The diagram provided shows a simple
experiment used to investigate the effect of IAA on shoot growth. The shoots are set up as outlined
in the diagram and then left for seven days with a supply of water and
nutrients. What would the expected result for
shoot B be? (A) The shoot will grow toward the
light. (B) The shoot will grow away from
the light. (C) The shoot will stop
growing. Or (D) the shoot will grow at a
IAA belongs to a group of chemical
messengers, or hormones, called auxins that play various roles in plants. IAA is produced mainly in the cells
of the apical bud, which is the top bud of the plant. Some of the numerous functions of
IAA are controlling growth by regulating cell division and elongation, sometimes in
response to certain stimuli such as light.
Let’s take a closer look at the
experiment shown in this diagram provided by the question. In shoot A, the cells in the apical
bud should produce IAA as normal. IAA will diffuse from cell to cell
down the plant shoot and accumulate in cells on the shaded side of the shoot. A high concentration of IAA in the
cells on the shaded side of the shoot causes these cells to elongate comparatively
more than the cells on the illuminated side of the shoot, resulting in directional
growth of the shoot toward the light source.
In shoot B, the apical bud has been
removed from the plant, which means that the cells that produce IAA have also been
removed. This means that IAA cannot diffuse
down the shoot to stimulate cell elongation in the cells below the apical bud. So this shoot is unlikely to grow
In shoot C, the tip is removed and
replaced with an agar block that contains IAA. This should result in directional
growth of the shoot in a similar manner to what we observed in shoot A. IAA will diffuse down from the agar
block to the cells below and accumulate on the shaded side of the shoot. This would stimulate them to
elongate in the same asymmetrical manner as we saw in shoot A, causing shoot C to
also bend in the direction of the light source.
Having explored the theory behind
this experiment and its likely results, we can answer this question correctly. The expected result for shoot B is
(C): the shoot will stop growing.