Weed Seed Dispersal Mechanisms

Weed Management in Horticulture Crops (1+1) Many seeds are well adapted to wind travel. Cottony coverings and parachute-like structures allow seeds to float with the wind. Examples of Weed Seed Dispersal Differences in time: -seed ripens on parent plant -seed retained on parent before its dispersed (seed shattering) -example: weeds with long flowering period and Weed Seed Dispersal Mechanisms Reading assignment : Harper: Ch. 2: pp. 33-60; summary p. xiv Introductory concepts Seed rain as a process in time Characteristic seed

Weed Management in Horticulture Crops (1+1)

Many seeds are well adapted to wind travel. Cottony coverings and parachute-like structures allow seeds to float with the wind. Examples of wind-dispersed seeds include common milkweed ( Asclepias syriaca ), common dandelion, Canada thistle, and perennial sowthistle ( Sonchus arvensis ). Weed seeds and fruits that disseminate through wind possess special organs to keep them afloat. Such organs are:

  • Pappus – It is a parachute like modification of persistent calyx into hairs e.g. Asteraceae family weeds – Tridax procumbens
  • Comose – Some weed seeds are covered with hairs, partially or fully e.g. Calotropis sp.
  • Feathery, persistent styles – Styles are persistent and feathery e.g. Anemone sp.
  • Baloon – Modified papery calyx that encloses the fruits loosely along with entrapped air e.g. Physalis minima
  • Wings – One or more appendages that act as wings e.g. Acer macrophyllum
  • seed weight
  • seed shape
  • structures (wings or pappus)
  • height of release
  • wind speed and turbulence

Aquatic weeds disperse largely through water. They may drift either as whole plants, plant fragments or as seeds with the water currents. Terrestrial weed seeds also disperse through irrigation and drainage water. Weed seed often moves with surface water runoff into irrigation water and ponds, where it is carried to other fields. Weeds growing in ditch banks along irrigation canals and ponds are the major source of weed seed contamination of irrigation water.

Weed seed often remains viable in water for several years, creating a “floating seed bank” and allowing weeds to disperse over large areas in moving water. Field bindweed seed, for example, remains over 50 per cent viable after being submerged in water for more than 4 years. Some seeds have special adaptations that aid in water travel. The seed pod of curly dock, for example, is equipped with pontoons that carry the floating seed.

Several weed species produce seeds with barbs, hooks, spines, and rasps that cling to the fur of animals or to clothing and then can be dispersed to long distances. Farm animals carry weed seeds and fruits on their skin, hair and hooves. This is aided by special appendages such as Hooks ( Xanthium strumarium ), Stiff hairs ( Cenchrus spp), Sharp spines ( Tribulus terrestris ) and Scarious bracts ( Achyranthus aspera ). Even ants carry a huge number of weed seeds. Donkeys eat Prosophis julifera pods.

Weed seeds are often ingested and passed through the digestive tracts of animals. Animal droppings provide an ideal nutrient and moisture environment for weed germination. Only a small percentage of the seed remains viable after exposure to an animal’s digestive enzymes. The ingested weed seeds are passed in viable form with animal excreta (0.2% in chicks, 9.6% in calves, 8.7% in horses and 6.4% in sheep), which is dropped wherever the animal moves. This mechanism of weed dispersal is called endozoochory e.g. Lantana seeds by birds, Loranthus seeds stick on beaks of birds. Viable weed seeds are present in the dung of farm animals, which form part of the FYM. Besides, addition of mature weeds to compost pit as farm waste also act as source.

Man disperses numerous weed seeds and fruits with raw agricultural produce. Weeds mature at the same time and height along with crop. Due to their similar size and shape as that of crop seed man unknowingly harvest the weeds also, and aids in dispersal of weed seeds. Such weeds are called “Satellite weeds” e.g. Avena fatua, Phalaris minor.

See also  Most Expensive Weed Strain Seeds

Weed seeds often are dispersed by tillage and harvesting equipments. Seeds move from field to field on the soil that sticks to tractor tires, and vegetative structures often travel on tillage and cultivation equipment and latter dropping them in other fields to start new infestation. Disc-type cultivation equipment is less likely to drag vegetative plant parts than are shovels or sweeps.

Introduction of weeds from one continent to another is through crop seed, feed stock, packing material and nursery stock e.g. Parthenium hysterophorus

Weed seed adaptations to look like crop seed: plant body or seed same size, shape, and morphology as crop e.g. barnyard grass bio-type looking like rice escapes hand weeding and is dispersed with rice. Nightshade fruit (berries) are same size, shape as dry beans, harvested and dispersed with beans.

Weeds probably are spread more commonly during the seeding of a new crop or in animal feed and bedding than by any other method. Seed labels often indicate a tiny percentage of weed seed, but consider this example. If a legume seed contains 0.001 percent dodder (a parasitic annual; Cuscuta campestris ) seed by weight, there will be eight dodder seeds per 2 kg of legume seed. If the legume seed is sown in a field despite an extremely low dodder seed percentage by weight, the small size of the seed, combined with rapid early-season growth, could result in an infested legume field within a single season.

Weed Seed Dispersal

Differences in time:
-seed ripens on parent plant
-seed retained on parent before its dispersed (seed shattering)
-example: weeds with long flowering period and corresponding long period of ripening and release: seed shatters immediately after ripening on parent
-example: weeds flowering and seed production in narrower time period, often require harvesting activity to release seed: evolved with crop, maximize chance of dispersal with crop

Selection for time of seed release: adaptation for quick or slow release from parent plant?
-quick release: oversaturate predator demands of feeding and leave enough after that for new colonizers
-slow release: maximize chance some seeds land on ground during favorable time for quick germination or escape from predators

Seed Dispersal: Non-animal

Gravity dispersal: most of our common, unspecialized weed seeds don’t move very far from mother plant without animals, humans
-preventative weed control is very important
-invasion from the outside overemphasized

Poverty of seed fall in the immediate neighborhood of the parent characteristic of of plants in isolation; e.g. mullein, musk thistle is pasture

Wind dispersal of weed seed is a function of:
-how fast seed falls: weight, density, ability to float in air
-height of release: may be most important factor in distance spread; mullein seed
-speed, direction and turbulence of wind between release point and ground
-very light, dust-like seed (poppies; fungal spores)
-species with specialized wind dispersal mechanisms generally don’t colonize as a horizon but as isolated individuals over a greater distance

Wind-blown seed, or seeds on the soil surface moved by water:
-accumulate near obstacles (fences, furrows, etc.), or fall in soil crevices, dried soil cracks
-Composite family weeds with pappus (or other similar attached seed structure) on the seed are held at a constant distance in the crack (soil) by the pappus and do not enter the crack.

“Tumble Weeds: movement of parent plant (with seeds) on ground after dispersal, -blow with wind;
-seeds dispersed in rolling action;
-examples: kochia on mother plant, forms rolling ball;
-tumbleweed;
-velvetleaf capsules

See also  Seed Stuck On Seedling Weed

Seed movement by water:
-surface water, irrigation, falling in rivers, lakes, etc.
-movement with water on soil surface, runoff
-specialized structures:
a. low specific gravity seed float easily (milkweed)
b. flattened seed shape for floating on water surface
c. “corky” seed wings: curled dock seed

Specialized seed structures
-plumed seeds or fruits: e.g. dandelion, pappus; milkweed, plume; thisles, pappus; sowthistles; pappus is characteristic of Compositae family seeds
-winged seeds: maple seeds: lift provided by wings; asymetrical wing can be an advantage

Specialized mechanisms of some weed species to bury themselves: wild oat hygroscopic awns that twist themselves into the soil (self-seeding)

Seed ejaculation
-example: mistletoe seed propelled by parent plant
-sticky seed surface to adhere to tree host, bird foot vector

The Role of Animals in Dispersal

Animal Dispersal: affected by animal feeding patterns: specific weeds fed on by specific animal feeders

Animal behavior affects where the moved seed ends up, and its success thereafter:
-the territorial and migratory behavior of animals dispersing seed
-animal feeding (eating, digestion) affects seed viability
-animal storage of seed: distance, concentration, location;

Birds:
-ones with hard gizzards that destroy seeds;
-soft gizzards that pass on viable seed

Ants play important role, especially in storage concentrations

Specialized seed structures affect whether, and how, animals spread seed:
-burrs: cocklebur, sandbur seed
-appendages: beggarticks seed

Human dispersal
-new continent, region invasions
-local dispersal: machinery, crop seed contamination

Crop mimicry dispersal
-weed seed adaptations to look like crop seed: plant body or seed same size, shape, morphology as crop
-example: barnyardgrass biotype looking like rice escapes handweeding and is dispersed with rice
-example: nightshade fruit (“berries”) same size, shape as dry beans, harvested and dispersed with beans
This information is optional, not required, but if you are interested:

Weed Seed Dispersal Mechanisms

Reading assignment: Harper: Ch. 2: pp. 33-60; summary p. xiv

  • Introductory concepts
  • Seed rain as a process in time
  • Characteristic seed dispersal patterns and mechanisms: Non-animal
  • The role of animals in dispersal
  • Post-Dispersal movement of seed

Two demographic ways of looking at dispersal: two parts of the same whole

the expanding range and increasing population size of of an invading weed species into a new area

the part of the process by which an established and stabilized weed species in an area maintains itself within that area

  • dispersal (leaving mother plant)
  • post-dispersal events (subsequent movement)

Terminology:

androchorous seed: dispersed by the agency of humans; synonymous terms: anthropochorous; brotochorous

anemochorous seed: dispersed by the agency of wind

barachorous seed: dispersed by its own weight; synonymous term: clitochorous

bolochorous: seed dispersed by the agency of propulsive mechanisms

endozoochorous
: seed dispersed by the agency of animals; synonymous terms: synzoochorous; zoochorous

entomochoric: seed dispersed by the agency of insects

hydrochoric
: seed dispersed by the agency of water

myrmecochorous
: seed dispersed by the agency of ants

Seed rain as a process in time

Dispersal of seed occurs in 4 dimensions (not just 2-D)

2-D: Land/habitat/soil surface area phenomena: distance, width

  1. Differences in time seed ripens on mother plant
  2. Differences in time seed retained on mother before its dispersed: seed shattering
    a. example: weeds with long flowering period and corresponding long period of ripening and release: seed shatters immediately after ripening on mother
    b. weeds flowering and seed production in narrower time period, often require harvesting activity to release seed: evolved with crop, maximize chance of dispersal with crop
  3. Weed seed ripening over short time not imply release and dispersal in corresponding short time
  4. Selection for time of seed release: adaptation to release quick or retain on mother plant?
    a. quick release: oversaturate predator demands of feeding and leave enough after that for new colonizers
    b. slow release: maximize chance some seeds land on ground during favorable time for quick germination or escape from predators
  5. Dormancy: delay in conjunction with dispersal, post-dispersal mechanisms to optimize time between seed maturity and germination
See also  Strawberry Cough Weed Seeds

Characteristic seed dispersal patterns
and mechanisms: non-animal

Gravity dispersal:

many of our common, unspecialized weed seeds don’t move very far from mother plant without animals, humans

preventative weed control is very important

invasion from the outside overemphasized

  1. Wind dispersal is a function of:
    a. how fast seed falls: weight, density, ability to float in air
    b. height of release: may be most important factor in distance spread; mullein seed
    c. speed, direction and turbulence of wind between release point and ground
  2. Very light, dust-like seed (poppies; fungal spores)
  3. Specialized seed structures
    a. plumed seeds or fruits: e.g. dandelion, pappus; milkweed, plume; thisles, pappus; sowthistles; pappus is characteristic of Compositae family seeds
    b. winged seeds: maple seeds: lift provided by wings; asymetrical wing can be an advantage
  4. Movement of mother plant (with seeds) on ground after dispersal, blow with wind; seeds dispersed in rolling action; examples: kochia on mother plant, forms rolling ball; tumbleweed; velvetleaf capsules

Seed ejaculation; example: mistletoe seed propelled by mother plant; also has sticky seed surface to adhere to tree host, bird foot vector

  1. Analogy of invasion of a habitat by colonizing plant (weed) with spread of pathogenic infection in crop
    a. Crop infection: logarithm of infection density to the logarithm of distance from source
    b. If the slope is that of the inverse square law, or steeper, a population will spread into a colonizable territory as an advancing front: “horizon of infection”; the steeper the slope the more sharply defined is the infection front
    c. Dispersal curves with slopes less than than that of the inverse square law will lead to a spread-out pattern of isolated colonists, which may subsequently act as new foci for new infections
  2. Poverty of seed fall in the immediate neighborhood of the parent characteristic of of plants in isolation; e.g. mullein, musk thistle is pasture
  3. Species with specialized wind dispersal mechanisms generally don’t colonize as a horizon but as isolated individuals over a greater distance
  4. Dense stands will colonize as an advancing front

The role of animals in dispersal

Animal Dispersal:

Species-specific feeding patterns

Territorial and migratory behavior

Eating, digestion, viability changes

Animal storage of seed: distance, concentration, location

Birds: ones with hard gizzards that destroy seeds; soft gizzards that pass on viable seed

Ants play important role, especially in storage concentrations

Specialized structures:
a. burrs: cocklebur, sandbur seed
b. appendages: beggarticks seed

  1. New continent, region invasions
  2. Local dispersal: machinery, crop seed contamination
  3. Crop mimicry dispersal
    a. weed seed adaptations to look like crop seed: plant body or seed same size, shape, morphology as crop
    b. example: barnyardgrass biotype looking like rice escapes handweeding and is dispersed with rice
    c. example: nightshade fruit (“berries”) same size, shape as dry beans, harvested and dispersed with beans

Post-Dispersal movement of seed

Wind-blown, or soil surface movement by water: accumulate near obstacles (fences, furrows, etc.) or fall in soil crevices, dried soil cracks: composite family weeds with pappus, etc., seed held at constant distance in crack (soil) by pappus not entering crack

Specialized mechanisms of some weed species to bury themselves: wild oat hygroscopic awns

Dynamic interactions with soil surface roughness, by species