9.4 Plant Reproduction

• 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)

  • This involves three distinct phases – pollination, fertilization and seed dispersal
• 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 involves 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

  • The flowering plant gains a means of sexual reproduction (via the transference of pollen between plants)
  • The animal gains a source of nutrition (plants secrete a sugar-rich substance called nectar to attract pollinators
• Common examples of pollinators include birds, bats and insects (including bees and butterflies)

  • Flowers may be structured to optimise access for certain pollinators (e.g. tube-shaped flowers for birds with long beaks)
• 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)
• Flowers are the reproductive organs of angiospermophytes (flowering plants) and contain male and female structures

  • Most flowers possess both male and female structures (monoecious), but some may only possess one structure (dioecious)
• The male part of the flower is called the stamen and is composed of:

  • 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)
• The female part of the flower is called the pistil (or carpel) and is composed of:

  • 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)
• In addition to these reproductive structures, flowers possess a number of other support 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
• The purpose of flowering is to enable the plant to sexually reproduce via pollination, fertilisation and seed dispersal

  • Consequently, flowers need to bloom when pollinators are most active and abundant – this is dependent on seasons
  • Some plants bloom in long day conditions (summer), whereas other plants bloom in short day conditions (autumn / winter)

  The critical factor responsible for flowering is the length of light and dark periods, which is detected by phytochromes
• Phytochromes are 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)
• Because sunlight contains more red light than moonlight, the active form is predominant during the day

  • Similarly, as the active form is reverted in darkness, the inactive form is predominant during the night
• Only the active form of phytochrome (Pfr) is capable of causing flowering, however 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
  • Crysanthemums are an example of a short-day plant
• 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
• A typical seed will possess the following features:

  • 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
• Germination is the process by which a seed emerges from a period of dormancy and begins to sprout
• For germination to occur, a seed requires a combination of:

  • 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 conditions for germination:

  • 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