9.4 Plant reproduction

Three ways of reproduction in plants

* Vegetative propagation (asexual from a plant cutting)
* Spore formations
* Pollen transfer

Three stages of sexual reproduction in flowering plants

1. Pollination
2. Fertilisation
3. Seed dispersal

Stages of pollen transfer: 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
* Improves genetic diversity

Stages of pollen transfer: 2. Fertilization

* Fusion of a male gamete nuclei with a female gamete nuclei to form a zygote
* In plants the male gamete is stored in pollen grain and female gamete is found in ovule

Stages of pollen transfer: 3. Seed dispersal

* Fertilization of gametes results in the formation of a seed, which moves away from plant
* seed dispersal reduces competition between parent plant and germinating seed
* Seeds can spread through wind, water, fruits and animals

Relationship between pollinator and flowering plant

Mutualistic relationship

* Flowering plant = sexual reproduction
* Animal = source of nutrition (nectar)

Reproductive organs of angiospermophytes

Flowers

How are flowers developed in angiospermophytes

* Due to a change in gene expression → triggers enlargement of the shoot apical meristem
* This differentiates to form the different flower structures - sepals, petals, stamen and pistil.

When will this change in gene expression take place?

Influenced by abiotic factors :

* Seasons
* When a suitable pollinator is most abundant
* day/night length

Flower structure with both male and female structures

monoecious

Flower structure with one structure (female or male)

dioecious

Function anther

Male part and pollen producing organ of the flower

Filament function

Male part and stalk supporting the anther

Stigma function

Female part and is responsible for catching the pollen

Style function

Female part and is tube connection between stigma and ovule

Function ovule

Female part and contains female reproductive cells

Petal function

Attracts pollinators

Sepal function

protects flower when in bud

Function peduncle

Stalk of flower

phytochromes

leaf pigments that are used by the plant to detect periods of light and darkness

Photoperiodism

Response of the plant to the relative lengths of light and darkness

Inactive form of phytochrome

Represented as Pr and is converted into active form when it absorbs red light (sun contains red light)

Active form of phytochrome

Represented as Pfr and is broken down into the inactive form when it absorbs far red light or in absence of light

Short-day plants and phytochromes levels

In short-day plants, flowering requires low levels of Pfr so that it inhibits flowering

Long day plants and phytochrome levels

* In long-day plants, Pfr *activates* flowering and hence flowering requires high levels of Pfr (i.e. resulting from short nights)

How can long day plants be manipulated to flower in the winter?

By exposing them to light during the night

How can short day plants be manipulated to flower in the summer?

Horticulturalists can trigger flowering in these plants by covering the plant with an opaque black cloth for \~12 hours a day

Function testa seed

Outer seed coat that protects the embryonic plant

Function microphyle seed

Small pore in outer covering of the seed, that allows for the passage of water

Function cotyledon seed

Contains the food stores for the seed and forms the embryonic leaves

Function plumule (epicotyl) seed

Embryonic shoot

Function radicle seed

Embryonic root

Conditions for germination of a seed

* oxygen - aerobic respiration
* Water - metabolically activate seed
* Temperature - optimal for enzymes
* pH - optimal for enzymes

Additional conditions for germination

* fire
* freezing
* digestion
* washing
* scarification