Plant Responses to Internal and External Signals
 Plants, being rooted to the ground, must respond to
whatever environmental change that comes their way.
HORMONE:
1.
2.
3.
 The discovery of plant hormones came from work with tropisms
TROPISM:
 Positive Tropism:
 Negative Tropism:
IMPORTANT PLANT HORMONES
1. AUXINS (
)
 Responsible for the ___________________________ involved in
phototropism (response to light) and gravitropism (response to gravity)
 Slow leaf abscission
 Inducing cell division in vascular _________________and promotes
differentiation in _______________.
 Sstimulates _________________ in cell membrane which weakens cell
wall so cells can expand.
 PHOTOTROPISM:
o positive = _______________________; negative = ____________
________________
o Differential rate of elongation
 Shoot bends towards light due
to asymmetrical distribution of
auxins
 Light stimulates movement of
auxin to dark side so cells on
dark side elongate faster than
cells on light side.
 GRAVITROPISM:
o Roots grow down and shoots grow up
 THIGOMOTROPISM:
o Vines curl around supports (example of ____________________)
o Do Mimosa plants show negative thigomotropism?
2. CYTOKININS (
)
 Produced in actively growing tissues such as _______, __________ and
__________
 Works together with auxin
 Plays an important role in cell division, growth and differentiation by
___________________________________________________
 Stimulates germination
 Delays senescence (_______________________________________)
3. GIBBERELLINS (
)
 Promotes seed/bud germination, leaf
growth
 Stimulates ____________________
_____________________________
 Stimulates stem elongation
Untreated
Treated




Many “dwarf” varieties don’t have working gibberellins
Stimulates _________________________
Delays _____________________ (aging of leaves and fruits)
Gibberellins are used commercially in the spraying of Thompson
seedless grapes
Aleurone
Endosperm
-amylase
Sugar
GA
Water
GA
cotyledon
Germination of a Seed
4. ABSCISIC ACID (ABA) (
)
 Often antagonistic to other hormones
 Slows growth and promotes ________________________
 Seed dormancy has great survival value because __________________
_________________________________________________________
 LEAF ABSCISSION: _______________________________________
o Prevents deciduous trees from desiccation (drying out) during
winter when roots cannot absorb water from frozen ground
o Shortening days and cooler temperatures cause ABA levels to
spike, which signals the closing of _______________ in leaves
5. ETHYLENE GAS (
)
 Controls senescence (aging) by playing a role in __________________
(programmed cell death) in leaves and flowers
 _________________________________________
 Example of _________________________ – ethylene triggers ripening;
ripening triggers more ethylene production
o Why should put fruit in a paper bag to ripen “One bad apple spoils
the whole bunch”