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Water Balance in Terrestrial Plants. Water Regulation on Land - Plants. W ip = W r + W a - W t - W s W ip = Plant’s internal water W r =Roots W a = Air W t = Transpiration W s = Secretions. Water Regulation on Land - Plants. Water Balance in Terrestrial Plants.

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water regulation on land plants
Water Regulation on Land - Plants

Wip= Wr + Wa - Wt - Ws

  • Wip= Plant’s internal water
  • Wr =Roots
  • Wa = Air
  • Wt = Transpiration
  • Ws = Secretions
water balance in terrestrial plants1
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
water movement between soils and plants
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.
variation in water availability
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.

variation in water availability1
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.

variation in water availability2
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).
water movement between soils and plants1
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.
variation in water availability3
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.
variation in water availability4
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.
variation in water availability5
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.
classification of plants according to habitat type
Classification of Plants According to Habitat Type
  • Mesophytes
  • Phreophytes
  • Halophytes
  • Xerophytes
  • Hydrophytes
  • 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
  • Long roots to reach water table
    • e.g. mesquite shrubs may have roots 175 feet long
    • Prairie grasses and forbs
  • 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

Desert Holly


Salt glands exuding salt droplets

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

California poppies and other ephemerals from the Mojave Desert of the American Southwest

Blue Phacelia from the Sonoran and Mojave Deserts

Seed Bank

common adaptations seen in desert plants
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.


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.
common adaptations seen in desert plants1
Common Adaptations Seen in Desert Plants
  • Succulent leaves reduce the surface-to-volume ratio and favor water conservation.
common adaptations seen in desert plants2
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.
common adaptations seen in desert plants3
Common Adaptations Seen in Desert Plants
  • Small, hard leaves
  • Drought-deciduous
    • Drop leaves/twigs when soil dries up.
      • Ocotillo
common adaptations seen in desert plants4
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.
common adaptations seen in desert plants5
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.
common adaptations seen in desert plants6
Common Adaptations Seen in Desert Plants
  • Use shady microhabitats
  • Stomates regulate exchange of gases
    • Recessed and reduced stomates which decreases water loss.
shade microhabitats
Shade Microhabitats

Aloes in Namib Desert

Lichens on rock in Big Bend Nat’l Park


Hornwort stomate (wet habitat)

Xerophyte stomates

Note countersunk guard cells and thick cuticle

  • 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
c3 photosynthesis
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.

C3 Photosynthesis

CO2 converted to a 3 C compound

Occurs in palisade mesophyll cells

c4 photosynthesis
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)

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.

c 4 plants
C4 Plants
  • Grasses, corn, sugar cane
  • C4 photosynthesis
    • CO2 fixed by mesophyll cells as a C4 compound
    • C4 cpd is transported to adjacent bundle sheath cells
    • C4 cpd is split, and CO2 is refixed by C3 pathway
    • Keeps CO2 level high in bundle sheath cells
      • CO2 doesn’t leak out through stomates
    • Since stomates don’t have to open so much don’t lose so much water
  • Very efficient; C4 plants do better at high temps but not when temps are below about 40oC
c4 photosynthesis1
C4 Photosynthesis
  • Adaptive Value:
    • Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.
    • Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.
c4 photosynthesis2
C4 Photosynthesis
  • Stomata can be more closed and decrease water loss while photorespiration is kept low --spatial separation
  • but, this costs extra energy, 2 ATP
    • ATP used to split C4 compound
cam photosynthesis
CAM Photosynthesis
  • CAM stands for Crassulacean Acid Metabolism
  • Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis.
  • Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis
  • CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads

CAM Photosynthesis

C4 pathway used at night when water loss is low

Stomata completely closed during day

Crassulacean acid metabolism plants have a temporal separation of C4 and C3 pathways of carbon fixation.


Stomates open

Take up CO2

Produce crassulacean acid

stores CO2 as a C4 acid


Stomates closed

Use stored CO2 for standard C3 photosynthesis

Crassulacean acid metabolism (CAM)Light and dark reactions of photosynthesis are uncoupled so stomates are closed during the day
cam photosynthesis1
CAM Photosynthesis
  • Adaptive Value:
    • Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
cam photosynthesis2
CAM Photosynthesis
  • Plants may CAM-idle.
    • When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis.
    • This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells).
  • Aquatic plants
    • Plants may be
      • submerged and free-floating
      • Submerged and anchored to the substrate
      • anchored to the substrate with the upper leaf surfaces exposed to the air
      • Free-floating
Hydrophytes maintain buoyancy by developing intercellular spaces that can trap gas bubbles.

Air spaces in a water lily stem


HETEROPHYLLY Condition where the same organ has a change in form.

The submerged aquatic leaf is simple, (upper diagram) and only three cells thick, while the floating leaf (lower diagram) contains numerous intercellular airspaces and has a columnar mesophyll arrangement.