Plant Responses to Abiotic
Environment
Biological orientation of plants
• Tropisms: when growth toward stimulus it
is positive and vice versa.
• Taxes: occur in algae which swim with
flagella and swim toward light.
• Nastic responses: turgor response to
diffuse stimulus. Not fully understood.
• Tropisms are growth responses and are
generally not reversible, nasty’s are
reversible.
Control of plant growth
• Achieved by hormones and chemicals.
• Auxin controls bending of plant due to
light.
Series of experiments done:
• Found that: auxin
produced in tip of shoot
• Is water soluble and
diffuses through agar
blocks but not through
mica
• Causes elongation of
cells thus promoting
upward growth
• If light shone on side,
cells on dark side
elongate and grow
faster so shoot bends.
– Auxin migrates to dark side of stem
– Main auxin is IAA (indole acetic acid), it is
turned into actively growing plant tissue.
– Artificial auxins can be used as weed killers.
Low concentrations of auxins stimulate roots and
lateral buds, high conc. Inhibit. The reverse is true
for stems.
Apical Dominance
• Auxin at top of plant is produced in high
concentrations so shoot grows fast but
lateral buds stay dormant unless they are
lower down the plant where the
concentration is lower. This gives many
plants and trees their triangular shape.
• Cytokinins from roots stimulate lateral
buds making them grow more at the
bottom of the plant.
Geotropism in seeds
• Gravity causes an uneven movement of
auxin to bottom of root and top of shoot to
cause bend.
• Plants can tell up from down by using
statoliths which move from side to side.
Other tropisms
• Hydrotropism: very strong in roots,
stronger than geotropism as a root will
grow to surface if that is where water is.
• Thigmotropisim: tendrils of climbing plant
bend round any object they touch, growth
is slowed on the side that touches an
object.
• Chemotropism: roots often move towards
or away from certain chemicals.
Other effects of auxin
• Initiates root formation (used in powders to
make roots form).
• Stops roots growing long
• Suppression of lateral buds
• Stimulate mitosis in stems
• Start flowering of some plants
• Stops premature abscission of leaves and
fruit.
Other plant hormones
• Gibberellins
– Produced in growing tip and cause rapid
elongation, can promote germination.
• Cytokinins
– Work with auxin to promote cell division. High
auxin + low cytokinin = roots, low auxin + high
cytokinin = stems, equal amounts = callus
tissue.
• Ethene gas
– Ripens fruit
• Abscisic acid (ABA)
– Tends to inhibit all other hormones. Induces
leaf fall and winter dormancy in seeds.
• Etiolation
– Response to light, leaves grow small and
yellow.
Biological timing responses
• Daily rhythms (circadian)
– Some phytoplankton have three daily
rhythms: when agitated by waves it will glow
but only at night, photosynthesis only occurs
during the day, all cells divide during a one
hour period just before dawn.
– Opening of flowers in day and closing at night.
– Nectar secreted only at certain times of day
– Flowers give out perfume at different times of
day
– Flowers which turn face to follow sun
– Leaves of many plants droop at night.
• Circamonthly
– No obvious ones in plants
• Circatidal
– Release of egg and sperm for reproduction.
• Circannual
– Flowering, dormancy, growth patterns, abscission of
leaves
Flowering of plants
• Many show photoperiodism: the response
to changes in day length such as flowering
or dropping leaves. The most improtant
factor in when a plant flowers is length of
darkness not light. This means plants can
be divided in 3 groups.
– Short Day plants: require a short day and a
long night, these plants flower in winter, early
spring and autumn.
– Long Day plants: require a long day and short
night so flower in summer.
– Day neutral plants: relatively unaffected by
daylight hours e.g. tomato.
Phytochrome System
• The ability of plants to activate the photoperiod is
controlled by the pigment phytochrome. This pigment
detects light. It exisits in two forms coreresponding to
the two wavelengths of light: P665 and P725.
• When P665 absorbs red light it is quickly changed to P725.
• When P725 absorbs far red light it is quickly turned into P665.
• In the dark P725 is slowly converted into P665
•
The significance is that P725 is biologically active while P665 is inactive. It
seems that P665 is the way the plant stores the potentially active form of
the phytochrome.
Diag.
Other plant responses to abiotic
environment
• Vernalisation
– Seeds that require a period of cold before
germinating
• Dormancy
– Seeds that are metabolically inactive after
forming, dries out to 5% water of body weight
then will only germinate in the right conditions
• Stratification
– A moist seed exposed to low temperatures for
many days breaks the dormancy.
• Scarrification
– Some seeds need coat scratched or
burnt/treated to germinate.
• Ephemerals
– Many desert plants contain chemicals that
stop germination and until there is enough
rain to get rid of the chemicals there will be
no growth..
• Abcission
– Leaf fall prevents water loss in the cold
months and is triggered by auxin at certain
temperatures.
• Autumn colour in leaf
– In autumn the chlorophyll breaks down
exposing the other pigments
Growth forms of plants
– To cope with seasonal changes there are
annuals, biennials, perennials (which
overwinter by having bulbs etc).