Photoperiodism
Essential Knowledge 2C2: Organisms respond to changes in their external environment.
Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals. Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of the light and dark periods. Here it should be emphasized that photoperiodic effects relate directly to the timing of both the light and dark periods.
Many flowering plants (angiosperms) use a photoreceptor protein, such as phytochrome or cryptochrome, to sense seasonal changes in night length, or photoperiod, which they take as signals to flower.
In 1920, W. W. Garner and H. A. Allard published their discoveries on photoperiodism and felt it was the length of daylight that was critical, but it was later discovered that the length of the night was the controlling factor. Photoperiodic flowering plants are classified as long-day plants or short-day plants, even though night is the critical factor, because of the initial misunderstanding about daylight being the controlling factor. Each plant has a different length critical photoperiod, or critical night length.
Long-day plants flower when the day length exceeds their critical photoperiod. These plants typically flower in the northern hemisphere during late spring or early summer as days are getting longer. In the northern hemisphere, the longest day of the year is on or about 21 June (solstice). After that date, days grow shorter (i.e. nights grow longer) until 21 December (solstice). This situation is reversed in the southern hemisphere (i.e. longest day is 21 December and shortest day is 21 June). In some parts of the world, however, "winter" or "summer" might refer to rainy versus dry seasons, respectively, rather than the coolest or warmest time of year. Examples are oat, carnation, and clover.
Short-day plants flower when the day lengths are less than their critical photoperiod. They cannot flower under long days or if a pulse of artificial light is shone on the plant for several minutes during the middle of the night; they require a consolidated period of darkness before floral development can begin. Natural nighttime light, such as moonlight or lightning, is not of sufficient brightness or duration to interrupt flowering. Examples are coffee, strawberry, and tobacco.
In general, short-day plants flower as days grow shorter after 21 June in the northern hemisphere, which is during summer or fall. The length of the dark period required to induce flowering differs among species and varieties of a species.
Photoperiod affects flowering when the shoot is induced to produce floral buds instead of leaves and lateral buds. Note that some species must pass through a "juvenile" period during which they cannot be induced to flower—common cocklebur is an example of a plant species with a remarkably short period of juvenility and plants can be induced to flower when quite small.
**Thank you Wikipedia for the information**
Photoperiodism Analysis
Directions: Answer the questions below using complete sentences.
1) What is photoperiodism?
2) How do plants know there are seasonal changes?
3) Initially, what did Dr. Garner and Dr Allard think was controlling the flowering time of plants? Why were they wrong?
4) Describe how long day plants operate.
5) Describe how short day plants operate.
6) Why are “long day” and “short day” bad names for this process? What should they be named instead?
7) If a plant is flowering, what structure will produce the flower buds instead of the lateral shoots?