Video Transcript
The graph provided shows how the
stimulation or inhibition of cell growth in the roots and stems of plants changes
with an increase in IAA concentration. What is happening in the plant root
at point X? (A) IAA is stimulating cell
growth. (B) IAA is having no overall effect
on cell growth. Or (C) IAA is inhibiting cell
growth.
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. In the graph provided by the
question, we can see the effect that different concentrations of IAA have on cell
growth in the roots and shoots, which are sometimes called stems of plants,
respectively.
Let’s start by interpreting the
trend that can be observed in the roots, as this is the plant part this particular
question concerns. In the plant root, IAA has a weakly
stimulating effect when it is at low concentrations. This means that low concentrations
of IAA in the roots may encourage cell growth. However, as the concentration of
IAA in the root increases, its stimulating effects on cell growth decrease and it
actually begins to have an inhibitory effect.
Interestingly, IAA also stimulates
cell growth in the shoots. And its stimulating effect
increases to a higher value as IAA concentration increases compared to in the
roots. In fact, at the concentration
labeled X on the graph, the concentrations of IAA are so high that they are strongly
inhibiting cell growth in the roots and therefore preventing cell growth. In the shoots, the same
concentration of IAA strongly encourages cell growth.
When the concentration increases
part of concentration X however, the stimulatory effect on the shoots begins to
decrease. Very high concentrations of IAA
even have an inhibitory effect on cell growth in the shoot. Now we know what is happening in
the plant root at point X. The correct answer is (C). IAA is inhibiting cell growth.
