Transport in plants occurs across a network of vessels and over long distances

1. Transport in plants occurs across a network of vessels and over long distances

2. Lecture 6 Outline (Ch. 36 & 37) I. Plant Transport Overview • Driving Forces • Water potential • Transpiration & Bulk Flow in Xylem • Stomata Control • Positive Pressure & Bulk Flow in Phloem III. Mineral Acquisition IV. Essential Nutrients V. Relationships with other organisms VI. Preparation for next lecture

3. Physical forces drive the transport of materials in plants over a range of distances Transport occurs on three scales Within a cell – cellular level Short-distance cell to cell –tissue level Long-distance in xylem & phloem - whole plant level Transport in Plants • Transport occurs by 3 mechanisms: • Osmosis & Diffusion • Active Transport • Bulk Flow

4. Transport in Plants – Water Potential Roots  xylem  stomata

5. Water Potential To survive • Plants must balance water uptake and loss • What is Osmosis? What is diffusion? • Water potential : predicts water movement due to solute concentration & pressure • designated as psi (ψ) Water molecules are attracted to: •  Each other (cohesion) •  Solid surfaces (adhesion)

6. Water Potential • Free water flows from regions of high water potential to regions of low water potential Ψ changes with: • Adding solutes • Adding pressure Water potential = Potential energy of water = Energy per volume of water in megapascals (MPa) ψTotal= ψsolute + ψpressure

7. (a) 0.1 M solution Pure water H2O P= 0 S= 0.23 = 0.23 MPa = 0 MPa Water Potential • Solutes added •  decreases ψ • (water less likely to cross membrane) (in an open area, no pressure, so ψp = 0)

8. (b) (c) H2O H2O P= 0.23 S= 0.23 = 0 MPa P= 0.30 S= 0.23 = 0.07 MPa = 0 MPa = 0 MPa Water Potential • Application of physical pressure  increases ψ (water more likely to cross membrane)

9. Water Potential Water Potential ψ = ψs + ψp Which direction will water move? ψcell = – 0.7 MPa + 0.5 MPa = – 0.2 MPa ψsolution = –0.3 MPa (solution has no pressure potential)

10. Water Potential • 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 0.4 M sucrose solution: Initial flaccid cell: P= 0 S= 0.7 Plasmolyzed cell at osmotic equilibrium with its surroundings = 0.7 MPa P= 0 S= 0.9 = 0.9 MPa P= 0 S= 0.9 = 0.9 MPa

11. Water Potential Uses of turgor pressure: • Inexpensive cell growth • Hydrostatic skeleton • Phloem transport

12. Water Route Most plant tissues - cell walls and cytosol are continuous cell to cell (via?) - cytoplasmic continuum called the symplast apoplast = continuum of cell walls plus extracellular spaces

13. Water Route How do water and minerals get from the soil to vascular tissue? Symporters (cotransporters) contribute to the gradient that determines the directional flow of water. H2O Soil Soil Cytosol Symporter Here, pumps in H+ and mineral ions H+ Mineral ions Water enters plants via the roots. Water

14. 14 Water Potential Minerals & ions pumped into root cells, then moved past endodermis What happens to ψ between soil and endodermis? Where is osmosis occurring?

15. Water Potential Once water & minerals cross the endodermis, they are transported through the xylem to upper parts of the plant.

16. 16 Xylem Water exits plant through stomata. Smooth surface Rippled surface H2O Water film that coats mesophyll cell walls evaporates. Water moves up plant through xylem. Adhesion to xylem cells Cohesion between water molecules 16

17. 17 Transpiration = loss of water from the shoot system to the surrounding environment. Bulk Flow = movement of fluid due to pressure gradient • Transpiration drives bulk flow of xylem sap. • Water is PULLEDup a plant. • Ring/spiral wall thickening protects against vessel collapse

18. Xylem Ascent by Bulk Flow The movement of xylem sap is against gravity maintained by the transpiration-cohesion-tension Stomata help regulate the rate of transpiration Leaves generally have broad surface areas These characteristics Increase photosynthesis Increase water loss through stomata 20 µm

19. 19 Xylem What happens if rate of transpiration nears zero? i.e. – at night, water pressure builds up in the roots • Guttation

20. H+ pumped out K+ flow in H2O flow in stomata open Stomata Control Why? Why? K+ channels, aquaporins and radially oriented cellulose fibers play important roles. Cues for opening stomata: Light Depleted CO2 Internal cell “clocks”

21. 21 Phloem • Direction is source to sink • Near source to near sink • Phloem under positive pressure Phloem sap composition: • Sugar (mainly sucrose) • amino acids • hormones • minerals • enzymes Phloem tissue Aphid Are tubers and bulbs sources or sinks?

22. Vessel (xylem) Sieve tube (phloem) Source cell (leaf) 1 H2O Sucrose 1 H2O 2 2 3 Transpiration stream Pressure flow 4 Sink cell (storage root) 4 3 Sucrose H2O Phloem Pressure Flow Hypothesis Where are sugars made? Sugars actively transported into companion cells  plasmodesmata to sieve tube elements Via H+/sucrose cotransporters Water follows (WHY?!) Water potential increased, turgor pressure increased, sap PUSHED through phloem Sugars removed (actively) at sink  water potential decreased, water leaves phloem

23. Overview: A Nutritional Network Every organism Continually exchanges energy and materials with its environment The branching root and shoot system provides high SA:V to collect resources Plants’ resources are diffuse (scattered, at low concentration) What are these diffuse resources?

24. What’s in dirt?! Mineral Acquisition

25. After heavy rainfall, water drains away from the larger spaces in soil But smaller spaces retain water attraction to surfaces, clay and other particles The film of loosely bound water available to plants Mineral Acquisition Soil particle surrounded by film of water Root hair Water available to plant Air space

26. Mineral Acquisition Soil particle – – K+ K+ – – – – – Cation Exchange •  Makes cations available for uptake. – – Ca2+ Mg2+ Cu2+ K+ H+ CO2 H+ Root hair H2O Steps: 1.  Roots acidify soil solution via respired CO2 and H+/ATPase pumps 2.  H+ attracted to soil particle (-) which “releases” cations 3.  Roots absorb cations

27. 27 Essential Nutrients and Deficiencies • Plants require certain chemicals to thrive • Plants derive most organic mass from the CO2 of air • Also depend on soil nutrients like water and minerals Essential elements: Required for a plant to complete its life cycle

28. Healthy Phosphate-deficient Potassium-deficient Nitrogen-deficient Essential Nutrients and Deficiencies • Photosynthesis = major source of plant nutrition • Overall need • Macronutrients – used in larger amounts • Nine = C, O, H, N, K, Ca, Mg, P, and S • Micronutrients – used in minute amounts • Seven = Cl, Fe, Mn, Zn, B, Cu, and Mo Deficiency of any one can have severe effects on plant growth

29. 29 Relationship with other organisms • Mycorrhizae • Root nodulation • Parasitic plants • Carnivorous plants

30. Relationship with other organisms • Symbiotic associations with mycorrhizal fungi are found in about 90% of vascular plants • Substantially expand the surface area available for nutrient uptake • Enhance uptake of phosphorus and micronutrients The fungus gets: sugars from plant Agriculturally, farmers and foresters …Often inoculate seeds with spores of mycorrhizae to promote mycorrhizal relationships.

31. Nitrogen, Soil Bacteria and Nitrogen Availability Plants need ammonia (NH3) or nitrate (NO3–) for: Proteins, nucleic acids, chlorophyll… Nitrogen-fixing soil bacteria convert atmospheric N2 to nitrogenous minerals that plants can absorb N2 N2 Atmosphere Soil Nitrate and nitrogenousorganiccompoundsexported inxylem toshoot system Nitrogen-fixingbacteria N2 Denitrifyingbacteria H+ (From soil) NH4+ NH3 (ammonia) Soil NO3– (nitrate) NH4+ (ammonium) Nitrifyingbacteria Ammonifyingbacteria Organicmaterial (humus) Root Symbiotic relationships form between nitrogen-fixing bacteria and certain plants - Mainly legume family (e.g. peas, beans)

32. Nodules: Swellings of plant cells “infected” by Rhizobium bacteria Nodules Roots (a) Pea plant root Bacteroids within vesicle 5 m • Inside the nodule • Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell (b) Bacteroids in a soybean root nodule. In this TEM, a cell froma root nodule of soybean is filledwith bacteroids in vesicles. The cells on the left are uninfected.

33. Epiphytes, Parasitic, and Carnivorous Plants Staghorn fern, an epiphyte EPIPHYTES Anchored on another plant, self-nourished PARASITIC PLANTS Absorb sugar/minerals from host plant • Pitcher plants • cavity filled with digestive fluid • Venus flytrap • To gain extra nitrogen Mistletoe, a photosynthetic parasite

34. Things To Do After Lecture 6… Reading and Preparation: • Re-read today’s lecture, highlight all vocabulary you do not understand, and look up terms. • Ch. 36 Self-Quiz: #2, 3, 4, 6, 7, 8, 9 (correct answers in back of book) Ch. 37 Self-Quiz: #1, 2, 8, 9, 10 (correct answers in back of book) 3. Read chapters 36 & 37, focus on material covered in lecture (terms, concepts, and figures!) 4. Skim next lecture. “HOMEWORK” (NOT COLLECTED – but things to think about for studying): • Explain the two components of water potential – which of these is due to osmosis? • Diagram the movement of water in a plant via xylem versus sugar movement through phloem. List similarities and differences. • Discuss how mycorrhizae and Rhizobium are different and the benefits each provide to plants. • Think about what types of environments might be more likely to have carnivorous plants – what do plants gain by digesting insects?