Chapter 5 Water Relations

Water Content of Air Relative humidity: a measure of the water

content of air relative to its content at saturation, defined as:

Relative humidity(%)= (Water vapor density /Saturation water vapor density) *100

Note: Saturation water vapor density

changes with temp., warm air can hold more vapor than cold air.

Water vapor pressure: pressure exerted by water vapor in the air.

Saturation water vapor pressure: pressure exerted by water vapor in air that is saturated with water.

Vapor pressure deficit: difference between the actual water vapor pressure and the saturation water vapor pressure at a particular temp.

Diffusion: water or substrates in solution move down their concentration gradients.

Osmosis: Diffusion of water across a semipermeable membrane.

Isosmotic organism: Organism with body fluids containing the same concentration of water as the external environment.

Hypoosmotic organism: with a higher concentration of water (lower solute concentration) than the external environment.

Hyperosmotic organism: with a lower concentration of water (higher solute concentration) than the external environment.

Water Potential and Water Movement Between Soil & Plants

Plant water potential (ψplant)

Ψplant=ψsolution + ψmatris + ψpressure

Water potential gradient from soil to plant: Fig.5.5, Fig.5.6

Metabolism of water movement from soils to plants: (Fig.5.7)

Water Regulation on Land Water regulation of terrestrial animals Wia=Wd + Wf +Wa –We –Ws Wia: internal water of an animal; Wd: water taken by drinking; Wf: water taken in with food; Wa: water absorbed from the air; We: water lost by evaporation; Ws: water lost with various secretions and excretions including urine, mucus, and feces. Fig.5.8

Water regulation of terrestrial plants :

Wip=Wr+ Wa –Wt–Ws

Wip: internal water concentration of a plant;

Wr: water taken from soil by roots;

Wa: water absorbed from the air; Wt: water lost by transpiration; Ws: water lost with various secretions and reproductive structures including nectar,

fruit, and seeds.

Water Acquisition by Animals

Drinking water Absorbing water from the air. For

example, a desert beetle (Fig. 5.9). Taking water with foods (Fig.5.10,

Fig,5.11) (free water and metabolic water).

Water Acquisition by Plants

Rooting difference of a same species in dry and moist soils. More and deeper roots develop in drier soil than in moister soil. Fig.5.12

Species from different habitats represent rooting characteristics.

Digitaria (growing in dry habitat) →(19 days unwatering) more root biomass, higher leaf water potential.

Eleusine (growing in less dry habitat) → (19 days unwatering) less root biomass, lower leaf water potential

Fig.5.13, Fig.5.14

Water Conservation by Plants & Animals

Animals: Reducing water loss: 1). waterproofing

Waterproofing differs from one species of organism to another. In general, species evolving in drier environments lose water at a slower rate (Fig.5.15).

Another example: two species of tiger beetles, experiment & result Fig.5.16

2). Other water conservation mechanisms:

Producing concentrated urine or feces with low water content;

Condensing and reclaiming (回收 ) the water vapor in breath;

Restricting activity to times and places that decrease water loss.

Plants: 1). reducing leaf area Fig.5.18 2). Other adaptation: --Developing thick leaves; --Having fewer stomata on leaves; --Having structures on the stomata that impede the movement of water; --Dormancy during times when moisture is unavailable ; --Water-conserving pathway for photosynthesis; --Wilting at midday, ect. Fig.5.19

Dissimilar Organisms with Similar Approaches to Desert Life Camel and Cactus

--can store a lot of water when water is available and lose water up to 20% of body weight without harmfulness to their life.

--can conserve water effectively by means of reducing water loss, such as thick hair to reduce heat gain, close stomata or reduce sweating to reduce evaporation. (Fig.5.21)

Two Arthropods (节肢动物 ) with Opposite Approaches to Desert Life

Scorpions: Relative inactive during the day in burrow near the base of the tree where the temp. is lower and humidity is higher, (Fig.5.22).

Cicada: Remains active in the hottest desert day because of evaporative cooling which is supported by drinking the tree fluid. (Fig.5.25, Fig.5.26)

Marine Fish & Invertebrates

Hyperosmotic Fish & Invertebrates

With internal fluid higher in water potential than surrounding environment, such as Shark. They gain water through osmosis mainly across the gills, and pump out water by the kidneys and exits as urine; Na+ and Cl- diffuse into sharks from the surrounding environment, and salts are concentrated by the salt gland and excreted with the urine. (Fig.5.27)

Hypoosmotic Fish & Invertebrates:

With internal fluid lower in water potential than surrounding environment, such as salt water mosquitoes.

They gain water by drinking, and lose it by diffusion osmosis mainly across the gills. They get rid of salts by specialized cells in the gills with secreted Cl- and Na+, and by urine. (Fig.5.28)