9.4 Reproduction in plants

• Flowers are the reproductive organs of angiospermophytes and develop from the shoot apical meristem.
• Changes in gene expression trigger the enlargement of the shoot apical meristem where tissue then differentiates to form the different flower structures.
• Photoperiodism is the most common trigger for the gene expression change that causes flowering.
• Long day plants flower in summer when the nights have become short enough.
• The photoperiodism of plants is dependent on the length of darkness, not the length of daylight.
• Phytochrome is a pigment in leaves that plants use to measure the length of dark periods. It can switch between two forms Pr and Pfr.
• During the day, Pr absorbs red light of wavelength 660nm and is converted into Pfr.
• During the night and when absorbing far-red light of wavelength 730nm Pfr is converted to Pr.
• Pfr is the active form of phytochrome and receptor proteins are present in the cytoplasm to which Pfr but not Pr binds.
• In short-day plants, Pfr inhibits flowering and hence flowering requires low levels of Pfr (i.e. resulting from long nights).
• In long-day plants, Pfr activates flowering and hence flowering requires high levels of Pfr (i.e. resulting from short nights).
• Pollen lands on the stigma and a tube grows down the style to the ovary. The pollen tube carries male gametes to fertilise the female gamete found in the ovule.
• Fertilisation of gametes results in the formation of a seed which moves away from the parental plant.
• Seed dispersal reduces competition for resources between the germinating seed and parental plant.
• Seed structure varies depending on the mechanism of dispersal employed by the plant.
• Sexual reproduction in flowering plants depends on the transfer of pollen from the stamen to a stigma of another plant.
• Pollen is most commonly transferred by animals known as pollinators.
• Mutualism is a close association between two organisms where both organisms benefit from the relationship. Pollinators gain food from nectar and the plant gains a means to transfer pollen.
• Flower forcing is a procedure designed to get flowers to bloom out of season or at a specific time such as during holiday time.
• Long day plants can flower in periods of long night as long as additional light is provided in the middle of the night.
• Flower Parts:

  A) Stamen

  B) Anther

  C) Filament

  D) Stigma

  E) Style

  F) Ovary

  G) Pistil

  H) Receptacle

  I) Ovule

  J) Sepal

  K) Petal

  
• Iris is an example of a long day plant.
• There are Pfr thresholds for flowering.
• Pfr promotes flowering in long day plants.
• Pfr inhibits flowering in short day plants
• During winter, the Pfr in long day plants goes below the threshold, thus flowering does not occur.
• Chrysanthemum is a short day plant.