Water Relations

1. Water Relations Chapter 5

2. 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

3. 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.

4. 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.

5. Water Content of Air • Relative Humidity: Water Vapor Density Saturation 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.

6. 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.

7. Evaporative Water Loss

8. 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.

9. 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.

10. 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.

12. 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.

14. 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.

15. Water Regulation on Land • Terrestrial organisms face (2) major challenges: • Evaporative loss to environment. • Reduced access to replacement water.

16. 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

17. Water Regulation on Land - Animals

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

19. Water Regulation on Land - Plants

20. 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.

21. 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.

22. 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

23. 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.

25. 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.

26. 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.

27. Osmoregulation by Marine Organisms

28. 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.

29. Osmoregulation by Freshwater Organisms

30. 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