Water Balance in Terrestrial Plants

1. Water Balance in Terrestrial Plants

2. Water Regulation on Land - Plants Wip= Wr + Wa - Wt - Ws • Wip= Plant’s internal water • Wr =Roots • Wa = Air • Wt = Transpiration • Ws = Secretions

3. Water Regulation on Land - Plants

4. Water Balance in Terrestrial Plants • Gain water through roots • Lose water • through photosynthesis (<1% loss this way) • through transpiration (stomates open to allow exchange of CO2 and O2; water escapes when guard cells are open) • transpiration also provides • transport of nutrients • cooling

5. Water Movement Between Soils and Plants • Water moving between soil and plants flows down a water potential gradient. • Water potential (Ψ) is the capacity to perform work. • Dependent on free energy content. • Pure Water ψ = 0. • Ψ in nature generally negative. • Ψsolute measures the reduction in Ψ due to dissolved substances.

6. Variation in Water Availability Water flows along energy gradients. Gravity—water flows downhill. The associated energy is gravitational potential. Pressure—from an area of higher pressure, to lower. The associated energy is pressure (or turgor) potential.

7. Variation in Water Availability Osmotic potential—water flows from a region of high concentration (low solute concentration) to a region of low concentration (high solute concentration). Matric potential—energy associated with attractive forces on surfaces of large molecules inside cells or on surfaces of soil particles.

8. Variation in Water Availability • Water potential is the sum of all these energy components. It can be defined as: • Ψo = osmotic potential (negative value). • Ψp = pressure potential. • Ψm = matric potential (negative value).

9. Water Movement Between Soils and Plants • Ψplant = Ψsolute + Ψmatric + Ψpressure • Matric Forces: Water’s tendency to adhere to container walls. • Ψpressure is the reduction in water potential due to negative pressure created by water evaporating from leaves. • As long as Ψplant < Ψsoil, water flows from the soil to the plant.

10. Variation in Water Availability • Water always moves from a system of higher Ψ to lower Ψ, following the energy gradient. • Atmospheric water potential is related to relative humidity. If less than 98%, water potential is low relative to organisms. Terrestrial organisms must thus prevent water loss to the atmosphere.

11. Variation in Water Availability • Resistance—a force that impedes water movement along an energy gradient. • To resist water loss, terrestrial organisms have waxy cuticles (insects and plants) or animal skin.

12. Variation in Water Availability • Terrestrial plants and soil microorganisms must take up water from the soil to replace water lost to the atmosphere. • Water potential of soils is mostly dependent on matric potential. • Amount of water in soil is determined by balance of inputs and outputs, soil texture, and topography.

15. Classification of Plants According to Habitat Type • Mesophytes • Phreophytes • Halophytes • Xerophytes • Hydrophytes

16. Mesophytes • Grow where there is a moderate amount of water • May also have some of the xerophyte adaptations for drought conditions • Many of our midwest native trees • Oaks • Maples • Elms • Hickories

17. Phreophytes • Long roots to reach water table • e.g. mesquite shrubs may have roots 175 feet long • Prairie grasses and forbs

19. Halophytes • Adapted for high salt environments • are able to take up water from soils with high solute concentrations • many do most of their growing during rainy periods when salt conc is lowest • desert holly - uses accumulated salt as reflective surface on leaves

20. Desert Holly Salicornia Salt glands exuding salt droplets

21. Xerophytes • Plants adapted to dry conditions • Succulents: e.g. Cacti, euphorbias • Fleshy tissue in which water can be stored • Waxy leaves • Insulating hairs • Trichomes

22. Xerophytes • Desert Ephemerals • Annuals; adaptation is in life history strategy • Plant activity is limited to periods that are optimal for growth and development, i.e. After a heavy rain. • Plants die after flowering and producing seeds • Produce seed bank • Seeds remain dormant in the soil (seed bank) until the next rains. This may be many years away.

23. California poppies and other ephemerals from the Mojave Desert of the American Southwest Blue Phacelia from the Sonoran and Mojave Deserts Seed Bank

24. Common Adaptations Seen in Desert Plants Enhanced cuticle, a waxy covering, which prevents water loss. Leaves of plants like the Jojoba and Compass Plant face N-S, minimizing exposure to most intense sunlight. Spines and hairs discourage herbivores and help shade plant.

25. Common Adaptations Seen in Desert Plants • Spines are leaves • Small narrow leaves decrease heating from the sun, less surface area for water loss. • Rotating leaves enable the plant to orient its leaves away from maximum exposure to the sun. • Paired leaves of creosote bush can close to conserve water.

26. Common Adaptations Seen in Desert Plants • Succulent leaves reduce the surface-to-volume ratio and favor water conservation.

28. Common Adaptations Seen in Desert Plants • Trichomes, hair-like projections, that create a thick boundary layer which will deflect excess light as well as infra red wavelengths.

29. A crew of intrepid Biologists with a Haleakala Silversword

31. Common Adaptations Seen in Desert Plants • Small, hard leaves • Drought-deciduous • Drop leaves/twigs when soil dries up. • Ocotillo

32. Common Adaptations Seen in Desert Plants • Long vertical roots enabling a plant to reach water sources beneath the soil. • Shallow, radial roots, those which extend horizontally, which maximize water absorption at the surface.

34. Mesquite

35. Common Adaptations Seen in Desert Plants • Leaf polymorphism in which broad leaves are formed when soil moisture is high and narrow leaves follow as that water is used up. • Increased leaf surface area which increases the rate of heat dissipation.

36. Common Adaptations Seen in Desert Plants • Use shady microhabitats • Stomates regulate exchange of gases • Recessed and reduced stomates which decreases water loss.

37. Shade Microhabitats Aloes in Namib Desert Lichens on rock in Big Bend Nat’l Park

38. Hornwort stomate (wet habitat) Xerophyte stomates Note countersunk guard cells and thick cuticle

39. Adaptive Variations of Photosynthesis: C3, C4 & CAM

42. Photosynthesis • RUBISCO: key enzyme that catalyzes the reduction of CO2 to organic C, but also catalyzes the reverse rxn. • Photorespiration -- uses O2 and releases CO2 • CO2 enters the leaf through stomates. • Open stomata decrease photorespiration, but increase water loss

44. C3 Photosynthesis • Called C3 because the CO2 is first incorporated into a 3-carbon compound. • Stomata are open during the day. • RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2. • Photosynthesis takes place throughout the leaf. • Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy). • Most plants are C3.

45. C3 Photosynthesis CO2 converted to a 3 C compound Occurs in palisade mesophyll cells

49. C4 Photosynthesis • Called C4 because the CO2 is first incorporated into a 4-carbon compound. • Stomata are open during the day. • Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis. • Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)

50. C4 Photosynthesis CO2 converted to a 4C compound in mesophyll cell RUBISCO operates in bundle sheath cell where CO2 conc. is high. C4 plants have spatial separation of the C4 and C3 pathways of carbon fixation.