Course 11.3Lesson 2 of 3
Photoperiodism is the response of a plant to the relative lengths of day and night. Short-day plants flower when the dark period is longer than a critical length, long-day plants when it is shorter, and day-neutral plants whatever the length. The dark period is what counts, and a flash of light in the night breaks it. The leaf perceives it, and a flowering signal moves to the shoot tip.
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Chrysanthemums flower in autumn. Wheat flowers in spring. The weather changes from year to year, but each of these plants flowers at nearly the same time every year. How do they know the season?
Temperature is one clue, but it is not a good one. It can be hot or cold on any day. The length of the day is a much better clue. It changes in the same way every year. A plant that can measure it can tell the season.
In 1920 two American scientists, Garner and Allard, grew a tall tobacco plant called Maryland Mammoth. It grew very large in summer, but it did not flower. They moved it into a greenhouse in winter, and it flowered. Warmth was not the cause, as the greenhouse was warm in both seasons.
They then covered the plants for part of each summer day, so that the days became short. The plants flowered. They did the same with soybean. The plants were not answering to the amount of light they had received. They were answering to the length of the day.
This response of plants to the relative lengths of day and night is called photoperiodism. A photoperiod is the length of the light period in a day. Flowering is the best known photoperiodic response.
The critical day length is different for each plant. A short-day plant does not need a short day. It needs a day that is shorter than its own critical length. A long-day plant needs a day that is longer than its critical length. A plant may be a short-day plant and still flower in the long days of summer, if its critical length is very long.
At first people thought that the plant measured the light. Later work showed that the dark period is just as important. In fact it is the dark period that the plant measures.
Look at the picture. A short-day plant flowers when the dark period is longer than its critical dark period. A long-day plant flowers when the dark period is shorter than its critical dark period. So a short-day plant is really a long-night plant. A long-day plant is a short-night plant.
The strongest proof is the night break. Hamner and Bonner (1938) worked with cocklebur, a short-day plant. They gave the plant a long dark period and then broke the night in the middle with a short flash of light. The plant did not flower. The long dark period had been cut into two short ones. In the same way, a long-day plant flowers when a long night is broken by a flash.
When they broke the day with a short spell of darkness, nothing changed. The plants answered as they would to the long day. Only a break in the night has an effect.
So the plant measures an unbroken dark period.
Flowers form at the shoot tip. But is it the tip that measures the day? Look at the second picture.
So the leaves perceive the length of day and night. The tip is not the place of perception, although it is the place where the flowers form. A message must travel from the leaf to the tip.
In 1936 the Russian scientist Chailakhyan named this message florigen, the flowering hormone. Florigen is made in the leaf, in the right photoperiod. It then moves to the shoot tip and changes it into a flowering tip.
Grafting supports this idea. A plant that has been given its right photoperiod can be grafted to another plant that is kept in the wrong photoperiod. Both then flower, as in picture 4. The message crosses the graft. In some pairs it even passes between a short-day plant and a long-day plant. It moves through the phloem. If the phloem of the leaf stalk is killed by heat soon after the plant is treated, the message cannot leave, and the plant does not flower.
An experiment in this topic usually gives a table of treatments and results. Read it in this order.
Beyond the Class 11 book: the pigment that senses the light is phytochrome. It has two forms. Red light turns it into one form, and far-red light turns it back. A flash of red light is the most effective night break, and far-red light cancels it. Phytochrome is also behind the germination of lettuce seeds, which you saw in the last lesson.
You should find that the short-day plant flowers only when the dark period is 10 hours or more and no flash breaks it. The long-day plant flowers when the dark period is shorter than 10 hours, or when a flash breaks a long night.
The program has a function that tells whether a plant flowers. Read how it uses the dark period.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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