Please enable JavaScript.

Coggle requires JavaScript to display documents.

Seed Dormancy - Coggle Diagram

- - - - A condition in which the essential germination components (light, water, temp) are not available to the seed and this fails to germinate.
      - related to physical properties of the seed coat
      - Factors Responsible for Exogenous Dormancy
        
        Water
        
        impermeability of seed coats to water is typical
        
        due to the prescence of a cuticle and a well-developed layer of palisade cells or both.
        
        heavy deposits of suberin and lignin or cutin are common in the integuments (in legume seeds)
        
        Can be related to the structure of the hilum
        
        a mucilaginous layer beneath the cuticle which is hard and thick has been observed in legume seeds.
        
        seeds that exhibit water impermeability are known as Hard Seeds
        
        caused by both genetic and environmental factors (impermeability)
        
        Gases
        
        impermeability of gases through the seed coat has been described as a mechanism of dormancy governed by the seed coat.
        
        attributed to physical and biochemical barriers
        
        dormant seeds are less permeable to oxygen, this slows the aerobic metabolism required for germination.
        
        Mechanical Restriction
        
        dormancy has been attributed to the physical restraint by the seed coats on an enlarging seed coat
        
        the "thrust" developed during imbibition is inadequate to rupture the seed coat and permit germination.
      - Methods of Breaking Exogenous Dormancy
        
        in natural conditions, exogenous dormancy is overcome by freezing-thawing of the soil, ingestion by animals, microorganism activity, forest fires, natural soil acidity, and other factors.
        
        The integrity of the seed coats needs to be affected.
        
        overcoming this dormancy is accomplished by the mechanical and chemical removal of the seed coat, called scarification.
        
        Mechanical Scarification
        
        grinding with abrasives or sand, shaking them (impaction)
        
        heating, chilling and drastic temperature shifts, brielf immersion in boiling water, etc
        
        Need to know the proper duration of treatments otherwise you face damaging your seeds.
        
        Chemical Scarification
        
        treat the seeds with chemicals to cause degradation of the seed coat.
        
        chemical scarification has not been commercially popular bc the materials are hazardous to handle.
        
        seeds must be thoroughly washed and dried after treatment
        
        reduction of germination may occur from even slightly overscarification.
    - - the most prevalent dormancy found in seeds
      - due to inherent properties of the seed.
      - the seed may contain inhibitors that must be removed or reduced prior to germination.
      - Causes of Endogenous Dormancy
        
        environmental conditions during seed development and maturation influence the duration of endogenous dormancy
        
        Factors
        
        length, moisture status, position of the seed in the fruit or inflorescence, age of the mother plant, temperature during seed maturation.
        
        day-length
        
        effect is the greatest in the final stages of seed maturation
        
        plants differ in their response to photoperiod and their expression of seed dormancy
        
        short days- enhance the germination
        
        long days- increase seed coat thickness
        
        moisture status
        
        moisture status on the mother plant or developing seed influences the degree of seed dormancy
        
        water deficits increase seed dormancy when they occur close to flowering but decrease dormancy at the final stages.
        
        Position of the seed on mother plant
        
        effects are primarily attributed to difference in seed maturation which are often expressed by seed weight
        
        differences in dormancy serve as an effective mechanism for maintaining viable but dormancy seeds in the soil seed bank.
        
        Age of the mother plant
        
        age of the mother plant at the time of flowering induction can affect seed dormancy
        
        Environmental conditions
        
        water, nitrogen availability, mineral nutrition
        
        higher moisture content can also lead to a higher degree of seed dormancy
      - Rudimentary Embryo Dormancy
        
        seeds of some species are shed before they are morphologically rmature.
        
        this dormancy is bc the immature embryo is unable to germinate
        
        changes that occur in dry, dormant seeds after dispersal is known as after-ripening.
      - Physiological Dormancy
        
        dormancy in higher plants is generally believed to be regulated by a balance of endogenous growth inhibitors and promoters.
        
        dormancy is a result of the presence of growth inhibitors, the absence of growth promoters or a combination of both.
        
        endogenous compounds are controlled by certain environmental stimuli such as light and temperature.
      - Metabolic Inhibition
        
        certain compounds are present in seeds inhibit specific metabolic pathways.
        
        inhibitors- cyanide, low concentrations of cyanide can stimulate germination.
        
        ABA- endogenous hormone that is active at low concentrations. Reduce following stratification
      - Osmotic Inhibitions
        
        substances possessing high osmotic pressures can inhibit the germination of seeds
      - Methods of Breaking Physiological Dormancy
        
        seeds that are dormant due to osmotic inhibition can be germinated after removing the seed from the influence of the inhibitor or diluting the inhibitor from around the seed (this is called Leaching)
        
        requires exposing the seeds to an excess of water that dilutes or remove an inhibitor from the fruit wall
        
        seeds that possess metabolic inhibitors located in the seed coat can be relieved of dormancy by removal of the seed coat by mechanical or chemical scarification.
        
        conventional scarification to break down otherwise hard, impermeable seed coats to allow water penetration is usually accomplished by various procedures including the use of abrasives or acids.
        
        alleviating physiological seed dormancy
        
        seed coats have often been shown to be the source of inhibitors, when they are removed the inhibitor is also removed.
        
        seed coats can function as differentially permeable membrances, permitting the entry of water but slowing the loss of inhibitors.
        
        Temperature Requirement
        
        Stratification
        
        shifts in hormonal levels occur
        
        addition of exogenous gibberellins can substitute for the stratification requirement of some seeds.
        
        hormones can be a promotive agent
        
        increases in levels of endogenous GA have been observed during stratification.
        
        typically pre-conditioned.
        
        After-ripening
        
        the disappearance of dormancy in dry seeds over time at room temperature.
        
        Light Requirement
        
        light quantity and quality perceived by a seed depends on its position in the soil, vegetative covering and light absorption characteristics of the seed coat or comparable structures.
        
        shorter wavelengths of light are absorbed more by soils than longer wavelengths.
        
        light intensity, wavelength, and photoperiod are all known to affect the germination of seeds that have physiological dormancy.
        
        Duration of light exposure (photoperiod) can also affect germination.
        
        continuous light may inhibit germination of some seeds, this imposes a different type of dormancy.
        
        Circadian Rhythms
        
        influence the pattern of seed germination
        
        Interaction of Primary Dormancy Mechanisms
        
        More than one type of dormancy can be imposed on a single seed.
        
        exogenous and endogenous can be active.
        
        Embryo Excision
        
        embryos of dormant seeds could be germinated if they were excised and grown separately.