Resource Acquisition, Nutrition, and Transport in Vascular Plants 29 Resource Acquisition, Nutrition, and Transport in Vascular Plants

Important Topics Roots: mutualistic adaptations with bacteria and their role in the nitrogen cycle Stems: xylem and phloem collectively make up vascular plants Leaves: stomata/guard cells Transpiration: adhesion/cohesion and its role in the transport of water from roots all the way to the leaves Special plant adaptations for different circumstances

The evolution of plants requires key adaptations What might be the advantage of this plant structure? What might the disadvantages be? © 2014 Pearson Education, Inc. 3

Early nonvascular land plants lived in shallow water ancestors of land plants absorbed water, minerals, and CO2 directly from surrounding water Early nonvascular land plants lived in shallow water © 2014 Pearson Education, Inc. 4

Xylem transports water and minerals from roots to shoots Phloem transports photosynthetic products What benefit do xylem and phloem provide in the evolution of plants? © 2014 Pearson Education, Inc. 5

H2O H2O and minerals Figure 29.2-1 Figure 29.2-1 An overview of resource acquisition and transport in a vascular plant (step 1) H2O and minerals 6

O2 CO2 H2O O2 H2O and minerals CO2 Figure 29.2-2 Figure 29.2-2 An overview of resource acquisition and transport in a vascular plant (step 2) O2 H2O and minerals CO2 7

O2 CO2 Light Sugar H2O O2 H2O and minerals CO2 Figure 29.2-3 Figure 29.2-3 An overview of resource acquisition and transport in a vascular plant (step 3) O2 H2O and minerals CO2 8

Roots What is the purpose of roots in a plant? Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae Mycorrhizal fungi increase the surface area for absorbing water and minerals Mutualisms with fungi helped plants colonize land © 2014 Pearson Education, Inc. 9

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 © 2014 Pearson Education, Inc. 10

(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 29.11 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 11

(dead organic material) SOIL Figure 29.11a N2 ATMOSPHERE N2 Nitrate and nitrogenous organic compounds exported in xylem to shoot system Proteins from humus (dead organic material) SOIL Nitrogen-fixing bacteria Microbial decomposition Amino acids NH3 (ammonia) Denitrifying bacteria Ammonifying bacteria NH4+ H+ (from soil) NH4+ (ammonium) NO2− (nitrite) NO3− (nitrate) Figure 29.11a The roles of soil bacteria in the nitrogen nutrition of plants (part 1) Nitrifying bacteria Nitrifying bacteria Root 12

(dead organic material) SOIL Figure 29.11b N2 ATMOSPHERE Proteins from humus (dead organic material) SOIL Nitrogen-fixing bacteria Microbial decomposition Amino acids NH3 (ammonia) Ammonifying bacteria H+ (from soil) Figure 29.11b The roles of soil bacteria in the nitrogen nutrition of plants (part 2) NH4+ (ammonium) NO2− (nitrite) Nitrifying bacteria 13

N2 Nitrate and nitrogenous organic compounds exported in xylem to Figure 29.11c N2 Nitrate and nitrogenous organic compounds exported in xylem to shoot system Denitrifying bacteria NH4+ Figure 29.11c The roles of soil bacteria in the nitrogen nutrition of plants (part 3) NO2− (nitrite) NO3− (nitrate) Nitrifying bacteria Root 14

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 © 2014 Pearson Education, Inc.

Nitrogen is lost to the atmosphere when denitrifying bacteria convert NO3– to N2 © 2014 Pearson Education, Inc.

Symbiotic relationships with nitrogen-fixing Rhizobium bacteria provide some legumes with a source of fixed nitrogen Along a legume’s roots are swellings called nodules, composed of plant cells “infected” by nitrogen-fixing Rhizobium bacteria © 2014 Pearson Education, Inc. 17

Figure 29.12 Nodules Figure 29.12 Soybean root nodules Roots 18

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 © 2014 Pearson Education, Inc. 19

Fungi and Plant Nutrition Mycorrhizae are mutualistic associations of fungi and roots The fungus benefits from a steady supply of sugar from the host plant The host plant benefits because the fungus increases the surface area for water uptake and mineral absorption Mycorrhizal fungi also secrete growth factors that stimulate root growth and branching © 2014 Pearson Education, Inc. 20

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 Video: Sundew Traps Prey © 2014 Pearson Education, Inc. 21

Epiphytes grow on other plants and obtain water and minerals from rain, rather than tapping their hosts for sustenance © 2014 Pearson Education, Inc.

Some species parasitize the mycorrhizal hyphae of other plants 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 © 2014 Pearson Education, Inc. 23

Mistletoe, a photosynthetic parasite Dodder, 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 24

Carnivorous plants are photosynthetic but obtain nitrogen by killing and digesting mostly insects © 2014 Pearson Education, Inc.

Carnivorous plants Sundew Pitcher plants Venus flytraps Figure 29.15c Figure 29.15c Exploring unusual nutritional adaptations in plants (part 3: carnivorous plants) 26

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 © 2014 Pearson Education, Inc. 27

The endodermis: controlled entry to the vascular cylinder (stele) 5 Figure 29.16a Vessels (xylem) Casparian strip 1 Apoplastic route Plasma membrane Apoplastic route 1 2 Symplastic route 3 2 4 5 Figure 29.16a Transport of water and minerals from root hairs to the xylem (part 1) Symplastic route Root hair 3 Transmembrane route Epidermis Endodermis Vascular cylinder (stele) Cortex 4 The endodermis: controlled entry to the vascular cylinder (stele) 5 Transport in the xylem 28

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 © 2014 Pearson Education, Inc. 29

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 © 2014 Pearson Education, Inc. 30

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 Animation: Transpiration Animation: Water Transport in Plants Animation: Water Transport © 2014 Pearson Education, Inc. 31

Cuticle Xylem Upper epidermis Microfibrils in cell wall of Figure 29.17 Cuticle Xylem Upper epidermis Microfibrils in cell wall of mesophyll cell Mesophyll Air space Figure 29.17 Generation of transpirational pull Lower epidermis Cuticle Stoma Microfibril (cross section) Water film Air-water interface 32

Microfibrils in cell wall of mesophyll cell Air space Figure 29.17a Cuticle Xylem Upper epidermis Microfibrils in cell wall of mesophyll cell Mesophyll Air space Figure 29.17a Generation of transpirational pull (part 1) Lower epidermis Cuticle Stoma 33

Microfibrils in cell wall of mesophyll cell Microfibril Figure 29.17b Microfibrils in cell wall of mesophyll cell Figure 29.17b Generation of transpirational pull (part 2) Microfibril (cross section) Water film Air-water interface 34

Outside air  = −100.0 MPa Leaf  (air spaces) = −7.0 MPa Figure 29.18 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 29.18 Ascent of xylem sap Water molecule Trunk xylem  Root hair −0.6 MPa  Soil particle Water Soil  Water uptake from soil −0.3 MPa  35

Water molecule Root hair Soil particle Water Water uptake from soil Figure 29.18a Water molecule Root hair Soil particle Water Figure 29.18a Ascent of xylem sap (part 1: water uptake) Water uptake from soil 36

Adhesion by hydrogen bonding Xylem cells Cell wall Cohesion Figure 29.18b Adhesion by hydrogen bonding Xylem cells Cell wall Cohesion by hydrogen bonding Figure 29.18b Ascent of xylem sap (part 2: cohesion and adhesion) Cohesion and adhesion in the xylem 37

Xylem sap Mesophyll cells Stoma Water molecule Atmosphere Figure 29.18c Xylem sap Mesophyll cells Stoma Water molecule Atmosphere Transpiration Figure 29.18c Ascent of xylem sap (part 3: transpiration) 38

Cohesion makes it possible to pull a column of xylem sap 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 © 2014 Pearson Education, Inc. 39

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 © 2014 Pearson Education, Inc. 40

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 © 2014 Pearson Education, Inc. 41

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 © 2014 Pearson Education, Inc. 42

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed Figure 29.19a Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed Radially oriented cellulose microfibrils Cell wall Figure 29.19a Mechanisms of stomatal opening and closing (part 1: cell shape) Vacuole Guard cell (a) Changes in guard cell shape and stomatal opening and closing (surface view) 43

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed Figure 29.19b Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed H2O H2O H2O H2O H2O K+ H2O H2O H2O H2O H2O Figure 29.19b Mechanisms of stomatal opening and closing (part 2: K+ ions) (b) Role of potassium ions (K+) in stomatal opening and closing 44

Changes in turgor pressure result primarily from the reversible uptake and loss of potassium ions (K) by the guard cells © 2014 Pearson Education, Inc. 45

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 © 2014 Pearson Education, Inc. 46

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 protein denaturation © 2014 Pearson Education, Inc. 47

Adaptations That Reduce Evaporative Water Loss Xerophytes are plants adapted to arid climates © 2014 Pearson Education, Inc. 48

Oleander (Nerium oleander) Figure 29.20 Ocotillo (Fouquieria splendens) Oleander (Nerium oleander) Thick cuticle Upper epidermal tissue 100 m Trichomes (“hairs”) Crypt Stoma Lower epidermal tissue Figure 29.20 Some xerophytic adaptations Old man cactus (Cephalocereus senilis) 49

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. 50