IB DP Biology Topic 9: Plant biology: 9.4 Reproduction in plants: Study Notes

​Essential idea:

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

​Nature of science:

• Paradigm shift—more than 85% of the world’s 250,000 species of flowering plant depend on pollinators for reproduction. This knowledge has led to protecting entire ecosystems rather than individual species. (2.3)
  • Contrast traditional conservation efforts with newer strategies of conservation.

9.4 U 1 ​Flowering involves a change in gene expression in the shoot apex.

• Compare the vegetative and reproductive phases of the angiospermatophyta life cycle.
• State that flowers are produced from a shoot apical meristem.
• State two abiotic factors that may trigger flowering.
• Compare the timing of flowering in short-day plants and long-day plants.
• Outline the process by which changes in gene expression trigger flowering.

9.4 U 2 ​The switch to flowering is a response to the length of light and dark periods in many plants ​

• State the role of the pigment phytochrome.
• Describe the conversions between the two forms of phytochrome.
• Describe role of phytochrome in controlling flowering in long and short day plants.

Photoperiodism is a biological response to a change in the proportions of light and dark in a 24‐hour daily cycle. Plants use it to measure the seasons and to coordinate seasonal events such as flowering.

The control of flowering is achieved through a process called photoperiodism. The critical factor is not actually day length – it is night length.

• Flowering in long day and short day plants is controlled by the plants biological clock
• The length of the darkness/night is the critical factor

Short days / long nights (fall, winter, spring)

Long days / short nights (summer)

Photochromes

• Combination of a protein and pigment molecule
• Can change its shape (active site) – will change the way chemicals around it react to it. –> can change into 2 different shapes
• Blue-green plant pigments that are produced in the leaf
• Present in low concentration
• Is a combination of protein molecule + pigment molecule
• Is able to absorb light of a certain wavelength and change its structure / form
  • P(r) absorbs mainly red light
  • P(fr) absorbs mainly far red light
• The structure / form influences its reaction with surrounding molecules

Plants use phytochromes to measure the length dark period / critical period

• During the day (presence of light) P(r) converts to P(fr) (faster if exposed to only red light)
• During the night (absence of light), P(fr) converts to P(r) (faster if exposed to only far red light)

The ACTIVE form is P(fr) – stimulates some growth and inhibits other growth.

9.4 U 3 ​Success in plant reproduction depends on pollination, fertilization and seed dispersal

• ​Define pollination, fertilization and seed dispersal.
• State the changes to the ovule and ovary that result from fertilization.
• List mechanisms of seed dispersal.

Plants can reproduce in a number of different ways:

• Vegetative propagation (asexual reproduction from a plant cutting)
• Spore formations (e.g. moulds, ferns)
• Pollen transfer (flowering plants – angiospermophytes)

Sexual reproduction in flowering plants involves the transfer of pollen (male gamete) to an ova (female gamete)

​Pollination

• The process in plants in which pollen grains (male gametes) are transferred to the female gametes (ovules contained within the carpel), thereby enabling fertilization and sexual reproduction.

Fertilization

• The fusion of male and female gametes to produce a diploid zygote. In flowering plants, a double fertilization occurs producing the embryo and the endosperm nucleus.

Seed dispersal

• The movement or transport of seeds away from the parent plant. This decreases competition between parents and offspring and promotes diversity within the species. Seeds can be dispersed through gravity, wind, water and by animals.

9.4 U 4 ​Most flowering plants use mutualistic relationships with pollinators in sexual reproduction.

• Define mutualism.​
• Explain an example of mutualism between a flowering plant and its pollinator.

​Pollination is the act of transferring pollen grains from the male anther of a flower to the female stigma. The goal of every living organism, including plants, is to create offspring for the next generation. One of the ways that plants can produce offspring is by making seeds.

• Mutualism is the relationship between two organisms, where both organisms benefit
• Sexual reproduction depends on the transfer of pollen stamen from one plant to the stigma of another plant
• Pollen can be transferred by wind and possibly water, but more commonly pollen is transferred by animals known as pollinators such as bees, butterflies, birds, and bats
• Pollinators gain food from nectar and the plant gains a method to transfer pollen to another plant to allow for sexual reproduction

9.4 A 1 ​Methods used to induce short-day plants to flower out of season.

• State how plants can be manipulated to force flowering out of season.​

Flowering can be controlled by:

• Greenhouses (temperature and humidity)
• Light conditions

Short day plants are kept in the:

• Dark during daylight
• Given bursts of far red light
• Keeps P(fr) low so flowering isn’t inhibited

Long day plants:

• Artificially lit during night
• Short bursts of light using red light can interrupt darkness and promote flowering
• It’s possible to expose plants to light for short periods to keep costs down but long enough to interrupt the dark period

9.4 S 1 ​Drawing internal structure of seeds.

• Draw and label the structure of seeds, including: Embryo root, Embryo shoot, Cotyledons, Testa, Micropyle, and Hilum
• State the function of the different parts of the seed.

9.4 S 2 ​Drawing of half-views of animal-pollinated flowers.

• Draw and label an animal pollinated flower, including: Nectar-secreting glands, Petals, Sepals, Anthers, Filaments, Carpel, Stigma, Style, Ovary and Ovule
• State the function of the different parts of the animal-pollinated flower

9.4 S 3 ​Design of experiments to test hypotheses about factors affecting germination.

• Define germination.
• Outline why water, oxygen and warmth are required for germination.
• Outline the role of gibberellin during germination.
• Write five example problem questions for experiments that could test factors affecting germination.

The experiment linked shows an experiment that gives results for all factors however when making your experiment make sure to:

• Pick a type of seed
• Vary one factor for investigation
• Mention how you will keep other factors constant
• How results will be collected and how it will be decided if a plant germinated or not

GERMINATION PROCESS 

Seeds need water for germination. Water rehydrates the seed’s tissues causing the cells to expand and metabolism to become reactivated. Once metabolism has been reactivated embryonic growth can begin. The appropriate temperature is needed to allow enzymes to work efficiently during metabolic reactions. Oxygen is needed for aerobic respiration to take place in the seed. Some plants have seeds that require specific variables in order for germination to take place such as fire or smoke, disruption of the seed coat, or removal of surface inhibitors by water.

• Water is absorbed by the seed through the micropyle and the seed coat.
• Water causes the release of a hormone called gibberellin or gibberellic acid (GA).
• GA causes the cells to expand and elongate, eventually allowing the root to break through the seed coat (testa).
• GA also stimulates the production of enzymes (specifically amylase) that hydrolyze starch located in the seed’s endosperm into maltose.
• Maltose is further broken down into glucose which can be transported to areas of growth in the cell.
• Glucose is used in aerobic respiration to produce energy for growth of the embryonic root and shoot