1. Chapter 7 Lecture Outline Water in Plants Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Outline  Molecular Movement  Water and Its Movement Through the Plant  Regulation of Transpiration  Transport of Food Substances (Organic Solutes) in Solution  Mineral Requirements for Growth

3. Molecular Movement  Diffusion Movement of molecules from a region of higher concentration to a region of lower concentration – Molecules move along concentration gradient. – Moving from lower to higher concentration is against concentration gradient. – State of equilibrium - Molecules distributed throughout available space – Rate of diffusion depends on pressure, temperature and density of medium.

4. Molecular Movement  Solvent - Liquid in which substances dissolve  Semipermeable membranes - Membranes in which different substances diffuse at different rates All plant cell membranes  Osmosis - Diffusion of water through a semipermeable membrane from a region where water is more concentrated to a region where it is less concentrated

5.  Osmotic pressure - Pressure required to prevent osmosis Osmotic potential balanced by resistance of cell wall. Water potential of cell = osmotic pressure + pressure potential – Pressure potential (Turgor Pressure) - Pressure that develops against walls as result of water entering cell – Turgid cell - Firm cell due to water gained by osmosis – Water moves from cell with higher water potential to cell with lower water potential. Turgid cell Molecular Movement Osmosis

6.  Osmosis is primary way water enters plants from environment. Pathway of water through plant: – Enters from soil into cell walls and intercellular spaces of root hairs and roots – Crosses differentially permeable membrane and cytoplasm of endodermis, then into xylem – Flows through xylem to leaves and diffuses out through stomata Molecular Movement Osmosis

7. Molecular Movement  Plasmolysis - Loss of water through osmosis Accompanied by shrinkage of protoplasm away from the cell wall Normal cells versus plasmolyzed cells

8. Molecular Movement  Imbibition Large molecules, such as cellulose and starch, develop electrical charges when wet, and thus attract water molecules. Water molecules adhere to large molecules. Results in swelling of tissues Imbibition is first step in germination of seed. Seeds before and after imbibition

9. Molecular Movement  Active Transport - Process used to absorb and retain solutes against a diffusion, or electrical, gradient by expenditure of energy Involves proton pump - Enzyme complex in plasma membrane energized by ATP molecules – Transport proteins - Facilitate transfer of solutes to outside and to inside of cell

10. Water and Its Movement Through the Plant  Transpiration - Water vapor loss from internal leaf atmosphere More than 90% of the water entering a plant is transpired.  Water needed for: Cell activities Cell turgor Evaporation for cooling – If more water is lost then taken in, stomata close.

11. Water and Its Movement Through the Plant  The Cohesion-Tension Theory - Transpiration generates tension to pull water columns through plants from roots to leaves. Water columns created when water molecules adhere to tracheids and vessels of xylem and cohere to each other.

12. Water and Its Movement Through the Plant  The Cohesion-Tension Theory When water evaporates from mesophyll cells, they develop a lower water potential than adjacent cells. Water moves into mesophyll cells from adjacent cells with higher water potential. Process is continued until veins are reached. Creates tension on water columns, drawing water all the way through entire span of xylem cells Water continues to enter root by osmosis.

13. Regulation of Transpiration  Stomatal apparatus regulates transpiration and gas exchange. Stomatal apparatus = 2 guard cells + stoma (opening). Transpiration rates influenced by humidity, light, temperature, and carbon dioxide concentration.

14. Regulation of Transpiration  When photosynthesis occurs, stomata open. Guard cells expend energy to acquire potassium ions from adjacent epidermal cells. Causes lower water potential in guard cells Water enters guard cells via osmosis. Guard cells become turgid and stomata opens.

15. Regulation of Transpiration  When photosynthesis does not occur, stomata close. Potassium ions leave guard cells. Thus, water leaves. Guard cells become less turgid and stomata close.

16. Regulation of Transpiration  Stomata of most plants are open during day and closed at night.  Water conservation in some plants: Stomata open only at night - Desert plants – Conserves water, but makes carbon dioxide inaccessible during day. o Thus, undergo CAM photosynthesis « Carbon dioxide converted to organic acids and stored in vacuoles at night. « Organic acid s converted to carbon dioxide during day. Stomata recessed below surface of leaf or in chambers. – Desert plants, pines

17. Regulation of Transpiration  Guttation - Loss of liquid water If cool night follows warm, humid day, water droplets are produced through hydathodes at tips of veins. In absence of transpiration at night, pressure in xylem elements forces water out of hydathodes. Guttation in barley plants

18. Transport of Organic Solutes in Solution  Important function of water is translocation of food substances in solution by phloem.  Pressure-Flow Hypothesis - Organic solutes flow from source, where water enters by osmosis, to sinks, where food is utilized and water exits. Organic solutes move along concentration gradients between sources and sinks.

19. Transport of Organic Solutes in Solution  Specifics of Pressure-Flow Hypothesis: Phloem loading - Sugar enters by active transport into sieve tubes. Water potential of sieve tubes decreases and water enters by osmosis. Turgor pressure develops and drives fluid through sieve tubes toward sinks. Food substances actively removed at sink and water exits sieve tubes, lowering pressure in sieve tubes. Mass flow occurs from higher pressure at source to lower pressure at sink. Water diffuses back into xylem.

20. Mineral Requirements for Growth  Essential Elements - Essential as building blocks for compounds synthesized by plants

21. Mineral Requirements for Growth  Macronutrients - Used by plants in greater amounts Nitrogen, potassium, calcium, phosphorus, magnesium and sulfur  Micronutrients - Needed by the plants in very small amounts Iron, sodium, chlorine, copper, manganese, cobalt, zinc, molybdenum and boron  When any required element is deficient in soil, plants exhibit characteristic symptoms.

22. Review  Molecular Movement  Water and Its Movement Through the Plant  Regulation of Transpiration  Transport of Food Substances (Organic Solutes) in Solution  Mineral Requirements for Growth