Chapter 39 Plant Responses to Internal and External Signals

Experiments with Light and the coleoptile Conclusion: Tip of coleoptile responsible for sensing light, but growth occurred below tip  some signal was sent from tip to elongating region of coleoptile

Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Offset blocks cause curvature Control (agar block lacking chemical) has no effect Control Cells on darker side elongate faster than cells on brighter side AUXIN = chemical messenger that stimulates cell elongation

Hormones: chemical messengers that coordinate different parts of a multicellular organism Important plant hormones: 1. Auxin – stimulate cell elongation; fruit development; apical dominance; phototropism & gravitropism 2. Cytokinins – stim. cell division & growth; germination; delay senescence (aging) 3. Gibberellins – promotes seed & bud germ.; stem elong. & leaf growth; stim. flowering & fruit development 4. Abscisic Acid – inhibit growth; closes stomata during H 2 O stress; encourage dormancy 5. Ethylene – promote fruit ripening; inhibit/promote growth & dev. depending on species

The effects of gibberellin on stem elongation and fruit growth

Ethylene gas: fruit ripening (videos) Canister of ethylene gas to ripen bananas in shipping container Untreated tomatoes vs. Ethylene treatment

Plant Movement 1. Tropisms: growth responses  SLOW  Phototropism – light  Gravitropism – gravity  Thigmotropism – touch 2. Turgor movement: allow plant to make relatively rapid & reversible responses  Venus fly trap, mimosa leaves, “sleep” movement

Positive gravitropism in roots: the statolith hypothesis.

Thigmotropism: rapid turgor movements by Mimosa plant  action potentials

Plant Responses to Light Plants can detect direction, intensity, & wavelenth of light Phytochromes: light receptors, absorbs mostly red light ◦ Two forms: P r (red light) and P fr (far-red light) ◦ P r  P fr : switches depending on light in greatest supply ◦ P fr aids in detection of sunlight ◦ Regulate seed germination, shade avoidance

Biological Clocks Circadian rhythm: biological clocks  Persist w/o environmental cues  Frequency = 24 hours Phytochrome system + Biological clock = plant can determine time of year based on amount of light/darkness

Sleep movements of a bean plant. Caused by reversible changes in turgor pressure of cells on opposing sides of the pulvini, motor organs of the leaf.

Photoperiodism: physiological response to the relative length of night & day (i.e. flowering)  Short-day plants: flower when nights are long (mums, poinsettia)  Long-day plant: flower when nights are short (spinach, iris, veggies)  Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions) Night length is a critical factor!

How does interrupting the dark period with a brief exposure to light affect flowering?

Plant Response to Stress Causes of stress: 1.Drought (H 2 O deficit) 2.Flooding (O 2 deprivation) 3.Salt excess 4.Heat 5.Cold 6.Herbivores 7.Pathogens

1. H 2 O deficit:  close stoma  release abscisic acid to keep stoma closed  Inhibit growth  roll leaves  reduce SA & transpiration  deeper roots 2. Flooding (O 2 deprivation):  release ethylene  root cell death  air tubes formed to provide O 2 to submerged roots

3. Salt:  cell membrane – impede salt uptake  produce solutes to ↓ ψ - retain H 2 O 4. Heat:  evap. cooling via transpiration  heat shock proteins – prevent denaturation 5. Cold:  alter lipid composition of membrane ( ↑ unsat. fatty acids, ↑ fluidity)  increase cytoplasmic solutes  antifreeze proteins

6. Herbivores:  physical (thorns)  chemicals (garlic, mint)  recruit predatory animals (parasitoid wasps) 7. Pathogens:  1st line of defense = epidermis  2nd line = pathogen recognition, host-specific

It is time for me to leaf you

I have taught you well young grasshopper

And remember…

STUDY … if not for you… then for..

Captain Pengiun ♥