Answer: Decreased auxin and increased ethylene and ABA.
- A A sharp rise in cytokinin levels delivered from the root system
- B Decreased auxin and increased ethylene and ABA
- C An isolated drop in ambient temperature unaccompanied by hormonal change
- D An increase in available soil water supply to the root system
Correct answer: B. Decreased auxin and increased ethylene and ABA
Explanation: Leaf senescence: triggered by decrease in auxin/cytokinin and increase in ethylene and ABA. Chlorophyll breaks down, nutrients are reclaimed.
Geometric growth (where both daughter cells keep dividing) produces this classic S-shaped curve: a slow lag phase, a rapid exponential phase, and a levelling-off stationary phase as nutrients or space become limiting - the same fundamental pattern seen in population growth.
Concept context
Phases of plant growth, the five classical plant hormones plus newer ones, photoperiodism, vernalization, and seed dormancy.