Resource Acquisition &
Transport in Vascular
Plants
Campbell and Reece
Chapter 36

genus of plants (Lithrops, known as stone
plants) found in Kalahari Desert of
southern Africa has mostly subterreanean
existence
◦ tips of 2 succulent leaves above ground
◦ clear, lens-like cells allow light  cells
underground
◦ conserve moisture (~20 cm rain/yr), hide from
grazing tortoises, avoid high temperatures (up to
45ºC, 113 ºF,) & high light intensity
◦ overall reduces water loss but inhibits
photosynthesis,  grow very slowly
Underground Plants
 nonvascular
◦ earliest land plants
◦ grew photosynthetic, leafless
shoots above the shallow water in
which they lived
◦ most had waxy cuticles & few
stomata
Early Land Plants

anchoring & absorbing functions done by base
of stem or threadlike rhizoids
Early Land Plants

typical land plant inhabits 2 worlds:
◦ under ground
◦ above ground
Adaptations of Vascular Plants
 as
competition for light, water, &
nutrients grew:
◦ plants with broader leaves had advantage for
light but then lost more water by
evaporation as surface area increased
◦ larger shoots required more of an anchor
which favored production of multicellular,
branching roots
◦ as shoots grew higher, needed long-distance
transport of water, minerals, products of
photosynthesis
Evolution of Plants

evolution of vascular tissue meant;
◦ Xylem: tubular dead cells that conduct most
of the water & minerals upward from roots
 rest of plant
◦ Phloem: vascular plant tissue consisting of
living cells arranged into elongated tubes
that transport sugar & other organic material
thru out plant
Xylem & Phloem
transpiration creates
a force thru leaves
that pulls xylem sap
upward
 water & minerals
up as xylem sap
 phloem sap flows
up & down
delivering sugars
 water & minerals in
soil absorbed by
roots

Xylem & Phloem

function:
◦ gather light
◦ take in CO2
LEAVES

arrangement of leaves on a stem called:
phyllotaxy
LEAVES

most angiosperms (flowering plants) have
alternate phyllotaxy
◦ each successive leaf emerges 137.5º from site
of previous leaf
◦ this angle minimizes shading of lower leaves
by upper leaves
◦ plants in intense sun: opposite phylloxy
which increase shading & so water loss
LEAVES
affects amt light capture
 leaf area index: ratio of total upper leaf surface
of a single plant or entire crop ÷ surface area of
land on which it grows
◦ values up to 7 possible for mature crops
◦ not much agricultural benefit to having
higher values
◦ more leaves increases shading of lower
leaves to pt. where respiring >
photosynthesizing

LEAF NUMBERS or SIZE

affects amt light captured
LEAF ORIENTATION

function:
◦ supporting structures for leaves
◦ conduit for long-distance transport of water
& nutrients
STEMS

generally. Enables plants to more effectively
capture sunlight
◦ only finite amt of nrg to give to shoot growth
◦ more nrg to shoot growth the less there is for
height which may compromise their chances
for capturing sunlight
◦ if lots nrg goes into being tall, plant not
optimizing resources above ground
◦ species have variety of branching patterns
BRANCHING PATTERNS
BRANCHING PATTERNS

function:
◦ mine the soil for
water & minerals
◦ anchor whole
plant
◦ evolution of
branching roots
enabled plants to
be more efficient
& more anchored
ROOTS
tallest plants typically have longest taproot &
most branches
 fibrous roots don’t anchor as well so those
plants generally not as tall
 fewer branches as root grows thru soil with
fewer nutrient; more branching in nitrogenrich areas

ROOTS
ROOT GROWTH

mutualistic associations formed between roots
& some soil fungi that aid in absorption of
minerals & water
MYCORRHIZAE
important ass’c in evolution of land plants
 ~80% land plants
 fungi provides increased surface area to root
system  more water & mineral absorption

◦ especially phosphates
Mycorrhizae
both active & passive transport controls
movement of substances in/out of cells
 plant tissues have 2 major compartments:
1. Apoplast: everything external to plasma
membrane of living cells

◦
◦
2.
cell walls, interior of dead cells, tracheids (long tapered
water-conducting cell in xylem in most vascular plants
extracellular spaces
Symplast: all cytosol of all living cells in plant
Transport in Plants
Apoplastic Route
1.
◦
water & solutes  cell walls & extracellular spaces
Symplastic Route
2.
◦
water & solutes  cytosol  plasma membrane 
plasmodesmata  next cell
Transmembrane Route
3.
◦
out of 1 cell  cell wall  neighboring cell
3 Routes for Transport in Plants
plant plasma membranes have same types of
transmembrane proteins as other cells
 some differences:
1. H+ pumps

◦
◦
◦
◦
(not Na+) play primary role in basic transport
processes
maintains membrane potential
H+ often ½ cotransporter (Na+ in animals)
part of absorption of neutral solutes, ions, &
sucrose
Short-Distance Transport Across
Plasma Membranes
Solute Transport across Plant Cell
Membranes

free water (not bound with other particle)
moves down its concentration gradient across
semipermeable membranes = osmosis

Water Potential: physical property that
predicts direction in which water will flow
based on water pressure & solute
concentration
Osmosis & Water Potential





free water moves from areas of higher water
potential  areas of lower water potential if
no barrier to its flow
as water moves it can perform work
“potential” refers to its PE
Ψ (psi) represents water potential
measured in a unit of pressure: megapascal
MPa
Water Potential
the Ψ of pure water in open container under
standard conditions (sea level, room
temperature) = 0MPa
 1 Mpa ~ 10x atmospheric pressure @ sea level
 internal pressure of living plant cell due to
osmotic uptake of water is ~ 0.5 MPa

Water Potential

Water Potential equation:
How Solutes & Pressure Affect Water
Potential
directly proportional to its molarity
 aka osmotic potential

◦ solutes affect direction water moves in osmosis

plant solutes
◦ mineral ions
◦ sugars
Solute Water Potential
in pure water the Ψs = 0
 as add solute they bind with water so there is
less free water molecules which decreases
water’s capacity to move & do work


reason Ψs always a (-) #

as concentration of solute increases Ψs
becomes more (-)
How Solutes & Pressure Affect Water
Potential
Ψp = physical pressure on a solution
 can be (+) or (-) relative to atmospheric
pressure

Pressure Potential

force directed against a plant cell wall after
the influx of water & swelling of the cell due
to osmosis
Turgor Pressure

Turgor Pressure
critical for plant
function: helps
maintain stiffness of
plant tissues & is
driving force for cell
elongation
Wilting in Nonwoody Plant
difference in water potential determines
direction water will flow
 How does water get in/out of plant cells?
◦ some molecules diffuse thru lipid bilayer

 does not affect the rate water moves
◦ transport proteins called aquaporins affect
the rate water molecules move across the
membrane
Aquaporins
Aquaporins
on cellular level diffusion effective but too
slow for long-distance transport w/in plant
 Long-distance transport occurs thru
 bulk flow
◦ movement of liquid in response to a pressure
gradient (always high  low)

Long-Distance Transport
occurs in tracheids & vessel elements of xylem
& w/in sieve-tube elements of the phloem
 tracheid: long, tapered water-conducting cell
found in xylem of nearly all vascular plants;
functioning tracheids are no longer living

Bulk Flow

diffusion, active transport, & bulk flow act
together transporting resources thru out whole
plant

water & minerals from soil enter plants thru
epidermis of roots
◦ cells here permeable to water
◦ many cells differentiate into root hairs:
modified cells that absorb most of water
plant uses
Root Cells

absorb water and “soil solution” (mineral ions
not bound to soil particles)
◦ crosses cell walls  pass freely along cell
walls & extracellular spaces  cortex
◦ allows for greater surface area for absorption
than epidermal cells alone
Root Cells

soil solution generally has low concentration
of mineral ions but root cells use active
transport to absorb & store higher
concentrations of mineral ions (ex. K+)
Mineral Ions

to get to the rest of plant water & minerals
must get to the endodermis: the innermost
layer of cortex, surrounds vascular cylinder
Transport into Xylem

serves as last “checkpoint” for selective
passage of minerals from cortex  vascular
cylinder
Endodermis
1.
Apoplectic Route:
uptake of soil
solution by root hair
cells  apoplast 
diffuse to cortex
along cell walls &
extracellular spaces
Transport of Water & Minerals
2. Symplastic Route:
minerals & water
cross plasma
membranes of root
hairs  symplast
Transport of Water & Minerals
3. Transmembrane
Route: a soil solution
moves along
apoplastic route
individual cells of
epidermis & cortex
take in what they
need. Then water &
minerals can move
toward endodermis
via symplastic route
Transport of Water & Minerals
4. Endodermis: cells
contain the Casparian
strip: a belt of waxy
material that blocks
passage of soil
solution. Only
minerals already in the
symplast or entering
thru plasma
membrane of an
endodermal cell can
detour around the
Casparian strip 
vascular cylinder =
stele
Transport of Water & Minerals
5. Transport in the
Xylem: endodermal
cells & living cells
w/in vascular
cylinder discharge
soil solution by bulk
flow into shoot
system
Transport of Water & Minerals
material flowing in xylem = xylem sap 
moves by bulk flow  veins in leaves
 peak velocities in xylem 15 – 45 m/hr for trees
with wide vessel elements
 transpiration: loss of water vapor from leaves
& other aerial parts of the plant
◦ transporting xylem sap involves loss of water
thru transpiration

Bulk Flow Transport via Xylem
@ nite when almost no transpiration roots still
actively pumping in soil solution
 Casparian strip prevents backward flow into
cortex or soil
 as result accumulation of minerals lowers
water potential w/in vascular cylinder
 water flows in from the root cortex generating
root pressure: a push of xylem sap

Xylem Sap Pushed by Root Pressure

when root pressure causes more water to enter
leaves than is transpired  exudation of water
droplets on edges of leaves
Guttation
in most plants: root pressure too weak to
overcome gravity
 even in plants that display guttation, root
pressure cannot keep up with transpiration
during sunlight hrs

Root Pressure
Cohesion-Tension Hypothesis: movement of
xylem sap driven by a water potential
difference @ leaf end of the xylem by
evaporation of water from leaf cells
 evaporation lowers the water potential @ airwater interface so generates (-) pressure that
pulls water thru xylem

Pulling Xylem Sap
Pulling Xylem Sap
Role of K+ in Stomatal Opening
3 cues contribute to opening of stomata @
dawn:
1. Light
2. CO2 depletion
3. Circadian rhythm in guard cells

Stimuli for Stomatal Opening &
Closing

Light increase K+ intake  guard cell become
turgid
◦ blue—light receptors in plasma membranes
of guard cells
◦ when these receptors activated  increases
activity of proton pumps in plasma
membrane  increases intake of K+
Light Controlling Guard Cells

as [CO2] stored in air-spaces used in
photosynthesis decreases during day hrs 
stomata slowly open if there is sufficient water
CO2 Depletion

a physiological cycle of about 24 hours that
persists even in absence of external cues

all eukaryotic organisms have internal clocks
that regulate cyclic processes

stomata will continue open/close cycle even in
darkness
Circadian Rhythm
Circadian Rhythm in Plants

close stomata even midday:
plant is dry: guard cells lose turgor  close
stoma
 plant hormone, abscisic acid (ABA) made in
roots & leaves in response to water shortage
signals guard cells to close stomata  reduces
wilting * restricts CO2 absorption  slows
photosynthesis

◦ turgor needed for cell growth  growth ceases thru
out plant
Environmental Stresses

greatest when temps moderate, sunny days
with light wind (all increases evaporation)
in drought: stoma close but still some water
loss thru cuticle  plant wilts
 prolonged drought  leaves severely wilted &
irreversibly damaged


evaporative cooling can lower leaf’s temp 10 ºC
compared to surrounding air: prevents
denaturation of proteins
Transpiration Effects
Xerophytes: plants adapted to arid climates
 dry soils unproductive not just because plants
need free water for photosynthesis but also
because freely available water allows plants to
keep stomata open so take in more CO2

Adaptations that Reduce Evaporative
Water Loss

some plants in arid conditions complete their
life cycle during short rainy season
Adaptations that Reduce Evaporative
Water Loss
reduced leaves decrease water loss (cacti)
 photosynthesis done in stems

Adaptations that Reduce Evaporative
Water Loss
stomata recessed in cavities called crypts
(protects stoma from hot dry wind  less
transpiration)
 cuticle thick, many layers of epidermis

Adaptations that Reduce Evaporative
Water Loss
stems of many xerophytes able to store water
for use during dry periods
 some desert plants have very deep (20 feet or
more) roots
 some white to reflect light

Adaptations that Reduce Evaporative
Water Loss

CAM (crassulacean acid metabolism) take in
CO2 during nite so stomata can close during
day when evaporative losses greatest

*stomata are most important mediators of
conflicting demands for CO2 & water
retention
Adaptations that Reduce Evaporative
Water Loss



mature leaves are main sugar sources
storage organs can be seasonal sources of sugar
sugar sinks: growing organs, stems, roots, fruit
transport of sugars (phloem sap) called
translocation carried out by phloem
 phloem sap carries sugars, minerals, a.a.,
hormones
 transport is not unidirectional like xylem

Phloem Moves Sugars

sugars made in mesophyll cells travel via
symplast (blue arrows below) to sieve-tube
elements. In some plants sucrose leaves the
symplast near sieve tubes & travels thru
apoplast (red arrow)
Loading of Sucrose into Phloem

A chemoosmotic mechanism is responsible for
the active transport of sucrose into companion
cells & sieve-tube elements Proton pumps
generate a H+ gradient which drives sucrose
accumulation with help of a cotransport
protein that couples sucrose transport to
diffusion of H+ back into cell
Loading of Sucrose into Phloem

phloem sap moves
from source to sink @
up to 1m/hr
◦ it moves thru sieve
tube by bulk flow
driven by (+)
pressure called
pressure flow
Bulk Flow by (+) Pressure
Plasmodesmata:
 change in permeability & number
 when dilated, they provide passageway for the
symplastic transport of proteins, RNAs, &
other macromolecules over long distances


Phloem also conducts nerve-like signals that
help integrate whole-plant function
Plant Communication