Question

What is pollination? Discuss mechanism favoring self and cross-pollination in crop plants.

Hint:

Remember: Self-pollination mechanisms include bisexuality, cleistogamy, homogamy while cross-pollination mechanisms include dichogamy, self-incompatibility, unisexuality, and herkogamy. Plants evolve these adaptations based on their reproductive strategy and environmental conditions.

Answer 1

Step 1: Definition of pollination.
Pollination is the process of transfer of pollen grains from the anther (male part) to the stigma (female part) of a flower. It is the first step in the reproductive process that leads to fertilization and seed formation. Pollination is essential for fruit and seed production in flowering plants.
Step 2: Types of pollination.
There are two main types of pollination:

• Self-pollination (Autogamy): Transfer of pollen from anther to stigma of the same flower or another flower on the same plant
• Cross-pollination (Allogamy): Transfer of pollen from anther of one flower to stigma of another flower on a different plant of the same species

Step 3: Mechanisms favoring self-pollination.
Plants have developed various adaptations that promote self-pollination:

• Bisexuality:
  • Flowers contain both male and female reproductive organs (stamens and pistil) in the same flower
  • Examples: Wheat, rice, tomato, chickpea, beans
• Cleistogamy:
  • Flowers never open and pollination occurs inside the closed flower
  • Ensures complete self-pollination
  • Examples: Groundnut, pansy, some grass species
• Homogamy:
  • Anthers and stigma mature at the same time
  • Ensures pollen is available when stigma is receptive
  • Examples: Wheat, rice, tomato
• Position of floral parts:
  • Anthers are positioned above or around the stigma so pollen naturally falls on stigma
  • Stigma and anthers are in close proximity
  • Examples: Tomato, brinjal
• Bud pollination:
  • Pollination occurs in the bud stage before the flower opens
  • Common in some legumes
  • Examples: Pea, bean

Step 4: Mechanisms favoring cross-pollination.
Plants have developed various adaptations to promote cross-pollination and avoid self-pollination:

• Dichogamy:
  • Anthers and stigma mature at different times
  • Protandry: Anthers mature before stigma (Sunflower, cotton, carrot)
  • Protogyny: Stigma matures before anthers (Pear, cabbage, fig)
• Self-incompatibility:
  • Pollen from the same flower or plant fails to germinate or grow on the stigma
  • Genetic mechanism prevents self-fertilization
  • Examples: Cabbage, radish, pear, apple, many grasses
• Unisexuality (Dicliny):
  • Flowers are either male or female
  • Monoecious: Male and female flowers on same plant (Maize, castor, cucumber)
  • Dioecious: Male and female flowers on different plants (Papaya, date palm, spinach)
• Herkogamy:
  • Physical barrier between anthers and stigma
  • Stigma is positioned away from anthers preventing contact
  • Examples: Many orchids, some Brassicaceae members
• Heterostyly:
  • Different flower types with different lengths of style and stamens
  • Pollination occurs only between different types
  • Examples: Primrose, buckwheat
• Wind pollination (Anemophily):
  • Flowers are adapted for pollen dispersal by wind
  • Large feathery stigmas, abundant light pollen
  • Examples: Maize, wheat, grasses, pines
• Insect pollination (Entomophily):
  • Flowers attract insects with color, scent, nectar
  • Insects carry pollen from flower to flower
  • Examples: Sunflower, mustard, apple

Step 5: Advantages and disadvantages.
Self-pollination advantages:

• Maintains pure lines and genetic uniformity
• No need for external agents
• Ensures reproduction even in absence of pollinators
• Desirable traits are preserved

Self-pollination disadvantages:

• No genetic variation, leading to inbreeding depression
• Less adaptable to changing environments
• Vigor may decrease over generations

Cross-pollination advantages:

• Creates genetic diversity and variation
• Produces more vigorous offspring (hybrid vigor)
• Better adaptability to environmental changes
• Evolution of new traits

Cross-pollination disadvantages:

• Dependent on external agents (wind, insects)
• Risk of pollination failure
• Wastage of pollen grains
• Undesirable traits may be introduced