Chapter 6: Water relations

ways plants prevent water loss (7)

1) having less leaf surface per length of root = less water loss 2) drop leaves in response to drought / produce leaves only in response to rain 3) thick leaves (which have less transpiring leaf surface area per unit volume of photosynthesizing tissue 4) fewer stomata on leaves than many 5) structures on the stomata that impede the movement of water 6) dormancy during times when moisture is unavailable 7) alternative pathways for photsysthesis (like C4 and CAM)

additional mechanisms, besides waterproofing, that animals adapted to dry conditions use (3 additional ones)

1) producing concentrated urine or feces with low water content, 2) condensing and reclaiming the water vapor in breath, and 3) restricting activity to times and places that decrease water loss.

stable isotope analysis

A major contributor to recent progress in belowground ecology has been the development of new tools. One of the most important of those tools is stable isotope analysis, which involves the analysis of the relative proportions of stable isotopes, such as the stable isotopes of carbon C13 and C,12 in materials. Different organisms contain different ratios of light and heavy stable isotopes because they use different sources of these elements, because they preferentially use (fractionate) different stable isotopes, or because they use different sources and fractionate.

ψpressure

At the level of the whole plant, another force is generated as water evaporates from the air spaces within leaves into the atmosphere. Evaporation of water from leaves generates a negative pressure, or tension, on the column of water that extends from the leaf through the plant all the way down to its roots. This negative pressure reduces the water potential of plant fluids still further.

D (hydrogen) isotope analysis

Because different sources of water often have different ratios of D to H,1 for example, shallow soil moisture versus deep soil moisture, hydrogen isotope analyses have helped identify where plants acquire their water.

how are cicadas able to sweat? won't that make them lose water?

So, it turns out that these cicadas can sing in the hottest hours of the desert day because they sweat! Diceroprocta is able to maintain this seemingly impossible lifestyle because it has tapped into a rich supply of water. Cicadas are members of the order Homoptera and distant relatives of the aphids. Like aphids, cicadas feed on plant fluids. So, though the cicada lives in the same macroclimate as the scorpion, it has tapped into a totally different microclimate. The cicada's scope for water acquisition is extended up to 30 m deep into the soil by the taproots of its mesquite host plant, Prosopis juliflora. accounts for high We (evaporation) by balancing it with high Wd (water acquisition)

geographic differences in rooting depth

However, there were pronounced geographic differences in rooting depth. Schenk and Jackson found that rooting depth increases from 80° to 30° latitude—that is, from Arctic tundra to Mediterranean woodlands and shrublands and deserts. However, there were no clear trends in rooting depth in the tropics. Consistent with our present discussion, deeper rooting depths occur mainly in water-limited ecosystems.

if δX =+X 0/00, then

If δX =+X 0/00, then the concentration of the heavier isotope is higher in the sample compared to the standard.

If δX=0, then

If δX=0, then the ratios of the isotopes in the sample and the standard are the same;

water movement in aquatic environments

In aquatic environments, water moves down its concentration gradient. If the internal environment of the organism and the external environment differ in concentrations of water and salts, these substances will tend to move down their concentration gradients. This movement is the process of diffusion. We give the diffusion of water across a semipermeable membrane a special name, however: osmosis.

differences between the approach taken by desert cicadas and scorpions to desert life

Scorpions: The scorpion's approach is to slow down, conserve, and stay out of the sun. Scorpions are relatively large and long-lived arthropods with very low metabolic rates. A low rate of metabolism means that they can subsist on low rations of food and lose little water during respiration. In addition, desert scorpions are well waterproofed; hydrocarbons in their cuticles seal in moisture. cicada: Diceroprocta apache, is active on the hottest days because they are capable of evaporative cooling; To verify this hypothesis, Eric Toolson and Neil Haley placed cicadas in the environmental chamber and then raised the relative humidity to 100%. At 100% relative humidity, the body temperatures of the cicadas quickly increased to the temperature of the environmental chamber. When Toolson reduced relative humidity to 0%, the cicadas cooled approximately 4°C within minutes. by raising the humidity of the air surrounding the cicadas to 100%, Toolson shut off any evaporative cooling that might be taking place.

sharks -TMAO -osmosis

Sharks, skates, and rays generally elevate the concentration of solutes in their blood to levels slightly hyperosmotic to seawater. However, inorganic ions constitute only about one-third of the solute in shark's blood; the remainder consists of the organic molecules urea and trimethylamine oxide, or TMAO. As a consequence of being slightly hyperosmotic, sharks slowly gain water through osmosis; that is, Wo is slightly positive. The water that diffuses into the shark, mainly across the gills, is pumped out by the kidneys and exits as urine. Sodium, because it is maintained at approximately two-thirds its concentration in seawater, diffuses into sharks from seawater across the gill membranes and some sodium enters with food. Sharks excrete excess sodium mainly through a specialized gland associated with the rectum called the salt gland. The main point here is that sharks and their relatives reduce the costs of osmoregulation, regulation of internal salt and water concentrations, by decreasing the osmotic gradient between themselves and the external environment (fig. 6.2

use of C-13

Stable isotope analysis can also measure the relative contribution of C3 and C4 plants (see chapter 7, section 7.1) to a species' diet (see chapter 1, section 1.2). This is possible because C4 plants are relatively richer in C.13

water regulation of animals on land (equation)

Terrestrial plants and animals regulate their internal water by balancing water acquisition against water loss. We can summarize water regulation by terrestrial animals as: Wia = Wd + Wf + Wa - We - Ws where Wia= internal water of an animal Wd= water intake by drinking Wf= water taken in through 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

water vapor density -what is it -units -saturation water vapor density

The actual amount of water in air is measured directly as the mass of water vapor per unit volume of air. This quantity, the water vapor density, is given either as milligrams of water per liter of air (mg H2O/L) or as grams of water per cubic meter of air (g H2O/m3). The maximum quantity of water vapor that air at a particular temperature can contain is its saturation water vapor density, the denominator in the relative humidity equation. Saturation water vapor density increases with temperature, as you can see from the red curve in figure 6.2.

The concentrations of stable isotopes are generally expressed as + equation

The concentrations of stable isotopes are generally expressed as differences in the concentration of the heavier isotope relative to some standard. The units of measurement are differences (±) in parts per thousand (± 000/). These differences are calculated as: δX =[(Rsample/Rstandard)−1]× 1000 where X= the relative concentration of the heavier isotope, for example, D, 13C, 15N, or 34S in 0/00 Rsample= the isotopic ratio in the sample, for example, D:1H, 13C : 12C or 15N:14N Rstandard = the isotopic ratio in the standard, for example, D:1H, 13C:12C, or 15N:14N

vapor pressure deficit

We can also use water vapor pressure to represent the relative saturation of air with water. You calculate this measure, called the vapor pressure deficit, as the difference between the actual water vapor pressure and the saturation water vapor pressure at a particular temperature.

water regulation by plants equation

Wip = Wr + Wa - Wt - Ws where Wip= internal water concentration of a plant Wr= water taken in from soil 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

-matric forces

Within small spaces, such as the interior of a plant cell or the pore spaces within soil, other forces, called matric forces, are also significant. Matric forces are a consequence of water's tendency to adhere to the walls of containers such as cell walls or the soil particles lining a soil pore. Matric forces lower water potential.

Piper auritum (tropical region plant water adaptation)

a large-leafed, umbrella-shaped plant, grows in clearings of the rain forest. Because it grows in clearings, the plant often faces drying conditions during midday. However, it reduces the leaf area it exposes to the midday sun by wilting. shows that even rain forests have microclimates requiring water conservation

in Merriam's kangaroo study: impact of acclimation on water conservation

acclimating animals to laboratory conditions did not eliminate the differences in water conservation among populations. In other words, even after being kept in the laboratory under controlled conditions, Merriam's kangaroo rats from the driest study site continued to lose water at a lower rate. The evidence from these studies supports the conclusion that these three populations differ in their degree of adaptation to desert living.

if δX =−X 0/00, then

if δX =−X 0/00, (is negative) then the concentration of the heavier isotope is lower (e.g., 15N) in the sample compared to the standard

saltwater mosquitoes -osmosis

like marine bony fish, saltwater mosquitoes are hypoosmotic to the surrounding environment, to which they lose water and replace it also by drinking and rid the excess salt specialized cells near the rectum. Here, saltwater mosquitoes do something that marine bony fish cannot. They excrete a urine that is hyperosmotic to their body fluids, which reduces water loss through the urine.

kangaroo rats

live in desert; don't have to drink at all - can survive entirely on metabolic water and food moisture

marine bony fish -osmosis

marine bony fish have body fluids that are strongly hypoosmotic to the surrounding medium. As a consequence, they lose water to the surrounding seawater, mostly across their gills. Marine bony fish make up these water losses by drinking seawater. However, drinking seawater increases salt gain. The fish rid themselves of excess salts in two ways. Specialized "chloride" cells at the base of their gills secrete sodium and chloride directly to the surrounding seawater, while the kidneys excrete magnesium and sulfate ions, which are expelled with urine. The urine, because it is hypoosmotic to the body fluids of the fish, represents a loss of water. However, the loss of water through the kidneys is low because the volume of urine is low.

how do desert cicadas have evaporative cooling? how was this shown?

placed a live Diceroprocta in an environmental chamber with a humidity sensor just above its cuticle. If Diceroprocta evaporatively cools, then this sensor would detect higher humidity as the temperature of the environment was increased. This is exactly what occurred. three areas on the dorsal surface with large pores that might be involved in evaporative cooling. When they plugged these pores, Diceroprocta could no longer cool itself.

relative humidity

the percent water content relative to a potential maximum: relative humidity = (water vapor density / saturation water vapor density) x 100

using stable isotopes to identify plant water sources - James Ehleringer

used deuterium:hydrogen (D:1H) ratio in Standard Mean Ocean Water, the 15N:14N ratios, or δD, to explore the use of summer versus winter rainfall by various plant growth forms in the deserts of southern Utah. They could use δD to determine the relative utilization of these two water sources since summer rains are relatively enriched with D and winter rains are relatively depleted of D. The δD of summer and winter rains in southern Utah at the time of Ehleringer's study were −250/00 and −900/00 respectively Ehleringer and his research team found that a succulent, several herbaceous perennials, and several woody perennials used winter moisture in the spring (see fig. 6.28). However, when summer rains fell, the succulent species shifted entirely to using soil moisture from summer rains that were stored mainly at shallow soil depths. Meanwhile, herbaceous and woody perennials continued to use significant amounts of deeper soil moisture that fell the previous winter. So, stable isotope analysis opens a window to the water relations of plants that would not be accessible without this innovative tool.

what provides waterproofing for terrestial insect cuticles?

usually provided by hydrocarbons, which include organic compounds, such as lipids and waxes. Fully saturated hydrocarbons are much more effective at waterproofing.

ψsolutes

we can express the water potential of a solution as: ψ(water potential of a solution) = ψsolutes ψsolutes is the reduction in water potential due to dissolved substances, which is a negative number. ψ is just a symbol for water potential in PSI

water potential for fluids within plant cells equation

ψplant = ψsolutes + ψmatric + ψpressure

desert animals and water absorption

Coastal deserts such as the Namib Desert of southwestern Africa receive very little rain but are bathed in fog. This aerial moisture is the water source for some animals in the Namib. beetles dig trenches on the face of sand dunes to condense and concentrate fog. The moisture collected by these trenches runs down to the lower end, where the beetle waits for a drink. Another tenebrionid beetle, Onymacris unguicularis, collects moisture by orienting its abdomen upward (Hamilton and Seely 1976). Fog Page 134condensing on this beetle's body flows to its mouth The remaining water is produced when the beetle metabolizes the carbohydrates, proteins, and fats contained in its food. (water is a product of cellular respiration- metabolic water)

most animals get their water from _____ while most plants get their water from___

Food and drinking their roots

use of N-14

For instance, the lighter isotope of nitrogen, N,14 is preferentially excreted by organisms during protein synthesis. As a consequence of this preferential excretion of N-14, an organism becomes relatively enriched in N15 compared to its food. Therefore, as materials pass from one trophic level to the next, tissues become richer in N.15

freshwater fish -osmosis

Freshwater fish are hyperosmotic; they have body fluids that contain more salt and less water than the surrounding medium. As a consequence, water floods inward and salts diffuse outward across their gills. Freshwater fish excrete excess internal water as large quantities of dilute urine. They replace the salts they lose to the external environment in two ways. Chloride cells at the base of the gill filaments absorb sodium and chloride from the water, while other salts are ingested with food.

water and salt balance in aquatic environments -equation

Marine and freshwater organisms use complementary mechanisms for water and salt regulation. Aquatic organisms, like terrestrial species, regulate internal water, Wi, by balancing water gain against water loss. We can represent water regulation in aquatic environments by modifying our equation for terrestrial water balance to: Wi = Wd - Ws +or- Wo where Wi= internal water Wd= drinking Ws= secretion of water withurine W0= osmosis (depends on the organism and the environment)

how do most marine invertebrates deal with an osmotic gradient in the surrounding water?

Most marine invertebrates maintain an internal concentration of solutes equivalent to that in the seawater around them. What does the animal gain by remaining isosmotic with the external environment? The isosmotic animal does not have to expend energy overcoming an osmotic gradient. however, Although the total concentration of solutes is the same inside and outside the animal, there are still differences in the concentrations of some individual solutes. These concentration differentials can only be maintained by active transport, which consumes some energy.

park found that the differences in root growth were _____ in the ______ soil layers

Park found that the differences in root growth were greatest in the deeper soil layers.

Tracy and Walsberg kangaroo rat study

Randall Tracy and Glenn Walsberg studied three populations of Merriam's kangaroo rats across a climatic gradient. Their main objective was to determine if different populations of the species vary in their degree of adaptation to dry environments Climatic differences at the three study sites are reflected in the vegetation. The habitat at the driest site consists of sand dunes with scattered shrubs; the intermediate site is a desert shrubland; and the vegetation at the moist site consists of temperate, pinyon-juniper woodland. results: kangaroo rats from the driest site lost water at lower rates; additionally, Tracy and Walsberg found that acclimating animals to laboratory conditions did not eliminate the differences in water conservation among populations.

comparison between camel and saguaro cactus

Both the camel and the saguaro cactus acquire massive amounts of water when water is available, store water, and conserve water. Camel: can drink up to 1/3 of its body weight in water when water is available; faces into the sun to reduce body surface in take with the su; thick hair insulates it from intense desert sun; rather than sweating, it allows its body temp to increase by up to 7 C cactus: dense network of shallow roots capture water and can store large quantities in the trunk and arms of the cactus; keeps it stomata closed during the day; internal temp of cactus can reach up to 50 c,which can actually be an advantage because, like the camel, this process reduces the rate of additional heating; also reduces rate of heating by the shape and orientation of its trunk and arms; also is insulated by a layer of plant hairs and a thick tangle of spines which reflect sunlight and shade the growing tip

osmotic pressure -isosmotic -hypoosmotic -hyperosmotic

In the aquatic environment, water moving down its concentration gradient produces osmotic pressure. Osmotic pressure, like vapor pressure, can be expressed in pascals. The strength of the osmotic pressure across a semipermeable membrane, such as the gills of a fish, depends on the difference in water concentration across the membrane. Those with body fluids containing the same concentration of water and solutes as the external environment are isosmotic. Organisms with body fluids with a higher concentration of water and lower solute concentration than the environment are hypoosmotic and tend to lose water to the environment. (hypo=low=low solute) Those with body fluids with a lower concentration of water and higher solute concentration than the external medium are hyperosmotic and are subject to water flooding inward. (hyper=high=high internal solute)

freshwater invertebrates -osmosis

Like freshwater fish, freshwater invertebrates are hyperosmotic to the surrounding environment and must expend energy to pump out the water that floods their tissues. They also expend energy by actively absorbing salts from the external environment. However, the concentration of solutes in the body fluids of freshwater invertebrates ranges from between about one-half and one-tenth that of their marine relatives. This lower internal concentration of solutes reduces the osmotic gradient between freshwater and the outside environment and so reduces the energy freshwater invertebrates must expend to osmoregulate.

root development and water availability

The extent of root development by plants often reflects differences in water availability. Studies of root systems in different climates show that plants in dry climates grow more roots than do plants in moist climates. In dry climates, plant roots tend to grow deeper in the soil and to constitute a greater proportion of plant biomass. The taproots of some desert shrubs can extend 9 or even 30 m down into the soil, giving them access to deep groundwater. Roots may account for up to 90% of total plant biomass in deserts and semiarid grasslands. In coniferous forests, roots constitute only about 25% of total plant biomass.

how plants get water to move from roots to leaves

The higher water potential of soil water compared to the water potential of roots induces water to flow from the soil into plant roots. As water enters roots from the surrounding soil, it joins a column of water that extends from the roots through the water-conducting cells, or xylem, of the stem to the leaves. Hydrogen bonds between adjacent water molecules bind the water molecules in this water column together. Consequently, as water molecules at the upper end of this column evaporate into the air at the surfaces of leaves, they exert tension, or negative pressure, on the entire water column. This negative pressure helps power uptake of water by terrestrial plants. Figure 6.6 summarizes the mechanisms underlying the flow of water from soil to plants. As plants draw water from the soil, they soon deplete the water held in the larger soil pore spaces, leaving only water held in the smaller pores. Within these smaller soil pores, matric forces are greater than in the larger pores. Consequently, as soil dries, soil water potential decreases and the remaining water becomes harder and harder to extract.

mosquito larvae

The larvae of approximately 95% of mosquito species live in freshwater, where they face osmotic challenges very similar to those presented to freshwater fish. Like freshwater fish, mosquito larvae must solve the twin problems of water gain and ion loss. In response, they drink very little water. They conserve ions taken with the diet by absorbing them with cells that line the midgut and rectum, and they secrete a dilute urine. Freshwater mosquito larvae replace the ions lost with urine by actively absorbing Na+ and Cl− from the water with cells in their anal papillae. Freshwater mosquitoes and fish use totally different structures to meet nearly identical environmental challenges

saturation water pressure

The pressure exerted by the water vapor in air that is saturated with water is called saturation water vapor pressure.

reference materials used as standards in the isotopic analyses of hydrogen, nitrogen, carbon, and sulfur are

The reference materials used as standards in the isotopic analyses of hydrogen, nitrogen, carbon, and sulfur are the D:1H ratio in Standard Mean Ocean Water, the 15N:14N ratio in atmospheric nitrogen, the 13C:12C ratio in PeeDee limestone, and the 34S:32S ratio in the Canyon Diablo meteorite.

R. Coupland and R. Johnson (1965) and roots in canada within microclimates

They found that microclimate affects root development in many grassland species. For instance, the roots of fringed sage, Artemesia frigida, penetrate over 120 cm into the soil on dry sites; on moist sites, its roots grow only to a depth of about 60 cm (fig. 6.11).

Y.-M. Park (1990) on why Digitaria can grow on coastal dunes while Eleusine cannot

Unwatered Digitaria and Eleusine responded differently. The root mass of Digitaria increased almost sevenfold over the 19 days of no watering, while the root mass of Eleusine increased about threefold. In addition, the roots of Digitaria were still growing at the end of the experiment, while those of Eleusine stopped growing about 4 days before the end of the experiment. Park's results suggest that Digitaria can be successful in the drier dune habitat because it grows longer roots, which exploit deeper soil moisture. With these deeper roots, Digitaria can keep the water potential of its tissues high even in relatively dry soils, where Eleusine suffers lowered water potential. In other words, Digitaria maintains higher leaf water potentials because its greater root development maintains a higher rate of water intake—higher Wr.

water vapor pressure

Using pressure as a common currency to represent water relations in very different environments helps us unify our understanding of this very important area of ecology. We usually think in terms of total atmospheric pressure, the pressure exerted by all the gases in air, but you can also calculate the partial pressures due to individual atmospheric gases such as oxygen, nitrogen, or water vapor. We call this last quantity water vapor pressure. The international convention for representing water vapor pressure, however, is in terms of the pascal (Pa), where 1 Pa is 1 newton of force per square meter. Using this convention, 760 mm of mercury, or one atmosphere of pressure, equals approximately 101,300 Pa, 101.3 kilopascals (kPa), or 0.101 megapascal (MPa = 106 Pa).

water potential -general -in aquatic environments

Water moving from the soil through a plant and into the atmosphere flows down a gradient of water potential, which is the capacity of water to do work. Flowing water has the capacity to do work, such as turning the water wheel of an old-fashioned water mill or the turbines of a hydroelectric plant. The capacity of water to do work depends on its free energy content. Water flows from positions of higher to lower free energy. Under the influence of gravity, water flows downhill from a position of higher free energy, at the top of the hill, to a position of lower free energy, at the bottom of the hill. water flows down its concentration gradient, from locations of higher water concentration (hypoosmotic) to locations of lower water concentration (hyperosmotic). The measurable "osmotic pressure" generated by water flowing down these concentration gradients shows that water flowing in response to osmotic gradients has the capacity to do work. We measure water potential, like vapor pressure deficit and osmotic pressure, in pascals, usually megapascals (MPa = Pa × 106). By convention, water potential is represented by the symbol ψ(psi), and the water potential of pure water is set at 0. In nature, water potentials are generally negative.