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Water RelationsChapter 5

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Outline

• Water Availability• Water Content of Air• Water Movement in Aquatic Environments• Water Movement Between Soils and Plants• Water Regulation on Land• Water Acquisition by Animals• Water Acquisition by Plants• Water Conservation by Plants and Animals• Water and Salt Balance in Aquatic

Environments

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Water Availability

• The tendency of water to move down concentration gradients, and the magnitude of those gradients, determine whether an organism tends to lose or gain water from its environment. Must consider an organism’s microclimate

in order to understand its water relations.

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Water Content of Air

• Evaporation accounts for much of water lost by terrestrial organisms. As water vapor in the air increases, the

water concentration gradient from organisms to air is reduced, thus evaporative loss is decreased.

Evaporative coolers work best in dry climates.

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Water Content of Air

• Relative Humidity:

Water Vapor DensitySaturation Water Vapor Density (x 100)

• Water vapor density is measured as the water vapor per unit volume of air.

• Saturation water vapor density is measured as the quantity of water vapor air can potentially hold. Changes with temperature.

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Water Content of Air

• Total Atmospheric Pressure Pressure exerted by all gases in the air.

• Water Vapor Pressure Partial pressure due to water vapor.

• Saturation Water Vapor Pressure Pressure exerted by water vapor in air

saturated by water.• Vapor Pressure Deficit

Difference between WVP and SWVP at a particular temperature.

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Evaporative Water Loss

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Water Movement in Aquatic Environments

• Water moves down concentration gradient. Water is more concentrated in freshwater

environments than in the oceans.• Aquatic organisms can be viewed as an

aqueous solution bounded by a selectively permeable membrane floating in an another aqueous solution.

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Water Movement in Aquatic Environments

• If two environments differ in water or salt concentrations, substances will tend to move down their concentration gradients. Diffusion

Osmosis: Diffusion through a semipermeable membrane.

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Water Movement in Aquatic Environment

• Isomotic: Body fluids and external fluid are at the same concentration.

• Hypoosmotic: Body fluids are at a higher concentration than the external environment.

• Hyperosmotic: Body fluids are at a lower concentration than the external environment.

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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.

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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.

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Water Regulation on Land

• Terrestrial organisms face (2) major challenges: Evaporative loss to environment. Reduced access to replacement water.

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Water Regulation on Land - Animals

• Wia= Wd + Wf + Wa - We - Ws

• Wia= Animal’s internal water

• Wd = Drinking

• Wf = Food

• Wa = Absorbed by air

• We = Evaporation

• Ws = Secretion / Excretion

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Water Regulation on Land - Animals

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Water Regulation on Land - Plants

• Wip= Wr + Wa - Wt - Ws

• Wip= Plant’s internal water

• Wr =Roots

• Wa = Air

• Wt = Transpiration

• Ws = Secretions

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Water Regulation on Land - Plants

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Water Acquisition by Animals

• Most terrestrial animals satisfy their water needs via eating and drinking. Can also be gained via metabolism

through oxidation of glucose: C6H12O6 + 6O2 6CO2 + 6H2O

Metabolic water refers to the water released during cellular respiration.

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Water Acquisition by Plants

• Extent of plant root development often reflects differences in water availability. Deeper roots often help plants in dry

environments extract water from deep within the soil profile.

Park found supportive evidence via studies conducted on common Japanese grasses, Digitaria adscendens and Eleusine indica.

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Water Conservation by Plants and Animals

• Many terrestrial organisms equipped with waterproof outer covering.

• Concentrated urine / feces.• Condensing water vapor in breath.• Behavioral modifications to avoid stress

times.• Drop leaves in response to drought.• Thick leaves• Few stomata• Periodic dormancy

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Dissimilar Organisms with Similar Approaches to Desert Life

• Camels Can withstand water loss up to 20%.

Face into sun to reduce exposure. Thick hair: Increased body temperature

lowers heat gradient.• Saguaro Cactus

Trunk / arms act as water storage organs. Dense network of shallow roots. Reduces evaporative loss.

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Two Arthropods with Opposite Approaches to Desert Life

• Scorpions Slow down, conserve, and stay out of sun. Long-lived Low metabolic rates

• Cicadas (Diceroprocta apache) Active on hottest days. Perch on branch tips (cooler microclimates). Reduce abdomen temp by feeding on xylem

fluid of pinyon pine trees.

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Water and Salt Balance in Aquatic Environments

• Marine Fish and Invertebrates Isomotic organisms do not have to expend

energy overcoming osmotic gradient. Sharks, skates, rays - Elevate blood

solute concentrations hyperosmotic to seawater.

Slowly gain water osmotically. Marine bony fish are strongly

hypoosmotic, thus need to drink seawater for salt influx.

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Osmoregulation by Marine Organisms

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Water and Salt Balance in Aquatic Environments

• Freshwater Fish and Invertebrates Hyperosmotic organisms that excrete

excess internal water via large amounts of dilute urine.

Replace salts by absorbing sodium and chloride at base of gill filaments and by ingesting food.

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Osmoregulation by Freshwater Organisms

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Review

• Water Availability• Water Content of Air• Water Movement in Aquatic Environments• Water Movement Between Soils and Plants• Water Regulation on Land• Water Acquisition by Animals• Water Acquisition by Plants• Water Conservation by Plants and Animals• Water and Salt Balance in Aquatic

Environments

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