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  Gain water through roots  Lose water  through photosynthesis (<1% loss this way)  through transpiration (stomates open to allow exchange of CO 2 and O 2 ; water escapes when guard cells are open)  transpiration also provides  transport of nutrients  cooling

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 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 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 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 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 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 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 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  Mesophytes  Phreophytes  Halophytes  Xerophytes  Hydrophytes

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

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

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

Desert Holly Salicornia Salt glands exuding salt droplets

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

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.

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  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.

 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 Plants

 Succulent leaves reduce the surface-to-volume ratio and favor water conservation.

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.

A crew of intrepid Biologists with a Haleakala Silversword

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

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.

Mesquite

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 Plants  Use shady microhabitats  Stomates regulate exchange of gases  Recessed and reduced stomates which decreases water loss.

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

Adaptive Variations of Photosynthesis: C 3, C 4 & CAM

Photosynthesis  RUBISCO: key enzyme that catalyzes the reduction of CO 2 to organic C, but also catalyzes the reverse rxn.  Photorespiration -- uses O 2 and releases CO 2  CO 2 enters the leaf through stomates.  Open stomata decrease photorespiration, but increase water loss

C3 Photosynthesis  Called C3 because the CO 2 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 CO 2.  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.

C 3 Photosynthesis CO 2 converted to a 3 C compound Occurs in palisade mesophyll cells

C4 Photosynthesis  Called C4 because the CO 2 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 CO 2. This enzyme allows CO 2 to be taken into the plant very quickly, and then it "delivers" the CO 2 directly to RUBISCO for photsynthesis.  Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)

C 4 Photosynthesis CO 2 converted to a 4C compound in mesophyll cell RUBISCO operates in bundle sheath cell where CO 2 conc. is high. C 4 plants have spatial separation of the C 4 and C 3 pathways of carbon fixation.

C 4 Plants  Grasses, corn, sugar cane  C 4 photosynthesis  CO 2 fixed by mesophyll cells as a C 4 compound  C 4 cpd is transported to adjacent bundle sheath cells  C 4 cpd is split, and CO 2 is refixed by C 3 pathway  Keeps CO 2 level high in bundle sheath cells  CO 2 doesn’t leak out through stomates  Since stomates don’t have to open so much don’t lose so much water  Very efficient; C 4 plants do better at high temps but not when temps are below about 40 o C

C4 Photosynthesis  Adaptive Value:  Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO 2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.  Has better Water Use Efficiency because PEP Carboxylase brings in CO 2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO 2 gain for photosynthesis.

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 stands for Crassulacean Acid Metabolism  Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO 2 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 CO 2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO 2 is released to RUBISCO for photosynthesis  CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads

CAM Photosynthesis C 4 pathway used at night when water loss is low Stomata completely closed during day Crassulacean acid metabolism plants have a temporal separation of C 4 and C 3 pathways of carbon fixation.

Crassulacean acid metabolism (CAM) Light and dark reactions of photosynthesis are uncoupled so stomates are closed during the day Night  Stomates open  Take up CO2  Produce crassulacean acid  stores CO2 as a C 4 acid Day  Stomates closed  Use stored CO2 for standard C 3 photosynthesis

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 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 CO 2 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).

Hydrophytes  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.

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