Plants ❮ 171down the aging of leaves. Supermarkets use synthetic cytokinins to keep their veggies
fresh.- Ethylene.This hormone initiates fruit ripening and causes flowers and leaves to drop
from trees (associated with aging). - Gibberellins.Another hormone group that assists in stem elongation. When you think
gibberellins, think “grow.” It is thought to induce the growth of dormant seeds, buds,
and flowers.
Plant Tropisms
Atropismis growth that occurs in response to an environmental stimulus such as sunlight
or gravity. The three tropisms you should familiarize yourself with are gravitropism, pho-
totropism, and thigmotropism.- Gravitropism.This is a plant’s growth response to gravitational force. Two of the hor-
mones mentioned earlier play a role in this movement: auxinandgibberellins.A plant
placed on its side will show gravitropic growth in which the cells on the upward-facing
side will not grow as much as those on the downward side. It is believed that the rela-
tive concentrations of these hormones in the various areas of the plant are responsible
for this imbalanced growth of the plant. - Phototropism.This is a plant’s growth response to light. Auxin is the hormone in
charge here. Auxin works its magic in the zone of elongation. While the mechanics
of the phototropism process may not be vital to this exam, it is still quite interesting
to know. When a plant receives light on all sides, auxin is distributed equally around
the zone of elongation and growth is even. When one half of a plant is in the sun,
and the other is in the shade, auxin (almost as if it feels bad for the shady portion)
focuses on the darker side. This leads to unequal growth of the stem with the side
receiving less light growing faster—causing the movement of the plant towardthe
light source. - Thigmotropism.This is a plant’s growth response to contact. One example involves
vines, which wind around objects with which they make contact as they grow.
How in the world did we figure out that auxin played such a large role in phototropism? A
series of experiments performed by two scientists proved vital to the understanding of this
process. Grass seedlings are surrounded by a protective structure known as the coleoptile.
Peter Boysen-Jensen performed an experiment in which a gelatin block permeable to chem-
ical signals was placed in between this coleoptile structure and the body of a grass seedling.
When the piece of grass was exposed to light on one side, it grew toward the light. When a
barrier impermeable to chemical signals was placed in between the two structures instead,
this growth toward light did not occur. Another scientist, F. W. Went, came onto the scene
and took Jensen’s experiment a step further. Went wanted to show that it was indeed a
chemical and not the coleoptile tip itself that was responsible for the phototropic response.
He cut off the tip and exposed it to light while the tip was resting on an agar block that
would collect any chemicals that diffused out. The block was then placed on the body of a
tipless grass seedling sitting in a dark room. Even in the absence of light, a block placed more
toward the right side of a seedling caused the seedling to bend to the left. A block placed
more toward the left side of a seedling caused the seedling to bend to the right. Because
there was no further light stimulation causing the growth, the agar block must indeed
have contained a chemical that induced a phototropic response. This chemical was given
the nameauxin.
KEY IDEABIG IDEA 3.B.2
The ethylene signal
causes changes in
production of dif-
ferent enzymes.
BIG IDEA 2.E.2
In plants, physio-
logical events
involve interactions
between environ-
ment (light) and
internal molecular
signals.
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