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A plant is kept in four treatments, each repeated every day…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A plant is kept in four treatments, each repeated every day. Light is on for the hours shown, and the rest of the day is dark.
| Treatment | What happens in 24 h | Result |
|---|---|---|
| 1 | 9 h light, then 15 h dark | flowers |
| 2 | 16 h light, then 8 h dark | no flowers |
| 3 | 9 h light, then 15 h dark, with a 2 minute flash in the middle of the dark | no flowers |
| 4 | 9 h light with a 15 minute dark spell in the middle of it, then 15 h dark | flowers |
- Is it a short-day plant or a long-day plant?
- Why does treatment 3 fail?
- Why does treatment 4 still work?
- Between which two lengths of dark period does the critical dark period lie?
Need a reminder? The key ideaShow
Photoperiodism is the response to the relative lengths of day and night. A short-day plant flowers when the unbroken dark period is longer than its critical dark period. A long-day plant flowers when it is shorter. A flash in the night breaks the dark period. The leaf perceives it, and florigen moves to the shoot tip.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) It flowers with the long night (treatment 1) and not with the short night (treatment 2). It is a short-day plant.
Step 2. (b) The flash cuts the h night into two shorter nights, about h each. Neither is long enough, so the plant does not flower.
Step 3. (c) A short dark spell in the day does not change the long night that follows. The plant still gets h of unbroken dark.
Step 4. (d) The h dark period worked and the h dark period failed. The critical dark period lies between h and h. Treatment 3 narrows it a little: halves of about h also failed.
Answer: (a) a short-day plant; (b) the flash breaks the night; (c) the night is not broken; (d) between h and h.
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