Water in Plants Chapter 9

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

Molecular Movement  Diffusion - movement of molecules from region of higher concentration to region of lower concentration Molecules move along concentration gradient State of Equilibrium - molecules distributed throughout available space Rate of diffusion depends on pressure, temperature and density of medium

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 H 2 O through semipermeable membrane from region where H 2 O more concentrated to region where less concentrated

 Osmotic Pressure - pressure required to prevent osmosis Osmotic potential balanced by resistance of cell wall H 2 O potential of cell = osmotic pressure + pressure potential – Pressure Potential (Turgor Pressure) - pressure that develops against walls as result of H 2 O entering cell – Turgid Cell - firm cell due to H 2 O gained by osmosis – H 2 O moves from cell with higher H 2 O potential to cell with lower H 2 O potential Turgid cell Molecular Movement Osmosis

 Osmosis main way H 2 O enters plants from environment Molecular Movement Osmosis

Molecular Movement  Plasmolysis - loss of H 2 O through osmosis Accompanied by shrinkage of protoplasm away from cell wall Normal cells versus plasmolyzed cells

Molecular Movement  Imbibition Large molecules (i.e., cellulose and starch) develop electrical charges when wet, and attract H 2 O molecules H 2 O molecules adhere to large molecules Results in swelling of tissues Imbibition 1 st step in germination of seed Seeds before and after imbibition

Molecular Movement  Active Transport - process used to absorb and retain solutes against 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

Water and Its Movement Through the Plant  Transpiration - H 2 O vapor loss from internal leaf atmosphere >90% of H 2 O entering plant transpired  H 2 O needed for: Cell activities Cell turgor Evaporation for cooling – If more H 2 O lost then taken in, stomata close

Water and Its Movement Through the Plant  Cohesion-Tension Theory - transpiration generates tension to pull H 2 O columns through plants from roots to leaves H 2 O columns created when H 2 O molecules adhere to tracheids and vessels of xylem and cohere to each other

Water and Its Movement Through the Plant  Cohesion-Tension Theory When H 2 O evaporates from mesophyll cells, they develop lower H 2 O potential than adjacent cells H 2 O moves into mesophyll cells from adjacent cells with higher H 2 O potential Process continued until veins reached Creates tension on H 2 O columns, drawing H 2 O all way through entire span of xylem cells H 2 O continues to enter root by osmosis

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 CO 2 concentration

Regulation of Transpiration  When photosynthesis occurs, stomata open Guard cells use energy to acquire K + from adjacent epidermal cells Causes lower H 2 O potential in guard cells H 2 O enters guard cells via osmosis Guard cells become turgid and stomata opens

Regulation of Transpiration  When photosynthesis does not occur, stomata close K + leave guard cells H 2 O follows Guard cells become less turgid and stomata close

Regulation of Transpiration  Stomata of most plants open during day/closed at night  H 2 O conservation in some plants: Stomata open only at night - Desert plants −Conserves H 2 O, but makes CO 2 inaccessible during day  Undergo CAM Photosynthesis » CO 2 converted to organic acids and stored in vacuoles at night » Organic acids converted to CO 2 during day Stomata recessed below surface of leaf or in chambers −Desert plants, pines

Regulation of Transpiration  Guttation - loss of liquid H 2 O If cool night follows warm, humid day, H 2 O droplets produced through hydathodes at tips of veins In absence of transpiration at night, pressure in xylem elements forces H 2 O out of hydathodes Guttation in barley plants

Transport of Organic Solutes in Solution  Important function of H 2 O is translocation of food substances in solution by phloem  Pressure-Flow Hypothesis - organic solutes flow from source, where H 2 O enters by osmosis, to sinks, where food utilized and H 2 O exits Organic solutes move along concentration gradients between sources and sinks

Transport of Organic Solutes in Solution  Specifics of Pressure-Flow Hypothesis: Phloem Loading - sugar enters by active transport into sieve tubes H 2 O potential of sieve tubes decreases and H 2 O enters by osmosis Turgor pressure develops and drives fluid through sieve tubes toward sinks Food substances actively removed at sink and H 2 O exits sieve tubes, lowering pressure in sieve tubes Mass flow occurs from higher pressure at source to lower pressure at sink H 2 O diffuses back into xylem

Mineral Requirements for Growth  Essential Elements - essential building blocks for compounds synthesized by plants

Mineral Requirements for Growth  Macronutrients - used by plants in greater amounts N, K, Ca, P, Mg, and S  Micronutrients - needed by plants in very small amounts Fe, Na, Cl, Cu, Mn, Co, Zn, Mo, and B  When any required element deficient in soil, plants exhibit characteristic symptoms

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