Resource Acquisition & Transport in Vascular

PlantsCampbell and Reece

Chapter 36

Underground Plants

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

Early Land 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

Adaptations of Vascular Plants

typical land plant inhabits 2 worlds:◦under ground◦above ground

Evolution of 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

Xylem & Phloem

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

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

LEAVES

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

LEAF NUMBERS or SIZE

affects amt light captureleaf 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 ORIENTATION

affects amt light captured

STEMS

function:◦supporting structures for leaves◦conduit for long-distance transport of water & nutrients

BRANCHING PATTERNS

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

ROOTS

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 nitrogen-rich areas

ROOT GROWTH

MYCORRHIZAE

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

Transport in Plants

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◦ cell walls, interior of dead cells, tracheids

(long tapered water-conducting cell in xylem in most vascular plants

◦ extracellular spaces

2. Symplast: all cytosol of all living cells in plant

3 Routes for Transport in Plants

1. Apoplastic Route◦ water & solutes cell walls &

extracellular spaces

2. Symplastic Route◦ water & solutes cytosol plasma

membrane plasmodesmata next cell

3. Transmembrane Route◦ out of 1 cell cell wall neighboring cell

Short-Distance Transport Across Plasma Membranes

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

Solute Transport across Plant Cell Membranes

Osmosis & Water Potential

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

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 PEΨ (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

How Solutes & Pressure Affect Water Potential

Water Potential equation:

Solute Water Potential

directly proportional to its molarityaka osmotic potential

◦solutes affect direction water moves in osmosis

plant solutes◦mineral ions◦sugars

How Solutes & Pressure Affect 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 (-)

Pressure Potential

Ψp = physical pressure on a solutioncan be (+) or (-) relative to

atmospheric pressure

Turgor Pressure

force directed against a plant cell wall after the influx of water & swelling of the cell due to osmosis

Turgor Pressure

critical for plant function: helps maintain stiffness of plant tissues & is driving force for cell elongation

Wilting in Nonwoody Plant

Aquaporins

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

Long-Distance Transport

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)

Bulk Flow

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

diffusion, active transport, & bulk flow act together transporting resources thru out whole plant

Root Cells

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

Mineral Ions

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+)

Transport into Xylem

to get to the rest of plant water & minerals must get to the endodermis: the innermost layer of cortex, surrounds vascular cylinder

Endodermis

serves as last “checkpoint” for selective passage of minerals from cortex vascular cylinder

Transport of Water & Minerals

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

Bulk Flow Transport via Xylem

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

Xylem Sap Pushed by Root Pressure

@ 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

Guttation

when root pressure causes more water to enter leaves than is transpired exudation of water droplets on edges of leaves

Root Pressure

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

Pulling Xylem Sap

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 @ air-water interface so generates (-) pressure that pulls water thru xylem

Pulling Xylem Sap

Role of K+ in Stomatal Opening

Stimuli for Stomatal Opening & Closing

3 cues contribute to opening of stomata @ dawn:

1. Light2. CO2 depletion3. Circadian rhythm in guard cells

Light Controlling Guard Cells

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+

CO2 Depletion

as [CO2] stored in air-spaces used in photosynthesis decreases during day hrs stomata slowly open if there is sufficient water

Circadian Rhythm

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 in Plants

Environmental Stresses

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

Transpiration Effects

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

Adaptations that Reduce Evaporative Water Loss

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

Phloem Moves Sugars

mature leaves are main sugar sourcesstorage organs can be seasonal sources of

sugarsugar 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

Loading of Sucrose into Phloem

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

Bulk Flow by (+) Pressure

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

Plant Communication

Plasmodesmata:change in permeability & numberwhen 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