Weed Seed Dormancy


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Weed Seed Dormancy Weeds are consistent problems in agriculture because of seed dormancy. Without dormancy, weed seeds would not survive in the soil for any period of time. Also critical are the Weed Seed Dormancy In this unit we will learn about seed dormancy as seen by many respected seed biologists, the way most scientists in the seed discipline view this complex area of plant

Weed Seed Dormancy

Weeds are consistent problems in agriculture because of seed dormancy. Without dormancy, weed seeds would not survive in the soil for any period of time. Also critical are the factors that influence dormancy. These factors which describe seed dormancy also serve to “determine” when the seed has the greatest potential to germinate successfully, and thus survive to replace the seed bank. Understanding the concepts of seed dormancy and factors that influence the continuation or termination of dormancy thus allowing germination are critical for the development of an effective weed-management program.

Weed Seed Dormancy

In this unit we will learn about seed dormancy as seen by many respected seed biologists, the way most scientists in the seed discipline view this complex area of plant biology. The concepts and organization of this unit follow this traditional way of looking at seed dormancy. It is essential that you understand these concepts, and see them from this perspective, if you are to have an understanding of weed seed biology and the scientific literature on seed dormancy.

Having said that, it is important that you realize that I am not in complete agreement with this established model of seed dormancy. Will will evaluate these different views in our classroom discussions. For starters, I view the term dormancy as the “biology of what isn’t“:

Dormancy: a state in which viable seeds, spores or buds fail to germinate under conditions favorable for germination and vegetative growth.

What this definition of dormancy obscures is what important phenomena are hidden from our view, it tells us nothing about what is happening in that seed, or its potential to germinate, its a “black box” definition. Many different seed phenomena, potentially caused by a multitude of different mechanisms, all fall under this vague term What we call dormant is a range of germinability states, from those right on the edge of germination and those profoundly dormant..

Germinability: the capacity of an seed, bud or spore to germinate under some set of conditions.

The dormant seed requires after-ripening for it to become capable of germination. After-ripening of weed seed usually occurs in the soil from the time it is shed in the growing season until it germinates, often over one or more winters (in the north temperate regions like Iowa). In the soil, physiological, chemical and physical changes occur and after-ripening proceeds.

After-ripening: period after dispersal when the seed, spores and bud cannot germinate, even under favorable conditions, and during which changes occur allowing it to germinate.

Imagine a seed at the end of this after-ripening period and process, it is still dormant but it is almost ready to germinate. If we were able to place this single seed in many slightly different but “favorable” germination conditions (parallel universes?), it might germinate in some and not others. This dormancy term just doesn’t tell us much. The ability to germinate is often a “window” of sensitivity to environmental conditions. As germination conditions in the soil become more favorable, the “window” widens and more weed seed germinate.

Weed seed, and their germination requirements, are very diverse. They range from the deeply and profoundly dormant (imagine a velvetleaf seed buried 18 inches in some cold Minnesota soil, its hard coat protecting it, low oxygen is preventing oxidative stress and germination: happy as a clam and able to last a hundred years) to viviparous (imagine a foxtail seed germinating right on the panicle in a warm, moist, foggy field along the edge of the Chesapeake Bay in Maryland).

Vivipary: germinating while still attached to the parent plant.
Viviparous: producing offspring from within the body of the parent.

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In this unit we will learn the established model of seed germination and dormancy. We will also discuss how a better model might be developed. Warning signs that you are headed in the wrong direction in your thinking can be signaled by the following terms, often used. BEWARE!:

Breaking Dormancy
Block to Dormancy
Dormancy Trigger
Biochemical Trigger
Dormancy Switch
Switch for Germination

Finally, it is important to realize there is very good reason for this confusion, why seed biologists like nothing better to do than invent new terms: dormancy is an extremely complicated area of biology. No one has figured out the mechanism of embryo dormancy. Most of the literature doesn’t differentiate between dormancy imposed by the embryo and surrounding (often inhibiting) envelopes when they discuss experimental seed germination results. And then there is the problem of figuring out how these seed behave in the soil: agricultural soil seed bank dynamics.

The Traditional Seed Dormancy Model

Reading assignment: Harper: Ch. 3: pp. 61-82; summary p. xiv-xv

  • Introductory concepts
  • Innate dormancy
  • Enforced dormancy
  • Induced dormancy

Introductory Concepts

  • Dormancy: 1. A state in which viable seeds (or buds; spores) fail to germinate under conditions of moisture, temperature and oxygen favorable for vegetative growth (Amen, 1968); 2. A state of relative metabolic quienscence
  • Seasonal dormancy: in an environment in which favorable growth conditions are seasonal, dormancy is usually clocked by solar rhythm; consequences to the population: predictive
  • Opportunistic dormancy: when there is only a small seasonal element in the occurrence of favorable conditions dormancy tends to be both imposed and released by the direct experience of the unfavorable or favorable conditions; consequences to the population: responsive
  • Innate dormancy: the condition of seeds as they leave the parent plant, and is a viable state but prevented from germinating when exposed to warm, moist aerated conditions by some property of the embryo or the associated endosperm or maternal structures
  • Induced dormancy: is an acquired condition of inability to germinate caused by some experience after ripening; in the opportunistic dormancy category
  • Enforced dormancy: is an inability to germinate due to an environmental restraint: shortage of water, low temperature, poor aeration, etc.; in the opportunistic dormancy category
  1. Dormancy occurs during periods of unfavorable conditions; is more resistant to environmental hazards
  2. Dormancy can be seen as “dispersal in time”
  3. Dispersal phase usually dormant; dehydrated seeds weight less (esp. wind dispersal) and are metabolically slower
  4. Dormancy:
    a. rhythmic adaptation of weeds to the temporal rhythms in the environment
    b. strategic alternative to dispersal
  5. Seed-Environment interactions: Seed dormancy a product of the interaction of the seed (embryo, envelopes like a seed coat, seed food reserves like the endosperm or cotyledons) and the environment (temperature, gases, water, light, soil, temperature)

Evolutionary and environmental context of weed seed dormancy as an adaptive strategy in the struggle for existence

1. Dormancy weak solution to problem of adaptation in changing environment: time lost in capturing resources, in struggle for existence with neighbors

  • Probability of suffering greater hardship by continuing growth; evolutionary solution: annual habit, dormancy
  • The effort or cost of a seasonally dimorphic phenotype; evolutionary solution: e.g. desert shrubs with somatic polymorphism, large leaves in wet season, small leaves and scales in dry season
  • The cost of producing a homeostatic growth form that is tolerant of the whole range of environmental conditions: cost of wide tolerance is great, obtained at cost of reduced efficiency at optimum conditions; evolutionary solution: e.g. evergreen trees
  • a. Grain: the way in which an individual plant experiences the heterogeneity of the environment
  • Fine-grained environment: individual exposed to environmental factors in small doses, short-termed flucuations; each individual in each year experiences the same range of environments in the same frequencies and there is no uncertainty; example: long-lived perennial tree experiences entire range of environmental factors in yearly changes
  • Coarse-grained environment: each individual spends its whole life, or a long period at least, in a single environmental alternative; example: annual weed gets a dry year, or a wet year
  • Optimal adaptive strategies in a heterogeneous environment:
    -if environments experienced are not too different, the optimal adaptive strategy is a single type of best suited to some intermediate environment
    -if the environments experienced are very different, and acts in a fine-grained way, the optimal strategy is a single type of dormancy adapted to the more common environment
    -if the environments experienced are very different, and act in a coarse-grained way, the optimal strategy is polymorphism; seed dormancy in weeds in temperate climates good example here
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Innate Dormancy

  • Innate dormancy conferred by the fact that the process of growth of an embryo to a stage fit for germination has not been completed while the the embryo was still borne on the parent plant, it is shed morphologically incomplete
  • Example: Heracleum sphondylium: development of embryo continues at the expense of extra-embryonic food reserves for several months after seed is shed
  • This process imposes a necessary time lag between dispersal and germination

Control by a biochemical trigger

1. A biochemical process may need to be stimulated before the germination process can begin

2. Often this trigger is a seasonally related stimulus which can switch on germination at an adaptively appropriate time of year

  • Cues and triggers involved in breaking innate dormancy do not produce a clear “all or nothing” effect: only a portion of the seed germinate at one time; a spectrum of requirements by seeds in a single sample which may reflect:
    -different genotypes
    -different maternal influences
    -different ages and ripening conditions (influence of different environmental conditions at different times during reproductive phase, in same plant: Cavers)
  • Light and phytochrome
    1. Example Betula pubescens (UK)
    a. require light and long days for germination
    b. length of dark period critical: germination declines with increasing dark period length
    c. temperature dependence complicated light dependence
    -at 20C light dependence lost
    -with chilling treatment light dependence lost
    2. Several other species follow variations on this same theme: e.g. many dicot weeds (Isikawa, 1954)
  • Temperature: chilling or flucuating temperature-
    example: Papaver spp.: diurnal flucuation between 10 and 30C breaks dormancy; occurs in upper layers of UK soils in April and May, fixes time of germination
  • Nitrate ion: NO3-
    -nitrate concentration of the soil solution often rises quite sharply as the soil temperature increases in the spring (Russell, 1962)
    -stimulation of Chenopodium album seed germination in the field, and several other species, stimulated by nitrate
  • Germination stimulants
    -e.g. ethanol, anesthetics, etc.
    -[ecological, agricultural relevance?]
  • Triggering of biochemical process which destroys a germination inhibitor: breakdown process of inhibitor which occurs within the tissues of the seed
  • Physical leaching, or removal of the source of, and inhibitor: leaching or destruction of inhibitor by an external agent

Physical restriction of gas exchange and growth

  • Impermeable (or relatively impermeable) seed or fruit coat may prevent water or gas uptake by seed and prevent germination until physical damage occurs to this barrier
    -example: Avena fatua (wild oat) seed dormancy broken easily by pricking pericarp
  • Common innate dormancy in species inhabiting sand dunes; abrasion by sand
  • Scarification: seeds that require abrasion tend to break dormancy at different times rather than in a sudden flush
    -example: Avena fatua (wild oat) seed dormancy broken easily by pricking pericarp
    -example: Abutilon theophrasti hard seed coat: germination broken readily with treatments cracking hard seed coat (50C for 15 minutes; 10-15 minutes in sulfuric acid); hard seed coat confers very long dormancy and viability in soil seed bank

2. Dormancy caused by mechanical restriction of growth by embryo coverings (pericarp, testa, perisperm, endosperm)
-example: cocklebur: upper seed (of two in capsule) radicle is restricted, insufficient thrust to rupture testa and germinate

  • Innate dormancy appears to be under strict and simple genetic control; often modified by maternal effects (i.e. endosperm effects from mother; maternal origin of ovary)
  • Commercial crop seed have lost dormancy present in wild relatives in process of domestication; some dormancy left as protection from precocious germination of crop seed while still in inflorescence (?) in wet weather near harvest time
  • Genetically controlled polymorphism: distinctly different dormancy genotypes
    -example: Spergula arvensis: 3 different seed coats, each control different levels of seed dormancy
    -example: Nicandra physaloides: presence or absence of isochromosomes determines whether the seed is non- dormant (2n = 20) or dormant (2n = 19)
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Somatic polymorphism and innate dormancy

Somatic polymorphism: Production of seeds of different morphologies or behavior (phenotypes) on different parts of the same plant; not a genetic segregation but a somatic differentiation

  • Adaptive advantage to producing seed on one plant with different qualities
  • Common adaptation limited to weedy species usually
  • Seed dormancy somatic polymorphism is common in weedy species of Gramineae, Compositae, Chenopodiaceae and Cruciferae families
  • A quality lacking in genetic polymorphisms: continuum of responses, not just a few genetic alternatives
  • Proportion of morphs can be subtly and directly altered by environmental conditions experienced by the parent plant
  • Water stress in mature leaves plus short days may induce abscissic acid production
  • ABA may have an effect on developing seeds as they differentiate histologically in developing seed
  • Differences in dormancy in seed may be a function of water stress at time of seed development
  • Germinability of seeds as a function of maternal environment (Gutterman, Y. 1978. Acta Horticulurae 83:41-55)
  • 1. example: Rumex crispus (curled dock)
    -progeny of individual plants vary enormously in ability to germinate in darkness or at common temperature
    -variation is greater between plants than between habitats, no one germination response
  • Example: Xanthium spp. (Cocklebur);
    -seed borne in pairs in capsule: large and small seed dispersed together
    -upper seed in capsule usually dormant, lower germinates first due to differences in testa permeability to water entry, leaching of endogenous germination inhibitors
    -dormancy breaking requirements different for 2: 12 month difference insurance second will become established if first year unfavorable
  • Example: Avena fatua (wild oat), and Avena ludoviciana
    -grains borne on different parts of the spikelet have different germination requirements
    -first grain of spikelet lacks dormancy, remainder have deep dormancy
  • Example: Compositae germination behavior differentiated by seed size, seed formed in ray versus disc flowers
  • Example: Chenopodium album (common lambsquarters) may produce 4 different kinds of seed on same plant
    -two color categories: brown and black; two seed coat categories: reticulate and smooth
    -brown: thin-walled, larger, germinate quicker than black, even at low temperatures; killed by winter, but if they survive have the capacity to produce very large plants with high reproductive output; only 3% of seed on a plant; among the first to be produced by a plant
    -black: require cold treatment, nitrate to break dormancy
    -ratio of brown:black govered by environmental conditions
  • Example: common purslane seed varied from nondormant to dormant on same plant (Egley, 1974)

Enforced Dormancy

  • Imposed dormancy: state of seed dormancy maintained by the absence of necessary conditions for germination
    -E.g. shortage of water, temperature, unfavorable soil atmosphere, etc.
    -E.g. seed buried deeply in soil by tillage, etc., commonly has enforced dormancy
  • Carbon dioxide narcosis in soils common factor in enforced dormancy; e.g. high respiration sites in soil elevate CO2 (seed respiration, soil microorganisms)
  • Lowered oxygen tension in the soil also important here; e.g. severe oxygen starvation in waterlogged or compacted soils
  • Temperate agricultural regions: low temperature enforces dormancy

Induced Dormancy

A seed has acquired dormancy which is not innate and which does not require continued enforcement

CO2 narcosis: example: Brassica alba dormancy induced by high CO2 treatment

  • ilum acts as a hygoscopically activated valve
  • when air is dry the hilum valve opens and allows water loss from seed
  • in wet air it closes,
  • embryo progressively dries to a value equal to that of the driest environment it experienced
  • hard seed dormancy only broken by seed coat scarification
  • white and red clover seed
  • collected seed from soil in dark after burial treatment
  • later seed of many species (buckhorn plantain, corn spurry, field poppy) tested had light requirement for germination which was not needed when freshly harvested

Cold treatment induced light requirement for germination of Stellaria media (Chickweed); one way autumn shed seed acquire light requirement by spring

High temperature exposure of imbibed seeds coupled with restriction of oxygen availability induced dormancy (Villiers, 1972).

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