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

Niklas 1992, 1997 Lecture adapted from Enquist

(UA), personal communication 4/04

How to be big…

Review: Form and Function in Evolutionary Context

1. interspecific variation 2. represent adaptations 3. convergent evolution

--> natural selection constrains variability for suite of interrelated traits

--> patterns of universal tradeoffs

Reich et al. 1997

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) 6. Mechanical support structure

Chl a & b

Innovation in cell division

Water and sap conducting tissue

Indeterminant growth of sporophyte - ability to grow large (but took awhile to happen!

Archegonium and antheridium

Niklas 1992, 1997

“Biologically Engineered” Designs

• Much of plant evolution and the relationships between plant form and function can be understood from a biomechanical perspective.

• Quantitative evaluation of plant anatomy, morphology, and development as each of these relate to the various biological functions that plants must perform to grow, survive, and reproduce.

• Underlying all of this work is a belief that plants are biologically engineered to perform vital functions, and that evolution has resulted in more efficient "designs".

Niklas website at Cornell

Some basic definitions

Biomechanical Stresses

1. Apply force --> internal forces develop in different parts

2. Describe as force per unit area3. Mechanical properties of plants

vary depending on1. Age2. Relative moisture content3. Diversity of material in the

structure (plants are composite materials after all)

Different Stresses

Sheer stress - pull at an angle or twist

Tensile stress - uniform pull from both ends

Compression stress - uniform push at both ends

Responses to Forces

The way materials deform depends on nature of chemical bonds - in plants this has a LOT to do with cellulose

1. Elastic - material that deforms when subjected to a force, but can regain original shape when stress is released

2. 2. Plastic - deforms under stress and cannot regain its original shape - permanent molecular reorganization (e.g., collenchyma)

Example plant responses to forces

1. Elastic - plant tissue stretches and regains original shape

--> Sclerenchyma tissue--> Phloem cells can have up to 10%

change in radius of cell; may influence capacity to contain sap

-->Branches - deflect under pressure of wind and snap back when it subsides

2. Plastic - plant tissue permanently deformed--> Collenchyma tissue

--> buckling at grass internode under stress

3. Break - can be really bad for plant (but is it every “good”?)

Should Plants always be Strong?

1. Mechanical strength over time reflects the selective pressures of a given environment

2. “Stiff and strong” strategy - avoid deleterious effects of relatively strong forces

3. “Flexible, stretchy” strategy - stretchy tissues act like shock absorbers through infrequent stressful forces

4. Case study of aquatic plants along gradients

1. --> velocity gradient2. --> in a lake this corresponds

to depth gradient

Decreasing water

movement

Plant Functional Category

Tensile Strength (MN m-2)

Extensibility (%)

Shallow plants

17.7 5.5

Deep plants

8.8 8.7

Brewer and Parker 1990

Plants living in wave exposed habitats are more than 2x stronger than plants in deep water - but what happens when water levels are lowered?

Decreasing water

movement

Stiff and strong in shallow water

Stretchy and flexible in deeper water

Water Lily - stiff, strong reinforced petiole, flexible blade

Bladderwort - stretchy, flexible stem

When Selective Pressures Change…

1. Selective pressure can change community structure

2. In lake with large drawdowns, plants that fragment rapidly disperse.

3. In Jackson Lake, these tended to be introduced species or those not terribly valuable to wildlife (e.g., Elodea canadensis, Myriophyllum spicatum)

4. Species of value to wildlife (Potamogeton sp.) were either left high and dry or broke

1. The way plants are distributed in aquatic habitats reflects (among other things) the mechanical properties of their stems and leaves.

2. Communities can change when nature of mechanical forces in the environment changes.

3. What materials in plants influence ability to deal with mechanical forces?

Cell wall

1. All plant cells (except sperm and some egg cells) have walls

--> Once thought of as being ‘dead’

--> Integral part of plant metabolism - an organelle

1. Cell membrane lays down a 1o cell wall (young cell) then (in not all cells) the 2o cell wall.

Origin of Cell Wall

2. Secondary wall - most modifications are made here

Cell Walls - Key Units of Biomechanical Structure

Function of Cell Wall

3. Shapes morphology (and strength) of the plant and tissue

1. Provides supporta. allows turgor pressure to build upb. mechanically protective (animals cannot digest!)

2. Barrier to large molecules - but not to small molecules (water and plant hormones are small)

Structural building blocks at the cellular level

1. Cellulose Polymer of glucose molecules linked by -1,4 bonds Flat/ribbon-like molecules lie parallel to each other = microfibrils One of the most abundant bio-polymers on earth!

glucose molecules linked by -1,4 bonds

‘Anatomy’ of cell walls

Structural building blocks at the cellular level - continued

1. Cellulose Polymer of glucose molecules linked by -1,4 bonds Flat/ribbon-like molecules lie parallel to each other = microfibrils One of the most abundant bio-polymers on earth!

2. Hemicellulose Mixture of polymers - highly branched ‘Glues’ microfibrils together

3. Proteins (amino acids) Function is unknown

4. Pectin substances Likely responsible for ‘plastic’ nature of the wallAbility to withstand stretching and pulling!

Another Important Plant Material in Cell Walls - Lignin

1. A strong waterproof polymer

2. Complex substance – lots of carbon rings ( carbon content)

3. Characteristic of secondary walls (Sclerenchyma)

4. Associated with tissues/organs that persist for long-time

5. Dramatically alters nature of secondary wall

1. Provides strength and rigidity of wall Forms extensive, cross linked-network. Important forsafety of hydraulics and transport.

2. Stable, resistant protective coating Shielding from chemical, physical, biological attack

3. Waterproof barrier

Regulates hydration; makes cell walls more hydrophobic

Resistant to decay

NOTE: Lignin is not found in algae or mosses;

Closely associated with evolution of vascular plants

LIGNIN - Dramatically alters nature of secondary wall

Putting the pieces together into a large plant

1. What is the trend in plant size over evolutionary time?

2. What are the evolutionary constraints?

3. What “problems” needed to be solved?

Most clades do in general (Cope’s Rule 1896)

1. Plants with vascular tissuehave increased in size over time

Evolution of Tracheophytes

Evolution of Angiosperms

2. Recall: “Constraints” on evolutionary diversification = any physical or biological process which ‘limits’ the phenotype possible for organic evolution

1. Physical constraints - Limit to what is physically possible

2. Organisms must obey the laws of physics and chemistry

3. Can use engineering model to predict how plants can change their structure within physical limits

Euler equation for the biomechanics of stem height and diameter:

Plants must obey this equation if they are to become larger

Hmax CE

1/ 3

D2 / 3

Maximum height to which a vertical cylindrical stem can grow before it elastically buckles under its weight

Young’s Elastic Modulus

Tissue density

Stem diameter

Photo of an old growth cedar taken at the turn of the century. The circumference is ~100 ft!!!

Anatomically, what does the equation predict the inside of the tree will look like?

Euler equation for the biomechanics of stem height and diameter:

Plants must obey this equation if they are to become larger

Hmax CE

1/ 3

D2 / 3

Maximum height to which a vertical cylindrical stem can grow before it elastically buckles under its weight

Young’s Elastic Modulus

Tissue density

Stem diameter

Spongy mesophyll

parenchyma parenchyma

collenchyma collenchyma sclerenchyma

Air sac

Vacuole

cytoplasm

Dinking around with density - Relative volume of apoplast, symplast, and cell wall in different tissues

Pulling Plant Form and Function Together:Synthesis Questions

1. What have been the central principles guiding the evolution of plant form and function?

2. How can physical laws and models help in understanding plant adaptations? What are the weaknesses of such an approach?

3. What have been the important constraints /principles which have shaped the evolution of plant form, physiology, and plant life histories?

4. What are the abiotic constraints on how plants operate in their environments? How does this relate to diversity?