PLANT RESPONSES TO
STIMULI
Chapter 21
A. Plant Growth
Regulated by the action of hormones.
Hormone = a chemical messenger
produced in one part of a plant &
usually transported to another,
where it elicits a response.
Plants have 5 major classes of hormones:
auxins, gibberellins, cytokinins,
ethylene & abscisic acid.
1. Auxins
First group of plant hormones to be
discovered (phototrophic experiments of
Charles & Francis Darwin, 1870s).
Auxins are synthesized in shoot tips,
young leaves & seed embryos.
Major Actions
 promote
elongation of
cells (shoots,
leaves & embryos)
 inhibit
growth of lateral buds
 inhibit leaf & fruit abscission
 stimulate synthesis of ethylene
IAA (indolacetic acid) is the most
active naturally occurring auxin.
Commercial uses:
 stimulate adventitious root growth in
cuttings
 stimulate some plants to produce fruit
(seedless) without being fertilized
 2,4-D (synthetic auxin) kills broadleaf
weeds, but not grasses
2. Gibberellins
Discovered by Japanese botanists
studying “foolish seedling disease” in
rice (1926).
Gibberellins are synthesized in young
shoots & developing seeds.
Major Actions
 promote elongation of cells (shoots,
leaves & seeds)
 stimulate
flowering & fruit development
 stimulate seed germination
Commercial uses:
 elongate flower stems
of cyclamen plants
 lengthen
grapes
stems of
3. Cytokinins
Name derived from fact that
cytokinins stimulate cytokinesis.
Cytokinins are synthesized in roots,
embryos & fruits.
Major Actions
 stimulate cell division (shoots, roots,
leaves & seeds)
 stimulate
growth of lateral buds
 delay leaf senescence
Effects of cytokinins are influenced
by auxin concentration.
Apical meristem intact - auxin
suppresses lateral bud
growth [apical dominance].
Apical meristem removed cytokinin concentration
increases, stimulating lateral
bud growth.
Commercial uses:
 extend shelf life of leafy vegetables
 keep cut flowers fresh
 promote branching in Christmas trees
4. Ethylene
Only plant hormone that is a gas.
Ethylene is synthesized in all parts of
plants, especially ripening fruits, nodes
of stems, & dying leaves.
Major Actions
 promotes fruit ripening
 stimulates leaf & flower
senescence
 stimulates
abscission
leaf & fruit
Stem
Leaf stalk
Abscission layer
Commercial uses:
 ripens fruits that are picked green
5. Abscisic acid (ABA)
Referred to as the “stress hormone”
because it helps plants cope with
adverse conditions (severe drought,
onset of winter).
ABA is synthesized in mature leaves &
plants under stress.
Major Actions
 inhibits growth
 closes stomata
 induces & maintains seed dormancy
Commercial uses:
 inhibits the growth of plants that are
to be shipped.
B. Plant Movement
1. Tropic Movements
Plant growth directed toward or away
from an environmental stimulus.
 Phototropism
Growth in response to
unidirectional light.
Plant shoots are positively phototropic.
Bending results from auxin accumulation on
shaded side of plant.
 Gravitropism
Growth in response
to gravity.
Plant roots are positively gravitropic.
Plant shoots are negatively gravitropic.
Believed that amyloplasts in root cells
function as statoliths (gravity detectors).
Shift in statoliths
signals
redistribution
of auxin.
 Thigmotropism
Growth in response
to touch.
Passion vine tendrils
exhibit positive
thigmotropism.
Coiling is controlled by
auxin & ethylene.
2. Nastic Movements
Plant movements that are not oriented
with respect to a stimulus.
 Thigmonasty
A nastic response to touch.
Ex. leaflet folding of “sensitive plant”
 Photonasty (“sleep movement”)
A nastic response to daily rhythms of
light & dark.
Ex. movement of prayer plant leaves
C. Response to Seasonal Changes
Many plant processes (flowering,
seed germination, senescence,
dormancy) occur at specific times
of the year.
Plants track seasons by measuring
photoperiod (relative lengths of
daylight & darkness).
1. Flowering
 Long-day
plants bloom when light periods
are longer than some critical length.
 Short-day
Flower in spring
or early
summer
plants bloom when light
periods are shorter than some critical
length.
Flower in late
summer or fall
 Day-neutral
plants do not rely on
photoperiod to stimulate flowering.
Flower spring,
summer &
fall
Long-day & short-day plants actually
respond to length of night rather than
length of day.
Thus, short-day plants require a specific
period of uninterrupted darkness to
flower.
1-minute
flash of light
Plants measure photoperiod with help of
phytochrome, a blue pigment molecule
that exists in 2 forms:
 Pr = red-absorbing (660nm)
 Pfr = far-red-absorbing (730nm)
In daylight:
Red light**

Pr  Pfr
(rapid conversion)
Far-red light

Pr
 Pfr
(rapid conversion)
In darkness:
Pr <--------- Pfr
(slow spontaneous conversion)
Effects of alternate flashes of red & far-red light
2. Seed Germination
Phytochrome affects seed germination.
In many weeds:
 red light stimulates germination
Pr  Pfr
 far-red light inhibits germination
Pfr  Pr
If seeds are buried too deeply in soil, Pfr
is lacking & germination does not occur.
D. Circadian Rhythms
Plant responses that occur daily.
Circadian rhythms are regulated by
biological clocks, which are
controlled:
 internally
by genes
 externally by environmental factors
Examples of circadian rhythms include:
 flowering of the evening primrose
 photonastic movements of prayer plant
 opening of stomata
 secretion of nectar
 solar tracking (heliotropism) of
sunflowers