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

7. 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

9. 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

10. 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

11. 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

12. 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

13. 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)
stomata

14. 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

15. 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

16. 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

17. 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

18. 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

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

20. 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

21. 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

23. 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

24. 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

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

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

27. 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

28. 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

29. 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

30. 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

31. 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

33. 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

34. 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

35. 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

36. 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

37. 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

39. 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)

40. 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)

41. 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