Plant Ecology - Chapter 3

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