NOTES – CH 39: Plant Responses to Internal and External Signals

Factors Affecting Plant Development:

1) The plant senses and responds to .

2) The plant’s encodes for enzymes that take part in development.

3) The plant uses , such as , that .

4) HORMONES () regulate the effects of environmental cues on receptors.

PLANT HORMONES:

• HORMONES =

at sites distant from where they are produced.

• they mediate/regulate developmental phenomena, such as:

--

Some important plant hormones:

• Abscisic acid: inhibits growth; & winter dormancy; closes stomata during stress

• Auxin: promotes stem elongation & fruit growth; ; gravitropism

• Gibberellins: , stem growth, flowering, & fruit development

•Cytokinins: ; cell division and growth; seed germination; (aging / death)

•Ethylene: (gaseous hormone) ; promotes leaf abscission(autumn) & enhances rate of senescence

Daily and Seasonal Responses

● Circadian rhythm ()

 Transpiration & synthesis of certain enzymes are plant processes that

Responses to environmental changes in: light levels, temperature, relative humidity, however…

these cyclic processes often continue even when the environmental cues are removed

(“”)

●Photoperiodism (a response to a change in the )

 Seasonal events, such as: , , the onset and breaking of bud dormancy – all occur at specific times of the year

Daily and Seasonal Responses:

FROM SEED TO SEEDLING:

● recall: seeds may remain dormant for weeks, months, years, or centuries!

● the mechanisms of dormancy include:

 mechanical restraint of embryo by tough seed coat

 chemical / hormonal inhibition of embryo development

Seed dormancy can be broken by…

● seed coat is weakened from tumbling across the ground, or passing through an animal’s digestive tract;

● ;

● fire (can release a mechanical restraint or remove the waterproofing of the seed coat)

● leaching (prolonged exposure to water)

Germination begins with:

1) IMBIBITION ()

-hydration causes the seed to swell and rupture the seed coat

-triggers metabolic changes in embryo  resume growth

-storage nutrients are digested by enzymes and nutrients are transferred to growing regions of embryo.

Germination continues as the:

2) ;

3) shoot tip breaks through soil surface

*in many dicots:

-the hypocotyl is in shape of a hook (pushed above ground)

-light stimulates the hypocotyl to straighten

-hypocotyl raises the

-epicotyl then spreads the first leaves which

become green and begin photosynthesis

**Many seeds will remain quiescent (dormant) until suitable environmental conditions are available; other seeds await a specific environmental cue (e.g. heavy rainfall; brush fire; exposure to cold or sunlight; passage through an animal’s digestive system) before they will break dormancy.

Flowering…

• FLOWERING =

• May be initiated by:

 (sensed by the length of night)…photoreceptors involved!

It is likely that a “flowering hormone” is sent from the leaves to where the flowers form

Fruit formation…

• As already studied, fruits form following

• Fruit ripening is under hormonal control

Plant death…

● Some plants are PERENNIALS:

their buds typically enter a state of winter dormancy during the cold season

the hormone ABSCISIC ACID

● many plans undergo SENESCENCE of certain cells / organs / entire plant…during LEAF ABSCISSION, leaves die & fall off at the end of the growing season

● both of these processes involve turning on specific genes leading to apoptosis ();

 newly formed enzymes break down many chemical components (DNA, RNA, proteins, membrane lipids) that the plant will salvage for a later date…

Plant Responses to STRESS:

ABIOTIC STRESSES:

● DROUGHT: ; wilt / roll up;

● FLOODING: ethylene production stimulates apoptosis of root cortex cells, producing air tubes (“snorkels”) to provide O2to roots

● SALT STRESS

● HEAT STRESS / COLD STRESS

BIOTIC STRESSES:

● HERBIVORY:

physical defenses: ,

chemical defenses:

 “recruit” predatory animals to kill the insect parasites

● PATHOGENS: (bacteria, virus)