Chapter 39 Plant Responses LongDay and Short Day
- Slides: 36
Chapter 39 Plant Responses: Long-Day and Short Day Plants üEssential Idea: Reproduction in flowering plants is influenced by the biotic and abiotic environment.
ü Essential Idea: Reproduction in flowering plants is influenced by the biotic and abiotic environment. 2
Response to Stimuli ü Plants are sensitive to a wide range of stimuli. ü They elicit a response. ü They use a signal transduction pathway.
Signal Reception ü Signals are detected by receptors. ü Proteins change in response to the stimulus.
Signal Transduction ü Second messengers are small, internally produced chemicals. ü They transfer and amplify signals from the receptor to the other proteins causing a response. ü One signal receptor protein can give rise to hundreds of specific enzymes. ü In this way, 2 nd messenger signal transduction leads to rapid amplification of the signal.
Response ü Signal transduction leads to one or more cellular pathways being regulated. ü Usually, this leads to an increase in the activity of certain enzymes.
Response: 2 Main Mechanisms ü 1. Stimulating transcription of m. RNA. ü 2. Activating existing enzyme molecules.
1. Stimulating Transcription of m. RNA ü This is called transcriptional regulation. ü These transcription factors bind directly to DNA molecules and control the transcription of specific genes.
2. Activating Existing Enzyme Molecules ü This gives rise to hundreds of second messenger molecules which leads to hundreds of activated enzymes. http: //www. utm. utoronto. ca/~w 3 bio 315/lecture 10. htm 12
The Change in Phytochrome ü Light causes the conformation of phytochrome to change. ü This leads to an increase in c. GMP (2 nd messenger) and Ca 2+. ü c. GMP activates protein kinases
The Change in Phytochrome ü Protein kinases are activated by c. GMP and Ca 2+, and can act to phosphorylate and activate other enzymes. ü These can be used to stimulate or shut down transcription. ü When transcription is affected, the enzymes can now synthesize proteins needed for the specific response.
The Change in Phytochrome ü The mechanism by which a signal promotes a new developmental course depends on the activation of positive or negative control factors.
Light ü Light is an important environmental factor in the growth and development of plants. ü Photomorphogenesis is the effect of light on plant morphology. ü The ability of a plant to perceive light allows plants to measure the passage of days and seasons.
Photoperiodism ü Photoperiodism is the physiological response of plant due to a change in the lengths of night and day--a photoperiod.
Different Types of Plants ü There are 3 general varieties of plants classified according to their light requirements for flowering: ü 1. Short-day plants ü 2. Long-day plants ü 3. Day-neutral plants.
Short-Day Plants ü Respond the long nights. ü A. k. a. long-night plants. ü They usually flower in the late summer, fall, or winter as the light period is shorter than 14 hours, for example.
Long-Day Plants ü Respond to short nights. ü A. k. a. short-night plants. ü They flower when the light period is longer than 14 hours, for example.
Day-Neutral Plants ü These are unaffected by the light period, and flower when they reach maturity. ü Tomatoes, rice, and dandelions.
Classic Experiments ü In the 1940’s scientists began experimenting with photoperiods. ü They looked at the length of the night and day. ü In these experiments, they found that short-day plants flower when days are 16 hours or shorter (nights are 8 hours or longer).
Classic Experiments ü They looked at flowering: ü They found that if the daytime portion of photoperiod is broken by a brief period of darkness, there is no effect--that is, the plant still flowers. ü However, if the nighttime portion of the photoperiod is interrupted by a short period of dim light, the plant doesn’t flower.
Classic Experiments ü The opposite is true for long-day plants. ü When long day plants are grown in a photoperiod of a long night, flowering doesn’t occur. ü However, if the long night portion of the experiment is interrupted by a brief period of dim light, flowering will occur.
From These Experiments ü Red light is most effective at interrupting the nighttime portion of the photoperiod. ü Scientists have demonstrated that phytochrome is the pigment that measures the photoperiod.
Extending the Experiments ü Scientists at the USDA conducted these experiments. ü Phytochrome was demonstrated to be the pigment responsible for seed germination. ü From this, they were able to elucidate the flowering cycle.
USDA Flowering Experiments ü Seeds were subjected to a variety of monochromatic light. ü Red and far-red light opposed each other in their germinating ability. ü One induced germination, the other inhibited it.
USDA Flowering Experiments ü It was determined that the two different forms of light switched the phytochrome back and forth between two isomeric forms.
USDA Flowering Experiments ü One form caused seed germination, the other inhibited the germination response.
USDA Flowering Experiments ü The question: How do plants in nature illicit a response to light and begin germination?
USDA Flowering Experiments ü If seeds are kept in the dark, they synthesize Pr. ü When seeds are illuminated with sunlight, they begin to be converted to Pfr. ü The appearance of Pfr is one of the ways plants detect sunlight. ü Adequate sunlight converts Pr to Pfr and triggers germination.
USDA Flowering Experiments ü In the flowering response, scientists were able to show the effects of the red and far red light on the flowering ability in plants. ü Again, the 2 forms of light canceled each other.
Other Stimuli ü There also a wide variety of stimuli other than light that effects plant growth. ü Gravity, mechanical stimuli, and environmental stress also play a role in plant growth and development.
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