Plant Adaptations

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Plant Adaptations

• Outline
• Photosynthesis and respiration
• Environmental controls on photosynthesis
• Plant adaptations to
• High and low light
• Water limitation
• Nutrient availability
• Readings Chapter 6

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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,

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Plant Resources

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

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PHOTOSYNTHESIS Conversion of carbon dioxide into
simple sugars
6CO2 12H2O ? C6H12O6 6O2 6H2O
LIGHT
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Light reactions
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Dark reactions
carboxylation
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Photosynthetically Active Radiation, PAR
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RESPIRATION
C6H12O6 6O2 ? 6CO2 6H2O ATP
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Net photosynthesis Photosynthesis -
Respiration
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Photosynthesis involves gas exchange
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• Controls on photosynthesis
• Light
• Water
• Nutrients
• Temperature

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1. Light
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PAR
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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
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Tradeoff

• Shade plants survive well in either sun or shade
• Sun plants cannot tolerate shade

Shade plant
Sun plant
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• 9 tree species of Macaranga from Borneo, Malaysia

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Phenotypic plasticity

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

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Phenotypic plasticity

• Sun and shade leaves can exist within the same
  tree

More deeply lobed --gt More rapid heat loss
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• Sun leaf
• thicker
• more cell layers
• more chloroplasts

• Shade leaf
• flat
• thin
• larger surface area / unit weight

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• Shade leaves
• Horizontal leaves, single layer
• Low saturation point
• Low compensation point
• Produce less RUBISCO
• Low respiration
• More chlorophyll
• Light availability limits photosynthesis rate

• Sun leaves
• Leaves at many angles
• High saturation point
• High compensation point
• Produce more RUBISCO
• High respiration
• Less chlorophyll
• RUBISCO availability limits photosynthesis rate

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2. Water
Transpiration
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For transpiration to occur

• ?atmosphere lt ?leaf lt ?root lt ?soil

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Water potential

• ?w ?p ?? ?m
• ?p hydrostatic pressure
• ?? osmotic pressure
• ?m matric pressure

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Stomata

• Reduction in soil ? --gt stomata close
• Species differ in tolerance to drying soils

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Strategies for drought

• Avoiders
• Short lifespan
• Wet season
• Seeds survive drought
• Drought deciduous species
• Leaves shed in dry season

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Strategies for drought

• Tolerators
• Leaves transpire slowly
• Change orientation of leaves
• Sunken stomata
• E.g. pines
• More efficient photosynthesis
• E.g. C4 --gt reduces photorespiration
• E.g. CAM --gt stomata open at night

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CAM photosynthesis
C4 photosynthesis
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C4
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CAM
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CAM
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of grasses that are C4
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Water absorption

• Root hairs increase surface area

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• Structure of the root system varies between
  species, depending on the amt. of soil moisture
  in their envt
• Individual species show phenotypic plasticity
• wet soil --gt shallow roots near surface (greater
  oxygen availability)
• dry soil --gt deep roots

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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

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Table 6-1
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Nutrient uptake rates

• Reach plateau with increasing nutrient
  concentration

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

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

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4. Effects of temperature

• Condition
• Increase temperature --gt increase biochemical
  reaction rate
• At high temperature,
• enzymes denature
• --gt death

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• 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

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Leaf temperature

• gt 95 of sunlight absorbed by a leaf becomes heat
• Cooling of leaves
• Transpiration
• Convection (movement of cool air around a leaf)

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C4 plants

• Have higher temperature optima than C3

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Phenotypic plasticity

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

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• Response to cold
• Chilling injury - near, gt 0 oC
• - cell membranes rupture
• Freezing - lt 0 oC
• - ice inside cells death
• - ice outside cells dehydration
• (may survive)
• may kill juveniles only

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Saguaro cacti (S.W. United States) store large
amounts of water they can tolerate short periods
of freezing temperatures
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• 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 cant survive freezing
• Costly in energy, nutrients to rebuild leaves
• Chilling breaks seed dormancy for
  temperate/boreal spp.
• Germinates only in spring

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Plants are phenotypically plastic