9.4 notes
9.4 Reproduction in plants
9.4.1 Plant reproduction
How plants reproduce:
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 transfer of pollen grains from an anther (male plant structure) to a stigma (female plant structure)
Many plants possess both male and female structures (monoecious) and can potentially self-pollinate
From an evolutionary perspective, cross-pollination is preferable as it improves genetic diversity
Fertilisation:
Fusion of a male gamete nuclei with a female gamete nuclei to form a zygote
In plants, the male gamete is stored in the pollen grain and the female gamete is found in the ovule
Seed dispersal:
Fertilisation of gametes results in the formation of a seed, which moves away from the parental plant
This seed dispersal reduces competition for resources between the germinating seed and the parental plant
There are a variety of seed dispersal mechanisms, including wind, water, fruits and animals
Seed structure will vary depending on the mechanism of dispersal employed by the plant
Cross pollination: transferring pollen grains from one plant to the ovule of a different plant
Pollen can be transferred by wind or water, but is commonly transferred by animals (called pollinators)
Pollinators are involved in a mutualistic relationship with the flowering plant – whereby both species benefit from the interaction
Flowers are the reproductive organs of angiospermophytes (flowering plants) and develop from the shoot apex
Changes in gene expression trigger the enlargement of the shoot apical meristem
This tissue then differentiates to form the different flower structures – sepals, petals, stamen and pistil
The activation of genes responsible for flowering is influenced by abiotic factors – typically linked to the seasons
Flowering plants will typically come into bloom when a suitable pollinator is most abundant
The most common trigger for a change in gene expression is day/night length (photoperiodism)
9.4.2 Flower Structure
Flowers are the reproductive organs of angiospermophytes (flowering plants) and contain male and female structures
Most plants contain both structures (monoecious) but some only contain one (dioecious)
9.4.3 Flower structures
Stamen (male)
Anther – pollen producing organ of the flower (pollen is the male gamete of a flowering plant)
Filament – slender stalk supporting the anther (makes the anther accessible to pollinators)
Pistil/carpel (female
Stigma – the sticky, receptive tip of the pistil that is responsible for catching the pollen
Style – the tube-shaped connection between the stigma and ovule (it elevates the stigma to help catch pollen)
Ovule – the structure that contains the female reproductive cells (after fertilisation, it will develop into a seed)
Other supporting structures
Petals – brightly coloured modified leaves, which function to attract pollinators
Sepal – Outer covering which protects the flower when in bud
Peduncle – Stalk of the flower
9.4.4 Photoperiodism
The purpose of flowering is to enable the plant to sexually reproduce via pollination, fertilisation and seed dispersal
They need to bloom when the pollinators are active – so its seasonal
Some plants bloom in long day conditions (summer), whereas other plants bloom in short day conditions (autumn / winter)
The critical factor is light periods – that is detected by phytochromes
Phytochromes: leaf pigments which are used by the plant to detect periods of light and darkness
The response of the plant to the relative lengths of light and darkness is called photoperiodism
Phytochromes exist in two forms – an active form and an inactive form:
The inactive form of phytochrome (Pr) is converted into the active form when it absorbs red light (~660 nm)
The active form of phytochrome (Pfr) is broken down into the inactive form when it absorbs far red light (~725 nm)
Additionally, the active form will gradually revert to the inactive form in the absence of light (darkness reversion)
Sunlight contains more red light than moonlight so active form is during the day
Inactive form is predominant during the night
Only the active form of phytochrome (Pfr) is capable of causing flowering but its action differs in certain types of plants
Plants can be classed as short-day or long-day plants, however the critical factor in determining their activity is night length
Short-day plants flower when the days are short – hence require the night period to exceed a critical length
In short-day plants, Pfr inhibits flowering and hence flowering requires low levels of Pfr (i.e. resulting from long nights)
Long-day plants flower when the days are long – hence require the night period to be less than a critical length
In long-day plants, Pfr activates flowering and hence flowering requires high levels of Pfr (i.e. resulting from short nights)
Horticulturalists can manipulate the flowering of short-day and long-day plants by controlling the exposure of light
The critical night length required for a flowering response must be uninterrupted in order to be effective
Long-day plants:
require periods of darkness to be less than an uninterrupted critical length
These plants will traditionally not flower during the winter and autumn months when night lengths are long
Horticulturalists can trigger flowering in these plants by exposing the plant to a light source during the night
Carnations are an example of a long-day plant
Short-day plants
require periods of darkness to be greater than an uninterrupted critical length
These plants will traditionally not flower during the summer months when night lengths are short
Horticulturalists can trigger flowering in these plants by covering the plant with an opaque black cloth for ~12 hours a day
Chrysanthemums are an example of a short-day plant
9.4.5 Seed Structure
When fertilisation occurs, the ovule will develop into a seed (which may be contained within a fruit)
The seed will be dispersed from the parental plant and will then germinate, giving rise to a new plant
Inside Seeds:
Testa – an outer seed coat that protects the embryonic plant
Micropyle – a small pore in the outer covering of the seed, that allows for the passage of water
Cotyledon – contains the food stores for the seed and forms the embryonic leaves
Plumule – the embryonic shoot (also called the epicotyl)
Radicle – the embryonic root
9.4.6 Germination
Germination: the process by which a seed emerges from a period of dormancy and begins to sprout
Basic requirements for germination:
Oxygen – for aerobic respiration (the seed requires large amounts of ATP in order to develop)
Water – to metabolically activate the seed (triggers the synthesis of gibberellin)
Temperature – seeds require certain temperature conditions in order to sprout (for optimal function of enzymes)
pH – seeds require a suitable soil pH in order to sprout (for optimal function of enzymes)
Additional germination requirements
Fire – some seeds will only sprout after exposure to intense heat (e.g. after bushfires remove established flora)
Freezing – some seeds will only sprout after periods of intense cold (e.g. in spring, following the winter snows)
Digestion – some seeds require prior animal digestion to erode the seed coat before the seed will sprout
Washing – some seeds may be covered with inhibitors and will only sprout after being washed to remove the inhibitors
Scarification – seeds are more likely to germinate if the seed coat is weakened from physical damage
Experiments can be developed using any of these factors as an independent variable
Germination can be measured by the rate of seed growth over a set period of time