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