Plant Hormones
How do we know they exist?
Why do the shoots grow up,
but the roots grow down?
Plant Hormones
Phototropism experiments by Charles and Francis Darwin:
Plant Hormones
Boysen-Jensen’s experiments:
Mechanisms of bending:
Plant Hormones
Frits Went discovered the hormone Auxin. Concentration of auxins in different parts of roots
and shoots determines growth rates of the cells… Since distribution of auxins is affected by
both light and gravity, auxins are involved in plants’ phototropic and gravotropic growth
patterns.
Plant Hormones
Major plant hormones and their action:
Abscisic acid
closing of stomata; seed dormancy
Auxins
elongation of shoots and roots, gravotropism,
phototropism
Cytokinins
promotion of sprouting of lateral buds
Ethylene
ripening of fruit
Gibberellins
germination of seeds and sprouting of buds;
elongation of stems; stimulation of flowering;
development of fruit
Plant Hormones
Hormones can work with
each other to regulate various
aspects of plant biology.
Here, auxin and cytokinin work
together in regulating lateral
bud formation… the sprouting
of lateral buds depends on the
ratio of cytokinin to auxin…
Plant Hormones
Plants use starch-filled organelles called statoliths to detect gravity… this is very similar
to how jellyfish detect gravity, and in some ways, it is even similar to how
humans keep themselves balanced…
Plant Hormones
A hormone called florigen both stimulates and inhibits flowering. Since flowering is
sometimes a function of day length, pigments called phytochromes are involved in
“measuring” day length. The ratio of different forms of this pigments change as a function
sunlight exposure and can thus be used by the plants to set their “internal biological clocks”.
Chemical Communication in Plants
Plant Communication
The Venus flytrap catches flies as a source of nitrogen.
Insects are attracted to the nectar in the trap, but when they trigger sensory “haris”
within the trap, they are captured…
What is the mechanism?
Plant Communication
The mechanism is based on “irreversible, differential growth”.
Triggering of the hairs creates an electrical potential that pumps H+ into outer cell walls.
The acidic condition created by the H+ induces enzymes to loosen the cellulose fibers of
the walls, causing water to rush in… the “growing” or swelling of the outer cells then
causes the rapid closing of the “trap”…