Transcript Plant nutrition, transport and adaptation to stress

Chapter 17: Plant nutrition, transport
and adaptation to stress
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Nutrition of plants
• The nutritional requirements of plants are relatively
simple
• Light, carbon dioxide and water for photosynthesis
and certain mineral elements that are also required
for growth
• In multicellular plants, light and carbon dioxide are
obtained above ground, while water and mineral
nutrients are generally taken up from the soil
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Nutrition of plants (cont.)
• Plant nutrients may be required in large
(macronutrient) or small (micronutrient) quantities
• Fourteen mineral elements are essential for plant
growth
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Table 17.1: Essential mineral elements
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Nutrition of plants (cont.)
• Plants growing on soils that are deficient in a
certain essential element may have stunted
growth or be more susceptible to disease
• For example, plants growing on iron-deficient
soils typically become yellow (chlorotic)
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Fig. 17.2: Seedlings grown on
alkaline calcareous soil
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Nutrition of plants (cont.)
• Mineral nutrients in the form of ions serve general
functions in plants by moderating the ionic balance
of cells and regulating water balance
• Minerals may also have specific functions, e.g.
– Mg2+ is a component of chlorophyll
– K+ affects the conformation of certain proteins
– Ca2+ is vital in maintaining the physical properties of
membranes and as a component of primary cell
walls
• Many trace elements form part of enzymes that are
essential in metabolic processes
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Nutrition of plants (cont.)
• Plants obtain essential nutrients from the soil, which
is formed from the weathering of underlying rocks
• However, such nutrients may also be obtained from
the decomposition of dead plant and animal matter
by the action of bacteria and fungi
• Nutrients move in cycles among pools of available
sources: the amount of a specific nutrient depends
more on its amounts in various pools and the rates
of movement among them than on the very slow
release of the nutrient from the underlying rocks
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Nutrition of plants (cont.)
• Nitrogen is absent from lithospheric rocks but
abundant in the atmosphere
• Nitrogen (N2) is ‘fixed’ by certain bacteria to form
ammonium (NH4+) and eventually nitrate (NO3–),
which are then absorbed by plant roots
• Such bacteria may live freely in the soil or in
association with plants, such as Rhizobium bacteria
in root nodules of legumes, or actinomycetes in the
roots of casuarinas (she-oaks)
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Question 1:
How are many Australian native plants able to
survive on low-nutrient soils?
A mechanism commonly found is:
a) A shorter life-cycle, leading to avoidance of the
stress
b) The development of fine roots, which increases root
surface area and phosphorus uptake
c) Increased shoot growth and a reduction in root
growth
d) Specialised structures in the leaves that are able to
trap airborne phosphorus particles
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Pathways and mechanisms of
transport
• Unicellular organisms do not require systems to
transport nutrients and water—the small distances
across which materials move means that simple
diffusion is adequate
• In contrast, tall vascular plants require transport
systems to distribute nutrients and water
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Transport pathways
• Transport pathways in plants include those
– between the soil and plant root
– between cells either along the apoplastic or
symplastic pathways
– between compartments within a cell
– involved in long-distance transport, i.e. the xylem
and phloem
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Fig. 17.4: Vacuolated plant cells
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Transport mechanisms
• Include both passive and active processes
• Passive processes include diffusion, mass flow
and osmosis
• Mass flow transport occurs in xylem and phloem,
and involves the carrying of solutes in solution,
driven by gradients of hydrostatic pressure
• Active processes, which require the expenditure of
energy, involve the uptake or movement of ions or
sugars
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Water transport
• The movement of water molecules from the soil
into roots or from leaf mesophyll cells through
stomata into the atmosphere occurs down
gradients of water potential (Ψ, psi)
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Fig. 17.5: Gradients of free energy of
water
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Factors that affect water potential
• In soil and within a plant, Ψ is less than zero, and
thus is negative. This is because the free energy of
water molecules decreases, due to the presence of
solutes and solids that absorb water molecules, to
below that of pure water
– e.g. a 1.0 M sucrose solution has a water potential
(Ψsolution) of –3.5 MPa
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Fig. 17.6: Net flow of water
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Factors that affect water potential
(cont.)
• Water potential is affected by hydrostatic pressure,
which is decreased when a fluid is under tension
(negative Ψ), as in the xylem, and increased by
application of positive pressure, such as occurs in a
turgid plant cell
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Fig. 17.7: Water potential of a plant
cell
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Factors that affect water potential
(cont.)
• The elastic properties of a plant cell wall mean that
it can swell and contract as the water content and
volume of the cell alter
• The cell wall is thus able to exert a positive
hydrostatic or turgor pressure on the cell contents
• Thus, the water potential of a plant cell (Ψcell) is the
sum of the turgor pressure (pressure potential, ΨP)
and the negative osmotic effect (osmotic potential,
Ψ) of solutes
Ψcell = ΨP + Ψ
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Water uptake by plant cells
• When plant cells exchange water with their
environment, they shrink or swell to a limit imposed
by the cell wall
• Both the pressure (ΨP) and osmotic (Ψ) potentials
change in response to changes in cell water content
• If cell water content and volume increase, cell walls
distend and ΨP increases, the solutes are diluted, and
both Ψ and Ψcell become less negative
• When a cell is fully turgid, ΨP = Ψ and Ψcell = 0
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Fig. 17.9: Leaves of a cucumber plant
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Water uptake by plant cells (cont.)
• On a hot day, water vapour that diffuses through
stomata often exceeds that taken up by roots
• If this occurs, cells lose water, their volume
decreases, ΨP decreases and both Ψ and Ψcell
become more negative
• The capacity of the cell to absorb water from
surrounding cells is greatly increased and water
moves into the cell
• During drought conditions, plants use changes in
turgor to adjust their water-retaining and waterabsorbing capacities
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Osmotic adjustment in plant cells
• Plants inhabiting areas that have persistent soil
water deficits over days or weeks (i.e. drought
stress) may respond by increasing the amount of
solute in cell vacuoles
• This has the effect of decreasing Ψ and Ψcell
without adversely affecting cell turgor and growth
• This response to drought is osmotic adjustment,
and it allows photosynthesis and continued growth
in drier conditions
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Transpiration
• Transpiration is the loss of water by evaporation
from leaves using energy from incoming solar
radiation to vaporise water
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Fig. 17.11: Movement of CO2 and
water vapour into and out of a leaf
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Transpiration (cont.)
• In a well-watered plant, transpired water is replaced
by water drawn up from the roots through the xylem
• On sunny days, plants may lose large amounts of
water
– e.g. the amount of water transpired by a mountain ash
tree, Eucalyptus regnans, growing in a moist habitat,
may be as much as 300 litres/day
• Along an increasingly negative gradient of water
potential, water enters the roots and moves through
the apoplast by mass flow, but at the endodermis it
must travel via the symplast to enter the root stele
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Water movement in xylem
• In flowering plants, xylem includes vessels and
tracheids
• Vessels consist of elements joined end-to-end to
form a tube that may extend up to 15 m in length
• Xylem sap, which forms a continuous column from
the root to the leaf veins, comprises a dilute solution
of inorganic ions and nitrogenous compounds
• The continuous column of sap is under tension, due
to the narrow diameter of xylem and the cohesive
nature of water  cohesion theory
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Fig. 17.13: Xylem sap is tapped from roots of
mallee gums by Aboriginal Australians at Yalata
Reserve, SA
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Water movement in xylem (cont.)
• When the tension in xylem sap becomes too high,
the water column may break or cavitate
• Under conditions of high humidity and soil
moisture, root pressure may drive the flow of xylem
sap
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Fig. 17.14: Cavitation in a xylem vessel
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Water movement from leaves
• The loss of water vapour from leaves is
dependent on two factors
(i) the cuticle, which is highly resistant to water loss,
and stomata, through which moves 90 per cent of
water vapour flux
(ii) the leaf boundary layer—that layer of still air just
outside the leaf surface
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Fig. 17.15: Transpiration
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Water movement from leaves (cont.)
• Stomatal pores occupy only a small proportion of
the leaf surface area
• By controlling the number of stomata and the size
of stomatal pores, a plant can regulate the
exchange of CO2, O2 and water vapour
• The size of stomatal pores is dependent on the
turgor of guard cells
• Guard cells are turgor-regulated valves that expand
outward to open the pore when turgid, but which
deflate and close under conditions of turgor loss
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Control of stomatal opening and closing
• Stomatal opening and closing is the result of
movement of solutes, particularly K+ and Cl–, into
and out of guard cells
• Stomata open and close in response to a number of
stimuli, including light intensity, CO2 concentration,
air humidity, and soil and leaf water deficits
• During drought, plant roots generate the hormone
abscisic acid, which induces stomatal closure
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Question 2:
What effect do you think a hot, dry wind would
have on transpiration rate and stomatal opening?
a) Slow down and open up
b) Slow down and close
c) Speed up and open up
d) Speed up and close
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Translocation of assimilates
• The end products (or assimilates) of photosynthesis
are translocated from the leaves (the source) to other
parts (the sinks) of the plant
• Assimilates move via the phloem to actively growing
parts of a plant, such as the roots, the youngest
expanding leaves and the shoot tip
• Phloem sap is a concentrated solution of solutes,
predominantly sucrose
• The rate of phloem transport ranges from 40 to
100 cm per hour
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Fig. 17.21: Ring-barking
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Fig 17.23: Phloem-feeding aphids
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Adaptations to stress
• During uptake of CO2 for photosynthesis, plants
lose water through stomata
• When water loss is greater than that which may be
taken up from the soil, plants may undergo water
stress, which may impair growth and normal plant
functioning
• Annual plants escape drought by germinating,
growing, flowering and setting seed only during
periods when water is available
• In arid habitats, perennial plants must be drought
tolerant, and achieve this by either avoiding or
tolerating dehydration
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Ways that plants cope with drought
• Plants avoid dehydration by either increasing water
absorption, reducing transpirational loss, or both
• Deep, extensive root systems are able to extract
water from a large soil volume
• High root:shoot ratios are common among arid zone
taxa, and others may shed leaves to reduce the leaf
area across which water may be lost
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Ways that plants cope with drought
(cont.)
• Succulent species such as cacti are able to store
water in specialised cells
• Leaf hairiness or waxiness is often higher for arid
zone taxa, as these increase reflectance of solar
radiation, thereby reducing leaf temperature and the
need for evaporative cooling
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Salinity and mineral stress
• High soil salt concentrations can cause water stress
and sodium (Na+) toxicity to plants
• Plant adaptations to salinity include
– separation and storage of ions within specialised cells
– ability to exclude salt at roots or excrete it from the
leaves
– maintenance of a balance between ion uptake and
transpiration and growth
• Ion toxicity can result from an increase in
concentration of ions at low soil pH or from the
effects of pollutants that facilitate entry of toxic ions
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Lack of oxygen around roots
• Lack of oxygen in waterlogged soil decreases
cellular respiration by root cells, which impairs root
growth and water and nutrient uptake
• Plants such as mangroves possess adaptations to
low soil oxygen
– These include anatomical features such as air canals in
roots or the production of lateral roots on the soil surface
• Biochemical adaptations to low soil oxygen include
an increased ability to sustain aerobic fermentation
in roots and an increased resistance to toxic
compounds produced in anaerobic soils
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Temperature stress
• Different parts of a plant may be subject to quite
different temperature regimes
• Temperature affects biochemical reactions due to
its effects on the kinetic energy of reactants and the
tertiary structure of enzymes and membranes
• In cold environments, plants are able to survive
very low temperatures by preventing intracellular
ice formation
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Summary
• Plants require light, carbon dioxide, water and
inorganic nutrients for growth
• Xylem and phloem are the transport pathways
between different parts of a plant
• Water movement in a plant is down gradients of
water potential
• Translocation of assimilates in phloem requires
energy
• Individual plants can respond to environmental
stress within limits defined by their genotype
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