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V.C.E. Biology Unit 2
Plant Tropisms and Hormonal control
Hormonal Systems
 Hormones are protein based chemicals
produced by cells in one part of an organism
that cause an effect in another part of the
organism.
 Plant hormones are less specific than animal
hormones. Some hormones work together to
produce an effect.
 Hormones are slow to act as they need to
travel via the phloem.
 The effect of hormones are long lasting.
Environmental Cues
 Plant hormones react to a number of
environmental conditions:
- Light – plants respond to the amount
of light, the quality of light and the
length of day, influencing when they
flower, set fruit and disperse seeds.
Growing toward a source of light is
called phototropism.
- Temperature – seed dormancy and
bud dormancy are influenced by the
temperature.
Environmental Cues
- Gravity – plant roots grow
toward the source of gravity,
the centre of the earth – this
is called positive geotropism.
- Touch – plants that send out
tendrils, such as climbing
peas, respond to the touch of
sticks or branches and grow
toward the object seeking
support. This is called
thigmotropism.
Hormones
 Auxins – Auxins are essential for cell growth,
affecting both cell division and celluar
expansion in shoots, roots and fruit. Auxins
directly stimulate or inhibit the expression of
specific genes (a signalling molecule).
 Auxins are responsible for the growth of tips
toward light. The auxin moves away from the
direction of light, causing the cells on the
‘dark’ side to elongate under turgor pressure.
It also promotes initiation of adventitious roots
 Auxins act with or against other hormones:
Hormones
 Ratio of auxin to cytokinin determines
initiation of root versus shoot growth
 In large amounts will stimulate production of
ethylene
 If gibberellin is present then cell elongation is
stronger.
 If cytokinins are present then auxin will
stimulate cell division
 Auxins play a minor role in flower inititation.
Hormones
 Gibberellins initiate cell growth and are
therefore responsible for the regulation of
growth of the plant, sex determination,
dormancy, enzyme induction, germination of
seeds, flowering and fruit enlargement.
 Produced in fruit, seeds, growth buds,
elongating stems
Hormones
 Cytokinins – antagonise auxins on leaf buds,
promoting cell division and differentiation and
the growth of lateral branches. Involved in
shoot and root growth, chloroplast
maturation, cell enlargement, auxillary bud
release and senescence.
 Produced in roots and developing fruits.
Hormones
 Abscisic acid – inhibits growth. It is
responsible for seed dormancy, vernalisation
and drought tolerance in plants. ABA
produced in terminal buds in response to the
onset of winter slows plant growth. ABA
produced in roots in response to low soil
water potential causes the closure of stomata.
 Produced in Chloroplasts.
Hormones
 Ethylene – is a gas
produced by plants
that causes fruit
ripening. Interestingly
it is produced by
ripening fruit and
accelerates the
ripening of adjacent
fruit making seed
dispersal easier.