Chapter 29- Resource Acquisition,Nutrition, and Transport in
Vascular Plants Following the Big Ideas
Energy and Homeostasis- Acquisition of water and nutrients by
plants involve specialized mechanisms and structures. Overview:
Underground Plants
The success of plants depends on their ability to gather and
conserve resources from their environment The transport of
materials is central to the integrated functioning of the whole
plant Stone plants (Lithops) are adapted to life in the desert Two
succulent leaf tips are exposed above ground; the rest of the plant
lives below ground 3 Concept 29.1: Adaptations for
acquiringresources were key steps in the evolution of vascular
plants
The evolution of adaptations enabling plants to acquire resources
from both above and below ground sources allowed for the successful
colonization of land by vascular plants The algal ancestors of land
plants absorbed water, minerals, and CO2 directly from surrounding
water Early nonvascular land plants lived in shallow water and had
aerial shoots Natural selection favored taller plants with flat
appendages, multicellular branching roots, and efficient transport
4 Why does over-watering kill a plant?
Transport in plants-The evolution of xylem and phloem in land
plants made possible the development of extensive root and shoot
systems that carry out long-distance transport H2O & minerals
transport in xylem transpiration evaporation, adhesion &
cohesion negative pressure (tension) Sugars transport in phloem
bulk flow Calvin cycle in leaves loads sucrose into phloem positive
pressure Gas exchange photosynthesis CO2 in; O2 out stomates
respiration O2 in; CO2 out roots exchange gases within air spaces
in soil Why does over-watering kill a plant? Shoot Architecture and
Light Capture
Stems serve as conduits for water and nutrients and as supporting
structures for leaves Shoot height and branching pattern affect
light capture There is a trade-off between growing tall and
branching Phyllotaxy, the arrangement of leaves on a stem, is
specific to each species Most angiosperms have alternate phyllotaxy
with leaves arranged in a spiral The angle between leaves is and
likely minimizes shading of lower leaves 6 Emerging phyllotaxy of
Norway spruce
Figure 29.3 24 32 42 29 40 16 11 19 21 27 3 34 8 6 14 13 26 Shoot
apical meristem 1 22 5 9 18 Buds 10 4 2 31 17 23 7 12 Figure 29.3
Emerging phyllotaxy of Norway spruce 15 20 25 28 1 mm Emerging
phyllotaxy of Norway spruce 7 Leaf orientation affects light
absorption
The productivity of each plant is affected by the depth of the
canopy, the leafy portion of all the plants in the community
Shedding of lower shaded leaves when they respire more than
photosynthesize, self-pruning, occurs when the canopy is too thick
Leaf orientation affects light absorption In low-light conditions,
horizontal leaves capture more sunlight In sunny conditions,
vertical leaves are less damaged by sun and allow light to reach
lower leaves 8 Root Architecture and Acquisition of Water and
Minerals
Soil is a resource mined by the root system Root growth can adjust
to local conditions For example, roots branch more in a pocket of
high nitrate than in a pocket of low nitrate Roots are less
competitive with other roots from the same plant than with roots
from different plants Roots and the hyphae of soil fungi form
mutualistic associations called mycorrhizae 9 Concept 29.2:
Different mechanisms transport substances over short or long
distances- The Apoplast and Symplast: Transport Continuums The
apoplast consists of everything external to the plasma membrane It
includes cell walls, extracellular spaces, and the interior of
vessel elements and tracheids The symplast consists of the cytosol
of the living cells in a plant, as well as the plasmodesmata Three
transport routes for water and solutes are The apoplastic route,
through cell walls and extracellular spaces The symplastic route,
through the cytosol The transmembrane route, across cell walls 10
Cell wall Apoplastic route Cytosol Symplastic route
Figure 29.4 Cell wall Apoplastic route Cytosol Symplastic route
Transmembrane route Key Figure 29.4 Cell compartments and routes
for short-distance transport Plasmodesma Apoplast Plasma membrane
Symplast 11 Short-Distance Transport of Solutes Across Plasma
Membranes
Plasma membrane permeability controls short-distance movement of
substances Both active and passive transport occur in plants In
plants, membrane potential is established through pumping H by
proton pumps In animals, membrane potential is established through
pumping Na by sodium-potassium pumps 12 Solute transport across
plant cell plasma membranes
Figure 29.5 Solute transport across plant cell plasma membranes
CYTOPLASM EXTRACELLULAR FLUID S H+ H+ H+ H+ H+ Hydrogen ion H+ H+ S
H+ S H+ H+ H+ H+ H+ H+ H+ S H+ S H+ S Proton pump H+ H+ H+/sucrose
cotransporter Sucrose (neutral solute) (a) H+ and membrane
potential (b) H+ and cotransport of neutral solutes H+ H+ NO3 NO3
H+ H+ H+ K+ Potassium ion H+ K+ Nitrate H+ K+ Figure 29.5 Solute
transport across plant cell plasma membranes K+ H+ NO3 NO3 NO3 K+
NO3 K+ K+ H+ H+/NO3 cotransporter H+ H+ Ion channel (c) H+ and
cotransport of ions (d) Ion channels Plant cells use the energy of
H+ gradients to cotransport other solutes by active transport Plant
cell membranes have ion channels that allow only certain ions to
pass 13 Short-Distance Transport of Water Across Plasma
Membranes
To survive, plants must balance water uptake and loss Osmosis
determines the net uptake or water loss by a cell and is affected
by solute concentration and pressure 2014 Pearson Education, Inc.
14 Water potential determines the direction of movement of
water
Water potential is a measurement that combines the effects of
solute concentration and pressure Water potential determines the
direction of movement of water Water flows from regions of higher
water potential to regions of lower water potential Potential
refers to waters capacity to perform work Water potential is
abbreviated as and measured in a unit of pressure called the
megapascal (MPa) 0 MPa for pure water at sea level and at room
temperature 15 How Solutes and Pressure Affect Water
Potential
Both pressure and solute concentration affect water potential This
is expressed by the water potential equation: S P The solute
potential (S) of a solution is directly proportional to its
molarity Solute potential is also called osmotic potential 16
Pressure potential (P) is the physical pressure on a solution
Turgor pressure is the pressure exerted by the plasma membrane
against the cell wall, and the cell wall against the protoplast The
protoplast is the living part of the cell, which also includes the
plasma membrane 17 (a) Initial conditions: cellular
environmental
Figure 29.6 Initial flaccid cell: P S 0.7 0.4 M sucrose solution:
0.7 MPa Pure water: P S Plasmolyzed cell at osmotic equilibrium
with its surroundings 0.9 P S Turgid cell at osmotic equilibrium
with its surroundings 0 MPa 0.9 MPa 0.9 P S 0.7 P S 0.7 0.9 MPa 0
MPa Figure 29.6 Water relations in plant cells (a) Initial
conditions: cellular environmental (b) Initial conditions: cellular
environmental 18 Long-Distance Transport: The Role of Bulk
Flow
Efficient long-distance transport of fluid requires bulk flow, the
movement of a fluid driven by pressure Water and solutes move
together through tracheids and vessel elements of xylem and
sieve-tube elements of phloem Efficient movement is possible
because mature tracheids and vessel elements have no cytoplasm, and
sieve-tube elements have few organelles in their cytoplasm 19
Concept 29.3: Plants roots absorb essential elements from the
soil
Water, air, and soil minerals contribute to plant growth 8090% of a
plants fresh mass is water 96% of plants dry mass consists of
carbohydrates from the CO2 assimilated during photosynthesis 4% of
a plants dry mass is inorganic substances from soil Macronutrients
and Micronutrients More than 50 chemical elements have been
identified among the inorganic substances in plants, but not all of
these are essential to plants There are 17 essential elements,
chemical elements required for a plant to complete its life cycle
20 Soil Management Ancient farmers recognized that crop yields
would decrease on a particular plot over the years Soil management,
by fertilization and other practices, allowed for agriculture and
cities In natural ecosystems, nutrients are recycled through
decomposition of feces and humus, dead organic material Soils can
become depleted of nutrients as plants and the nutrients they
contain are harvested Fertilization replaces mineral nutrients that
have been lost from the soil 21 Adjusting Soil pH Soil pH affects
cation exchange and the chemical form of minerals Cations are more
available in slightly acidic soil, as H+ ions displace mineral
cations from clay particles The availability of different minerals
varies with pH For example, at pH 8 plants can absorb calcium but
not iron At present, 30% of the worlds farmland has reduced
productivity because of soil mismanagement Soil Texture Soil
particles are classified by size; from largest to smallest they are
called sand, silt, and clay Topsoil is formed when mineral
particles released from weathered rock mix with living organisms
and humus Soil solution consists of water and dissolved minerals in
the pores between soil particles After a heavy rainfall, water
drains from the larger spaces in the soil, but smaller spaces
retain water because of its attraction to clay and other particles
Loams are the most fertile topsoils and contain equal amounts of
sand, silt, and clay 23 Topsoil Composition A soils composition
refers to its inorganic (mineral) and organic chemical components
Inorganic components of the soil include positively charged ions
(cations) and negatively charged ions (anions) Most soil particles
are negatively charged Anions (for example, NO3, H2PO4, SO42) do
not bind with negatively charged soil particles and can be lost
from the soil by leaching Cations (for example, K, Ca2, Mg2) adhere
to negatively charged soil particles; this prevents them from
leaching out of the soil through percolating groundwater During
cation exchange, cations are displaced from soil particles by other
cations Displaced cations enter the soil solution and can be taken
up by plant roots Soil particle Root hair Cell wall
Figure 29.10 Soil particle K+ K+ Ca2+ Ca2+ Mg2+ K+ H+ H2O + CO2
H2CO3 HCO3 + H+ Figure Cation exchange in soil Root hair Cell wall
26 Organic components of the soil include decomposed leaves, feces,
dead organisms, and other organic matter, which are collectively
named humus Humus forms a crumbly soil that retains water but is
still porous It also increases the soils capacity to exchange
cations and serves as a reservoir of mineral nutrients Living
components of topsoil include bacteria, fungi, algae and other
protists, insects, earthworms, nematodes, and plant roots These
organisms help to decompose organic material and mix the soil
Concept 29.4: Plant nutrition often involves relationships with
other organisms
Plants and soil microbes have a mutualistic relationship Dead
plants provide energy needed by soil-dwelling microorganisms
Secretions from living roots support a wide variety of microbes in
the near-root environment Soil bacteria exchange chemicals with
plant roots, enhance decomposition, and increase nutrient
availability 28 Rhizobacteria The soil layer surrounding the plants
roots is the rhizosphere Rhizobacteria thrive in the rhizosphere,
and some can enter roots The rhizosphere has high microbial
activity because of sugars, amino acids, and organic acids secreted
by roots Rhizobacteria known as plant-growth-promoting
rhizobacteria can play several roles Produce hormones that
stimulate plant growth Produce antibiotics that protect roots from
disease Absorb toxic metals or make nutrients more available to
roots Bacteria in the Nitrogen Cycle
Nitrogen can be an important limiting nutrient for plant growth The
nitrogen cycle transforms atmospheric nitrogen and
nitrogen-containing compounds Plants can only absorb nitrogen as
either NO3 or NH4 Most usable soil nitrogen comes from actions of
soil bacteria 30 (dead organic material)
Figure 29.11 ATMOSPHERE N2 SOIL N2 ATMOSPHERE N2 Nitrate and
nitrogenous organic compounds exported in xylem to shoot system
SOIL Proteins from humus (dead organic material) Nitrogen-fixing
bacteria Microbial decomposition Figure The roles of soil bacteria
in the nitrogen nutrition of plants Amino acids NH3 (ammonia)
Denitrifying bacteria Ammonifying bacteria NH4+ H+ (from soil) NH4+
(ammonium) NO2 (nitrite) NO3 (nitrate) Nitrifying bacteria
Nitrifying bacteria Root 31 Conversion to NH4+ Conversion to
NO3
Ammonifying bacteria break down organic compounds and release
ammonium (NH4+) Nitrogen-fixing bacteria convert N2 gas into NH3
NH3 is converted to NH4+ Conversion to NO3 Nitrifying bacteria
oxidize NH4+ to nitrite (NO2) then nitrite to nitrate (NO3)
Different nitrifying bacteria mediate each step Nitrogen is lost to
the atmosphere when denitrifying bacteria convert NO3 to N2
Bacterial root nodules
Found on roots of legumes- symbiotic relationship! Inside the root
nodule, Rhizobium bacteria assume a form called bacteroids, which
are contained within vesicles formed by the root cell The plant
obtains fixed nitrogen from Rhizobium, and Rhizobium obtains sugar
and an anaerobic environment Each legume species is associated with
a particular strain of Rhizobium 34 Mycorrhizae increase
absorption
The hyphae of mycorrhizal fungi extend into soil, where their large
surface area and efficient absorption enable them to obtain mineral
nutrients, even if these are in short supply or are relatively
immobile. Mycorrhizal fungi seem to be particularly important for
absorption of phosphorus, a poorly mobile element, and a proportion
of the phosphate that they absorb has been shown to be passed to
the plant. Mycorrhizae Symbiotic relationship between fungi &
plant
symbiotic fungi greatly increases surface area for absorption of
water & minerals increases volume of soil reached by plant
increases transport to host plant The hyphae of mycorrhizal fungi
extend into soil, where their large surface area and efficient
absorption enable them to obtain mineral nutrients, even if these
are in short supply or are relatively immobile. Mycorrhizal fungi
seem to be particularly important for absorption of phosphorus, a
poorly mobile element, and a proportion of the phosphate that they
absorb has been shown to be passed to the plant. Epiphytes,
Parasitic Plants, and Carnivorous Plants
Some plants have nutritional adaptations that use other organisms
in nonmutualistic ways Three unusual adaptations are Epiphytes
Parasitic plants Carnivorous plants 37 Epiphytes grow on other
plants and obtain water and minerals from rain, rather than tapping
their hosts for sustenance Parasitic plants absorb water, sugars,
and minerals from their living host plant Some species also
photosynthesize, but others rely entirely on the host plant for
sustenance Some species parasitize the mycorrhizal hyphae of other
plants Carnivorous plants are photosynthetic but obtain nitrogen by
killing and digesting mostly insects Staghorn fern, an
epiphyte
Figure 29.15a Figure 29.15a Exploring unusual nutritional
adaptations in plants (part 1: epiphytes) Staghorn fern, an
epiphyte 39 Mistletoe, a photosynthetic Indian pipe, a
nonphoto-
Figure 29.15b Parasitic plants Figure 29.15b Exploring unusual
nutritional adaptations in plants (part 2: parasitic plants)
Mistletoe, a photosynthetic parasite Dodder, a nonphoto- synthetic
parasite (orange) Indian pipe, a nonphoto- synthetic parasite of
mycorrhizae 40 Carnivorous plants Sundew Pitcher plants Venus
flytraps
Figure 29.15c Carnivorous plants Sundew Pitcher plants Venus
flytraps Figure 29.15c Exploring unusual nutritional adaptations in
plants (part 3: carnivorous plants) 41 Concept 29.5: Transpiration
drives the transport of water and minerals from roots to shoots via
the xylem Plants can move a large volume of water from their roots
to shoots Most water and mineral absorption occurs near root tips,
where root hairs are located and the epidermis is permeable to
water Root hairs account for much of the absorption of water by
roots After soil solution enters the roots, the extensive surface
area of cortical cell membranes enhances uptake of water and
selected minerals The concentration of essential minerals is
greater in the roots than in the soil because of active transport
42 Water flow through root
Porous cell wall water can flow through cell wall route & not
enter cells plant needs to force water into cells Casparian strip
The endodermis, with its Casparian strip, ensures that no minerals
can reach the vascular tissue of the root without crossing a
selectively permeable plasma membrane. If minerals do not enter the
symplast of cells in the epidermis or cortex, they must enter
endodermal cells or be excluded from the vascular tissue. The
endodermis also prevents solutes that have been accumulated in the
xylem sap from leaking back into the soil solution. The structure
of the endodermis and its strategic location in the root fit its
function as sentry of the border between the cortex and the
vascular cylinder, a function that contributes to the ability of
roots to transport certain minerals preferentially from the soil
into the xylem. Controlling the route of water in root
Endodermis cell layer surrounding vascular cylinder of root lined
with impermeable Casparian strip forces fluid through selective
cell membrane filtered & forced into xylem cells Water &
mineral absorption
Water absorption from soil osmosis aquaporins Mineral absorption
active transport proton pumps active transport of H+ aquaporin root
hair proton pumps H2O Mineral absorption Proton pumps
active transport of H+ ions out of cell chemiosmosis H+ gradient
creates membrane potential difference in charge drives cation
uptake creates gradient cotransport of other solutes against their
gradient The most important active transport protein in the plasma
membranes of plant cells is the proton pump , which uses energy
from ATP to pump hydrogen ions (H+) out of the cell. This results
in a proton gradient with a higher H+ concentration outside the
cell than inside. Proton pumps provide energy for solute
transport.By pumping H+ out of the cell, proton pumps produce an H+
gradient and a charge separation called a membrane potential. These
two forms of potential energy can be used to drive the transport of
solutes. Plant cells use energy stored in the proton gradient and
membrane potential to drive the transport of many different
solutes. For example, the membrane potential generated by proton
pumps contributes to the uptake of K+ by root cells. In the
mechanism called cotransport, a transport protein couples the
downhill passage of one solute (H+) to the uphill passage of
another (ex. NO3). The coattail effect of cotransport is also
responsible for the uptake of the sugar sucrose by plant cells. A
membrane protein cotransports sucrose with the H+ that is moving
down its gradient through the protein. The role of proton pumps in
transport is an application of chemiosmosis. Transport in Plants
Transpiration pull-cohesion tension theory states that for every
molecule of water evaporated from the leaf, another molecule is
drawn in at the root Pulling Xylem Sap: The Cohesion-Tension
Hypothesis
According to the cohesion-tension hypothesis, transpiration and
water cohesion pull water from shoots to roots Xylem sap is
normally under negative pressure, or tension Transpirational pull
is generated when water vapor in the air spaces of a leaf diffuses
down its water potential gradient and exits the leaf via stomata As
water evaporates, the air-water interface retreats farther into the
mesophyll cell walls and becomes more curved Due to the high
surface tension of water, the curvature of the interface creates a
negative pressure potential 48 This negative pressure pulls water
in the xylem into the leaf
The pulling effect results from the cohesive binding between water
molecules The transpirational pull on xylem sap is transmitted from
leaves to roots 49 Transpiration pull generated by
evaporation
Ascent of xylem fluid Transpiration pull generated by evaporation
from the leaf Cohesion and adhesion in the ascent of xylem sap:
Water molecules are attracted to each other through cohesion
Cohesion makes it possible to pull a column of xylem sap Water
molecules are attracted to hydrophilic walls of xylem cell walls
through adhesion Adhesion of water molecules to xylem cell walls
helps offset the force of gravity Thick secondary walls prevent
vessel elements and tracheids from collapsing under negative
pressure Drought stress or freezing can cause cavitation, the
formation of a water vapor pocket by a break in the chain of water
molecules 52 Xylem Sap Ascent by Bulk Flow: A Review
Bulk flow is driven by a water potential difference at opposite
ends of xylem tissue Bulk flow is driven by evaporation and does
not require energy from the plant; like photosynthesis, it is solar
powered Bulk flow differs from diffusion It is driven by
differences in pressure potential, not solute potential It occurs
in hollow dead cells, not across the membranes of living cells It
moves the entire solution, not just water or solutes It is much
faster 53 Concept 29.6: The rate of transpiration is regulated by
stomata
Leaves generally have broad surface areas and high
surface-to-volume ratios These characteristics increase
photosynthesis and increase water loss through stomata Guard cells
help balance water conservation with gas exchange for
photosynthesis About 95% of the water a plant loses escapes through
stomata Each stoma is flanked by a pair of guard cells, which
control the diameter of the stoma by changing shape Stomatal
density is under genetic and environmental control 54 water moves
into guard cells water moves out of guard cells
Control of Stomates Uptake of K+ ions by guard cells proton pumps
create a membrane potential that drives the uptake of K+ ions from
epidermal cell surrounding the guard cells water enters by osmosis
guard cells become turgid Loss of K+ ions by guard cells water
leaves by osmosis guard cells become flaccid Epidermal cell Guard
cell Chloroplasts Nucleus K+ K+ H2O H2O H2O H2O K+ K+ K+ K+ H2O H2O
H2O H2O K+ K+ Thickened inner cell wall (rigid) H2O H2O H2O H2O K+
K+ K+ K+ Stoma open Stoma closed water moves into guard cells water
moves out of guard cells Blue light causes stomates to open Control
of transpiration
Balancing stomate function always a compromise between
photosynthesis & transpiration leaf may transpire more than its
weight in water in a daythis loss must be balanced with plants need
for CO2 for photosynthesis Stimuli for Stomatal Opening and
Closing
Generally, stomata open during the day and close at night to
minimize water loss Stomatal opening at dawn is triggered by Blue
Light CO2 depletion An internal clock in guard cells All eukaryotic
organisms have internal clocks; circadian rhythms are 24-hour
cycles Drought stress can cause stomata to close during the daytime
The hormone abscisic acid (ABA) is produced in response to water
deficiency and causes the closure of stomata 57 Osmoregulation in
Hydrophytes (Aquatic plants)
poorly developed root systems and supportive xylem tissues no
stomata (for submerged leaves) thin & finely divided leaves no
cuticle Adaptations That Reduce Evaporative Water Loss
Xerophytes are plants adapted to arid climates Some desert plants
complete their life cycle during the rainy season Others have leaf
modifications that reduce the rate of transpiration Some plants use
a specialized form of photosynthesis called crassulacean acid
metabolism (CAM) where stomatal gas exchange occurs at night 2014
Pearson Education, Inc. 59 Oleander (Nerium oleander) Upper
epidermal tissue
Figure 29.20 Ocotillo (Fouquieria splendens) Oleander (Nerium
oleander) Thick cuticle Upper epidermal tissue 100 m Trichomes
(hairs) Crypt Stoma Lower epidermal tissue Figure Some xerophytic
adaptations Old man cactus (Cephalocereus senilis) 60
Osmoregulation- Plant Responses to Water Limitations
Wilting or curling leaves reduces sunlight exposure and water
evaporation Stomata are on the underside of the leaf, thick waxy
cuticle is on the top Adaptations like needles and fat
photosynthesizing stems that store water Deep tap roots or shallow
fibrous roots Hairs or scales on leaves that reduce wind
evaporation Pits on underside of leaf where stomata are to reduce
water loss by raising water potential around stoma area Thick
fleshy leaves that store water Small leaves Stomata that only open
at night Multiple layered epidermis and small intercellular spaces
Concept 29.7: Sugars are transported from sources to sinks via the
phloem
The products of photosynthesis are transported through phloem by
the process of translocation In angiosperms, sieve-tube elements
are the conduits for translocation Phloem sap is an aqueous
solution that is high in sucrose It travels from a sugar source to
a sugar sink A sugar source is an organ that is a net producer of
sugar, such as mature leaves A sugar sink is an organ that is a net
consumer or storer of sugar, such as a tuber or bulb 62 A storage
organ can be both a sugar sink in summer and sugar source in
winter
Sugar must be loaded into sieve-tube elements before being exported
to sinks Depending on the species, sugar may move by symplastic or
both symplastic and apoplastic pathways Companion cells enhance
solute movement between the apoplast and symplast 63 In most
plants, phloem loading requires active transport
Proton pumping and cotransport of sucrose and H+ enable the cells
to accumulate sucrose At the sink, sugar molecules diffuse from the
phloem to sink tissues and are followed by water Sometimes there
are more sinks than can be supported by sources Self-thinning is
the dropping of sugar sinks such as flowers, seeds, or fruits 64
Transport of sugars in phloem
Loading of sucrose into phloem flow through cells via plasmodesmata
proton pumps cotransport of sucrose into cells down proton gradient
Pressure flow in phloem
Bulk flow hypothesis source to sink flow direction of transport in
phloem is dependent on plants needs phloem loading active transport
of sucroseinto phloem increased sucrose concentration decreases H2O
potential water flows in from xylem cells increase in pressure due
to increase in H2O causes flow In contrast to the unidirectional
transport of xylem sap from roots to leaves, the direction that
phloem sap travels is variable. However, sieve tubes always carry
sugars from a sugar source to a sugar sink. A sugar source is a
plant organ that is a net producer of sugar, by photosynthesis or
by breakdown of starch. Mature leaves are the primary sugar
sources. A sugar sink is an organ that is a net consumer or storer
of sugar. Growing roots, buds, stems, and fruits are sugar sinks. A
storage organ, such as a tuber or a bulb, may be a source or a
sink, depending on the season. When stockpiling carbohydrates in
the summer, it is a sugar sink. After breaking dormancy in the
spring, it is a source as its starch is broken down to sugar, which
is carried to the growing tips of the plant. A sugar sink usually
receives sugar from the nearest sources. Upper leaves on a branch
may send sugar to the growing shoot tip, whereas lower leaves
export sugar to roots. A growing fruit may monopolize sugar sources
around it. For each sieve tube, the direction of transport depends
on the locations of the source and sink connected by that tube.
Therefore, neighboring tubes may carry sap in opposite directions.
Direction of flow may also vary by season or developmental stage of
the plant. Bulk flow by negative pressure Bulk flow by positive
pressure
Figure 29.22 Sieve tube (phloem) Pressure Flow: The Mechanism of
Translocation in Angiosperms Phloem sap flows from source to sink
at rates as great as 1 m/hr, much too fast to be accounted for by
either diffusion or cytoplasmic streaming. In studying angiosperms,
researchers have concluded that sap moves through a sieve tube by
bulk flow driven by positive pressure (thus the synonym pressure
flow. The building of pressure at the source end and reduction of
that pressure at the sink end cause water to flow from source to
sink, carrying the sugar along. Xylem recycles the water from sink
to source. The pressure flow hypothesis explains why phloem sap
always flows from source to sink. Source cell (leaf) Vessel (xylem)
1 Loading of sugar H2O 1 Sucrose H2O 2 can flow 1m/hr 2 Uptake of
water Bulk flow by negative pressure Bulk flow by positive pressure
3 Unloading of sugar Sink cell (storage root) Figure Bulk flow by
positive pressure (pressure flow) in a sieve tube 4 Recycling of
water 4 3 Sucrose H2O 67 Experimentation Testing pressure flow
hypothesis
using aphids to measure sap flow & sugar concentration along
plant stem Pressure Flow: The Mechanism of Translocation in
Angiosperms Phloem sap flows from source to sink at rates as great
as 1 m/hr, much too fast to be accounted for by either diffusion or
cytoplasmic streaming. In studying angiosperms, researchers have
concluded that sap moves through a sieve tube by bulk flow driven
by positive pressure (thus the synonym pressure flow. The building
of pressure at the source end and reduction of that pressure at the
sink end cause water to flow from source to sink, carrying the
sugar along. Xylem recycles the water from sink to source. The
pressure flow hypothesis explains why phloem sap always flows from
source to sink. Maplesugaring Connecting the Concepts With the Big
Ideas
Energy and Homeostasis- Plants depend on waters special properties
of adhesion and cohesion to accomplish movement of water via
transpiration pull when stomata are open. Plants employ stomata and
root hairs to facilitate gas exchange. Negative feedback allows
plants to regulate their transpiration based on water availability
Related methods of osmoregulation are used throughout the plant
kingdom, showing their common ancestry. Important cooperative
relationships that assist plants in their acquisition of nutrients
include mycorrhizae and bacterial root nodules.