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 Vascular tissues to transport water, nutrients Changed reproduction, development to survive dry environment (pollen, seed)

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 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 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 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

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 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 Loss

Transpiration losses controlled mostly by stomata High conductance of water vapor when stomata are open, low when closed Conductance to water vapor, CO 2 closely linked stomata

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

Transpiration - Water Loss

Problem develops when soils dry Stomata closed to conserve water shuts out CO 2 , ends photosynthesis starvation Stomata open to allow CO 2 risks desiccation stomata

Coping with Availability

stomata 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 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

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

Ratio of carbon gain to water loss during photosynthesis (WUE) Water loss greater than CO 2 uptake Steeper gradient, smaller molecules, shorter pathway

Water Use Efficiency

CAM plants have highest water use efficiencies decoupling of carbon uptake and fixation C 4 C 3 plants more efficient than plants - efficiency of C 4 step in capturing CO 2 C 3 WUE highest when stomata partially open, concentrations of photosynthetic enzymes high

Whole-Plant Adaptations

Desert annuals avoidance drought 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 Adaptations

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

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 Adaptations

True xerophytes tolerant drought 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

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 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

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 Plants

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

Flooding

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

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

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

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, CO 2 concentration, water availability

Anatomical Adaptations

Light causes guard cells to open in C plants 3 and C 4 Close in response to high CO 2 inside leaf, open when CO 2 is low CAM plants open stomata at night as CO 2 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, CO 2 ) Protecting against desiccation more important than maintaining photosynthesis

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 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 Adaptations

Xerophytes also have more stomata per leaf area, but less pore area per leaf area Allows tighter regulation of water loss while allowing CO 2 the most direct access to cells

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 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 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 CO 2

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 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 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 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 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 overheating Also reduces rate of photosynthesis

Energy Balance

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