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.
What happens after the sperm and egg fuse inside to form a diploid embryo?
-The ovule develops into a seed containing the embryonic plant and a supply of food for the embryo during germination.
-The ovary containing the ovule or ovules develops into the fruit.
What is a fruit?
-Any plant structure containing seeds is considered a fruit in scientific terms.
-For example, a bell pepper, aubergine (eggplant) and tomato are all fruits.
-Carrots and potatoes are true vegetables because they are not seed-containing structures and do not develop from the ovule.
What is the difference between pollination and fertilization?
-Pollination occurs when pollen grains attach to the stigma .
-Fertilisation occurs after pollination when the sperm and egg fuse in the ovule.
The pollen produced by wind-pollinated plants is often smaller than the pollen produced by animal-pollinated plants. What could be a reason for this?
Small pollen grains can be carried further by the wind.
Which letter shows most specifically where fertilisation takes place?
G
Name the process by which pollen is transferred from the anther of one flower to the stigma of another.
Pollination
How can plants reproduce asexually?
Using roots, stems, leaves and other vegetative structures.
What does sexual reproduction require?
Gametes produced by meiosis.
How are angiosperms unique?
They have reproductive structures called flowers that produce gametes and provide a site for fertilisation.
How are flowers produced?
-They are produced by the meristems in the shoot and come in an enormous array of shapes and sizes.
-However, the bright petals and perfume-like scents that we associate with flowers are primarily found only when there is a need to attract animal pollinators.
What is an advantage of animal pollination?
It allows targeted delivery of pollen; they pick up the male gametes (found inside the pollen grains) and transport them to the female part of another flower where they can fertilise an egg.
What is the appearnace and smell of the flowers of wind-polllinated plants
They often have little scent and reduced or absent petals.
What are the structure of a typical animal-pollinated flower?
-Pistil (carpel)
-Stamen
-Petals
-Sepals
-Receptacle
Describe the structure and function of the pistil (carpel)
This is the female part of the flower, made up of the stigma, style and ovary, with the ovary containing the ovule or ovules.
What is the stigma?
The sticky area at the tip of the carpel where pollen grains are deposited.
What is the style?
The stalk that supports the stigma and connects it to the ovary; the pollen tube grows through the style to allow sperm to reach the egg.
What is the ovary and ovule?
-The ovary is the base of the style, containing one or more ovules; in most species it develops into the fruit.
-The ovule contains an egg (female gamete) and, after fertilisation, develops into a seed.
What is the stamen?
The male part of the flower, made up of the anther and filament.
What is the anther?
The part of the stamen that produces pollen.
What is the filament?
The stalk that supports the anther.
Describe the structure and function of the petals
These are the structures surrounding the reproductive parts of the flower; evolved from leaves and often coloured to attract pollinators by contrasting strongly with green leaves.
Describe the structure and function of the sepals
These are the structures that enclose and protect the developing flower bud; they are found beneath the petals and may offer support when the flower is in bloom; they are often green but are brightly coloured in some species.
Describe the structure and function of the receptacle
This is the thickened part of the stem at the base of the flower.
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.
Single-sex flowers vs. both male and female
-Some flowers are single-sex, having the carpel or stamen only.
-When a single flower has both male and female structures, there are usually adaptations to prevent self-pollination, which would limit genetic diversity.
What can happen in plants to promote cross-pollination?
The carpel and stamen may mature at different times or the stigma may have chemical methods of rejecting its own pollen.
What is cross-pollination?
Pollination by a different individual.
How are animal-pollinated flowers special (reword)
Many animal-pollinated flowers have modified or missing structures and additional adaptations specific to their pollination relationship.
Give an example of how any animal-pollinated flowers have modified or missing structures and additional adaptations specific to their pollination relationship
For example, in orchids the sepals are as brightly coloured as the petals and have a modified petal that forms a passageway for the pollinator.
A labelled Phalaenopsis sp. orchid.
Which letter specifically labels the part of the flower that produces pollen?
J
What is a plant seed and what does it contain?
-A plant seed is a multicellular ripened ovule that has the potential to grow into a new plant.
-It contains an embryonic plant as well as structures to protect and nourish the embryo.
Give examples of how different the sizes of seeds can be
-Seeds can be massive.
-The largest seed is produced by the coco de mer, Lodoicea maldivica, and may weigh over 15 kilograms.
-Other seeds, like those of orchids, are so light they weigh only a few hundred micrograms, about the same as a typical grain of pollen.
What structures does a dicotyledonous seed normally contain?
-The embryonic plant
-Testa
-Hilum
-Micropyle
What does the embryonic plant comprise?
-Radicle – an embryonic root
-Plumule – an embryonic shoot
-Hypocotyl – a shoot above the root and below the cotyledons
-Cotyledons – modified leaves that store food for the embryo
What is the testa?
A seed coat that protects the embryo and food stores
What is the hilum?
A scar where the seed was attached to the ovary
What is the micropyle?
A small pore above the hilum where the pollen tube entered to allow fertilisation
Diagram showing the structure of a typical dicot seed
Picture of the structure of a peanut seed (dicot)
How are monocotyledonous seeds different from dicotyledonous seeds?
-Monocotyledonous seeds, like corn, have a slightly different structure.
-There is only one cotyledon and it does not store food.
-Instead, the cotyledon has a role in producing hormones and protecting the embryo.
-The food reserves are outside the embryo in a tissue called the endosperm.
Diagram of the structure of a corn seed (monocot)
Drawing seeds in the exam
-You should be able to make labelled drawings of the internal structure of seeds.
-If you are asked to draw a seed, you should draw and label a dicotyledonous seed like a bean. It is possible, but unlikely, that you would be expected to know the anatomy of a monocotyledonous seed.
Why do seeds need to be dispersed?
-Seeds contain the offspring of plants produced by sexual reproduction.
-In order to colonise new areas and reduce competition for light and nutrients, seeds must be dispersed, which means taken to new areas.
How can seeds be dispersed?
Like pollen, seeds may be carried on the wind or by animals; seeds also travel on the water more commonly than pollen.
What is seed dispersal an important factor in?
The reproductive success of flowering plant species.
How have seeds adapted to suit their needs?
-Wind- and water-dispersed seeds have structures that are evolved to suit their needs.
-Water-dispersed seeds are buoyant.
-Many variations on wind dispersal have evolved, including gliders, helicopters, tumbleweeds, cottony fluff, and the parachute-like dandelion seeds.
Describe the dispersion of animal-dispersed seeds
-In animal-dispersed seeds, the plant and the animal often have a mutualistic relationship.
-The fruit, or ripened ovary, offers nourishment and may attract animals using bright colours and scent.
-When the ovary is consumed, the testa (seed coat) protects the seed from digestion.
-The animal receives the benefit of food from the fruit, and the indigestible seeds are carried to new areas in the digestive tract of animals, and later ‘planted’ in nutrient-rich feces.
What animals are the most common agents of seed dispersal?
Birds and mammals are the most common agents of seed dispersal by animals, but ants and other animals also disperse some seeds.
How are seeds adapted when they are in a relationship that is not mutualistic?
They ay have barbs or needles designed to catch on the fur or flesh of animals.
Picture of seeds that have small hooks in the burs that are able to attach to the fur of animals
What is seed dispersal?
Transport of seeds to new locations
What are the major vectors for seed dispersal?
-Major vectors are wind, water, and animals
-Mammals and birds are the most common animal seed-dispersal agents
Why must seeds be dispersed?
To reduce competition between closely related individuals and to colonize new areas.
What is the name of the part labelled D?
Testa / seed coat
A plant produces flowers with several brightly coloured petals and a strong scent. The seeds are small and embedded in a mass of cottony fibers. What vector is most likely to disperse the seeds?
The wind
A and D in the figure above refer to:
A: plumule and D: cotyledon
What happens once a seed has been produced?
-Once a seed has been produced, the embryo does not immediately begin to grow into a new individual.
-Many seeds have a period of dormancy, when their metabolic activities are suspended and there is no active growth.
What is germination?
The process in which an embryo grows and emerges from the seed coat.
Why do seeds require particular conditions for germination?
Plants have evolved many means to ensure that seeds germinate under the conditions that offer the best chance for survival.
How do light and heat affect germination?
-Light does not affect germination in most seeds, though some do require light, and some require darkness.
-Some seeds need to pass through periods of cold or extreme heat while others cannot grow until the testa is weakened by passage through a digestive tract.
What are the three requirements for germination that seeds share?
-Water
-Oxygen
-Temperature
Explain how water is a requirement for germination
-For metabolic reactions to occur, the seed must first rehydrate by taking in water through imbibition (Imbibition is a special type of diffusion that takes place when water is absorbed by solids-colloids causing an increase in volume.
-Examples include the absorption of water by seeds and dry wood).
-This is a passive process, because the solutes in the seed are highly concentrated.
Explain how oxygen is a requirement for germination
-The embryonic plant will need a great deal of oxygen because it will have a high rate of growth and metabolism.
-This means a high rate of oxygen-consuming cellular respiration.
-The plant does not have mature leaves during germination and thus does not perform photosynthesis or produce oxygen.
Explain how temperature is a requirement for germination
-Different seeds have different optimal growth temperatures, but they all require temperatures that allow enzymes to function well.
-Temperatures that are too high can denature enzymes, while cold temperatures can slow enzyme activity substantially.
Experiment into the effects of different factors on germination- dependent variable
Germination can be measured by the rate of seed growth over a set period of time
Flowering involves a change in gene expression in the ___
Shoot apex
