POLLINATION

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What is pollination?

Pollination is the fertilization step in plants. It is performed by insects and birds in their search for nectar and pollen.

 

 Nectar and pollen

The main constituents of nectar are the carbohydrates; glucose, fructose and sucrose (10-70%). Common amino acids and lipids are also present (~0.05%). Pollen (carrier of the male gamete) serves as a nutrient and contains protein (15-30%), carbohydrates (10%) and fats (5%). These values vary considerably according to species.

Fragrances

Flowering plants emit volatile low molecular weight substances that attract pollinators (e.g. insects) from a distance. Insects and bats that are active at night rely entirely on the fragrances. There are biosynthetically 3 types of pollination stimulants.

  •  Fatty acid derivatives which are products of the polyketide pathway
  • Aromatics from the shikimic acid pathway
  • Mono and sesquiterpenoids  

Fatty acid derivatives (a) and (b) occur in Magnolia and Cypripedium fragrance respectively.

                         CH3(CH2)13CH3                            CH3(CH2)7OCOCH3

                                  (a)                                            (b)

The monoterpenoids  3-carene occur in  Actaea spp. and β-ionone in violets.

     

Fragrances sometimes contain substances with dual effects. They act as feeding attractants and as sex pheromones. For example the orchid (e.g Ophrys) flower which contain compounds also identified in the odour glands of Andrena female bees, attract male bees to the flower.

Colours

The insects respond to the colors of flowers at short distance. Butterflies generally prefer lively bright colours and bees white, yellow and blue colors. Animals with weak colors sense such as bats, moths, flies and beetles pollinate flowers with dull, green and whitish colours. Birds prefer vivid scarlet flowers.

The group of compounds is responsible for the colour pigments of the flowers.

                Flavonoids
               Carotenes
               Quinones

The highly conjugated anthocyanidin ions are found over the whole colour range from orange, red, blue to purple black. The polyconjugated carotenes give yellow-orange colours to flowers.

The quinones (plumbagin, primin) have been identifies as flower pigments in a few cases. They are yellow to black coloured compounds, widespread in higher plants, lichens, insects and microorganisms.





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