Chapter 30
Plant Nutrition and Transport
Nutrients and Their Availability in
Soils
• Nutrients are elements needed for growth
and survival
– “Big three” elements are oxygen, hydrogen,
and carbon
– Thirteen others are taken in water through the
roots
• Six elements have deemed macronutients
(e.g. N, K, Ca); seven others are
micronutrients (e.g. Cl, Fe, Zn)
Properties of Soil
• Soil consists of particles of minerals mixed
with humus (dead organisms and their
litter)
– Particles come three sizes: Sand, silt and
Clay
– Clay: the smallest – holds nutrients as water
percolates through the soil
– Plants do best in loam, soil with nearly equal
proportions of the three particle type.
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• Layers of soil can be classified by profile
properties; topsoil, the uppermost, is the
most essential layer for plant growth.
Leaching and Erosion
• Leaching refers to the removal of some of
the nutrients from soil as water percolates
through it
• Erosion is the movement of land under the
force wind, running water, and ice.
How Do Roots Absorb Water and
Mineral Ions?
• Absorption Routes
• Water moves from the soil across the root
epidermis to the vascular cylinder, a
column of vascular tissue in the center of
the root
– Sheetlike layer of cells, the endodermis,
surrounds the column
– Water-repellant Casparian strip forces water
to move through the cytoplasm of the cells of
the endodermis
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– Therefore, membrane transport proteins help
control the types of absorbed solutes that will
become distributed throughout the plant
• Most flowering plants also have an
exodermis, a layer of cells just inside the
roots which also has a Casparian strip that
functions just like the one next to the root
vascular cylinder
Specialized Absorptive Structure
• Root hairs, extensions of the root
epidermal cells, greatly increase the
absorptive surface.
• Root nodules of legumes harbor bacteria
that covert gaseous nitrogen to forms
useful in the growth of the plants.
(Symbiotic relationship: Mutualism)
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• Mycorrhizae (Fungi growing around plant
roots) aid in absorbing minerals that are
supplied to the plant in exchange for
sugars, a symbiotic relationship that is
beneficial to both
Water Transportation
• Transpiration Defined
– Water moves from roots to stems and then to
leaves
– Some water is used for growth and
metabolism, but evaporates into the air by
transpiration
Cohesion-Tension Theory of Water
Transport
• Water moves through pipelines called
xylem, composed of cells (dead at
maturity) called tracheids and vessel
members
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• Cohesion-Tension Theory of water
explains water movement in plants:
– The drying power of air causes transpiration,
which puts water in the xylem in a state of
tension leading from leaves to stems to roots.
– Unbroken fluid columns of water show
cohesion (aided by the hydrogen bonds); they
resist rupturing as they are pulled upward
under tension
– As long as water molecules escape from the
plant, molecules are pulled up to replace
How do Stems and Leaves
Conserve Water?
• Of the water that moves through the leaf,
90% is lost by transpiration, only about 2
percent is used during photosynthesis
Water Conserving Cuticle
• Cuticle is translucent, water impermeable
layer secreted from epidermal cells
• It coats the outer walls, which are exposed
to air
– Waxes are embedded in a matrix of cutinm a
lipid polymer
– The cuticle does not bar the entry of light
rays, but does restrict water loss, inward
diffusion of CO2, and outward diffusion of
oxygen
Stomata: Controlled Water Loss
• Regulates the passage of water, carbon
dioxide, and oxygen.
• A pair of guard cells defines each opening
– In sunlight, a drop in carbon dioxide levels in
the guard cells causes potassium and water
to move into the guard cells causing them to
swell (turgor pressure); this creates an
opening for carbon dioxide entry (a benefit)
and water loss (detriment)
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• At night, potassium and water move out
and the guard cells collapse to close the
gap and conserve water
How are Organic Compounds
Distributed Through Plants?
• Phloem: distributes organic products of
photosynthesis throughout the plant.
– Sieve tubes cells are alive at maturity and are
interconnected from leaf to root
– Companion cells also participate in a supportive
role
– Storage forms of organic molecules (examples:
Starch, fats, and proteins) are unsuitable for
transport throughout the plant body
– They are therefore converted to more soluble
forms, sucrose
Translocation
• Most often used to signify the transport of
sucrose and other compounds through
phloem
• Observations of aphids feeding show that
sugars inside the sieve tubes are being
moved under pressure
Pressure Flow Theory
• Movement of molecules through phloem
from sources (mostly leaves) to sinks
(flowers and fruits)
• According to the pressure flow theory,
translocation depends on pressure
gradients
– Solutes are loaded by active transport into the
phloem from a source; water follows
– As pressure builds in the tubes it pushes the
surcose-laden fluid out of the leaf into the
stem, and on to the sink