Chapter 39

Plant Responses: Long-Day and Short Day Plants

Essential Idea: Reproduction in flowering plants is influenced by the biotic and

abiotic environment.

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Essential Idea: Reproduction in flowering plants is influenced by the biotic and abiotic environment.

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Response to Stimuli

Plants are sensitive to a wide range of stimuli.

They elicit a response.They use a signal transduction pathway.

Signal Reception

Signals are detected by receptors.Proteins change in response to the

stimulus.

Signal Transduction

Second messengers are small, internally produced chemicals.

They transfer and amplify signals from the receptor to the other proteins causing a response.

One signal receptor protein can give rise to hundreds of specific enzymes.

In this way, 2nd messenger signal transduction leads to rapid amplification of the signal.

Response

Signal transduction leads to one or more cellular pathways being regulated.

Usually, this leads to an increase in the activity of certain enzymes.

Response: 2 Main Mechanisms

1. Stimulating transcription of mRNA.2. Activating existing enzyme

molecules.

1. Stimulating Transcription of mRNAThis is called transcriptional regulation.These transcription factors bind directly

to DNA molecules and control the transcription of specific genes.

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2. Activating Existing Enzyme MoleculesThis gives rise to hundreds of second

messenger molecules which leads to hundreds of activated enzymes.

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The Change in Phytochrome

Light causes the conformation of phytochrome to change.

This leads to an increase in cGMP (2nd messenger) and Ca2+.

cGMP activates protein kinases

The Change in Phytochrome

Protein kinases are activated by cGMP and Ca2+, and can act to phosphorylate and activate other enzymes.

These can be used to stimulate or shut down transcription.

When transcription is affected, the enzymes can now synthesize proteins needed for the specific response.

The Change in Phytochrome

The mechanism by which a signal promotes a new developmental course depends on the activation of positive or negative control factors.

Light

Light is an important environmental factor in the growth and development of plants.

Photomorphogenesis is the effect of light on plant morphology.

The ability of a plant to perceive light allows plants to measure the passage of days and seasons.

Photoperiodism

Photoperiodism is the physiological response of plant due to a change in the lengths of night and day--a photoperiod.

Different Types of Plants

There are 3 general varieties of plants classified according to their light requirements for flowering:1. Short-day plants2. Long-day plants3. Day-neutral plants.

Short-Day Plants

Respond the long nights. A.k.a. long-night plants.They usually flower in the late summer,

fall, or winter as the light period is shorter than 14 hours, for example.

Long-Day Plants

Respond to short nights.A.k.a. short-night plants.They flower when the light period is

longer than 14 hours, for example.

Day-Neutral Plants

These are unaffected by the light period, and flower when they reach maturity.

Tomatoes, rice, and dandelions.

Classic Experiments

In the 1940’s scientists began experimenting with photoperiods.

They looked at the length of the night and day.

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

They looked at flowering: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.

However, if the nighttime portion of the photoperiod is interrupted by a short period of dim light, the plant doesn’t flower.

Classic Experiments

The opposite is true for long-day plants.When long day plants are grown in a

photoperiod of a long night, flowering doesn’t occur.

However, if the long night portion of the experiment is interrupted by a brief period of dim light, flowering will occur.

From These Experiments

Red light is most effective at interrupting the nighttime portion of the photoperiod.

Scientists have demonstrated that phytochrome is the pigment that measures the photoperiod.

Extending the Experiments

Scientists at the USDA conducted these experiments.

Phytochrome was demonstrated to be the pigment responsible for seed germination.

From this, they were able to elucidate the flowering cycle.

USDA Flowering Experiments

Seeds were subjected to a variety of monochromatic light.

Red and far-red light opposed each other in their germinating ability.

One induced germination, the other inhibited it.

USDA Flowering Experiments

It was determined that the two different forms of light switched the phytochrome back and forth between two isomeric forms.

USDA Flowering Experiments

One form caused seed germination, the other inhibited the germination response.

USDA Flowering Experiments

The question: How do plants in nature illicit a response to light and begin germination?

USDA Flowering Experiments

If seeds are kept in the dark, they synthesize Pr.

When seeds are illuminated with sunlight, they begin to be converted to Pfr.

The appearance of Pfr is one of the ways plants detect sunlight.

Adequate sunlight converts Pr to Pfr and triggers germination.

USDA Flowering Experiments

In the flowering response, scientists were able to show the effects of the red and far red light on the flowering ability in plants.

Again, the 2 forms of light canceled each other.

Other Stimuli

There are also a wide variety of stimuli other than light that effects plant growth.

Gravity, mechanical stimuli, and environmental stress also play a role in plant growth and development.