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Plant Ecology - Chapter 3. Water & Energy. Life on Land. Ancestors of terrestrial plants were aquatic Dependent on water for everything - nutrient delivery to reproduction. Life on Land. Evolution has involved greater adaptation to dry environments Coverings to reduce desiccation

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Plant Ecology - Chapter 3

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life on land
Life on Land
  • Ancestors of terrestrial plants were aquatic
  • Dependent on water for everything - nutrient delivery to reproduction
life on land1
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)
water potential
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
water potential1
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
water potential2
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
water potential3
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
transpiration water loss
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
transpiration water loss1
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
transpiration water loss2
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


transpiration water loss3
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


transpiration water loss4
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


coping with availability
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 >-1.5 MPa
  • Close stomata and wait out drier conditions (hours to days)


coping with availability1
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


coping with availability2
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
water use efficiency
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
water use efficiency1
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
whole plant adaptations
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
whole plant adaptations1
Whole-Plant Adaptations
  • Desert trees and shrubs - drought avoidance
  • Drought-deciduous - lose leaves during dry season, grow new leaves when rains return
whole plant adaptations2
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
whole plant adaptations3
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
physiological adaptations
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
physiological adaptations1
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
resurrection plants
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
resurrection plants1
Resurrection Plants
  • Synthesize drought-stable proteins
  • Add phospholipid-stabilizing carbohydrates into cell membranes
  • Cytoplasm may gel
  • Metabolism virtually stopped
  • Rehydration also step-by-step
  • Adaptation to flooding needed in some habitats
  • Variations: depth, frequency, season, duration
  • Adapted to predictable flooding
  • Not adapted to greater frequency, severity
  • Biggest problem - lack of oxygen
  • Plant roots need oxygen
  • Waterlogged soils inhibit oxygen diffusion
  • Toxic substances from bacterial anaerobic metabolism accumulate
  • Plants get stressed
  • 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
anatomical adaptations
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
anatomical adaptations1
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
anatomical adaptations2
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
anatomical adaptations3
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
anatomical adaptations4
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
anatomical adaptations5
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
anatomical adaptations6
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
anatomical adaptations7
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
anatomical adaptations8
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
anatomical adaptations9
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
anatomical adaptations10
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)
energy balance
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)
energy balance1
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
energy balance2
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
energy balance3
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.
energy balance4
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
energy balance5
Energy Balance
  • Plants are not simply passive receptors of heat
  • Can modify what they “experience” via short-term physiological changes and long-term adaptations