in analogousto a rubber band!

Enquist, U of A

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Xylem - Principle water-conducting system in vascular plants

--> Transport - ROOT TO LEAF--> Support

Primary Xylem: Derived from the procambium

Secondary Xylem: Derived from the vascular cambium

Complex Vascular Tissues - Review

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(1)Tracheids

Lower vascular plants and gymnosperms

(2) Vessel elements

--> elongated cells with secondary cell walls--> lack protoplast at maturity

Majority of Angiosperms have both

connected end to end

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Xylem conducting cells (Tracheary Elements)

Specialized type of sclerenchyma

€

dS

dt= −DiA

∂Ci

∂x

⎛

⎝ ⎜

⎞

⎠ ⎟

Rate of diffusion

Surface area across whichdiffusion occurs

Diffusion coefficient

Concentrationgradient

Case 1 - Passive diffusion: direct dependence of diffusionrate on surface area and inversely on distance (x)

dS/dt

x

Physical principles of fluid transport Fick’s first law of diffusion

What does Fick’s first law tell us . . .

3. Passive diffusion over long distances is VERY slow!!!

1. How much time required for 50% of a populationof small solute molecules to passively diffuse 5cm?

Di = 10-9 m2 s-1

2. A little algebra to rearrange Fick’s law…

time = distance2 /2.8 (coefficient of diffusion)

= (0.05 m)2/2.8 (10-9 m2 s-1) =~ 10 days!

IMPLICATION: Plants need a directed transport system to increase in size

Water moves vertically along a negative water Water moves vertically along a negative water potential gradientpotential gradient

Ability of xylem ‘tubes’ to permit the movement of water is conductance

(1) Moves according to water potential gradients

(2) Adhesive - interacts and ‘sticks’ to manysubstances

(3) Cohesive - via hydrogen bonds; molecules act as unit

(4) Heavy

Properties of Water - what do we know?

The Ascent of Water

1. Water has a high cohesive force…when confined in tubes with small diameters, you have to pull REALLY hard to snap the water column

2. Continuity of water in a plant…water columns in xylem are part of a continuous uninterrupted system going from water-saturated cells in the roots all the way to water-saturated cells in the leaves

3. Gradient of water potential in the plant body…evaporation of water from leaves reduces water potential and this causes water to be pulled through xylem cells

Cohesive forces in water

1. Hard to measure but…2. Experiments subjecting water column to

centrifugal force showed cavitations after 22MPa

3. Other experiments suggest only 0.015 - 0.020 MPa needed to lift water up trunks of rapidly transpiring trees

4. Or… ~2.0 MPa sufficient to overcome force of gravity and resistance to flow in a column of wood 100 m tall!

What engineering principles tell us about moving water - the most direct method is through ‘pipes’

Evolution of Xylem reaches similar answer: piping in the form of vessels and tracheids!

1. Vessels (and tracheids - sort of) are like capillary tubes

2.Long compared to diameter, and inner diameter very small (e.g., red maple vessel is ~45 µm wide and 1.2 cm long!)

3. Vessels differ in that inner wall not smooth (20 wall thickenings)

4. Movement of water through pipes depends on adhesion and cohesion

What are the tradeoffs???

Hagen - Poiseuille equation

What engineering principles tell us about moving water - the most direct method is through ‘pipes’

€

Zi =8ηli

ri4

li

ri

Length of pipe = li

Radius of pipe = ri

Resistance of fluid flow = Zi

= fluid viscosity

Evolution of Xylem reaches similar answer: vessels and tracheid piping!

What parameters can selection act on?

€

Zi =8ηli

ri4

Scenario 1 - What is the gain for decrease in length vs. increase in radius?

Resistance decreases linearly with decreases in length

But it decreases as r-4

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Consider the advantage of the evolution of vessels in angiosperms…

Diagram of flow through a capillary tube

li

ri

Predictive modeling… how does water move in a capillary tube?

1.Velocity of the flow is parabolic meaning

1. maximum flow in center of tube where resistance is zero

2. no flow at the wall where resistance is maximum

2.Geometry of flow due in part to adhesion of water to surface of tube

Niklas 1992

Diagram of flow through a capillary tube

li

ri

Predictive modeling… continued

3.Tubes with SMALL diameters have low Lp (conductance) BUT can draw water upwards due to adhesion and cohesion of water

4. Tubes with LARGE diameters have large Lp BUT cannot take good advantage of adhesion and cohesion….unless there is pressure in the tube

5. Thus, the flow of water through a capillary-like tube should be influenced by average diameter and pressure

Niklas 1992

Another way to look at this:

li

ri

Lp = π d4

128

Lp = conductance per length of capillary tube

d is the radius of the tube

= fluid viscosity

The take home message? Doubling the relative diameter of a conducting cell results in an 16-fold increase in the relative flow rate!!!

Niklas 1992

Relative Benefit of Conducting Systems vs Passive Diffusion

1. Flow increased dramatically as function of tube diameter and pressure

Ex: if diameter increased by factor of 4, flow increased by factor of 246 (or 93%!)

2. Specific ConductivityConifers: 20 ml hr-1 cm-2 MPa -1

Deciduous Broadleaf Trees: 65 - 128 ml hr-1 cm-2 MPa -1

Some vines: 1,273 ml hr-1 cm-2 MPa -1

Simple diffusion: 10 days to travel 5 cm

Xylem efficiency versus safety

1. Efficiency: xs area of xylem vs area where water is going

2. Safety: avoidance of cavitations (air embolism)

3. Water column can be broken by• injury• freezing• pulling to hard on it (drying out)

4. Preventing embolisms1. Localize and trap the bubble2. Maintain a small diameter

Niklas 1992

Anatomy of Pits in Tracheids

1. Tracheid pits - between 2 cells

2. Site of water passage

3. Cells separated by pit membrane - resistance to water flow

4. GREATLY restricts passage of air bubbles due to very small size

5. “Seal off” an embolism and restrict it to one cell

Niklas 1992

Throughout the evolution of the Tracheophytes . . .

. . . with increases in transport distance (size) tracheid and vessel dimensions have changed. Selection to lower resistance of transport likely enabled plants to increase in size.

Tra

chei

d di

amet

er

Niklas 1992

Thought Questions

1. What are the key structural innovations that distinguish the tracheophytes?

2. Why would natural selection favor these structural traits?

3. What does the Hagen- Poiseuille equation tell us about the evolution of the angiosperms?