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Plant Responses to Internal &
External Signals
Plant Hormones
Hormones are chemical signals that
coordinate the various parts of an
organism
A hormone is a compound produced in one part
of the body which is then transported to other
parts of the body, where it triggers responses in
target cells and tissues
Examples of human hormones:
Adrenaline, testosterone, estrogen, epinephrine…
Plant Hormones
There are 5 major classes of plant
hormones, each with specific functions:
Auxin
Cytokinins
Gibberellins
Abscisic acid
Ethylene
Auxin
Stimulates stem elongation
Stimulates development of fruit
Involved in phototropism and gravitropism
Cytokinins
Stimulate cell division and growth
Stimulate cytokinesis
Stimulate germination and flowering
Gibberelins
 Trigger seed and
bud germination
 Promote stem
elongation and leaf
growth
 Important in the
growth of fruit
Ethylene
 Promotes fruit
ripening
 Senescence (aging)
is a progression of
irreversible change
that eventually leads
to death
Caused, at least in part,
by ethylene
 “One bad apple spoils
the whole bunch”
Abscisic Acid
 Induces seed dormancy
 Anti-gibberellin
 Inhibits cell growth
 Anti-cytokinin
 Inhibits fruit ripening
 Anti-ethylene
 Closes stomata during
water stress, allowing
many plants to survive
droughts
Tropisms
Tropisms are growth responses that
result in curvatures of whole plant organs
toward or away from a stimuli
There are three major stimuli that induce
tropisms
Light (Phototropism)
Gravity (Gravitropism)
Touch (Thigmotropism)
Phototropism
 Phototropism is the
growth of a shoot
towards light
This is primarily due to
the action of auxin
Auxin elongates the
cells on the non-light
side
Biological Clocks/Circadian Rhythms
A physiological cycle with a frequency of
about 24 hours is called a circadian
rhythm
Even without external, environmental
cues, circadian rhythms persist in humans
and in all eukaryotes
Example: jet lag in humans
Photoperiodism
 A physiological response to day length (differs in winter,
summer, spring, and fall) is known as photoperiodism
 Short-day plants
 Require a shorter light period
 Flower in later summer/fall/winter
 Example: poinsettias
 Long-day plants
 Require a longer light period
 Flower in late spring/early summer
 Example: spinach
 Day-neutral plants
 Are unaffected by photoperiod
 Example: tomatoes
 But it’s actually the night that matters!!
Plant Defenses
Plants defend themselves against
herbivores in several ways
Physical defenses, such as thorns
Chemical defenses, such as
producing distasteful/toxic
compounds