Water, drought, adaptation…

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Water, drought, adaptation

• From presentations by
• Marek Neuberg and Jan Pokorný
• compiled by Jan Kvet

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Daily course of transpiration
Effect of stomatal regulation on the daily course
of transpiration in plants growing with different
water supply, from 1 to 5 (www.ictinternational.co
m.au)
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Control of transpiration
Prevzato z http//www.mhhe.com/biosci/pae/botany/
vrl/images1.htm (20.6. 2004)
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Water flows within the plant along awater
potential gradient

• Water flow in plant cells is a passive process.
• That is, water moves in response to physical
  forces, towards regions of low water potential.
• Consequently, there are no metabolic pumps for
  water movement (such as reactions driven by ATP)
  that would push water from one place to another.

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Aquaporins increasing cell membrane permeability
There are proteins in the cell membranes of
many plant cells, and these proteins are
sensitive to the water status of the cells they
increase the overall permeability of water flow
through the cell membranes
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What happens when the external solute
concentration is increased?
Increasing the solution concentration to 0.3 M
sucrose causes the cell to lose water, to the
point where the membrane is just about to pull
away from the cell wall (called incipient
plasmolysis) The increased solute potential of
the external solution draws water out the cell,
reducing the cells turgor pressure Again,
water moves from a high to a low water potential.
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Water potential in plants
Water potential strongly affects the growth of
cells and photosynthesis in plants. Thus, like
measurements of body temperature of humans, water
potential is a good overall indicator of plant
health. Plant physiologists know a wide variety
of ways to measure water potential in plants
(e.g., plasmolysis, pressurizing of leaves)
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Plant water status and the operation of
physiological processes
Because of transpirational water loss, plants are
seldom fully hydrated Plants thus readily cope
with conditions of negative pressure, or water
deficit, up to a point where different
physiological processes become inhibited Thus
by measuring the water potential of plants we can
get an idea of how stressed they are.
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The whole plant water transportnetwork
Transport of water through the whole plant
involves several interacting components, each
with different driving forces and rates of
transport We will consider the following
components of water transport from roots to
shoots Soil transport properties (remember
plants are in contact with the soil!) Root
hydraulic transport, water flow across roots
Water flow within the xylem (stems and leaf
veins) Evaporation (transpiration) from the
leaf intercellular air spaces to the atmosphere
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Soil water transport an over-looked dimension
Water supply to the plant is critically influenc
ed by how water moves through the soil what are
the driving forces for water flow in soil and
which soil characteristics affect water flow?
Soil type and texture determine how easily water
flows in the soil by affecting the soil
porosity Sandy soils have large particles
and large pores in between (1 mm or more) Water
IS NOT held tightly by this soil Clay soils
have much smaller particles and pores in between
(2 microns) Water IS tightly held by this soil
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How does water move in soil?
Water moves through the soil predominantly by
bulk flow, that is down a pressure gradient under
NO influence of solutes As plants absorb water
from the soil, they deplete the soil of
water near the surface of the roots. This
reduces ?P of the water near the root surface and
establishes a pressure gradient from the
soil into the root. Because water-filled pore
spaces of the soil are INTERCONNECTED, water
moves through the soil by bulk flow down a
pressure gradient.
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Water uptake by roots thegatekeepers of whole
plant water flow

• Water enters the plant typically first at the
  root level
• Water moves in the root through several
  interconnected and
• complicated pathways
• Unlike the soil where water moves only by bulk
  flow, water
• flow through the root is more complicated and
  occurs by
• osmosis
• Water flow through roots takes two major
  pathways through the epidermis and cortex of the
  root towards the central cylinder with vascular
  bundles
• Apoplastic
• Symplastic
• The relative importance of one or the other
  path is not clear.

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Pathways of water movement in roots
Apoplastic and symplastic paths
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Root pressure and guttation
Root pressure occurs when soil water potentials
are very high and transpiration rates are low
If transpiration rate is high, water is taken up
so rapidly that a positive pressure cannot
develop Plants that produce high root pressure
frequently form liquid droplets on the edges of
their leaves a process called guttation
Positive (root) pressure causes the water
exudation
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Water transport in the xylem
In most plants, the xylem constitutes the longest
part of the whole plant water transport pathway
For a 1 m tall plant, more than 99.5 of the
water transport pathway will occur within the
xylem. Compared to the root, xylem
water transport is a fairly simple low resistance
transport pathway. We will consider now The
role of negative pressures in xylem transport
And the importance of safe xylem transport under
negative pressure
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Negative or positive pressure drivenwater flow?

• Pressure gradients needed to move water through
  the xylem could come about in two ways
• Positive pressures
• - Hypothetically positive pressure could drive
  water to the tops of trees, but
• - Root pressures are generally less than 0.1 MPa
  (lift water only 1 m above the ground) and
  pressures of at least 2 MPa are required
• to supply some of the tallest trees on the planet
• - Root pressure is energetically expensive...
• - Also, evaporation will easily collapse positive
  pressure gradients...
• - Negative pressures
• - Solar power! Plants use the evaporative power
  of solar energy to pull water through the
  vascular system.
• - So, water at the tops of trees develops a large
  tension to pull
• water upwards through the xylem

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Xylem water transport under tension isrisky
The large tensions that develop in the xylem of
trees can create some problems First, tension
results in an INWARD pull on the tracheary cell
system. The development of lignified walls is
necessary to allow resistance to implosion from
this force... The next problem is that water
is METASTABLE and very sensitive to slight
changes in gas content Recall that pure
degassed water is very strong, but with gas
added, the water column can become increasingly
easily broken
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Metastable state of water undertension

• The other problem is that water is METASTABLE and
  very sensitive to slight changes in gas content
• Recall that pure degassed water is very strong,
  but with gas added, the water column can become
  increasingly easily broken
• As tensions increase, there is an increased
  tendency for air to be pulled in from microscopic
  pores in the xylem wall that contain air (from
  respiring living cells or just close to
  lenticels, etc.)
• This is called air-seeding.

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Drought survival
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Adaptation

• Plants optimize their dispersal, probably by root
  competition so as to obtain the maximum amount of
  growth per unit rainfall.

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Water stress due to water shortage
Mostly combined with stress due to excessive
irradiance and overheating, often also with
stress due to high soil salinity
Possible reasons of water shortage

• High rate of evapotranspiration at low rainfall
• Water bound osmotically in soil (saline soils)
• Frozen soil
• Thin soil layer, underdeveloped root system

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Symptoms of water stress in plants?

• Reduction of cell volume, dehydration of
  protoplasm
• Decreased turgor and slower growth, esp.
  elongation
• Metabolic changes osmotically active
  substances, etc.
• Disturbed protein metabolism and aminoacid
  synthesis
• Slower meristematic cell divisions, anomalous
  meiosis
• Enhanced synthesis of abscissic acid closure
  of stomata
• Changed allocation of assimilates, increased
  root/shoot ratio
• Characteristic morphogenetic alterations

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Osmoregulation and stress metabolites

• Cumulation of osmotically active substances
• Maintenance of turgor
• Non-structural saccharids (carbohydrates) and
  aminoacids
• Stress metabolites accumulate in cytoplasm as a
  result of stress (betaines and the aminoacid
  proline)
• Proline Its source can be either the synthesis
  from glutamate or hydrolysis of proteins
• Heat shock proteins

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Water stress

• Poikilohydric plants, which maintain an
  equilibrium between atmospheric humidity and the
  hydrature of their protoplasm (algae?, fungi,
  lichens and bryophytes)
• Homoiohydric plants - hydratation of protoplasm
  is independent of air humidity (higher plants)
• Xerophytes (irrespective whether poikilohydric
  or homoiohydirc) which are non-succulent and
  non-halophytic, obtain their water supply from
  local precipitation and atmospheric moisture.
• Therophytes vs. Geophytes - two adaptations to
  drought
• Time of flowering and fruiting.

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Poikilohydric xerophytes

• Plants of this type lose water rapidly in the dry
  season and survive the adverse period in a state
  of intense desiccation.
• Parkinsonia microphyla 250 years of drought

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Grimmia pulvinata - derkavka poduškovitá
Bazzania stolonifera - Játrovka rohozec
Tortula ruralis - rourkatec obecný
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Selaginella lepidophylla
Ceterach officinarum kyvor lékarský
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The greatest numbers of poikilohydric angiosperms
occur in South Africa various species of
Scrophulariaceae, Myrothamnaceae, Velloziaceae,
Cyperaceae, Poaceae
Myrothamnus flabellifolia
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Vascular plants both dehydration and
rehydration must be slower
After 2-3 years of almost absolute dryness they
can completely recover, damaged root tips and
trichomes quickly regenerate
Ramonda myconi
Ramonda serbica
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Homoiohydric xerophytes 1

• Malacophyllous species (soft-leaved) which are
  characteristic of semi-desert conditions where
  winter or seasonal rains ensure periodic
  alleviation of drought.
• When exposed of drought, they shed their leaves.
• Cistus, Lavendula, Rosmarinus

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Homoiohydric xerophytes
Malacophytes Can live under semidesert
conditions Their leaves wilt, or are shed off
during the dry period Cistus, Lavendula, Thymus,
Rosmarinus, Artemisia, etc.
Rosmarinus officinalis
Lavendula officinalis
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Homoiohydric xerophytes 2

• Sclerophyllus species (hard-leaved) which are
  able to maintain a favourable water balance by
  reducing their transpiring surface area during
  predictable drought periods.
• Well-developed root system.
• Quercus ilex, Pinus pinea, Spartium junceum

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Plants with tough leaves or entirely leafless
Reduced transpiring surface Leaf turnover of
several years Extensive and deep root systems
Sclerophytes
Araucaria araucana leaves persist up to 25 years
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Quercus ilex
Pinus pinea
Schinus molle
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Spartium junceum Vítecník sítinolistý
Olea europaea
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Succulent xerophytes

• Water-storing species
• Crassulacean acid metabolism (CAM)
• Nananthus, Conophytum, Opuntia

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Opuntia puberula
Succulent xerophytes
Water accumulation during (mostly rare) wet
periods. Shallow-rooted, sometimes lacking roots,
e.g., epiphytic tillandsias.
Or their roots die off during dry periods and
regenerate when water is again available.
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Succulents
CAM metabolism (Bromeliaceae, Orchideaceae,
Cactaceae, Aizoaceae, Euphorbiaceae) Leaves
cylindrical, sphaerical, often plants
leafless Photosynthesis rather in stems Reduction
of number of stomata, thick epidermis, stomata
immerged
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Desert conditions long periods without water,
some plants completely buried in soil Efficient
control of water output (loss)
Lithops francisci
Lithops salicola
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Drought tolerance 1

• The maintenance of sufficient physiological
  integrity so that metabolism can be reactivated
  quickly upon rehydration.
• A repair mechanism that can be put into effect on
  rehydration and which can repair any damage
  caused to membranes and membrane-bound organelles.

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Drought tolerance 2

• Density of protoplasm (sugars), water-stress
  resistant proteins?
• Seeds have water-repellent layer(s) importance
  of hardness and morphology of seed coat

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Reduction of water loss 1

• In a drying out soil profile, roots not only grow
  faster than in a well-watered one but also
  achieve a substantially greater total length.
• Aerial roots of epiphytic orchids.
• Specially adapted water-absorbing leaf hairs
  (trichomes) of the Bromeliaceae
• Stomatal pores

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Tillandsia usneoides( Spanish moss)
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Reduction of water loss 2

• Cuticular adaptations to water stress
• Osmoregulation and stress metabolism, proline
  accumulation

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Reduction of water loss 3

• Diurnal variation
• in CO2 uptake,
• PEP carboxylase,
• CAM metabolism

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Conclusion

• The principle of optimization with regard to
  water loss can be summarized as balancing the
  risks of water output against the benefits of
  carbon fixation.

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Thank you and see you again!