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

Auxin3) Functions (along with cytokinens) in apical dominance Auxins produced by apical meristemInhibits growth of lateral branches

If apical meristem damaged, auxin production stopsNo longer inhibits lateral branch growth

lateral buds ---> branches

Auxin4) 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!

Gibberellins1) 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.,germinationof barley(beerproduction)

Gibberellins2) Fruit development Seedless fruit crops (e.g., grapes) may be

artificially sprayed with gibberellins - make fruits bigger

Gibberellins3) Stem growth (elongation)

Induce “bolting” - elongation of flowering stem of some plants

gibberellins added

Ethylene (a gas)

1) Involved in fruit ripeningapplied 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

Phytochromes1) Involved in seed

germinationOnly P fr form

(after exposure to red light) will cause seeds to germinate:

Fig. 39.18

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 atthe 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)

WATERFLOW

WATERFLOW

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)