The tip of the main shoot was cut from the plant pictured below.

A hormone was present at very high levels in this tip. This hormone may be

Question

The tip of the main shoot was cut from the plant pictured below.

A hormone was present at very high levels in this tip. This hormone may be

BlackTom AI · Accepted Answer

Auxin is produced in the shoot tip and mediates phototropism, so this hormone is the best fit.

1) Relevant knowledge points and concepts
- Auxin (indole-3-acetic acid) is synthesized in the apical meristem and young leaves of shoots.  
- Phototropism: plants bend toward light because light causes auxin to redistribute to the shaded side of the stem; higher auxin concentration on the shaded side stimulates cell elongation there, producing curvature toward the light.  
- Cytokinins promote cell division and shoot formation (and can break apical dominance), ethylene is associated with stress responses, fruit ripening and inhibition of stem elongation in some contexts, and erythropoietin is an animal hormone (stimulates red blood cell production) and irrelevant to plants.

2) Step-by-step reasoning
- The tip of the main shoot is the primary site of auxin production.  
- When a plant sits in a mostly dark room but with light entering the window, photoreceptors detect directional light.  
- Auxin is actively transported away from the light-exposed side and accumulates on the shaded side of the shoot tip.  
- Cells on the shaded side elongate more because of elevated auxin, causing the stem to bend toward the window (visible in the picture).  
- Therefore the removed tip would have contained very high auxin levels.

3) Calculations / intermediate steps
- No numerical calculations are required. The mechanistic “steps” are: auxin synthesis in tip → photoreceptor-triggered lateral auxin redistribution → differential cell elongation → bending toward light.

4) Why the correct answer is correct
- The picture shows classic phototropic bending toward the window; auxin’s location (shoot tip) and mode of action (redistribution to shaded side causing elongation) explain both the high hormone level in the tip and the observed bend.

5) Why other options are incorrect
- Erythropoietin: an animal hormone, not found in plants.  
- Cytokinin: promotes cell division and lateral bud growth, not the differential elongation causing phototropism.  
- Ethylene: involved in ripening and stress responses and often inhibits elongation; it does not cause the directional bending seen here.

6) Conclusion
- The correct hormone is auxin; this matches the provided answer: "Auxin, because the plant has been placed in a dark room near a window."

The tip of the main shoot was cut from the plant pictured below. A hormone was present at very high levels in this tip. This hormone may be单项选择题

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