Plant Adaptations

Outline:

•Photosynthesis and respiration

•Environmental controls on photosynthesis

•Plant adaptations to:–High and low light–Water limitation–Nutrient availability

Readings: Chapter 6

Conditions and Resources

• Conditions are physical / chemical features of the environment – E.g. Temperature, humidity, pH, etc. Not consumed by living organisms (but may

still be important to them)

• Resources are consumed – Once used, they are unavailable to other

organisms– Plants: sunlight, water, mineral nutrients, …– Animals: prey organisms, nesting sites, …

Plant Resources

• Plants are autotrophs - make their own organic carbon form inorganic nutrients– Need light, ions, inorganic molecules

• Plants are sessile– Grow towards nutrients

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

PHOTOSYNTHESISConversion of carbon dioxide into simple sugars

LIGHT

Light reactions

carboxylation

Dark reactions

Photosynthetically Active Radiation, PAR

C6H12O6 + 6O2 6CO2 + 6H2O + ATP

RESPIRATION

Net photosynthesis = Photosynthesis - Respiration

Photosynthesis involves gas

exchange

Controls on photosynthesis

•Light •Water•Nutrients•Temperature

1. Light

PAR

Tradeoff

• Shade plants grow better in the sun than in the shade,

• but sun plants grow faster than shade plants in direct sun

Shade plant

Sun plant

Tradeoff

• Shade plants survive well in either sun or shade

• Sun plants cannot tolerate shade

Shade plant

Sun plant

• 9 tree species of Macaranga from Borneo, Malaysia

Phenotypic plasticity

• Most plants have the ability to alter their morphology (within limits) in response to light conditions

Phenotypic plasticity

• Sun and shade leaves can exist within the same tree

More deeply lobed --> More rapid heat loss

Sun leaf• thicker• more cell

layers• more

chloroplasts

Shade leaf• flat• thin• larger

surface area / unit weight

Sun leaves

•Leaves at many angles•High saturation point

•High compensation point•Produce more RUBISCO

•High respiration•Less chlorophyll

•RUBISCO availability limits photosynthesis rate

Shade leaves

•Horizontal leaves, single layer•Low saturation point

•Low compensation point•Produce less RUBISCO

•Low respiration•More chlorophyll

•Light availability limits photosynthesis rate

2. Water

Transpiration

For transpiration to occur

atmosphere < leaf < root < soil

Water potential

w = p + + m

p= = hydrostatic pressure

= = osmotic pressure

m= = matric pressure

Stomata

• Reduction in soil --> stomata close

• Species differ in tolerance to drying soils

Strategies for drought

i. Avoiders • Short lifespan• Wet season• Seeds survive drought• Drought deciduous species

– Leaves shed in dry season

Strategies for drought

ii. Tolerators• Leaves transpire slowly• Change orientation of leaves• Sunken stomata

– E.g. pines

• More efficient photosynthesis• E.g. C4 --> reduces photorespiration• E.g. CAM --> stomata open at night

C4photosynthesis

CAMphotosynthesis

C4

CAM

CAM

% of grasses that are C4

Water absorption• Root hairs increase surface area

• Structure of the root system varies between species, depending on the amt. of soil moisture in their env’t

• Individual species show phenotypic plasticity

• wet soil --> shallow roots near surface (greater oxygen availability)

• dry soil --> deep roots

3. Nutrients

•Macronutrients – needed in large amounts (e.g. C, H, O, … N, P, K, Ca, Mg, S)

•Micronutrients – trace elements (e.g. Fe, Mn, B)

•Micro/macro refer to the quantity needed

Table 6-1

Nutrient uptake rates

• Reach plateau with increasing nutrient concentration

Maximum growth rate of a plant reflects N availability in its natural habitat. A. stolonifera occurs on more nitrogen-rich soils than A. canina.

Evergreen leaves

• Plants adapted to nutrient-poor conditions tend to have evergreen leaves

4. Effects of temperature

= Condition

• Increase temperature --> increase biochemical reaction rate

• At high temperature,

enzymes denature

--> death

• Gross photosynthetic rate increases up to a point with increasing temperature

• Respiration rate also increases with temperature.

• Net photosynthesis is maximal at a point slightly below that at which gross photosynthesis is maximal

Leaf temperature

• > 95% of sunlight absorbed by a leaf becomes heat

• Cooling of leaves:1. Transpiration

2. Convection (movement of cool air around a leaf)

C4 plants

• Have higher temperature optima than C3

Phenotypic plasticity

• Individual species can modify their Topt according to the changing seasons

= acclimatization

Saguaro cacti (S.W. United States) store large amounts of water; they can tolerate short periods of freezing temperatures

CLOSER TO HOME

•Freeze-tolerant plants: frost hardening•When T decreases – plants synthesize sugars, amino acids, other molecules to act as antifreeze.

•Winter – deciduous plants•Lose leaves in autumn•Leaves very efficient in summer – high photosynthesis rate•Leaves can’t survive freezing•Costly in energy, nutrients to rebuild leaves

•Chilling breaks seed dormancy for temperate/boreal spp.•Germinates only in spring

Plants are phenotypically plastic