Resource Acquisition and Transport in Vascular Plants Chapter 36 Resource Acquisition and Transport in Vascular Plants

Overview: Underground Plants 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 © 2011 Pearson Education, Inc.

Figure 36.1 Figure 36.1 Plants or pebbles?

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 © 2011 Pearson Education, Inc.

Concept 36.1: Adaptations for acquiring resources were key steps in the evolution of vascular plants The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the 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 © 2011 Pearson Education, Inc.

Xylem transports water and minerals from roots to shoots The evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis Xylem transports water and minerals from roots to shoots Phloem transports photosynthetic products from sources to sinks © 2011 Pearson Education, Inc.

H2O H2O and minerals Figure 36.2-1 Figure 36.2 An overview of resource acquisition and transport in a vascular plant. H2O and minerals

CO2 O2 H2O O2 H2O and minerals CO2 Figure 36.2-2 Figure 36.2 An overview of resource acquisition and transport in a vascular plant. O2 H2O and minerals CO2

CO2 O2 Light Sugar H2O O2 H2O and minerals CO2 Figure 36.2-3 Figure 36.2 An overview of resource acquisition and transport in a vascular plant. O2 H2O and minerals CO2

Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss © 2011 Pearson Education, Inc.

Shoot Architecture and Light Capture Stems serve as conduits for water and nutrients and as supporting structures for leaves There is generally a positive correlation between water availability and leaf size © 2011 Pearson Education, Inc.

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 137.5 and likely minimizes shading of lower leaves © 2011 Pearson Education, Inc.

Figure 36.3 24 32 42 29 40 16 11 19 21 27 3 34 8 6 14 13 26 Shoot apical meristem 1 5 22 9 18 Buds 10 4 2 31 17 7 12 Figure 36.3 Emerging phyllotaxy of Norway spruce. 23 15 20 25 28 1 mm

Figure 36.3a Figure 36.3 Emerging phyllotaxy of Norway spruce.

Figure 36.3b 24 32 42 29 40 16 11 19 21 27 34 8 3 6 14 13 26 Shoot apical meristem 1 22 5 9 18 10 4 2 31 17 Figure 36.3 Emerging phyllotaxy of Norway spruce. 7 12 23 15 20 25 28 1 mm

Light absorption is affected by the leaf area index, the ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows Self-pruning is the shedding of lower shaded leaves when they respire more than photosynthesize © 2011 Pearson Education, Inc.

Ground area covered by plant Figure 36.4 Ground area covered by plant Figure 36.4 Leaf area index. Plant A Leaf area  40% of ground area (leaf area index  0.4) Plant B Leaf area  80% of ground area (leaf area index  0.8)

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 © 2011 Pearson Education, Inc.

Shoot height and branching pattern also affect light capture There is a trade-off between growing tall and branching © 2011 Pearson Education, Inc.

Root Architecture and Acquisition of Water and Minerals Soil is a resource mined by the root system Taproot systems anchor plants and are characteristic of gymnosperms and eudicots Root growth can adjust to local conditions For example, roots branch more in a pocket of high nitrate than low nitrate Roots are less competitive with other roots from the same plant than with roots from different plants © 2011 Pearson Education, Inc.

Mutualisms with fungi helped plants colonize land Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae Mutualisms with fungi helped plants colonize land Mycorrhizal fungi increase the surface area for absorbing water and minerals, especially phosphate © 2011 Pearson Education, Inc.

Figure 36.5 Roots Figure 36.5 A mycorrhiza, a mutualistic association of fungus and roots. Fungus

Concept 36.2: Different mechanisms transport substances over short or long distances There are two major pathways through plants The apoplast The symplast © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Cell wall Apoplastic route Cytosol Symplastic route Figure 36.6 Cell wall Apoplastic route Cytosol Symplastic route Transmembrane route Key Figure 36.6 Cell compartments and routes for short-distance transport. Plasmodesma Apoplast Plasma membrane Symplast

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 © 2011 Pearson Education, Inc.

Figure 36.7 Solute transport across plant cell plasma membranes. CYTOPLASM EXTRACELLULAR FLUID (a) H+ and membrane potential  + ATP  H+ + Hydrogen ion  + H+ H+ H+ H+ H+  + H+ Proton pump H+  +  + (b) H+ and cotransport of neutral solutes S H+ H+  + H+ H+  + H+ H+ S H+ S H+ H+ H+ S S  + S H+  H+/sucrose cotransporter + Sucrose (neutral solute)  +  + (c) H+ and cotransport of ions H+ H+ NO3  + NO3  + H+ H+ H+ H+ Nitrate H+ NO3 H+ Figure 36.7 Solute transport across plant cell plasma membranes. NO3  + NO3 NO3  + H+ H+NO3 cotransporter H+  + H+  + (d) Ion channels K+ Potassium ion  + K+ K+  + K+ K+ K+ K+  + Ion channel  +

CYTOPLASM EXTRACELLULAR FLUID Hydrogen ion Proton pump  + H+  + ATP Figure 36.7a CYTOPLASM EXTRACELLULAR FLUID  + H+  + Hydrogen ion ATP  + H+ H+ H+ H+ H+ H+  + Figure 36.7 Solute transport across plant cell plasma membranes. Proton pump H+  + (a) H+ and membrane potential

Plant cells use the energy of H gradients to cotransport other solutes by active transport © 2011 Pearson Education, Inc.

H+/sucrose cotransporter Sucrose (neutral solute) Figure 36.7b  + H+ S H+  + H+ H+  + H+ H+ H+ S S H+ H+ H+ S S S  + H+  Figure 36.7 Solute transport across plant cell plasma membranes. + H+/sucrose cotransporter Sucrose (neutral solute)  + (b) H+ and cotransport of neutral solutes

Nitrate H+NO3 cotransporter  + H+ H+ NO3  + NO3  H+ + H+ H+ H+ Figure 36.7c  + H+ H+ NO3  + NO3  + H+ H+ H+ H+ Nitrate H+ H+ NO3 NO3  NO3 + NO3  + H+ Figure 36.7 Solute transport across plant cell plasma membranes. H+NO3 cotransporter H+  H+ + (c) H+ and cotransport of ions

Plant cell membranes have ion channels that allow only certain ions to pass © 2011 Pearson Education, Inc.

 + K+ Potassium ion  + K+  K+ + K+ K+ K+ K+  + Ion channel  + Figure 36.7d  + K+ Potassium ion  + K+  K+ + K+ K+ K+ K+  + Ion channel Figure 36.7 Solute transport across plant cell plasma membranes.  + (d) Ion channels

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 © 2011 Pearson Education, Inc.

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 water’s capacity to perform work © 2011 Pearson Education, Inc.

Ψ = 0 MPa for pure water at sea level and at room temperature Water potential is abbreviated as Ψ and measured in a unit of pressure called the megapascal (MPa) Ψ = 0 MPa for pure water at sea level and at room temperature © 2011 Pearson Education, Inc.

How Solutes and Pressure Affect Water Potential Both pressure and solute concentration affect water potential This is expressed by the water potential equation: Ψ  ΨS  ΨP The solute potential (ΨS) of a solution is directly proportional to its molarity Solute potential is also called osmotic potential © 2011 Pearson Education, Inc.

Pressure potential (ΨP) 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 © 2011 Pearson Education, Inc.

Consider a U-shaped tube where the two arms are separated by a membrane permeable only to water Water moves in the direction from higher water potential to lower water potential © 2011 Pearson Education, Inc.

Figure 36.8 Solutes have a negative effect on  by binding water molecules. Positive pressure has a positive effect on  by pushing water. Solutes and positive pressure have opposing effects on water movement. Negative pressure (tension) has a negative effect on  by pulling water. Pure water at equilibrium Pure water at equilibrium Pure water at equilibrium Pure water at equilibrium H2O H2O H2O H2O Adding solutes to the right arm makes  lower there, resulting in net movement of water to the right arm: Applying positive pressure to the right arm makes  higher there, resulting in net movement of water to the left arm: In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water: Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm: Figure 36.8 Effects of solutes and pressure on water potential () and water movement. Positive pressure Positive pressure Negative pressure Pure water Solutes Solutes Membrane H2O H2O H2O H2O

Solutes have a negative effect on  by binding water molecules. Figure 36.8a Solutes have a negative effect on  by binding water molecules. Adding solutes to the right arm makes  lower there, resulting in net movement of water to the right arm: Pure water at equilibrium Pure water Figure 36.8 Effects of solutes and pressure on water potential () and water movement. Solutes Membrane H2O H2O

Positive pressure has a positive effect on  by pushing water. Figure 36.8b Positive pressure has a positive effect on  by pushing water. Applying positive pressure to the right arm makes  higher there, resulting in net movement of water to the left arm: Positive pressure Pure water at equilibrium Figure 36.8 Effects of solutes and pressure on water potential () and water movement. H2O H2O

Solutes and positive pressure have opposing effects on water movement. Figure 36.8c Solutes and positive pressure have opposing effects on water movement. In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water: Positive pressure Pure water at equilibrium Figure 36.8 Effects of solutes and pressure on water potential () and water movement. Solutes H2O H2O

Pure water at equilibrium Figure 36.8d Negative pressure (tension) has a negative effect on  by pulling water. Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm: Negative pressure Pure water at equilibrium Figure 36.8 Effects of solutes and pressure on water potential () and water movement. H2O H2O

Water Movement Across Plant Cell Membranes Water potential affects uptake and loss of water by plant cells If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall Video: Plasmolysis © 2011 Pearson Education, Inc.

Figure 36.9 Water relations in plant cells. Initial flaccid cell: P  S 0.7  0.4 M sucrose solution:   0.7 MPa Pure water: P  P  S  0.9 S  Plasmolyzed cell at osmotic equilibrium with its surroundings  0.9 MPa    0 MPa Turgid cell at osmotic equilibrium with its surroundings P  P  0.7 S 0.9  S  0.7  0.9 MPa    0 MPa Figure 36.9 Water relations in plant cells. (a) Initial conditions: cellular   environmental  (b) Initial conditions: cellular   environmental 

Plasmolyzed cell at osmotic equilibrium with its surroundings  Figure 36.9a Initial flaccid cell: P  S 0.7  0.4 M sucrose solution:  0.7 MPa  P  S 0.9  Plasmolyzed cell at osmotic equilibrium with its surroundings  0.9 MPa  P  S 0.9  Figure 36.9 Water relations in plant cells.  0.9 MPa  (a) Initial conditions: cellular   environmental 

Turgid cell at osmotic equilibrium with its surroundings Figure 36.9b Initial flaccid cell: P  S 0.7   0.7 MPa  Pure water: P  S   0 MPa  Turgid cell at osmotic equilibrium with its surroundings P 0.7  S 0.7  Figure 36.9 Water relations in plant cells.  0 MPa  (b) Initial conditions: cellular   environmental 

If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid Turgor loss in plants causes wilting, which can be reversed when the plant is watered Video: Turgid Elodea © 2011 Pearson Education, Inc.

Aquaporins: Facilitating Diffusion of Water Aquaporins are transport proteins in the cell membrane that allow the passage of water These affect the rate of water movement across the membrane © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Concept 36.3: 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 © 2011 Pearson Education, Inc.

Absorption of Water and Minerals by Root Cells 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 surface area of roots After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals Animation: Transport in Roots © 2011 Pearson Education, Inc.

The concentration of essential minerals is greater in the roots than soil because of active transport © 2011 Pearson Education, Inc.

Transport of Water and Minerals into the Xylem The endodermis is the innermost layer of cells in the root cortex It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into the vascular tissue © 2011 Pearson Education, Inc.

Water can cross the cortex via the symplast or apoplast The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder Water and minerals in the apoplast must cross the plasma membrane of an endodermal cell to enter the vascular cylinder © 2011 Pearson Education, Inc.

Pathway along apoplast Figure 36.10 Casparian strip Endodermal cell Pathway along apoplast Pathway through symplast Plasma membrane Casparian strip Apoplastic route Figure 36.10 Transport of water and minerals from root hairs to the xylem. Vessels (xylem) Symplastic route Root hair Epidermis Endodermis Vascular cylinder (stele) Cortex

Vascular cylinder (stele) Figure 36.10a Plasma membrane Casparian strip Apoplastic route Figure 36.10 Transport of water and minerals from root hairs to the xylem. Vessels (xylem) Symplastic route Root hair Epidermis Endodermis Vascular cylinder (stele) Cortex

Pathway along apoplast Figure 36.10b Casparian strip Endodermal cell Pathway along apoplast Pathway through symplast Figure 36.10 Transport of water and minerals from root hairs to the xylem.

Water and minerals now enter the tracheids and vessel elements The endodermis regulates and transports needed minerals from the soil into the xylem Water and minerals move from the protoplasts of endodermal cells into their cell walls Diffusion and active transport are involved in this movement from symplast to apoplast Water and minerals now enter the tracheids and vessel elements © 2011 Pearson Education, Inc.

Bulk Flow Transport via the Xylem Xylem sap, water and dissolved minerals, is transported from roots to leaves by bulk flow The transport of xylem sap involves transpiration, the evaporation of water from a plant’s surface Transpired water is replaced as water travels up from the roots Is sap pushed up from the roots, or pulled up by the leaves? © 2011 Pearson Education, Inc.

Pushing Xylem Sap: Root Pressure At night root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential Water flows in from the root cortex, generating root pressure Root pressure sometimes results in guttation, the exudation of water droplets on tips or edges of leaves © 2011 Pearson Education, Inc.

Figure 36.11 Figure 36.11 Guttation.

Positive root pressure is relatively weak and is a minor mechanism of xylem bulk flow © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Transpirational Pull Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata As water evaporates, the air-water interface retreats further into the mesophyll cell walls The surface tension of water creates a negative pressure potential © 2011 Pearson Education, Inc.

This negative pressure pulls water in the xylem into the leaf The transpirational pull on xylem sap is transmitted from leaves to roots © 2011 Pearson Education, Inc.

Microfibrils in cell wall of mesophyll cell Mesophyll Air space Figure 36.12 Cuticle Xylem Upper epidermis Microfibrils in cell wall of mesophyll cell Mesophyll Air space Figure 36.12 Generation of transpirational pull. Lower epidermis Cuticle Stoma Microfibril (cross section) Water film Air-water interface

Outside air   100.0 MPa Leaf  (air spaces)  7.0 MPa Figure 36.13 Xylem sap Outside air  Mesophyll cells  100.0 MPa Stoma Leaf  (air spaces) Water molecule  7.0 MPa Atmosphere Transpiration Leaf  (cell walls) Adhesion by hydrogen bonding  1.0 MPa Xylem cells Cell wall Water potential gradient Trunk xylem  Cohesion by hydrogen bonding  0.8 MPa Cohesion and adhesion in the xylem Figure 36.13 Ascent of xylem sap. Water molecule Root hair Trunk xylem   0.6 MPa Soil particle Soil  Water Water uptake from soil  0.3 MPa

Water molecule Root hair Soil particle Water Water uptake from soil Figure 36.13a Water molecule Root hair Soil particle Figure 36.13 Ascent of xylem sap. Water Water uptake from soil

Adhesion by hydrogen bonding Xylem cells Figure 36.13b Adhesion by hydrogen bonding Xylem cells Cell wall Figure 36.13 Ascent of xylem sap. Cohesion by hydrogen bonding Cohesion and adhesion in the xylem

Xylem sap Mesophyll cells Stoma Water molecule Atmosphere Figure 36.13c Xylem sap Mesophyll cells Stoma Water molecule Atmosphere Figure 36.13 Ascent of xylem sap. Transpiration

Adhesion and Cohesion in the Ascent of Xylem Sap Water molecules are attracted to cellulose in xylem cell walls through adhesion Adhesion of water molecules to xylem cell walls helps offset the force of gravity Animation: Water Transport Animation: Transpiration © 2011 Pearson Education, Inc.

Water molecules are attracted to each other through cohesion Cohesion makes it possible to pull a column of xylem sap 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 © 2011 Pearson Education, Inc.

Xylem Sap Ascent by Bulk Flow: A Review The movement of xylem sap against gravity is maintained by the transpiration-cohesion-tension mechanism 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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Concept 36.4: 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 © 2011 Pearson Education, Inc.

Figure 36.14 Figure 36.14 An open stoma (left) and closed stoma (LMs).

Figure 36.14a Figure 36.14 An open stoma (left) and closed stoma (LMs).

Figure 36.14b Figure 36.14 An open stoma (left) and closed stoma (LMs).

Stomata: Major Pathways for Water Loss 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 © 2011 Pearson Education, Inc.

Mechanisms of Stomatal Opening and Closing Changes in turgor pressure open and close stomata When turgid, guard cells bow outward and the pore between them opens When flaccid, guard cells become less bowed and the pore closes © 2011 Pearson Education, Inc.

Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed Figure 36.15 Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell (a) Changes in guard cell shape and stomatal opening and closing (surface view) H2O H2O H2O H2O Figure 36.15 Mechanisms of stomatal opening and closing. H2O K H2O H2O H2O H2O H2O (b) Role of potassium in stomatal opening and closing

Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed Figure 36.15a Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed Radially oriented cellulose microfibrils Cell wall Figure 36.15 Mechanisms of stomatal opening and closing. Vacuole Guard cell (a) Changes in guard cell shape and stomatal opening and closing (surface view)

This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells © 2011 Pearson Education, Inc.

Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed Figure 36.15b Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed H2O H2O H2O H2O H2O K H2O H2O H2O Figure 36.15 Mechanisms of stomatal opening and closing. H2O H2O (b) Role of potassium in stomatal opening and closing

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 Light CO2 depletion An internal “clock” in guard cells All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles © 2011 Pearson Education, Inc.

Drought, high temperature, and wind can cause stomata to close during the daytime The hormone abscisic acid is produced in response to water deficiency and causes the closure of stomata © 2011 Pearson Education, Inc.

Effects of Transpiration on Wilting and Leaf Temperature Plants lose a large amount of water by transpiration If the lost water is not replaced by sufficient transport of water, the plant will lose water and wilt Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes © 2011 Pearson Education, Inc.

Adaptations That Reduce Evaporative Water Loss Xerophytes are plants adapted to arid climates © 2011 Pearson Education, Inc.

Oleander leaf cross section Figure 36.16 Ocotillo (leafless) Oleander leaf cross section Cuticle Upper epidermal tissue Ocotillo after heavy rain Oleander flowers 100 m Trichomes (“hairs”) Crypt Stoma Lower epidermal tissue Figure 36.16 Some xerophytic adaptations. Ocotillo leaves Old man cactus

Ocotillo (leafless) Figure 36.16a Figure 36.16 Some xerophytic adaptations.

Ocotillo after heavy rain Figure 36.16b Ocotillo after heavy rain Figure 36.16 Some xerophytic adaptations.

Ocotillo leaves Figure 36.16c Figure 36.16 Some xerophytic adaptations. Ocotillo leaves

Oleander leaf cross section Figure 36.16d Oleander leaf cross section Cuticle Upper epidermal tissue Figure 36.16 Some xerophytic adaptations. 100 m Trichomes (“hairs”) Crypt Stoma Lower epidermal tissue

Oleander flowers Figure 36.16e Figure 36.16 Some xerophytic adaptations.

Figure 36.16f Figure 36.16 Some xerophytic adaptations. Old man cactus

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 © 2011 Pearson Education, Inc.

Concept 36.5: Sugars are transported from sources to sinks via the phloem The products of photosynthesis are transported through phloem by the process of translocation © 2011 Pearson Education, Inc.

Movement from Sugar Sources to Sugar Sinks 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 © 2011 Pearson Education, Inc.

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 exposed 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 © 2011 Pearson Education, Inc.

Companion (transfer) cell High H concentration Cotransporter Figure 36.17 Key Apoplast Symplast Companion (transfer) cell High H concentration Cotransporter Mesophyll cell Proton pump H Cell walls (apoplast) Sieve-tube element S Plasma membrane Plasmodesmata Figure 36.17 Loading of sucrose into phloem. ATP Sucrose H H S Bundle- sheath cell Phloem parenchyma cell Mesophyll cell Low H concentration (a) (b)

Companion (transfer) cell Mesophyll cell Figure 36.17a Key Apoplast Symplast Companion (transfer) cell Mesophyll cell Cell walls (apoplast) Sieve-tube element Plasma membrane Plasmodesmata Figure 36.17 Loading of sucrose into phloem. Bundle- sheath cell Phloem parenchyma cell Mesophyll cell (a)

High H concentration Cotransporter Proton pump H S ATP Sucrose H H Figure 36.17b High H concentration Cotransporter Proton pump H S Figure 36.17 Loading of sucrose into phloem. ATP Sucrose H H S Low H concentration (b)

In many 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 © 2011 Pearson Education, Inc.

Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms Phloem sap moves through a sieve tube by bulk flow driven by positive pressure called pressure flow Animation: Translocation of Phloem Sap in Summer Animation: Translocation of Phloem Sap in Spring © 2011 Pearson Education, Inc.

1 Loading of sugar 2 Uptake of water 3 Unloading of sugar 4 Figure 36.18 Sieve tube (phloem) Source cell (leaf) Vessel (xylem) 1 Loading of sugar H2O 1 Sucrose H2O 2 2 Uptake of water Bulk flow by positive pressure Bulk flow by negative pressure 3 Unloading of sugar Figure 36.18 Bulk flow by positive pressure (pressure flow) in a sieve tube. Sink cell (storage root) 4 Water recycled 4 3 Sucrose H2O

The pressure flow hypothesis explains why phloem sap always flows from source to sink Experiments have built a strong case for pressure flow as the mechanism of translocation in angiosperms Self-thinning is the dropping of sugar sinks such as flowers, seeds, or fruits © 2011 Pearson Education, Inc.

Stylet in sieve-tube element Separated stylet exuding sap Figure 36.19 EXPERIMENT 25 m Sieve- tube element Sap droplet Figure 36.19 Inquiry: Does phloem sap contain more sugar near sources than sinks? Stylet Sap droplet Aphid feeding Stylet in sieve-tube element Separated stylet exuding sap

Sap droplet Aphid feeding Figure 36.19a Figure 36.19 Inquiry: Does phloem sap contain more sugar near sources than sinks? Aphid feeding

Stylet in sieve-tube element Figure 36.19b 25 m Sieve- tube element Stylet Figure 36.19 Inquiry: Does phloem sap contain more sugar near sources than sinks? Stylet in sieve-tube element

Separated stylet exuding sap Figure 36.19c Sap droplet Figure 36.19 Inquiry: Does phloem sap contain more sugar near sources than sinks? Separated stylet exuding sap

Concept 36.6: The symplast is highly dynamic The symplast is a living tissue and is responsible for dynamic changes in plant transport processes © 2011 Pearson Education, Inc.

Changes in Plasmodesmata Plasmodesmata can change in permeability in response to turgor pressure, cytoplasmic calcium levels, or cytoplasmic pH Plant viruses can cause plasmodesmata to dilate so viral RNA can pass between cells © 2011 Pearson Education, Inc.

Plasmodesma Virus particles Cell wall 100 nm Figure 36.20 Figure 36.20 Virus particles moving cell to cell through a plasmodesma connecting turnip leaf cells (TEM).

Phloem: An Information Superhighway Phloem is a “superhighway” for systemic transport of macromolecules and viruses Systemic communication helps integrate functions of the whole plant © 2011 Pearson Education, Inc.

Electrical Signaling in the Phloem The phloem allows for rapid electrical communication between widely separated organs For example, rapid leaf movements in the sensitive plant (Mimosa pudica) © 2011 Pearson Education, Inc.

Figure 36.UN01 Turgid Figure 36.UN01 In-text figure, p. 770

Figure 36.UN02 Wilted Figure 36.UN02 In-text figure, p. 770

H2O CO2 O2 O2 CO2 Minerals H2O Figure 36.UN03 Figure 36.UN03 Summary figure, Concept 36.1 O2 CO2 Minerals H2O

Figure 36.UN04 Figure 36.UN04 Test Your Understanding, question 10

Figure 36.UN05 Figure 36.UN05 Appendix A: answer to Test Your Understanding, question 10