9.4 Reproduction in plants Flashcards
What has to happen for a plant to flower?
A meristem that produces the stem and leaves in the shoot apex has to be converted into a floral meristem or inflorescence meristem, from which all the parts of the flower will be produced.
Gene expression in flowering
-The conversion of a shoot meristem to a floral meristem is controlled by a change in gene expression.
-The activation of flowering genes and the suppression of some vegetative genes is controlled by chemical pathways.
-Each pathway is triggered by certain conditions, and each plant species needs a particular set of conditions in order to flower.
Give examples of the sets of conditions each plant species needs in order to flower
-Internal factors such as age of the plant, molecular signalling regarding food storage and plant health, and the concentration of hormones like gibberellin.
-External factors such as temperature, and photoperiod – the relative length of day and night, that affects molecules sensitive to environmental stimuli. For example, phytochrome molecules are sensitive to light.
The conversion of a meristem into a floral meristem to promote flowering involves a change in gene expression in the ___
Shoot apex
What is the change in gene expression a response to?
Internal and external factors, often including the photoperiod (the relative length of light and dark periods.)
What is the main trigger for flowering for many plant species, especially outside of tropical climates?
Photoperiod.
How is the length of darkness measured by plants?
It is measured by photoreceptor molecules called phytochromes.
Long-day plants
-Some angiosperms, known as ‘long-day’ plants, develop flowers when phytochromes detect a period of darkness shorter than a critical length.
-As the days grow longer during the spring, the nights grow shorter, triggering the changes in gene expression that cause flowering.
-These flowers will bloom in late spring and summer.
When do long-day plants bloom?
In late spring and summer.
Short-day plants
-‘Short-day’ plants develop flowers when phytochromes detect a period of darkness longer than a critical length.
-Flowering is inhibited during the summer until the night grows long enough to activate the flowering genes.
-These plants bloom in late summer and autumn.
-Interestingly, a brief burst of light that separates the darkness into two lengths both shorter than the critical period will cause the plant to behave as if it is in a long day.
When do short-day plants bloom?
In late summer and autumn.
Day-neutral plants
Some plants are day-neutral and do not require any particular length of darkness to flower.
Diagram showing long-day plants flower in response to a short period of darkness and short-day plants flower in response to a long period of darkness.
How does phytochrome detect patterns of light and dark?
-It can do this because it changes back and forth between two forms, Pr and Pfr:
-In daylight, phytochrome rapidly converts to Pfr.
-In darkness, phytochrome slowly converts to Pr.
-Similar to grains of sand in an hourglass, the plant measures the length of darkness by the proportion of Pr phytochromes.
-The more Pr is present, the more time has passed in darkness.
-The plant can only measure duration of darkness; within a few minutes of daylight most of the phytochrome is in the Pfr form, so the duration of light cannot be determined.
Why can the duration of light not be determined by the phytochrome?
The plant can only measure duration of darkness; within a few minutes of daylight most of the phytochrome is in the Pfr form.
What do the names Pr and Pfr (little r and fr) refer to?
The wavelength of light that will convert the phytochrome to its other form.
Explain how Pr and Pfr work
-Pr is sensitive to red light.
-Absorbing a red photon (a wavelength of about 660 nm) will convert Pr to Pfr. Pfr can be converted in two ways. First, Pfr slowly and spontaneously changes to Pr in darkness.
-Pfr can also be rapidly converted to Pr by exposure to far-red light (photons with a wavelength of about 730 nm).
-Sunlight contains more red light than far-red light, so in sunlight most phytochrome is in the Pfr form.
Diagram showing interconversion between Pfr and Pr
What factor controls flowering?
Although plants are referred to as short-day or long-day plants, it is the length of darkness that controls flowering.
How does the length of darkness control flowering in long- and short-day plants?
-In long-day plants, a short night induces flowering.
-In short-day plants, a long night induces flowering.
How do commercial flower growers make use of the knowledge of photoperiodism to control when crops produce their flowers?
-For instance, chrysanthemums are short-day plants.
-When farmers want to retain vegetative plants in a season with long nights, they use a brief burst of light to interrupt the darkness, preventing flowering.
-When they wish to produce flowers during a season with short nights, they artificially extend the darkness by covering the plants or greenhouse with black cloth.
Picture of black cloth that prevents natural light can be used to induce flowering in short-day plants like chrysanthemums in commercial flower production
What is the molecule that detects light in plants and affects flowering?
Phytochrome
A short-day plant will flower when ___
The period of darkness is longer than a certain critical value.
What would cause a high proportion of Pr compared to Pfr?
Long exposure to darkness
Long exposure to darkness will allow most of the Pfr to convert back to Pr. Brief exposure to far-red light would have the same result.
What is pollination?
The transfer of pollen from the anther of a flower to the stigma.
What is pollen?
A multicellular structure containing one or two sperm.
What is the stigma?
A part of a flower’s female reproductive organ that receives the pollen.
How many pollen grains can a single anther produce and what are they?
A single anther may produce hundreds of thousands of pollen grains – microscopic, multicellular, haploid structures that contain male gametes in angiosperms (and gymnosperms).
Describe the structure of a pollen grain
-Each pollen grain has an outer shell with a distinct shape, size and pattern of proteins that helps a plant recognise pollen of its own species.
-Inside the shell are one or two sperm, and another haploid cell that grows a tube from the stigma through the style to the ovule.
-This allows the sperm to reach the egg for fertilisation.
A colourised micrograph of pollen grains from six species of plants.
How is pollen carried from the anther to the stigma?
By wind or animals, or very rarely, water.
Give examples of wind-pollinated plants
Many important species of grass and cereals.
What method of pollination does the vast majority of flowering plants use?
Animal-pollination
Describe the relationship between flowers and the animals that pollinate them
-Plants often have a close relationship with the animals that pollinate them.
-Some are parasitic, where the plant benefits at a cost to the animal, usually by taking its time and energy with no reward.
-However, most relationships between plant and pollinator are mutualistic, meaning that both species benefit.
What is Rafflesia arnoldii (italics) and how does it pollenate?
-One of the largest flowers in the world is produced by Rafflesia arnoldii.
-Not only does this flower parasitise a host plant for nourishment, it lures its pollinating flies with a stench similar to decaying animals.
-Rafflesia does not compensate the flies with nectar or edible pollen as most species do; it traps them inside the flower overnight before releasing them with a load of pollen.
Picture of Rafflesia arnoldii, one of the largest flowers in the world
Describe pollination in a carnivorous butterwort plant
-The leaves of the tiny carnivorous butterwort plant prey on non-pollinating insects; holding the flower on long stalks away from the leaves minimises the risk of trapping and digesting its flying pollinators.
-Although the butterwort genus preys on other insects, their relationship with pollinators is mutualistic.
-The butterworts benefit because their pollen is delivered to another flower of their species, allowing them to reproduce and maintain genetic diversity.
-The pollinating insects benefit by consuming the nutritious nectar supplied by the flower.
Picture of a flower of Pinguicula vulgaris, a carnivorous butterwort
.
Explain how mutualistic pollination relationships work
-In mutualistic pollination relationships, the plants receive the benefit of the animal’s delivery service, transporting pollen from the anther of one flower to the stigma of another.
-Some pollinators visit many types of flowers, while others are highly specific.
-Pollinating animals receive the benefit of nutritious sugar-rich nectar, edible protein-rich pollen, lipid-rich secretions for feeding larvae, or even collectible pheromone molecules to assist in attracting a mate.
What is obligate mutualism?
-Some flowers have such a specific structure that they can only be pollinated by a single species of insect.
-This is obligate mutualism, where one species cannot survive without the other. If the pollinating moth in Figure 4 were to go extinct, it would be unlikely that there would be another moth with a proboscis of the correct length to reach the nectar and pollen.
-Lacking a means of sexual reproduction, the orchid would probably go extinct as well.
Examples of insects that are pollinators
Bees, wasps, beetles, moths and butterflies.
Exaples of pollinators other than insects
-Other pollinators include birds, bats, mice and even lemurs.
-The marsupial honey possum is one of the only mammal species to live almost entirely on flower nectar.
How do most flowering plants carry out sexual reproduction?
Using mutualistic relationships with pollinators.
What does successful pollination allow?
This allows fertilisation to occur inside the ovule, when the sperm and egg fuse inside to form a diploid embryo.
Which letter shows most specifically where fertilisation takes place?
G
Diagram of an animal-pollinated flower
Drawing animal-pollinated flowers for exam
You should be able to draw and label an animal-pollinated flower with all of the structures shown in Figure 2.
A labelled Phalaenopsis sp. orchid.
Which letter specifically labels the part of the flower that produces pollen?
J
Diagram showing the structure of a typical dicot seed
Picture of the structure of a peanut seed (dicot)
Diagram of the structure of a corn seed (monocot)
Picture of seeds that have small hooks in the burs that are able to attach to the fur of animals
What is the name of the part labelled D?
Testa / seed coat
A and D in the figure above refer to:
A: plumule and D: cotyledon
