35 plantstructure text

Plant Structure, Growth, and Development The plant body has a hierarchy of organs, tissues, and cells Plants, like multicellular animals Have organs composed of different tissues, which are in turn composed of cells

The Three Basic Plant Organs: Roots, Stems, and Leaves Three basic organs of plants: roots, stems, and leaves They are organized into a root system and a shoot system Figure 35.2 Reproductive shoot (flower) Terminal bud Node Internode Terminal bud Vegetative shoot Blade Petiole Stem Leaf Taproot Lateral roots Root system Shoot system Axillary bud

Roots A root Is an organ that anchors the vascular plant Absorbs minerals and water Often stores organic nutrients In most plants The absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area of the root

Many plants have modified roots Figure 35.4a–e (a) Prop roots (b) Storage roots (c) “Strangling” aerial roots (d) Buttress roots (e) Pneumatophores

Stems A stem is an organ consisting of An alternating system of nodes, the points at which leaves are attached Internodes, the stem segments between nodes An axillary bud Is a structure that has the potential to form a lateral shoot, or branch A terminal bud Is located near the shoot tip and causes elongation of a young shoot

Many plants have modified stems Figure 35.5a–d Rhizomes. The edible base of this ginger plant is an example of a rhizome, a horizontal stem that grows just below the surface or emerges and grows along the surface. (d) Tubers. Tubers, such as these red potatoes, are enlarged ends of rhizomes specialized for storing food. The “eyes” arranged in a spiral pattern around a potato are clusters of axillary buds that mark the nodes. (c) Bulbs. Bulbs are vertical, underground shoots consisting mostly of the enlarged bases of leaves that store food. You can see the many layers of modified leaves attached to the short stem by slicing an onion bulb lengthwise. (b) Stolons. Shown here on a strawberry plant, stolons are horizontal stems that grow along the surface. These “runners” enable a plant to reproduce asexually, as plantlets form at nodes along each runner. (a) Storage leaves Stem Root Node Rhizome Root

Leaves The leaf Is the main photosynthetic organ of most vascular plants Leaves generally consist of A flattened blade and a stalk The petiole, which joins the leaf to a node of the stem

Monocots and dicots Differ in the arrangement of veins, the vascular tissue of leaves Most monocots Have parallel veins Most dicots Have branching veins

In classifying angiosperms Taxonomists may use leaf morphology as a criterion Figure 35.6a–c Petiole (a) Simple leaf. A simple leaf is a single, undivided blade. Some simple leaves are deeply lobed, as in an oak leaf. (b) Compound leaf. In a compound leaf, the blade consists of multiple leaflets. Notice that a leaflet has no axillary bud at its base. (c) Doubly compound leaf. In a doubly compound leaf, each leaflet is divided into smaller leaflets. Axillary bud Leaflet Petiole Axillary bud Axillary bud Leaflet Petiole

Some plant species Have modified leaves that serve various functions Figure 35.6a–e (a) Tendrils. The tendrils by which this pea plant clings to a support are modified leaves. After it has “lassoed” a support, a tendril forms a coil that brings the plant closer to the support. Tendrils are typically modified leaves, but some tendrils are modified stems, as in grapevines. (b) Spines. The spines of cacti, such as this prickly pear, are actually leaves, and photosynthesis is carried out mainly by the fleshy green stems. (c) Storage leaves. Most succulents, such as this ice plant, have leaves modified for storing water. (d) Bracts. Red parts of the poinsettia are often mistaken for petals but are actually modified leaves called bracts that surround a group of flowers. Such brightly colored leaves attract pollinators. (e) Reproductive leaves. The leaves of some succulents, such as Kalanchoe daigremontiana, produce adventitious plantlets, which fall off the leaf and take root in the soil.

The Three Tissue Systems: Dermal, Vascular, and Ground Each plant organ Has dermal, vascular, and ground tissues Figure 35.8 Dermal tissue Ground tissue Vascular tissue

The dermal tissue system Consists of the epidermis and periderm The vascular tissue system Carries out long-distance transport of materials between roots and shoots Consists of two tissues, xylem and phloem Ground tissue Includes various cells specialized for functions such as storage, photosynthesis, and support

Xylem Conveys water and dissolved minerals upward from roots into the shoots Phloem Transports organic nutrients from where they are made to where they are needed

Some of the major types of plant cells include Parenchyma –metabolic function (photosynthesis). Most abundant type. Collenchyma – grouped in cylinders and support growing part of the plant. Sclerenchyma – found in areas where there is no active growth. Have tough cell walls. Supportive cells. Water-conducting cells of the xylem (Tracheids and Vessels) Sugar-conducting cells of the phloem (Sieve-tube elements and companion cells)

Parenchyma, collenchyma, and sclerenchyma cells Figure 35.9 Parenchyma cells 60 m PARENCHYMA CELLS 80 m Cortical parenchyma cells COLLENCHYMA CELLS Collenchyma cells SCLERENCHYMA CELLS Cell wall Sclereid cells in pear 25 m Fiber cells 5 m

Water-conducting cells of the xylem and sugar-conducting cells of the phloem Figure. 35.9 WATER-CONDUCTING CELLS OF THE XYLEM Vessel Tracheids 100 m Tracheids and vessels Vessel element Vessel elements with partially perforated end walls Pits Tracheids SUGAR-CONDUCTING CELLS OF THE PHLOEM Companion cell Sieve-tube member Sieve-tube members: longitudinal view Sieve plate Nucleus Cytoplasm Companion cell 30 m 15 m

Meristems generate cells for new organs Apical meristems Are located at the tips of roots and in the buds of shoots Elongate shoots and roots through primary growth Lateral meristems Add thickness to woody plants through secondary growth

An overview of primary and secondary growth Figure. 35.10 In woody plants, there are lateral meristems that add secondary growth, increasing the girth of roots and stems. Apical meristems add primary growth, or growth in length. Vascular cambium Cork cambium Lateral meristems Root apical meristems Primary growth in stems Epidermis Cortex Primary phloem Primary xylem Pith Secondary growth in stems Periderm Cork cambium Cortex Primary phloem Secondary phloem Vascular cambium Secondary xylem Primary xylem Pith Shoot apical meristems (in buds) The cork cambium adds secondary dermal tissue. The vascular cambium adds secondary xylem and phloem.

In woody plants Primary and secondary growth occur simultaneously but in different locations Figure 35.11 This year’s growth (one year old) Last year’s growth (two years old) Growth of two years ago (three years old) One-year-old side branch formed from axillary bud near shoot apex Scars left by terminal bud scales of previous winters Leaf scar Leaf scar Stem Leaf scar Bud scale Axillary buds Internode Node Terminal bud

Primary growth lengthens roots and shoots Primary growth produces the primary plant body, the parts of the root and shoot systems produced by apical meristems The root tip is covered by a root cap, which protects the delicate apical meristem as the root pushes through soil during primary growth

Primary Growth of Roots Figure 35.12 Dermal Ground Vascular Key Cortex Vascular cylinder Epidermis Root hair Zone of maturation Zone of elongation Zone of cell division Apical meristem Root cap 100 m

The primary growth of roots Produces the epidermis, ground tissue, and vascular tissue

Organization of primary tissues in young roots Figure 35.13a, b Cortex Vascular cylinder Endodermis Pericycle Core of parenchyma cells Xylem 50 m Endodermis Pericycle Xylem Phloem Key 100 m Vascular Ground Dermal Phloem Transverse section of a root with parenchyma in the center. The stele of many monocot roots is a vascular cylinder with a core of parenchyma surrounded by a ring of alternating xylem and phloem. (b) Transverse section of a typical root. In the roots of typical gymnosperms and eudicots, as well as some monocots, the stele is a vascular cylinder consisting of a lobed core of xylem with phloem between the lobes. (a) 100 m Epidermis

Lateral roots Arise from within the pericycle, the outermost cell layer in the vascular cylinder Figure 35.14 Cortex Vascular cylinder Epidermis Lateral root 100 m 1 2 3 4 Emerging lateral root

Primary Growth of Shoots A shoot apical meristem Is a dome-shaped mass of dividing cells at the tip of the terminal bud Gives rise to a repetition of internodes and leaf-bearing nodes Figure. 35.15 Apical meristem Leaf primordia Developing vascular strand Axillary bud meristems 0.25 mm

Tissue Organization of Stems In gymnosperms and most eudicots The vascular tissue consists of vascular bundles arranged in a ring Figure 35.16a Xylem Phloem Sclerenchyma (fiber cells) Ground tissue connecting pith to cortex Pith Epidermis Vascular bundle Cortex Key Dermal Ground Vascular 1 mm (a) A eudicot stem. A eudicot stem (sunflower), with vascular bundles forming a ring. Ground tissue toward the inside is called pith, and ground tissue toward the outside is called cortex. (LM of transverse section)

In most monocot stems The vascular bundles are scattered throughout the ground tissue, rather than forming a ring Ground tissue Epidermis Vascular bundles 1 mm (b) A monocot stem. A monocot stem (maize) with vascular bundles scattered throughout the ground tissue. In such an arrangement, ground tissue is not partitioned into pith and cortex. (LM of transverse section) Figure 35.16b

Tissue Organization of Leaves The epidermal barrier in leaves Is interrupted by stomata, which allow CO 2 exchange between the surrounding air and the photosynthetic cells within a leaf The ground tissue in a leaf Is sandwiched between the upper and lower epidermis The vascular tissue of each leaf Is continuous with the vascular tissue of the stem

Leaf anatomy Key to labels Dermal Ground Vascular Guard cells Stomatal pore Epidermal cell 50 µm Surface view of a spiderwort ( Tradescantia ) leaf (LM) (b) Cuticle Sclerenchyma fibers Stoma Upper epidermis Palisade mesophyll Spongy mesophyll Lower epidermis Cuticle Vein Guard cells Xylem Phloem Guard cells Bundle- sheath cell Cutaway drawing of leaf tissues (a) Vein Air spaces Guard cells 100 µm Transverse section of a lilac ( Syringa ) leaf (LM) (c) Figure 35.17a–c

Secondary growth adds girth to stems and roots in woody plants Secondary growth Occurs in stems and roots of woody plants but rarely in leaves The secondary plant body Consists of the tissues produced by the vascular cambium and cork cambium

The Vascular Cambium and Secondary Vascular Tissue The vascular cambium Is a cylinder of meristematic cells one cell thick Develops from parenchyma cells Produces secondary xylem

Primary and secondary growth of a stem Figure 35.18a Vascular cambium Pith Primary xylem Secondary xylem Vascular cambium Secondary phloem Primary phloem Periderm (mainly cork cambia and cork) Pith Primary xylem Vascular cambium Primary phloem Cortex Epidermis Vascular cambium 4 First cork cambium Secondary xylem (two years of production) Pith Primary xylem Vascular cambium Primary phloem 2 1 6 Growth Primary xylem Secondary xylem Secondary phloem Primary phloem Cork Phloem ray 3 Xylem ray Growth Bark 8 Layers of periderm 7 Cork 5 Most recent cork cambium Cortex Epidermis 9 In the youngest part of the stem, you can see the primary plant body, as formed by the apical meristem during primary growth. The vascular cambium is beginning to develop. As primary growth continues to elongate the stem, the portion of the stem formed earlier the same year has already started its secondary growth. This portion increases in girth as fusiform initials of the vascular cambium form secondary xylem to the inside and secondary phloem to the outside. The ray initials of the vascular cambium give rise to the xylem and phloem rays. As the diameter of the vascular cambium increases, the secondary phloem and other tissues external to the cambium cannot keep pace with the expansion because the cells no longer divide. As a result, these tissues, including the epidermis, rupture. A second lateral meristem, the cork cambium, develops from parenchyma cells in the cortex. The cork cambium produces cork cells, which replace the epidermis. In year 2 of secondary growth, the vascular cambium adds to the secondary xylem and phloem, and the cork cambium produces cork. As the diameter of the stem continues to increase, the outermost tissues exterior to the cork cambium rupture and slough off from the stem. Cork cambium re-forms in progressively deeper layers of the cortex. When none of the original cortex is left, the cork cambium develops from parenchyma cells in the secondary phloem. Each cork cambium and the tissues it produces form a layer of periderm. Bark consists of all tissues exterior to the vascular cambium. 1 2 3 4 5 6 7 8 9 Secondary phloem (a) Primary and secondary growth in a two-year-old stem

Secondary phloem Vascular cambium Late wood Early wood Secondary xylem Cork cambium Cork Periderm (b) Transverse section of a three-year- old stem (LM) Xylem ray Bark 0.5 mm 0.5 mm Figure 35.18b

As a tree or woody shrub ages The older layers of secondary xylem, the heartwood, no longer transport water and minerals The outer layers, known as sapwood Still transport materials through the xylem

Figure 35.20 Growth ring Vascular ray Heartwood Sapwood Vascular cambium Secondary phloem Layers of periderm Secondary xylem Bark

Cork Cambia and the Production of Periderm The cork cambium Gives rise to the secondary plant body’s protective covering, or periderm Periderm Consists of the cork cambium plus the layers of cork cells it produces Bark Consists of all the tissues external to the vascular cambium, including secondary phloem and periderm

Morphogenesis and Pattern Formation Pattern formation Is the development of specific structures in specific locations Is determined by positional information in the form of signals that indicate to each cell its location

Polarity Is one type of positional information In the gnom mutant of Arabidopsis The establishment of polarity is defective Figure 35.26

Morphogenesis in plants, as in other multicellular organisms Is often under the control of homeotic genes Figure 35.27

Gene Expression and Control of Cellular Differentiation In cellular differentiation Cells of a developing organism synthesize different proteins and diverge in structure and function even though they have a common genome

Cellular differentiation To a large extent depends on positional information Is affected by homeotic genes Figure 35.28 When epidermal cells border a single cortical cell, the homeotic gene GLABRA-2 is selectively expressed, and these cells will remain hairless. (The blue color in this light micrograph indi- cates cells in which GLABRA-2 is expressed.) Here an epidermal cell borders two cortical cells. GLABRA-2 is not expressed, and the cell will develop a root hair. The ring of cells external to the epidermal layer is composed of root cap cells that will be sloughed off as the root hairs start to differentiate. Cortical cells 20 µm

Location and a Cell’s Developmental Fate A cell’s position in a developing organ Determines its pathway of differentiation Plants pass through developmental phases, called phase changes Developing from a juvenile phase to an adult vegetative phase to an adult reproductive phase

The most obvious morphological changes Typically occur in leaf size and shape Leaves produced by adult phase of apical meristem Leaves produced by juvenile phase of apical meristem Figure 35.29

Genetic Control of Flowering Flower formation Involves a phase change from vegetative growth to reproductive growth Is triggered by a combination of environmental cues and internal signals The transition from vegetative growth to flowering Is associated with the switching-on of floral meristem identity genes

Plant biologists have identified several organ identity genes That regulate the development of floral pattern Figure 35.30a, b (a) Normal Arabidopsis flower. Arabidopsis normally has four whorls of flower parts: sepals (Se), petals (Pe), stamens (St), and carpels (Ca). (b) Abnormal Arabidopsis flower. Reseachers have identified several mutations of organ identity genes that cause abnormal flowers to develop. This flower has an extra set of petals in place of stamens and an internal flower where normal plants have carpels. Ca St Pe Se Pe Pe Se Pe Se

Transport in Vascular Plants Physical forces drive the transport of materials in plants over a range of distances Transport in vascular plants occurs on three scales Transport of water and solutes by individual cells , such as root hairs Short-distance transport of substances from cell to cell at the levels of tissues and organs Long-distance transport within xylem and phloem at the level of the whole plant

A variety of physical processes Are involved in the different types of transport Minerals H 2 O CO 2 O 2 CO 2 O 2 H 2 O Sugar Light Sugars are transported as phloem sap to roots and other parts of the plant. Figure 36.2 Sugars are produced by photosynthesis in the leaves. 5 6 Through stomata, leaves take in CO 2 and expel O 2 . The CO 2 provides carbon for photosynthesis. Some O 2 produced by photosynthesis is used in cellular respiration. 4 Transpiration, the loss of water from leaves (mostly through stomata), creates a force within leaves that pulls xylem sap upward. 3 Water and minerals are transported upward from roots to shoots as xylem sap. 2 Roots absorb water and dissolved minerals from the soil. 1 Roots exchange gases with the air spaces of soil, taking in O 2 and discharging CO 2 . In cellular respiration, O 2 supports the breakdown of sugars. 7

Selective Permeability of Membranes: A Review The selective permeability of a plant cell’s plasma membrane Controls the movement of solutes into and out of the cell Specific transport proteins Enable plant cells to maintain an internal environment different from their surroundings

The Central Role of Proton Pumps Proton pumps in plant cells Create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work Contribute to a voltage known as a membrane potential Figure 36.3 CYTOPLASM EXTRACELLULAR FLUID ATP H + H + H + H + H + H + H + H + Proton pump generates membrane potential and H + gradient. – – – – – + + + + +

Plant cells use energy stored in the proton gradient and membrane potential To drive the transport of many different solutes Figure 36.4a + CYTOPLASM EXTRACELLULAR FLUID Cations ( , for example) are driven into the cell by the membrane potential. Transport protein K + K + K + K + K + K + K + K + – – – + + (a) Membrane potential and cation uptake – – + +

In the mechanism called cotransport A transport protein couples the passage of one solute to the passage of another Figure 36.4b H + H + H + H + H + H + H + H + H + H + H + H + NO 3 – NO 3 – NO 3 – NO 3 – NO 3 – NO 3 – – – – + + + – – – + + + NO 3 – (b) Cotransport of anions H + of through a cotransporter. Cell accumulates anions ( , for example) by coupling their transport to the inward diffusion

The “coattail” effect of cotransport Is also responsible for the uptake of the sugar sucrose by plant cells H + H + H + H + H + H + H + H + H + H + S S S S S Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. S H + – – – + + + – – + + – Figure 36.4c H + H + S + – (c) Contransport of a neutral solute

Effects of Differences in Water Potential To survive Plants must balance water uptake and loss Osmosis Determines the net uptake or water loss by a cell Is affected by solute concentration and pressure

Water potential Is a measurement that combines the effects of solute concentration and pressure Determines the direction of movement of water Water Flows from regions of high water potential to regions of low water potential Both pressure and solute concentration Affect water potential

The solute potential of a solution Is proportional to the number of dissolved molecules Pressure potential Is the physical pressure on a solution

Quantitative Analysis of Water Potential The addition of solutes Reduces water potential Figure 36.5a 0.1 M solution H 2 O Pure water  P = 0  S = 0.23  = 0.23 MPa  = 0 MPa (a)

Application of physical pressure Increases water potential Figure 36.5b, c H 2 O  P = 0.23  S = 0.23  = 0 MPa  = 0 MPa (b) H 2 O  P = 0.30  S = 0.23  = 0.07 MPa  = 0 MPa (c)

Negative pressure Decreases water potential Figure 36.5d H 2 O  P = 0  S = 0.23  = 0.23 MPa (d)  P = 0.30  S = 0  = 0.30 MPa

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 become plasmolyzed Figure 36.6a 0.4 M sucrose solution: Initial flaccid cell: Plasmolyzed cell at osmotic equilibrium with its surroundings  P = 0  S = 0.7  P = 0  S = 0.9  P = 0  S = 0.9  =  0.9 MPa  = 0.7 MPa  =  0.9 MPa

If the same flaccid cell is placed in a solution with a lower solute concentration The cell will gain water and become turgid Distilled water: Initial flaccid cell: Turgid cell at osmotic equilibrium with its surroundings  P = 0  S = 0.7  P = 0  S = 0  P = 0.7  S = 0.7 Figure 36.6b  = 0.7 MPa  = 0 MPa  = 0 MPa

Turgor loss in plants causes wilting Which can be reversed when the plant is watered Figure 36.7

Aquaporin Proteins and Water Transport Aquaporins Are transport proteins in the cell membrane that allow the passage of water Do not affect water potential

Bulk Flow in Long-Distance Transport In bulk flow Movement of fluid in the xylem and phloem is driven by pressure differences at opposite ends of the xylem vessels and sieve tubes Roots absorb water and minerals from the soil Water and mineral salts from the soil Enter the plant through the epidermis of roots and ultimately flow to the shoot system

Lateral transport of minerals and water in roots Figure 36.9 1 2 3 Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls. Minerals and water that cross the plasma membranes of root hairs enter the symplast. As soil solution moves along the apoplast, some water and minerals are transported into the protoplasts of cells of the epidermis and cortex and then move inward via the symplast. Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks the passage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder. Endodermal cells and also parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water and minerals upward into the shoot system. Casparian strip Pathway along apoplast Pathway through symplast Plasma membrane Apoplastic route Symplastic route Root hair Epidermis Cortex Endodermis Vascular cylinder Vessels (xylem) Casparian strip Endodermal cell 4 5 2 1

The Roles of Root Hairs, Mycorrhizae, and Cortical Cells Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are located Root hairs account for much of the surface area of roots

Most plants form mutually beneficial relationships with fungi, which facilitate the absorption of water and minerals from the soil Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hyphae Figure 36.10 2.5 mm

Once soil solution enters the roots The extensive surface area of cortical cell membranes enhances uptake of water and selected minerals

The Endodermis: A Selective Sentry The endodermis Is the innermost layer of cells in the root cortex Surrounds the vascular cylinder and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissue

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 ascend from roots to shoots through the xylem Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant The transpired water must be replaced by water transported up from the roots

Factors Affecting the Ascent of Xylem Sap Xylem sap Rises to heights of more than 100 m in the tallest plants At night, when transpiration is very low 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

Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism Water is pulled upward by negative pressure in the xylem Water vapor in the airspaces of a leaf Diffuses down its water potential gradient and exits the leaf via stomata

Transpiration produces negative pressure (tension) in the leaf Which exerts a pulling force on water in the xylem, pulling water into the leaf Evaporation causes the air-water interface to retreat farther into the cell wall and become more curved as the rate of transpiration increases. As the interface becomes more curved, the water film’s pressure becomes more negative. This negative pressure, or tension, pulls water from the xylem, where the pressure is greater. Cuticle Upper epidermis Mesophyll Lower epidermis Cuticle Water vapor CO 2 O 2 Xylem CO 2 O 2 Water vapor Stoma Evaporation At first, the water vapor lost by transpiration is replaced by evaporation from the water film that coats mesophyll cells. In transpiration, water vapor (shown as blue dots) diffuses from the moist air spaces of the leaf to the drier air outside via stomata. Airspace Cytoplasm Cell wall Vacuole Evaporation Water film Low rate of transpiration High rate of transpiration Air-water interface Cell wall Airspace  = –0.15 MPa  = –10.00 MPa 3 1 2 Figure 36.12 Air- space

Cohesion and Adhesion in the Ascent of Xylem Sap The transpirational pull on xylem sap Is transmitted all the way from the leaves to the root tips and even into the soil solution Is facilitated by cohesion and adhesion

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

Xylem Sap Ascent by Bulk Flow: A Review The movement of xylem sap against gravity Is maintained by the transpiration-cohesion-tension mechanism

Stomata help regulate the rate of transpiration Leaves generally have broad surface areas And high surface-to-volume ratios

Both of these characteristics Increase photosynthesis Increase water loss through stomata 20 µm Figure 36.14

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 absorption through the roots The plant will lose water and wilt

Transpiration also results in evaporative cooling Which can lower the temperature of a leaf and prevent the denaturation of various enzymes involved in photosynthesis and other metabolic processes About 90% of the water a plant loses Escapes through stomata

Each stoma is flanked by guard cells Which control the diameter of the stoma by changing shape Cells flaccid/Stoma closed Cells turgid/Stoma open Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Changes in guard cell shape and stomatal opening and closing (surface view). Guard cells of a typical angiosperm are illustrated in their turgid (stoma open) and flaccid (stoma closed) states. The pair of guard cells buckle outward when turgid. Cellulose microfibrils in the walls resist stretching and compression in the direction parallel to the microfibrils. Thus, the radial orientation of the microfibrils causes the cells to increase in length more than width when turgor increases. The two guard cells are attached at their tips, so the increase in length causes buckling. (a) Figure 36.15a

Changes in turgor pressure that open and close stomata Result primarily from the reversible uptake and loss of potassium ions by the guard cells H 2 O H 2 O H 2 O H 2 O H 2 O K + Role of potassium in stomatal opening and closing. The transport of K + (potassium ions, symbolized here as red dots) across the plasma membrane and vacuolar membrane causes the turgor changes of guard cells. (b) H 2 O H 2 O H 2 O H 2 O H 2 O Figure 36.15b

Xerophyte Adaptations That Reduce Transpiration Xerophytes Are plants adapted to arid climates Have various leaf modifications that reduce the rate of transpiration

The stomata of xerophytes Are concentrated on the lower leaf surface Are often located in depressions that shelter the pores from the dry wind Lower epidermal tissue Trichomes (“hairs”) Cuticle Upper epidermal tissue Stomata 100  m Figure 36.16

Organic nutrients are translocated through the phloem Translocation Is the transport of organic nutrients in the plant

Movement from Sugar Sources to Sugar Sinks Phloem sap Is an aqueous solution that is mostly sucrose Travels from a sugar source to a sugar sink

A sugar source Is a plant 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

Sugar must be loaded into sieve-tube members before being exposed to sinks In many plant species, sugar moves by symplastic and apoplastic pathways Figure 36.17a Mesophyll cell Cell walls (apoplast) Plasma membrane Plasmodesmata Companion (transfer) cell Sieve-tube member Mesophyll cell Phloem parenchyma cell Bundle- sheath cell Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube members. In some species, sucrose exits the symplast (red arrow) near sieve tubes and is actively accumulated from the apoplast by sieve-tube members and their companion cells. (a)

In many plants Phloem loading requires active transport Proton pumping and cotransport of sucrose and H + Enable the cells to accumulate sucrose A chemiosmotic mechanism is responsible for the active transport of sucrose into companion cells and sieve-tube members. Proton pumps generate an H + gradient, which drives sucrose accumulation with the help of a cotransport protein that couples sucrose transport to the diffusion of H + back into the cell. (b) High H + concentration Cotransporter Proton pump ATP Key Sucrose Apoplast Symplast H + H + Low H + concentration H + S S Figure 36.17b

Pressure Flow: The Mechanism of Translocation in Angiosperms In studying angiosperms Researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure Vessel (xylem) H 2 O H 2 O Sieve tube (phloem) Source cell (leaf) Sucrose H 2 O Sink cell (storage root) 1 Sucrose Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sieve-tube members. This causes the tube to take up water by osmosis. 2 4 3 1 2 This uptake of water generates a positive pressure that forces the sap to flow along the tube. The pressure is relieved by the unloading of sugar and the consequent loss of water from the tube at the sink. 3 4 In the case of leaf-to-root translocation, xylem recycles water from sink to source. Transpiration stream Pressure flow Figure 36.18

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 Aphid feeding Stylet in sieve-tube member Severed stylet exuding sap Sieve- Tube member EXPERIMENT RESULTS CONCLUSION Sap droplet Stylet Sap droplet 25 m Sieve- tube member To test the pressure flow hypothesis,researchers used aphids that feed on phloem sap. An aphid probes with a hypodermic- like mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different points between a source and sink. The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration. The results of such experiments support the pressure flow hypothesis. Figure 36.19

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