TOPIC 5 Surviving with limited water Flashcards
Describe the threat of dehydration posed by the atmosphere
- ‘Drying air’ - the atmosphere can hold very large quantities of water vapour, the capacity of which increases if the temperature of the air rises.
- Atmosphere rarely attains equilibrium - diurnal changes in air temp causes changes in humidity and will tend to lose water as precipitation or gain it by evaporation and transpiration
- A small change in air temp. increases the airs capacity to hold water substantially (vapour pressure gradient) therefore liquid water in contact with it will tend to evaporate, including the moisture in organisms.
-Water is essential for all living organisms as
a. Reagent for metabolic reactions
b. Solvent in which metabolic reactions take place
-Therefore terrestrial organisms have evolved adaptations to prevent dehydration and overcome the atmospheres moisture deficit.
-
DROUGHT AVOIDANCE STRATEGY
Maximising water supply
- Dual root systems: deep roots that extend to water table and shallower roots that can collect rainfall - Hydraulic lift; water taken from deeper in ground is lifted to shallower roots, where it is deposited and nutrients can then be taken from the nutritionally dense topsoil - eg. Acacias, desert trees
-Dormancy (growth pattern+morphology)
- Shed leaves in drought conditions or have polymorphic leaves that differ dependent on conditions. Leaves may grow opportunistically dependent on water availability
- Survive underground as tubers
- Survive as seeds - ephemereals have very short life cycles and produce seeds quickly so they have been shed by the time water shortage arrives. Seeds germinate only after persistent rainfall. Use C4 pathway to maximise growth in high light and efficiently use water
- eg desert shrubs, ephemereals
-Efficient storage and use (physiological+structure+biochemistry)
- Store water in specialised stem cells
- Have thick waxy cuticle
- Stomata close in day
- Shallow extensive root systems
- Use CAM pathway for efficient water usage
- Spines to protect from herbivores
- SUCCULENTS eg cacti, crassulaceae
- Water piracy
- Directly parasitic eg mistletoe
- May steal water from other plants who have hydraulically lifted water nearby
- e.g. grasses
Compare and contrast water conservation strategies used by animals with those employed by plants
Animals and plants share many similar water conservation adaptations (kangaroo rats in example)
- nocturnal activity to minimise vapour pressure gradients (corresponds to the CAM photosynthetic pathway)
- lipid coating on skin and hair to minimise evaporative losses direct from the surface (compare to plant cuticles)
- avoid direct solar radiation by staying below ground during day (plants try to achieve this by orientating their leaves vertically or shedding them altogether during water shortage)
- synchronisation of reproductive cycle with heavy rain events. The fertility of both males and female kangaroo rats increase immediately after heavy rain. With a gestation of one month, they can exploit plant growth as food when the young are born before the soil dries out again and plants become dormant (compare with reproduction cycles in desert ephemerals) - Additionally kidneys are highly adapted so nearly all water is recovered from urine before exctretion
- Kangaroo rats use respiratory water as opposed to getting it from food/drink
Describe how a hormone operates within an organism.
MODULE THEME HOMEOSTASIS and GROWTH
MODULE THEME GROWTH
Hormones are released in response to a TRIGGER that maybe internal (eg water potential in leaf) or external (water potential in soil)
CONCENTRATION AT RECEPTOR CELL MUST RESPOND TO ENVIRONMENTAL SIGNALS AT PERCEPTION LOCATION
Hormones may send positive signals eg. Hormone such as ABA present triggers response of stomatal closure
Or negative signals eg. When hormone Cytokinin is absent from plant stomatal pores close. (it encourages growth by cell division when present and opens stomatal pores)
Cytokinin and ABA are antagonistic - they work against each other so response is is determined by their relative concentrations
Multipilicity of effect - Hormones have more than one effect on physiology of an animal. ABA also inhibits growth by reducing elasticity in cell walls and triggers abscission in response to stressors.
EPIPHYTES
Use other plants as support structure; not parasitic
Have virtually no root system
Use water from droplets in fog/precipitation
Mosses, some ferns, crassulaceae, cacti - CAM plants
PHYSIOLOGICAL DROUGHT
Water is present but can not be accessed eg. in clay (matric pressure), saline (osmotic pressure) and waterlogged soil (lowers respiration and therefore energy available to take up water)
HORMONES
Chemical messengers that transmit messages from one cell to another or from one part of an organism to another part
Apoplastic
Outside the cell membrane
Free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, by air spaces between plant cells and by the plant cuticle.
Symplastic
The inner side of the cell membrane in which water (and low-molecular-weight solutes) can freely diffuse. The plasmodesmata allow the direct flow of small molecules such as sugars, amino acids, and ions between cells
Describe the availability of water within a system and explain its flux through the system in terms of water potential.
The presence of water is not the same as the availability of water. eg. Water may be present in soil but inaccessible due to matric pressure (in clay soil) or osmotic pressure in saline soil.
The concept of water potential allows the measurement of water availability and the prediction of water movement within a system
- Water potential is a measure using units of pressure (pascals) that denotes the availability of water compared with a reference state of pure water at atmospheric pressure (ψ=0)
- Transfer of water of water is PASSIVE and will move DOWN the water potential gradient (from high water potentials to low water potentials)
- Water potential comprises of two factors
a. OSMOTIC PRESSURE (π) - Solutes; The higher the concentration of solutes the LOWER the water potential.
b. HYDROSTATIC PRESSURE (P)
i. Inside the cell = turgor pressure (Positive pressure) High turgor pressure RAISES water potential
ii. Outside the cell = Tension eg. from transpiration (Negative pressure)
c. MATRIC PRESSURE - Water bound to surfaces or colloids in soil by absorption or adhesion so less available to the plant. High matric pressure LOWERS water potential
Water potential equation is ψ = P − π − m
At full turgor a cells water potential is 0 as it has reached equilibrium. P = π.
Transpiration is the dominant feature of water flux as plants lose most of their water this way. Water will move down a water gradient (from highest water potential in roots through to the lowest water potentials in the atmosphere). The force created by the large difference in water potential gradient is how water can get to the tops of trees up to 125 ft high. Transpiration is driven by the solar energy. The loss of water to transpiration is largely controlled by the opening and closing of stomata.
ISOHYDRIC PLANTS
Plants that maintain near constant water potential in leaves despite environmental changes via the use of ABA as a stomatal regulator
ABA
Plant hormone
- Acts as a regulator of stomatal closure and therefore helps to maintain water potential of plant (homeostasis)
- Most plant cells can synthesise ABA in response to a variety of environmental stressors
- Multipilicity of effect - Hormones have more than one effect on physiology of an animal. ABA also inhibits growth by reducing elasticity in cell walls and triggers abscission in response to stressors.
How does ABA trigger and close plant stomata?
Abscisic acid is an example of a plant hormone. It communicates stress to a range of tissues, particularly the guard cells. It is a regulator of stomatal closure (triggered by water shortage)
1) Biosynthesis is triggered by stressor in root (eg water shortage)
2) Trigger raises pH level of xylem which facilitates transport of ABA
3) Also negatively charges the ABA so it can not pass through guard cell membrane and accumulates at cell wall
4) Guard cells ABA receptors are on the outside of cell membrane so are sensitive to high apoplastic concentrations of ABA
5) Triggers a SIGNAL CASCADE - small amount of calcium ions are let through pores created at ABA receptor site, which triggers release of larger amount K+ ions and then the movement of vast amounts of water
Stomatal conductance
Measures the ease with which water vapour can flow out of a leaf. Measured in mmol m-2 s-1
DROUGHT TOLERANCE STRATEGIES
(biochemical+Physiological)
- Dessication
- Tissue dries out but resurrect upon being rewetted
- Biochemical reaction
- Mosses, some ferns, resurrection plant
- Xerophytes
- Make no physiological or biochemical changes to survive drought
- Highly adapted to tolerate drought conditions
- Small leaves with vertical orientation minimise water loss and reduce light absorption
- Relatively low leaf water potentials before stomata close and leaf shapes that protect stomata
- Biochemical adaptations that allow water content to fall without cell damage.
- Accumulate solutes to lower water potential
