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)

1. 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

2. 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

3. 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 (P_r) is converted into the active form when it absorbs red light (~660 nm)

• The active form of phytochrome (P_fr) 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 (P_fr) 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, P_fr inhibits flowering and hence flowering requires low levels of P_fr (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, P_fr activates flowering and hence flowering requires high levels of P_fr (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