Plant Ecology - Chapter 3

1
Plant Ecology - Chapter 3

• Water Energy

2
Life on Land

• Ancestors of terrestrial plants were aquatic
• Dependent on water for everything - nutrient
  delivery to reproduction

3
Life on Land

• Evolution has involved greater adaptation to dry
  environments
• Coverings to reduce desiccation
• Vascular tissues to transport water, nutrients
• Changed reproduction, development to survive dry
  environment (pollen, seed)

4
Water Potential

• Plants need to acquire water, move it through
  their structures
• Also lose water to the environment
• All these depend on water potential of various
  plant parts, immediate environment

5
Water Potential

• Water potential - difference in potential energy
  between pure water and water in some system
• Represents sum of osmotic, pressure, matric, and
  gravitational potentials

6
Water Potential

• Water always moves from larger to smaller water
  potentials
• Pure water has water potential of 0
• Soils, plant parts have negative water potentials
• Gradient in water potential drives water from
  soil, through plant, into atmosphere

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Water Potential

• Energy is required to move water upward through
  plant into atmosphere
• Energy not expended by plant itself
• Soil to roots - osmotic potential
• Up through tree and out - pressure potential
• Sunlight provides energy to convert liquid into
  vapor

8
Transpiration - Water Loss

• Plants transpire huge amounts of water
• Far more than they use for metabolism
• Needled-leaved tree - 30 L/day
• Temperate deciduous tree - up to 140 L/day
• Rainforest tree - up to 1000 L/day

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Transpiration - Water Loss

• Transpiration caused by huge difference in water
  potential between moist soil and air
• Huge surface area of roots, leaves produce much
  higher losses via transpiration than evaporative
  losses from open body of water

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Transpiration - Water Loss

• Transpiration losses controlled mostly by stomata
• High conductance of water vapor when stomata are
  open, low when closed
• Conductance to water vapor, CO2 closely linked

stomata
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Transpiration - Water Loss

• Transpiration losses have no negative effects on
  plants when soil water is freely available
• Benefits plants because process carries in
  nutrients with no energy expenditure

stomata
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Transpiration - Water Loss

• Problem develops when soils dry
• Stomata closed to conserve water shuts out CO2,
  ends photosynthesis - starvation
• Stomata open to allow CO2 risks desiccation

stomata
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Coping with Availability

• Mesophytes - plants that live in moderately moist
  (mesic) soils
• Experience only infrequent mild water shortages
• Typically transpire when soil water potentials
  are gt-1.5 MPa
• Close stomata and wait out drier conditions
  (hours to days)

stomata
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Coping with Availability

• Common temperate plants are mesophytes - forest
  trees and wildflowers, ag crops, ornamental
  species
• Drought-intolerant - begin to die after days to
  weeks of dry soils

stomata
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Coping with Availability

• Xerophytes are adapted for living in dry (xeric)
  soils
• Continue to transpire even when soil water
  potentials drop as low as -6 MPa
• Can survive/recover from low leaf water
  potentials that would kill mesophytes

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Water Use Efficiency

• Ratio of carbon gain to water loss during
  photosynthesis (WUE)
• Water loss greater than CO2 uptake
• Steeper gradient, smaller molecules, shorter
  pathway

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Water Use Efficiency

• CAM plants have highest water use efficiencies -
  decoupling of carbon uptake and fixation
• C4 plants more efficient than C3 plants -
  efficiency of C4 step in capturing CO2
• C3 WUE highest when stomata partially open,
  concentrations of photosynthetic enzymes high

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Whole-Plant Adaptations

• Desert annuals - drought avoidance
• Carry out entire life cycle during rainy season -
  germinate, grow, flower, set seed, die
• Experience desert only as a moist environment
  during their brief life

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Whole-Plant Adaptations

• Desert trees and shrubs - drought avoidance
• Drought-deciduous - lose leaves during dry
  season, grow new leaves when rains return

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Whole-Plant Adaptations

• Herbaceous perennials in xeric habitats (many
  grasses) - drought avoidance
• Go dormant, die back to ground level during dry
  seasons
• Major disadvantage - no photosynthesis for
  extended time periods

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Whole-Plant Adaptations

• True xerophytes - drought tolerant
• Physiology, morphology, anatomy adapted for life
  in dry conditions, continue to live and grow
• High root-to-shoot ratios - take up more water
  and lose less through transpiration
• Succulents - store large amounts of water

22
Physiological Adaptations

• Series of physiological events begin when soils
  dry
• Hormones signal changes in plant functions
• Cell growth, protein synthesis slow, cease
• Nutrients reallocated to roots, shoots
• Photosynthesis inhibited, leaves wilt, older
  leaves may die

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Physiological Adaptations

• Some plants synthesize more soluble nitrate
  compounds, carbohydrates to lower osmotic
  potential of plant cells
• Allows continued inflow of water via osmosis,
  prevents turgor loss, wilting

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Resurrection Plants

• Unusual adaptations to survive complete, extended
  desiccation
• Many different kinds of plants
• Various parts of world, but common in southern
  Africa
• Survive cellular dehydration by coordinated set
  of processes

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Resurrection Plants

• Synthesize drought-stable proteins
• Add phospholipid-stabilizing carbohydrates into
  cell membranes
• Cytoplasm may gel
• Metabolism virtually stopped
• Rehydration also step-by-step

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Flooding

• Adaptation to flooding needed in some habitats
• Variations depth, frequency, season, duration
• Adapted to predictable flooding
• Not adapted to greater frequency, severity

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Flooding

• Biggest problem - lack of oxygen
• Plant roots need oxygen
• Waterlogged soils inhibit oxygen diffusion
• Toxic substances from bacterial anaerobic
  metabolism accumulate
• Plants get stressed

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Flooding

• Plants have evolved physiological, anatomical,
  life history characteristics to function in
  flooded environments
• E.g., some plants able to use ethanol
  fermentation to generate some energy in absence
  of oxygen

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Anatomical Adaptations

• Most water regulation done by stomata
• Pore width controlled by guard cells -
  continually change shape
• Movement controlled by plant hormones
• Respond to changes in light, CO2 concentration,
  water availability

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Anatomical Adaptations

• Light causes guard cells to open in C3 and C4
  plants
• Close in response to high CO2 inside leaf, open
  when CO2 is low
• CAM plants open stomata at night as CO2 is used
  up, close during day when it builds up

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Anatomical Adaptations

• Declining water potential in leaf will cause
  stomata to close, overriding other factors
  (light, CO2)
• Protecting against desiccation more important
  than maintaining photosynthesis

32
Anatomical Adaptations

• Mesophyte, xerophyte stomata respond differently
  to changing moisture
• Mesophyte stomata close during middle of day, or
  whenever soil moisture drops
• Xerophyte stomata remain open during dry, hot
  conditions
• Related to capacities for maintaining different
  leaf water potentials

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Anatomical Adaptations

• Xerophytes typically are amphistomous - stomata
  on both sides of leaf
• Also often isobilateral - pallisade mesophyll on
  both upper and lower sides of leaf
• Adaptation to high light levels

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Anatomical Adaptations

• Xerophytes also have more stomata per leaf area,
  but less pore area per leaf area
• Allows tighter regulation of water loss while
  allowing CO2 the most direct access to cells

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Anatomical Adaptations

• Xerophytes may have sunken stomata, increasing
  resistance to water loss
• Leaves may also have thicker waxy cuticle
  covering, to reduce water loss when stomata are
  closed

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Anatomical Adaptations

• Root systems vary
• Fibrous root systems of monocots (grasses)
  especially good at obtaining water from large
  volume of soil
• Taproots can extend deep into soil, possible
  store food

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Anatomical Adaptations

• Plants adapted to growing in aquatic, flooded
  habitats may have aerenchyma (aerated tissues)
• Air channels (gas lacunae) allow gases to move
  into and out of roots
• Oxygen and CO2

38
Anatomical Adaptations

• Water-conducting vessels vary among plants
• Thin-walled, large-diameter xylem vessels best
  for conducting water under normal conditions
• But problems under low water conditions

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Anatomical Adaptations

• Thin walls collapse under extreme negative
  pressures in xerophytes (need thick-walled, small
  diameter)
• Big vessels prone to cavitation - break in water
  column caused by air bubbles (especially during
  freezing, low water conditions)

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Energy Balance

• Radiant heat gain from sun is balanced by
  conduction (transfer to cooler object) and
  convection (transport by moving fluid or air)
  losses and latent heat loss (evaporation)

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Energy Balance

• Large leaves in bright sunlight, still air, dry
  soils face problem
• Heat gained needs to be balanced by heat loss, or
  risk severe wilting, death
• Light breeze would be sufficient to cool leaf
  properly with normal soil moisture, stronger
  winds in drier soils

42
Energy Balance

• Plants can control latent heat loss, and leaf
  temperature, by controlling transpiration
• Adaptation to warm, dry habitats often involves
  developing smaller, narrower leaves that can
  remain close to air temperature even when stomata
  are closed

43
Energy Balance

• Holding leaves at steep angle reduces radiant
  heat gain (leaves of the desert shrub, jojoba)
• Some plants can change angle as leaf temperature
  changes - steeper at hotter temps.

44
Energy Balance

• Leaves with pubescence (hairs) or shiny, waxy
  coatings reduce absorption of radiant heat from
  sun and keep leaves from overheating
• Also reduces rate of photosynthesis

45
Energy Balance

• Plants are not simply passive receptors of heat
• Can modify what they experience via short-term
  physiological changes and long-term adaptations