Reproductive strategies in plants

Regeneration - can be likened to an obstacle race

• At each stage in the process, each individual has to overcome:
  • Environmental stress
  • Competition
  • Predation
  • Disease
• Species differ:
  • Pollination failure
  • Predation of ripening ovules
  • Competition during seedling establishment
• Seed vs vegetative reproduction
  • Flowering plants can reproduce sexually or asexually
  • Typical scenario:
    • Annuals = seeds
    • Water plants = vegetative
    • Herbaceous plants = sexual and vegetative
  • Adaptive value of reproductive methods
    • Seed reproduction
      • Produce large numbers
      • Dispersal to new ground
      • Survive adverse conditions
      • Genetically unique (crossover and recombination)
    • Vegetative reproduction
      • Genetically identical to parent
      • Dependability
  • Principle of allocation
    • Each organism has a finite amount of available resources
      • Partitioning of resources:
        1. Chances for survival
        2. Size of plants (inverse relationship)
        3. Devote lower proportion when stressed
        4. Increase allocation with increased density  
  • Reproductive effort
    • Varies depending upon the availability in the plant
      • The seed will be given preference for limited minerals
  • Fertility schedules
    • Unstable environments favor plants that produce seed quickly
    • Biennials are found less frequently in unstable environments
      • Produce greater number of seeds
  • Seed size and number
    • Size may be a compromise between requirements for dispersal and seedling establishment
    • Larger seeds are associated with drier environments
    • Very small seeds tend to be associated with:
      1. Parasitic or saprophytic plants for wide dispersal
      2. Plants that establish in persistent dormant seed banks
    • Seed weight is much more constant than size

Predispersal hazards

• Period between ovule formation and seed ripening (hazardous period)
  • Causes:
    • Seed losses due to pollination failure
      • Lack of insects
      • Competition with other species
    • Resource deficiency
      • Control by sink capacity
      • Eliminate defective fruits (selfing)
    • Predation
      • Ranges up to 94% due to insects
      • Defense mechanisms:
        1. Mechanical
        2. Chemical
        3. Flowering period
        4. Reduced flower size
        5. Masting (alternating production cycles)
    • Developmental failure
      • Lethal gene combinations

Dispersal

• Dispersal curves
  • Factors:
    1. Size and shape of the seed
    2. Height of the plant
    3. Speed of the wind
    4. Density of the surrounding vegetation
• Dispersal agents
  1. Exploding pods
  2. Wind dispersal
     • Dominant in temperate forests and prairie regions
       • Maple, ash, elm, Compositae
  3. External carriage by birds, animals and man
     • Achieved by burrs, hooks, sticky substances or mud
     • Female animals shown to carry more than males
  4. Insects
     • Ants are an important dispersal agent in temperate and tropical areas
  5. Digestive tracts
     • Includes a wide range of animals, birds and fish
     • Feces acts as a fertilizer
     • Length of time can range as high as 23 days
• Long distance dispersal
  • Factors:
    1. Distance to the nearest seed source
    2. Relationship to bird migration routes
    3. Direction of prevailing wind
    4. Frequency of hurricanes
    5. Size of the target area

Soil seed banks

• Factors:
  1. Seed content of soils
     • Range up to 1,000,000 m² in some grasslands
  2. Dynamics
     • Rates of input vs loss
  3. Life strategies
  4. Environment
     • Cold areas have lower densities of dormant seeds
• Field longevity experiments
  • Longevity dependent upon species
  • Deep planting tends to extend longevity
• Ecological significance
  • Evolutionary memory
    • Progeny represents extended time-period
  • Control methods
    • Based on knowledge of species involved
  • Erosion control

Reference:

Seed Ecology. M. Fenner. Chapman and Hall, New York, 1985.