Plant Growth in Angiosperms Plants have hormones: Substances produced in one part of body, transported to another part where it has a physiological effect (binds to receptor, triggers response in nearby cells/tissues)

Auxin = Indole Acetic Acid (IAA) 1)Functions in phototropism = movement toward light - Auxin concentrated on dark side - Induces cell expansion / elongation

Auxin 2) Functions in gravitropism = movement relative to gravity Shoots negatively gravitropic Roots positively gravitropic In both, auxin concentration induces cell expansion / elongation

Auxin Acts by loosening cellulose microfibrils of cell walls Causes cell expansion

Auxin 3) Functions (along with cytokinens) in apical dominance Auxins produced by apical meristem Inhibits growth of lateral branches If apical meristem damaged, auxin production stops No longer inhibits lateral branch growth lateral buds ---> branches

Auxin 4) Promotes secondary growth (& wood production)

Cytokinins 1)Promote cell division 2)Stimulate bud formation --> lateral branches 3)Inhibit stem elongation Balance of cytokinins & auxins ---> many plant growth processes!

Gibberellins 1) Function in seed germination - Embryo releases gibberellins - Causes aleurone layer (in seed coat) to release enzymes (alpha-amylase): break down starch in endosperm to sugars (e.g., maltose) E.g., germination of barley (beer production)

Gibberellins 2) Fruit development Seedless fruit crops (e.g., grapes) may be artificially sprayed with gibberellins - make fruits bigger

Gibberellins 3) Stem growth (elongation) Induce “bolting” - elongation of flowering stem of some plants gibberellins added

Ethylene (a gas) 1) Involved in fruit ripening applied commercially to ripen some fruits 2) Apical hook some Eudicot seedlings - ethylene inhibits growth on inside of hook - protects apical meristem

Phytochromes Photoreceptor proteins Two forms: P r = absorbs red light, converted to P fr P fr = absorbs far red light, converted to P r P r P fr red light far red light

Phytochromes 1) Involved in seed germination Only P fr form (after exposure to red light) will cause seeds to germinate: Fig

Transport of water and sugar in plants Chapter 36

How does water get up a tree? Transpiration Cohesion Tension Mechanism

Transpiration, evaporation of water from leaves, moves water molecules in xylem up from the roots Cohesion of water molecules for one another allows continuous water column to be maintained. Water follows a water potential (  ) gradient (affected by pressure, gravity, and solute concentration), from high to low. Water potential at the leaf is very low (-10 to -100 MPa), becoming increasingly higher at the roots (-0.6 MPa). Tension of water column is maintained by water flow and cohesion.

Low water potential (-10 to -100 MPs) Medium water potential (-0.8 MPs) High water potential (-0.3 MPs) WATER FLOW WATER FLOW

How is sugar transported? Movement occurs from a region of high concentration to low concentration: sugar source (where sugar produced, e.g., from leaf or by breakdown of starch) to a sugar sink (where it is utilized)

How is sugar transported? Water entering sieve elements causes positive osmotic pressure, greatest where sugars are most concentrated. Thus, sugars move by pressure flow, from a region of high concentration (high pressure) to a region of low concentration (low pressure)

Osmosis-movement of water across a membrane from low to high concentration of solutes – results in increased osmotic pressure.

Pressure Flow: Movement by osmotic pressure within sieve elements from high sugar concentration to low sugar concentration: sugar source (where sugar produced, e.g., from leaf or by breakdown of starch) to a sugar sink (where it is utilized)