Lecture Eleven - Adaptations to environmental stress I

Question 1

What are biological constraints?

Answer 1

‘Biological’ constraints are physical or biological processes that limit the phenotype possible for evolution.
 Organisms must obey the laws of chemistry and physics
 Biomechanics
 Thermodynamics
 Trade-offs among functions
 Allocation of energy
 Zero-sum dynamics (energy in = energy out)

Question 2

What is the paradox of plants - what makes us question how plants can be so diverse?

Answer 2

All plants must balance four basic functions.
Photosynthesis - Light interception and gas exchange.
Hydraulics - Absorption, transport and evaporation.
Reproduction - Spore release, pollen capture and see and fruit release.
Mechanical support - Static loads and dynamic loads.
 All plants need the same resources (light,
H2O, and nutrients.
 All plants are subject to same biomechanical, thermodynamic, and physical constraints.

Question 3

What wave lengths of light do plants generally utilise?

Answer 3

Plant and algal cells use light between 400 and 700 nm (Photosynthetically Active Radiation or ‘PAR’).

Question 4

What are adaptations to improve performance at low light intensities?

Answer 4

Adaptations to improve performance at low light include:
• more chlorophyll;
• increased stacking of grana thylakoids;
• thinner leaves.

Question 5

What are three ways in which plants can get to the canopy?

Answer 5

Three ways to get into the canopy:
 Grow from ground and support yourself (trees)
 Grow from ground and be supported by someone
else (lianas)
 Grow from within crown of tree (epiphytes and hemi-
epiphytes)

Question 6

What are hemi-epiphytes?

Answer 6

Establish in crown of other tree
Water and nitrate storage in tuberous aerial
roots
May not reproduce until roots have reached the ground
Eventually kill host and support themselves

Question 7

What can too much light do, and how do plants adapt to too much light?

Answer 7

Too much light
Causes photoinhibition and photo- oxidative damage

Adaptations to minimize high-light stress:
Reflective leaf surfaces
Hairy leaves (increases scattering)
Shiny leaves (increases reflectance)
Biochemical mechanisms to prevent photo-oxidation
Adjust leaf angle relative to incoming sunlight

Question 8

How much water is lost per gram of carbon assimilated?

Answer 8

For every gram of carbon assimilated into plant matter, 400-500 g of water is lost from the plant via transpiration.

Question 9

What are behavioural adaptations to water stress?

Answer 9

Behavioural
 Avoid the problem
 Only grow when water is available (ephemeral/opportunistic species)
 Only grow where water is always available (habitat specialists)
 Tolerate the problem
 Seed heteroblasty  variability in timing of
germination
 Drop leaves during dry periods
 Deeper, larger roots

Question 10

What are physiological adaptations to water stress?

Answer 10

Physiological
 In mesophytes (require conditions that are not too wet nor too dry) water loss is controlled by closure of stomata
 As water is lost from the plant, changes in turgor stimulate production of abscisic acid, causing stomatal closure

Question 11

What are CAM plants?

Answer 11

Crassulacean Acid Metabolism (CAM)
•Common in succulents
•Stomata open at night and store CO2
•Stored CO2 is used during the day for Ps

Question 12

What are structural adaptations to water stress?

Answer 12

Plants can lose water through leaf surface if diffusion gradient is strong enough
 Xerophytes have anatomical (and physiological) adaptations that allow them to survive when little water is available

 Waxy leaf surface
 Thick cuticle prevents water loss from epidermal cells
 Hairs
 Increased boundary layer of still air over leaf surface
 Sunken stomata
 Air directly above stoma is still (often protected by hairs)
 Leaf rolling
 Surrounds one leaf surface with a layer of still air
Enhanced root systems
 Deep tap roots to exploit water table
 Widespread shallow roots to take advantage of
small rainfall events Swollen stems and trunks
 Store water in stems and trunks for use when soil water supply is depleted

Question 13

What are plant adaptations to water uptake?

Answer 13

Critical because most physiological activities requires nutrients
Australian soils are typically nutrient-poor
Strategies to increase nutrient acquisition
 Root structure:
Root hairs increase the surface area for absorption of nutrients (21x in wheat).
 In some cases the root system is hugely enhanced  proteoid (cluster) roots

 Symbiotic associations
With bacteria -
Most common  Rhizobium spp. and legumes
Bacteria fix N2 from the atmosphere and pass on fixed-N to the plant in return for carbohydrate
With fungi -
Ecto- and endomycorrhizae allow plant root systems to exploit more soil
Ectomycorrhizae form a net or sheath around roots without entering plant cells
Endomycorrhizae actually penetrate the plant cells
In arbuscular mycorrhizas the fungus forms highly branched structures called arbuscules within the plant cells to aid exchange of materials between the plant and fungus
 Carnivory
If soil nutrients are limiting, carnivory can make significant contributions to plant nutrition and growth