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 II.Driving Forces A.Water potential B.Transpiration & Bulk Flow in Xylem C.Stomata Control D.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 1.Within a cell – cellular level 2.Short-distance cell to cell – tissue level 3.Long-distance in xylem & phloem - whole plant level Transport in Plants Transport occurs by 3 mechanisms: A.Osmosis & Diffusion B.Active Transport C.Bulk Flow

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

5 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 Potential Water molecules are attracted to: Each other (cohesion) Solid surfaces (adhesion)

6 Free water flows from regions of high water potential to regions of low water potential Water Potential Adding solutes Adding pressure Water potential = Potential energy of water = Energy per volume of water in megapascals (MPa) ψ Total = ψ solute + ψ pressure Ψ changes with:

0.1 M solution H2OH2O Pure water  P = 0  S =  0.23  =  0.23 MPa  = 0 MPa (a) 7 Solutes added  decreases ψ (water less likely to cross membrane) Water Potential (in an open area, no pressure, so ψ p = 0)

8 Application of physical pressure  increases ψ (water more likely to cross membrane) H2OH2O  P = 0.23  S =  0.23  = 0 MPa (b) H2OH2O  P = 0.30  S =  0.23  = 0.07 MPa  = 0 MPa (c) Water Potential

9 ψ cell = – 0.7 MPa MPa = – 0.2 MPa ψ = ψ s + ψ p ψ solution = –0.3 MPa (solution has no pressure potential) Water Potential Which direction will water move?

10 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: 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 Water Potential

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

12 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 Water Route

13 Symporters (cotransporters) contribute to the gradient that determines the directional flow of water. Soil H2OH2O Mineral ions Symporter Water Soil Cytosol H+H+ Water Route Water enters plants via the roots. How do water and minerals get from the soil to vascular tissue? Here, pumps in H+ and mineral ions

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

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

16 Water exits plant through stomata. Smooth surface Rippled surface Water film that coats mesophyll cell walls evaporates. Water moves up plant through xylem. Adhesion to xylem cells Cohesion between water molecules H2OH2O Xylem

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

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

20 Stomata Control H + pumped out K + flow in H 2 O flow in stomata open Why? K+ channels, aquaporins and radially oriented cellulose fibers play important roles. Cues for opening stomata: Light Depleted CO 2 Internal cell “clocks”

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

22 Vessel (xylem) H2OH2O H2OH2O Sieve tube (phloem) Source cell (leaf) Sucrose H2OH2O Sink cell (storage root) 1 Sucrose Transpiration stream Pressure flow Phloem Pressure Flow Hypothesis Where are sugars made? Sugars actively transported into companion cells  plasmodesmata to sieve tube elements Via H+/sucrose cotransporters Water potential increased, turgor pressure increased, sap PUSHED through phloem Sugars removed (actively) at sink  water potential decreased, water leaves phloem Water follows (WHY?!)

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?

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 Soil particle surrounded by film of water Root hair Water available to plant Air space Mineral Acquisition

26 H2OH2O Root hair K+K+ Cu 2+ Ca 2+ Mg 2+ K+K+ K+K+ H+H+ H+H+ Soil particle – – – – – – – – – Mineral Acquisition CO2 Steps: 1. Roots acidify soil solution via respired CO 2 and H+/ATPase pumps 2. H+ attracted to soil particle (-) which “releases” cations 3. Roots absorb cations Cation Exchange Makes cations available for uptake.

27 Essential Nutrients and Deficiencies Plants require certain chemicals to thrive Plants derive most organic mass from the CO 2 of air –Also depend on soil nutrients like water and minerals Essential elements: Required for a plant to complete its life cycle

28 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 Essential Nutrients and Deficiencies Phosphate-deficient Healthy Potassium-deficient Nitrogen-deficient Deficiency of any one can have severe effects on plant growth

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

30 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 Relationship with other organisms 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 (NH 3 ) or nitrate (NO 3 – ) for: Proteins, nucleic acids, chlorophyll… Nitrogen-fixing soil bacteria convert atmospheric N 2 to nitrogenous minerals that plants can absorb N2N2 Soil N2N2 N2N2 Nitrogen-fixing bacteria Organic material (humus) NH 3 (ammonia) NH 4 + (ammonium) H + (From soil) NO 3 – (nitrate) Nitrifying bacteria Denitrifying bacteria Root NH 4 + Soil Atmosphere Nitrate and nitrogenous organic compounds exported in xylem to shoot system Ammonifying bacteria 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 (a) Pea plant root Nodules Roots 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 from a root nodule of soybean is filled with bacteroids in vesicles. The cells on the left are uninfected. 5  m Bacteroids within vesicle

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

Things To Do After Lecture 6… Reading and Preparation: 1.Re-read today’s lecture, highlight all vocabulary you do not understand, and look up terms. 2.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): 1.Explain the two components of water potential – which of these is due to osmosis? 2.Diagram the movement of water in a plant via xylem versus sugar movement through phloem. List similarities and differences. 3.Discuss how mycorrhizae and Rhizobium are different and the benefits each provide to plants. 4.Think about what types of environments might be more likely to have carnivorous plants – what do plants gain by digesting insects?