Resource Acquisition and Transport in Vascular Plants (Ch. 36)

1. Resource Acquisition and Transport in Vascular Plants(Ch. 36)

3. Water and minerals are absorbed by the roots Transpiration creates a force within leaves that pulls xylem sap upward Water and minerals are transported upward as xylem sap Phloem sap can flow both ways; from site of sugar production or storage to site of sugar use or storage Roots exchange gases with air spaces in soil

4. Shoot Architecture & Light Capture Phyllotaxy • Arrangement of leaves on a stem • Immensely important for light capture • Angiosperms • Alternate/spiral, opposite, or whorled phyllotaxy • Ascending spiral around stem • At an angle to reduce shading of other leaves Self pruning • Programmed cell death of leaves that respire more than photosynthesize Leaf orientation, height, branching patterns

5. Root Architecture & Acquisition of Water and Minerals Root branching • Increase in nitrogen pockets in the soil • Adjust cells to absorb nitrogen more efficiently Same species will not show root competition Mycorhizzae • Seen in 80% of extant plant species

6. Three Levels of Transport Uptake & release of water & solutes • dependent on selectively permeable membranes • individual cells Cell-to-cell transport • tissue and organ level Long distance transport of sap • xylem and phloem

7. Apoplast & Symplast: Transport Continuums Symplast and apoplast function in transport within tissue and organs • across walls and membranes via plasmodesmata Symplast • Consists of the entire mass of cytosol of living cells in a plant as well as the plasmodesmata Apoplast • Consists of everything external to the plasma membrane including cell walls, extracellular spaces, and interior of dead cells

8. Apoplast & Symplast: Transport Continuums Apoplastic route • Water and solutes move along the continuum of cell walls and extracellular spaces Symplastic route • Water and solutes move along the continuum of cytosol • Requires substance to cross the cell membrane once then utilizes plasmodesmata Transmembrane route • Water and solutes move out of one cell, across the cell wall, and into the neighboring cell, which may pass then to the next cell in the same way • Repeated crossings of the plasma membrane

9. Short Distance Transport of Solutes Across Plasma Membranes Controlled by selective permeability of the cell membrane Passive Transport Active Transport Utilizes H+ • Creates membrane potential • Chemiosmosis • Utilizes ATP

10. Short Distance Transport of Water Across Membranes Osmosis Water potential Ψ = Ψp + Ψs • Predicts the direction water will flow • Flow from high to low potential • Solutes decreases potential • Decreases the amount of free water available • negative • Pressure increases potential • Turgor pressure • Turgid, flaccid, plasmolysis, wilting Aquaporins

11. Long Distance Transport: The Role of Bulk Flow Bulk Flow • Movement of liquid in response to a pressure gradient • Always occurs from high pressure to low pressure • Independent of solute concentration Occurs in • Tracheids • Vessel elements • Sieve tube elements

12. Absorption of Water and Minerals by Root cells Soil  Epidermis • near root tips; root hairs increase SA • mycorrhizae • plant roots and hyphae symbiotic relationship Epidermis  Root cortex • apoplastic routes • symplastic routes

13. Absorption by Roots Root cortex  Xylem • endodermis • Casparian strip • blocks minerals so that only selected ions may enter the xylem • Only way past this barrier is for water and minerals to enter via the symplastic route

14. 1 2 1 2 3 3 5 5 4 4 Lateral transport of minerals and water in roots Casparian strip Endodermis Pathway along apoplast Pathway through symplast 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. Casparian strip Plasma membrane Minerals and water that cross the plasma membranes of root hairs enter the symplast. Apoplastic route Vessels (xylem) Transmembrane Route…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. Root hair Symplastic route Epidermis Endodermis Vascular cylinder Cortex 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. 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.

15. Transpiration

16. Bulk Flow Transport via Xylem Xylem Sap • Water and dissolved minerals in the xylem • Gets transported via bulk flow to the veins in each leaf Relies on Transpiration • Evaporation of water from the aerial parts of a plant • Results in tremendous water loss • Guard cells and the photosynthesis-transpiration compromise • balance the requirements for photosynthesis with the need to conserve water • stomata concentrated on leaf underside

17. 3 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. Y = –0.15 MPa Y = –10.00 MPa Cell wall Air-water interface Airspace Low rate of transpiration High rate of transpiration Cuticle Upper epidermis Cytoplasm Evaporation Airspace Mesophyll Cell wall Lower epidermis Evaporation Water film Vacuole Stoma CO2 O2 Cuticle Xylem CO2 O2 Water vapor Water vapor Vacuole 1 2 In transpiration, water vapor (shown as blue dots) diffuses from the moist air spaces of the leaf to the drier air outside via stomata. At first, the water vapor lost by transpiration is replaced by evaporation from the water film that coats mesophyll cells. Transpirational Pull

18. Pushing Xylem Sap: Root Pressure Root pressure • A push of xylem sap upward • Minor influence because of gravitational force • May cause more water to enter the leaves than is transpired • Causes guttation (exudation of water droplets that can be seen in the morning on the tips or edges of some plant leaves)

19. Pushing Xylem Sap: Cohesion-Tension Hypothesis • Transpiration provides the pull of the ascent of xylem sap and the cohesion of water molecules transmits this pull along the entire length of the xylem from roots to shoots • Transpirational pull • Adhesion and Cohesion

20. 20 µm Rate of Transpiration is Regulated by Stomata Opening and closing of stomata • guard cells change shape • turgid cause cells to buckle and open the stomata • flaccid causes guard cells to sag and stomata close

21. Stimuli for Stomatal Opening and Closing Stomatal opening at dawn • light, decrease in CO2, internal clock Stomatal closing during the day • water low, abscisic acid, high temps Benefits • assists in mineral transfer • evaporative cooling protects enzymes

22. Adaptations that Reduce Evaporative Water Loss Xerophytes • Plants adapted arid environments

23. Sugars are Transported from Sources to Sinks via the Phloem

24. Movement from Sugar Sources to Sugar Sinks Translocation • Transport of products of photosynthesis to the rest of the plant • Phloem sap consists mainly of sucrose but can contain hormones, minerals, amino acids Sugar sources to sugar sinks • Sugar source…organ where sugar is produced • usually leaves • Sugar sink…organ that consumes or stores the sugar • roots, shoots, stems, fruits

25. Bulk Flow by Positive Pressure Loading and unloading • sugar must be loaded before it can be transferred to a sink Pressure flow (bulk flow) • mechanism of translocation • pressure builds up at source and releases pressure at the sink causing source to sink flow

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

27. Vessel (xylem) Sieve tube (phloem) Source cell (leaf) 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. Sucrose 1 1 H2O H2O 2 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. Transpiration stream Pressure flow In the case of leaf-to-root translocation, xylem recycles water from sink to source. Sink cell (storage Root) 4 4 3 3 Sucrose H2O Figure 36.18 Pressure flow in a sieve tube

28. 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. EXPERIMENT 25 m Sieve- Tube member Sieve-tubemember Sap droplet Sap droplet Stylet Stylet in sieve-tube member Severed stylet exuding sap Aphid feeding RESULTS The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration. CONCLUSION The results of such experiments support the pressure flow hypothesis. Figure 36.18 Tapping phloem sap with the help of an aphid

29. Symplast is Highly Dynamic Changes in Plasmodesmata • Permeability and numbers Phloem: An Information Superhighway • Sugars, macromolecules, viruses Electrical Signaling in the Phloem • Rapid leaf movements