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Essential Idea

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

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Chapter 38

Angiosperm Reproduction

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Flowers Flowers are the reproductive shoots of the

angiosperm sporophyte. Floral organs are the sepals, petals, stamens

and carpels.

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Flowering

Flowering involves a change in gene expression at the shoot apex.

This change in gene expression happens when meristems in the shoot apex produce flowers in stead of leaves.

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Flowers

Flowers are the sexual structures of plants.

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Flowering

Temperature and the length of day influence the formation of flowers.

Light plays a role in the production of inhibitors and activators of genes that control flowering.

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Flowering

It is the dark period, specifically, that is the main trigger for the production of a flower.

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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).

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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.

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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.

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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.

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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.

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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.

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USDA Flowering Experiments

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

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USDA Flowering Experiments

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

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Flowering

The active form of phytochrome results in the transcription of the FT gene (flowering time).

FT mRNA gets transported in the phloem to the shoot apical meristem where it is translated into FT protein.

FT protein binds to a transcription factor enabling the activation of flowering genes.

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USDA Flowering Experiments

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

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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.

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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.

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Pollination

Pollination is the first step in the chain of events which leads to fertilization.

It occurs when pollen from the stamen of one plant lands on the stigma of another plant.

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Pollination

Pollination often occurs when pollen is transferred by the wind and/or animals.

Animals such as birds, bats and insects are the main sources of pollination.

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Pollination

Pollinators are attracted to the plants by scents given off by plants.

The nectar in plants serve as a food source for pollinators.

When the pollinators are getting the nectar, they are picking up pollen and transferring it from one plant to another.

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Pollination

Through the course of time, plants and pollinators have developed a mutualistic relationship where both of them benefit.

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Pollination

Ideally plants don’t self-fertilize. This ensures variety and a good mix of

genes for future generations of plants.

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Mechanisms Preventing Self-Fertilization

Dioecious species can’t self-fertilize because they are either stamenate or capellate.

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Mechanisms Preventing Self-Fertilization

Monoecious plants often have floral organs that develop at different times.

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Mechanisms Preventing Self-Fertilization

Some flowers arrange floral parts so plants can’t self-fertilize.

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Mechanisms Preventing Self-Fertilization

The most common mechanism is self-incompatibility.

An individual rejects its own pollen and that of its close relatives.

Biochemical blocks prevent pollen development.

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Fertilization

Fertilization occurs after pollination. In this process, when a pollen grain lands

on the stigma of a plant, a pollen tube grows down the carpel.

The male gametes travel through this pollen tube and eventually make their way to the egg.

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Double Fertilization Double fertilization is unique

to angiosperms. One sperm fertilizes the egg. The second sperm fertilizes the

polar nuclei forming a triploid (3n) nucleus in the center of the large, central cell of the embryo sac.

This large cell gives rise to the endosperm--the food storing tissue of the seed.

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Double Fertilization

Double fertilization is an evolutionary mechanism which ensures the development of the endosperm only in ovules where the egg has been fertilized.

This helps prevent wasted resources.

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Double Fertilization

After double fertilization: – Each ovule develops into a

seed.– The ovary develops into the

fruit that encloses the seed. The triploid nucleus divides

forming a multi-nucleated supercell.

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Fruit Development

Once fertilization occurs and a zygote is formed.

The zygote gives rise to an embryo which grows inside the ovule that contains the developing seed.

The entire ovary develops into a fruit containing one or more seeds.

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Fruit Development While the seeds are developing, the walls of the

ovary are developing into a fruit. The pericarp is the thickened walls of the ovary.

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Fruits

Simple fruits-derived from a single carpel or several fused carpels.

Ex: peas, peach, nut.

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Fruits Aggregate fruits

result from a single flower that has more than one separate carpel. Each one grows a small fruit.

Example: raspberry.

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Fruits

Multiple fruits develop from inflorescence. A group of flowers tightly clustered together.

When the walls of the ovaries thicken, they fuse together forming the fruit.

Example: pineapple.

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Fruits

Fruit usually ripens around the time seeds complete their development.– Example: Peaches

Some fruit ripens and then ages and dries out.– Example: Soybeans

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Seed Maturation and Development

As a seed matures, it prepares for dormancy.

To break the dormancy, the seed has to find an optimal condition.

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Seed Production

The chances that a seedling will survive and produce offspring is very low.

This is why many plants produce so many seeds.

This is also why so many plants use asexual reproduction.

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Plant Reproduction

Some plants use both sexual and asexual reproduction.– Asexual reproduction

– Advantages and disadvantages:• Offspring more hearty than seedlings.

• Susceptible to catastrophe.

– Sexual reproduction:• Genetic variability to deal with changing environments.

• May not always have a mate.

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Seed Dispersal

Once seeds have been produced, there are a variety of mechanisms by which they are dispersed.

Wind and animals play a major role in seed dispersal.