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A short-day plant is kept in long days. Four different plants are treated as follows…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A short-day plant is kept in long days. Four different plants are treated as follows.
| Plant | Treatment | Result |
|---|---|---|
| A | all the leaves removed, shoot tip given short days | no flowers |
| B | one leaf given short days, the rest in long days | flowers |
| C | one leaf given short days, and its leaf stalk heated at once to kill the phloem | no flowers |
| D | the same as C, but the stalk is heated h after the short days | flowers |
- What do A and B together show?
- What does C show?
- What does D show?
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) Without leaves the plant cannot respond, even if the tip gets short days. With only one leaf in short days it flowers. So the leaf perceives the photoperiod, not the tip.
Step 2. (b) Killing the phloem at once stops the plant from flowering. So a message from the leaf moves through the phloem to the tip.
Step 3. (c) Heating the stalk h later did not stop it. By then the message had already left the leaf and reached the tip.
Answer: (a) the leaf perceives the photoperiod; (b) the message travels in the phloem; (c) the message had left the leaf before the stalk was killed.
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