Video Transcript
Which of the following is correct
about auxin production during phototropism? (A) Auxins are produced in the
shaded side of the stem. (B) Auxins are produced in the
illuminated side and diffuse to the shaded side of the stem. (C) Auxins are produced in the stem
tip and accumulate on the shaded side. Or (D) auxins are only produced in
the roots and diffuse to the stem.
Before answering this question,
let’s review some of the key terms that have been used. A tropism is the movement of an
organism toward or away from a stimulus. Many people might think of plants
as static, unmoving organisms, but they can actually respond to a huge range of
stimuli through tropisms. Remember, a stimulus is any
detectable change in an organism’s internal or external environment that causes an
effect in that organism.
This question asks us specifically
about phototropism. The prefix photo- indicates the
stimulus the organism is moving toward or away from through phototropic responses,
light. You may be thinking, “But how does
the plant do this?” It is achieved with the help of
hormones, specifically a group of hormones called auxins that, among their other
roles, control cell elongation.
Auxins are named after the Greek
word “auxein,” which means to grow or increase. These plant hormones specifically
play a role in stimulating directional growth, which is a growth movement either
toward or away from a stimulus. Auxins are usually produced by the
cells at the tips of shoots, which are sometimes called stems, and root tips. For instance, we know that auxins
are produced in the coleoptile, which is a sheath that surrounds the shoot tip in
the growing regions of plants.
The presence of auxin can either
stimulate or inhibit cell elongation, depending on where in the plant it is acting
and its concentration. If a light source is directly above
the plant shoot, auxin produced in the tip of the plant stem diffuses down both
sides of the stem equally. This causes symmetrical cell
elongation in the cells either side of the stem, causing the stem to grow directly
upward toward the light source. However, if a light source arrives
from one side, auxin produced in the shoot tip accumulates on the shaded side of the
stem. This causes these cells that are
not in direct sunlight to elongate comparatively more than the side that is in
direct sunlight, which causes the stem to bend in the direction of light.
We now have enough information to
answer our question about auxin production during phototropism correctly. The correct answer is (C). Auxins are produced in the stem tip
and accumulate on the shaded side.
