Resource Acquisition Transport in Vascular Plants Campbell and

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 CO 2 LEAVES

�arrangement LEAVES of leaves on a stem called: phyllotaxy

�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 nitrogen-rich 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 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) = 0 MPa � 1 Mpa ~ 10 x 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 Potential

� 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 Turgor Pressure

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

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 �material 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. CO 2 depletion 3. Circadian rhythm in guard cells Stimuli for Stomatal Opening & Closing

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 [CO 2] stored in air-spaces used in photosynthesis decreases during day hrs stomata slowly open if there is sufficient water CO 2 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: is dry: guard cells lose turgor close stoma hormone, abscisic acid (ABA) made in roots & leaves in response to water shortage signals guard cells to close stomata reduces wilting * restricts CO 2 absorption slows photosynthesis �plant ◦ turgor needed for cell 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 CO 2 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 CO 2 during nite so stomata can close during day when evaporative losses greatest �*stomata are most important mediators of conflicting demands for CO 2 & 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 1 m/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