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Plant Science

Plant Science. Stem cross section (Helianthus spp) . Plan Diagram of Dicot Stem. Cross-section shows: Epidermis – protective, often with waxy cuticle Cortex – often cells have thickenings which provide secondary support of plant Vascular bundles: contain xylem, phloem and cambium

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Plant Science

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  1. Plant Science

  2. Stem cross section (Helianthus spp) Plan Diagram of Dicot Stem • Cross-section shows: • Epidermis – protective, often with waxy cuticle • Cortex – often cells have thickenings which provide secondary support of plant • Vascular bundles: contain xylem, phloem and cambium • Cambium tissue: lateral meristem where cell division produces secondary xylem and phloem • Pith – thin-walled cells (fill space and sometimes degenerate if stem is hollow)

  3. Actual Dicot Stem XS

  4. Leaf Diagram

  5. Functions of Leaf Tissues • Absorption of light: mostly in palisade layer • Gas Exchange: mostly through stomata – gases stored and exchanged in spongy layer, usually on bottom of leaf • Support: xylem and phloem; veins • Water conservation: waxy cuticle on epidermis and stomata (closing) • Transport of water: xylem • Transport of Photosynthetic Products: phloem Does there seem to be a relationship between the placement of these tissues and their function?

  6. Monocots vs. Dicots

  7. Monocots: Taproot with adventitious extensions Dicots: Tap root with lateral extensions

  8. Modifications of Roots • Tubers: modified roots for storage of photosynthetic products • Eg. Potato stores starch (carbohydrate) (eyes are axillary buds)

  9. Modification of Roots • Tap roots can also be modified for other reasons • Eg. Carrot • Grow in sandy soils – anchor and store water for plant

  10.  . Roots • Large surface area provided by root hairs and branches - The extension of the cell wall increases the surface area for the absorption of water and minerals at the cellular level. The root hair cell provides both an increase in the cell wall (apoplastic pathway) and the cytoplasmic route (symplastic pathway) for the movement of water

  11. Movement of Minerals to the Root 3 Ways: • Diffusion – often not sufficient if minerals are in low concentration in soil • Fungal hyphae – mycorrhizal relationships – fungi increase surface area for absorption, gain access to plant carbohydrates • Mass flow… Fungal hyphae on roots

  12. Mass Flow Hypothesis • Minerals dissolved in water that the plant is absorbing form hydrogen bonds with water and are thus ‘dragged’ near to plant with the water • This concentrates the minerals for absorption

  13. Mineral Absorption in Roots By Active Transport • Clay particles are negatively charged – attract positively charged ions like potassium, sodium, calcium • proton pumps in roots pump protons ( H+) outside of the cell. This creates an electro-negative charge within thecell.2. When the root cells secrete protons into the surrounding soil water the hydrogen ions displace the mineral ions from the clay particle, freeing them into solution. 3. The mineral ions in the soil water are free to be absorbed by various pathways.

  14. Mineral Absorption in Roots By Active Transport • Plants can also directly actively transport minerals into roots • Experiments that metabolically poison the root (stop ATP production) causes all mineral absorption to stop.

  15. : Modifications of Stems • Bulbs: short vertical underground stems • Many fleshy highly modified leaves for the storage of nutrient. • Eg. Onion and Lily • Not to be confused with corms (daffodil, tulip, garlic) which are More solid in tissue.

  16. Stem Modification • Runners: eg. Strawberry – allow plant to form new plantlets (asexual reproduction) • Tendrils: used by climbing plants for support (eg. Peas) • Tendril Animation

  17. Meristems • Are areas of active cell division in plant body • Two main types: • Apical (up/down/sideways growth) • Lateral (growth in girth) Cambium forms secondary growth

  18. Apical Meristems (a) Shoot apical meristem (b) Leaf primordial (c) Axillary bud primordium (d) leaf (e) Stem tissue

  19. Apical Meristems

  20. Root Apical Meristem • (a) Root cap. • (b) Root apical meristem. • (c) Ground meristem. • (d) Protoderm. • (e) Epidermal tissue of the root. • (f) Vascular tissue (central stele).

  21. Primary Growth • Added by apical meristems This tissue diagram is a cross section of the stem of the primary plant body. This means that there has been no additional secondary thickening of the cell walls.

  22. Secondary Growth Secondary growth added by the Lateral meristem (cambium) has two types: 1. Vascular cambium that produces secondary xylem and phloem 2. Cork cambium produces some of the bark layer of a stem.

  23. Phototropism • Bending of plant toward light • Auxins are a group of plant hormones • Unlike animal hormones, plant hormones often have multiple target sites and actions • Most important auxin may be IAA:

  24. Darwin’s Experiments Charles Darwin studies of auxin effects are published a book called, 'The Power of movement'. The cylindrical shoot is enclosed in a sheath of cells called the coleoptile. " when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend".

  25. Phototropism • Stems may exhibit positive phototropism, roots negative phototropism • Auxins cause cell elongation and cell division on ‘dark’ side of plant • (a type of auxin was used as a herbicide “Agent Orange” in Viet Nam – dioxins that contaminated it caused many health problems)

  26. Plant Support • Plants can support themselves in 3 ways: • Thickening of cellulose walls • Lignified xylem vessels • Turgor pressure

  27. Thickened Cellulose Walls Cells deposit extra cellulose in areas that require more support

  28. Lignified Xylem Vessels • A polymer that binds to cellulose to strengthen cell walls • Often deposited in rings Rhubard with lignin rings

  29. Turgor Pressure • The pressure of a cell’s contents (mostly caused by the vacuole in plant cells) on the interior cell walls • Maintained by osmotic pressure • Low turgor pressure causes ‘wilting’

  30. Stomata • Regulate transpiration, thus osmotic pressure in cells • Regulated by abscisic acid • Dehydrated mesophyll cells release this hormone which stimulates stomata to close, no matter how much photosynthesis is inhibited

  31. Factors Affecting Transpiration • Humidity – high humidity reduces rate of transpiration – low water gradient • Wind – increases evap, increasing transpiration • Temperature – increasing temp. increases transpiration – more evap from leaf surface • Light – more photosynthesis stimulates the stomata to open, increasing transpiration

  32. (a) The guard cell absorbs light and produces ATP in the light dependent reaction. • (b) The ATP is used to drive proton pumps that pump out H+ . The inside of the cell becomes more negative. • (c) Potassium ions enter the cell which increases the solute concentration. • (d) Water moves from the surrounding tissue by osmosis.

  33. Xerophytes • Plants adapted to low water conditions • Adaptations include: • Reduced or rolled leaves • Spines • Deep roots • Thick cuticle • Stomata in pits surrounded by hairs • Water storage tissue • Low growth form • CAM • C4 physiology

  34. Photorespiration Maize: a C4 plant • Tendency of Rubisco to fix oxygen instead of carbon dioxide when carbon dioxide levels are low • Inefficient, but may have a safety function in prevention of oxygen free radical formation • Prevented by C4 and CAM

  35. CAM Plants • Crassulacian Acid Metabolism – plants “hold breath” all day! • At night carbon dioxide is combined with phosphoenol pyruvic acid (C3) to form Oxoloacetic acid (C4). • Oxoloacetic acid is changed to malic acid or aspartic acid. This stores the carbon dioxide until required for photosynthesis during the day • During the day, malic acid converted back to carbon dioxide for use in Ps.

  36. C4 Plants • Enzyme used to ‘fix’ carbon into a four carbon (instead of three carbon) compound • This is then is shuttled as oxaloacetate or malate to bundle sheath cells for storage

  37. Translocation • Phloem actively translocates sugars and amino acids from their source (usually leaves or storage areas) to the ‘sink’ (fruits, seeds, roots). • The source is where food is produced, this would be the leaves. They produce glucose which is then converted to sucrose which enter the phloem. This makes the water potential more negative making water from the surrounding xylem enter. • All of this extra material increases the pressure and forces the solution down and through the sieve plate. Then it gets to the sink where the sucrose is moved by active transport into the parenchyma; where it is made into insoluble starch so the water returns to the xylem.

  38. Translocation • Sugars can move in the opposite direction if no light (at night) when energy must come from storage areas (eg. Tubers)

  39. Combination of Transpiration/Translocation 1. Source produces organic molecules 2. Glucose from photosynthesis produced 3.Glucose converted to sucrose for transport 4. Companion cell actively loads the sucrose 5. Water follows from xylem by osmosis 6. Sap volume and pressure increased to give Mass flow 7. Unload the organic molecules by the companion cell 8. Sucrose stored as the insoluble and unreactive starch 9. Water that is released is picked up by the xylem 10. water recycles as part of transpiration to re supply the sucrose loading

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