Download Maintaining water balance

Maintaining water
balance - plants
Chapter 23
Transpiration Stream
 Passage of water from roots to leaves
 Root hairs have large absorbing surface
 Water moves into root cells and through cortex cells into xylem
along a concentration gradient
 Water also diffuses between the cells through the cell wall
fibres
 Roots also apply a root pressure
 Capillarity through xylem also helps
 Water leaves xylem to surrounding tissues through pits
 Transpiration – water lost by evaporation (mainly through
stomata)
 Concentration gradient carries water through leaves to stomata
 To replace loss, cells draw water from xylem – transpiration pull
 Cohesion-tension theory helps this pull
 Minerals (e.g. magnesium) actively taken up by root hairs
 Move from cell to cell via cytoplasmic connections
 Pumped into xylem and transported through the plant
Transpiration Stream
Xylem Pits
Capillarity
Transpiration Stream
Stomata
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Monocotyledons – equally distributed on leaf surfaces
Dicotyledons – mostly on lower epidermis
Have sausage-shaped guard cells – possess chloroplasts
Inner cell wall (lining stomatal opening) is thicker and less elastic
Stomata open/close as turgor changes in guard cells
Increased turgor = outer walls stretch out – inner walls pulled apart
More hypotonic environment = more water lost through guard cells
Advantages/Disadvantages
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Advantages
Supplies water for photosynthesis
Water also aids structural support (maintains turgor)
Minerals aid healthy growth & metabolic processes
Water evaporation also has a cooling effect
Disadvantages
Water loss through stomata can be excessive
More water lost through leaves than is gained through
roots
A major problem in hot weather
As a consequence, guard cells lose turgor and stomata
close
This conserves water loss – a type of osmoregulation
However water still lost directly through cuticle
Leads to wilting, and eventually death
Measuring transpiration
rates
 A bubble potometer measures rate of water uptake by
a shoot
 Movement of an air bubble along a scale (mm/min)
 Wind increases transpiration rate
 Increased humidity decreases transpiration
 A bubble atmometer measures rate of evaporation from
a non-living surface
 Proves that light increases increases transpiration rate
 Temperature increases rate, air pollution decreases it
Xerophytes
 Live in very dry, hot or windy environments
 Most normal plants (mesophytes) couldn’t survive – excessive
transpiration
 Xerophytes have fewer stomata & a thick cuticle
 Leaf often rolled and/or hairy
 Stomata are sunken in pits
 Both trap a layer of air
 Some have leaves with a very small surface area e.g. pine
needles/cacti
 Cacti have very long roots, & superficial roots
 Reverse stomatal rhythm (open at night, closed in the day)
 Some xerophytes cease vegetative activity – survive in a
dessicated (dried) state
Hydrophytes
 Live submerged in water
 E.g. water-millfoil, water lily
 Overcome problem of oxygen absorption, by air-filled
cavities
 These hold onto oxygen from photosynthesis for
assisting respiration
 Also provides buoyancy – aids light absorption
 Xylem tissue is reduced
 Submerged leaves are narrow and finely divided –
avoids damage from currents
 Floating leaves have long leaf stalks – prevents
stomata being flooded