Passage VI

Plant roots can respond to a stimulus. Response to light is phototropism; response to gravity is gravitropism. Growth toward a stimulus is a positive tropism; growth away from a stimulus is a negative tropism.

For 2 experiments with wild-type (WT) and mutant (M) Arabidopsis seeds, nutrient agar was put into each of 8 petri dishes (PD1-PD8).

Experiment 1

Six WT Arabidopsis seeds were placed in each of PD1 and PD2. Six M Arabidopsis seeds were placed in each of PD3 and PD4. Then, PD1 and PD3 were placed in the dark for 70 hr, and PD2 and PD4 were exposed to light from above for 70 hr. Figure 1 shows the growth of the hypocotyls (seedling stems) above the surface of the nutri­ent agar and the growth of the radicles (seedling roots) below the surface of the nutrient agar in each dish at 70 hr.

Experiment 2

Six WT Arabidopsis seeds were placed in each of PD5 and PD6. Six M Arabidopsis seeds were placed in each of PD7 and PD8. Then, PD5-PD8 were exposed to light from above for 70 hr. After 70 hr, each petri dish was turned 90° such that each dish was vertical and the seedlings in each dish were approximately horizontal (see Figure 2).

Then, PD5 and PD7 were exposed to light from above for 25 hr while PD6 and PD8 were exposed to light from below for 25 hr. At various times during the 25 hr, the downward curvature, in degrees (°), of the radicle (relative to its starting position) of each seedling in each dish was measured. The average downward curvature of the radicles in each dish at each measurement time is shown in Figure 3.

Figures adapted from Stanislav Vitha, Liming Zhao, and Fred David Sack, "Interaction of Root Gravitropism and Phototropism in Arabidopsis Wild-Type and Starchless Mutants."©2000 by Ameri­can Society of Plant Physiologists.

37. During the 25 hr in Experiment 2 that WT Arabidopsis seedlings were exposed to light from below, did the hypocotyls of the seedlings more likely exhibit positive phototropism or negative phototropism?

Answer and Explanation

Your Answer is

Correct Answer is B

Explanation

The hypocotyl is asked in the question, but the radicle is shown in figure 3, the answer should not be obtained from figure 3. This question is a general knowledge question, because the stems of most plants grow toward the light, so you should choose B.