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