Plant Ecology - Chapter 3 Water & Energy. Life on Land Ancestors of terrestrial plants were aquatic...
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Transcript of Plant Ecology - Chapter 3 Water & Energy. Life on Land Ancestors of terrestrial plants were aquatic...
Plant Ecology - Chapter 3
Water & Energy
Life on Land
Ancestors of terrestrial plants were aquaticDependent on water for everything - nutrient delivery to reproduction
Life on Land
Evolution has involved greater adaptation to dry environmentsCoverings to reduce desiccationVascular tissues to transport water, nutrientsChanged reproduction, development to survive dry environment (pollen, seed)
Water Potential
Plants need to acquire water, move it through their structuresAlso lose water to the environmentAll these depend on water potential of various plant parts, immediate environment
Water Potential
Water potential - difference in potential energy between pure water and water in some systemRepresents sum of osmotic, pressure, matric, and gravitational potentials
Water Potential
Water always moves from larger to smaller water potentialsPure water has water potential of 0Soils, plant parts have negative water potentialsGradient in water potential drives water from soil, through plant, into atmosphere
Water Potential
Energy is required to move water upward through plant into atmosphereEnergy not expended by plant itselfSoil to roots - osmotic potentialUp through tree and out - pressure potentialSunlight provides energy to convert liquid into vapor
Transpiration - Water Loss
Plants transpire huge amounts of waterFar more than they use for metabolismNeedled-leaved tree - 30 L/dayTemperate deciduous tree - up to 140 L/dayRainforest tree - up to 1000 L/day
Transpiration - Water Loss
Transpiration caused by huge difference in water potential between moist soil and airHuge surface area of roots, leaves produce much higher losses via transpiration than evaporative losses from open body of water
Transpiration - Water Loss
Transpiration losses controlled mostly by stomataHigh conductance of water vapor when stomata are open, low when closedConductance to water vapor, CO2 closely linked
stomata
Transpiration - Water Loss
Transpiration losses have no negative effects on plants when soil water is freely availableBenefits plants because process carries in nutrients with no energy expenditure
stomata
Transpiration - Water Loss
Problem develops when soils dryStomata closed to conserve water shuts out CO2, ends photosynthesis - starvationStomata open to allow CO2 risks desiccation
stomata
Coping with Availability
Mesophytes - plants that live in moderately moist (mesic) soilsExperience only infrequent mild water shortagesTypically transpire when soil water potentials are >-1.5 MPaClose stomata and wait out drier conditions (hours to days)
stomata
Coping with Availability
Common temperate plants are mesophytes - forest trees and wildflowers, ag crops, ornamental speciesDrought-intolerant - begin to die after days to weeks of dry soils
stomata
Coping with Availability
Xerophytes are adapted for living in dry (xeric) soilsContinue to transpire even when soil water potentials drop as low as -6 MPaCan survive/recover from low leaf water potentials that would kill mesophytes
Water Use Efficiency
Ratio of carbon gain to water loss during photosynthesis (WUE)Water loss greater than CO2 uptakeSteeper gradient, smaller molecules, shorter pathway
Water Use Efficiency
CAM plants have highest water use efficiencies - decoupling of carbon uptake and fixationC4 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
Desert annuals - drought avoidanceCarry out entire life cycle during rainy season - germinate, grow, flower, set seed, dieExperience desert only as a moist environment during their brief life
Whole-Plant Adaptations
Desert trees and shrubs - drought avoidanceDrought-deciduous - lose leaves during dry season, grow new leaves when rains return
Whole-Plant Adaptations
Herbaceous perennials in xeric habitats (many grasses) - drought avoidanceGo dormant, die back to ground level during dry seasonsMajor disadvantage - no photosynthesis for extended time periods
Whole-Plant Adaptations
True xerophytes - drought tolerantPhysiology, morphology, anatomy adapted for life in dry conditions, continue to live and growHigh root-to-shoot ratios - take up more water and lose less through transpirationSucculents - store large amounts of water
Physiological Adaptations
Series of physiological events begin when soils dryHormones: signal changes in plant functionsCell growth, protein synthesis slow, ceaseNutrients reallocated to roots, shootsPhotosynthesis inhibited, leaves wilt, older leaves may die
Physiological Adaptations
Some plants synthesize more soluble nitrate compounds, carbohydrates to lower osmotic potential of plant cellsAllows continued inflow of water via osmosis, prevents turgor loss, wilting
Resurrection Plants
Unusual adaptations to survive complete, extended desiccationMany different kinds of plantsVarious parts of world, but common in southern AfricaSurvive cellular dehydration by coordinated set of processes
Resurrection Plants
Synthesize drought-stable proteinsAdd phospholipid-stabilizing carbohydrates into cell membranesCytoplasm may gelMetabolism virtually stoppedRehydration also step-by-step
Flooding
Adaptation to flooding needed in some habitatsVariations: depth, frequency, season, durationAdapted to predictable floodingNot adapted to greater frequency, severity
Flooding
Biggest problem - lack of oxygenPlant roots need oxygenWaterlogged soils inhibit oxygen diffusionToxic substances from bacterial anaerobic metabolism accumulatePlants get stressed
Flooding
Plants have evolved physiological, anatomical, life history characteristics to function in flooded environmentsE.g., some plants able to use ethanol fermentation to generate some energy in absence of oxygen
Anatomical Adaptations
Most water regulation done by stomataPore width controlled by guard cells - continually change shapeMovement controlled by plant hormonesRespond to changes in light, CO2 concentration, water availability
Anatomical Adaptations
Light causes guard cells to open in C3 and C4 plantsClose in response to high CO2 inside leaf, open when CO2 is lowCAM plants open stomata at night as CO2 is used up, close during day when it builds up
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 Adaptations
Mesophyte, xerophyte stomata respond differently to changing moistureMesophyte stomata close during middle of day, or whenever soil moisture dropsXerophyte stomata remain open during dry, hot conditionsRelated to capacities for maintaining different leaf water potentials
Anatomical Adaptations
Xerophytes typically are amphistomous - stomata on both sides of leafAlso often isobilateral - pallisade mesophyll on both upper and lower sides of leafAdaptation to high light levels
Anatomical Adaptations
Xerophytes also have more stomata per leaf area, but less pore area per leaf areaAllows tighter regulation of water loss while allowing CO2 the most direct access to cells
Anatomical Adaptations
Xerophytes may have sunken stomata, increasing resistance to water lossLeaves may also have thicker waxy cuticle covering, to reduce water loss when stomata are closed
Anatomical Adaptations
Root systems varyFibrous root systems of monocots (grasses) especially good at obtaining water from large volume of soilTaproots can extend deep into soil, possible store food
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 rootsOxygen and CO2
Anatomical Adaptations
Water-conducting vessels vary among plantsThin-walled, large-diameter xylem vessels best for conducting water under normal conditionsBut problems under low water conditions
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
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 Balance
Large leaves in bright sunlight, still air, dry soils face problemHeat gained needs to be balanced by heat loss, or risk severe wilting, deathLight breeze would be sufficient to cool leaf properly with normal soil moisture, stronger winds in drier soils
Energy Balance
Plants can control latent heat loss, and leaf temperature, by controlling transpirationAdaptation to warm, dry habitats often involves developing smaller, narrower leaves that can remain close to air temperature even when stomata are closed
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 Balance
Leaves with pubescence (hairs) or shiny, waxy coatings reduce absorption of radiant heat from sun and keep leaves from overheatingAlso reduces rate of photosynthesis
Energy Balance
Plants are not simply passive receptors of heatCan modify what they “experience” via short-term physiological changes and long-term adaptations