4 intro inv wds weed biology reproduction dispersal germination establishment

Weed Biology

Weed Classifications

• Life cycle–annuals

• summer annuals• winter annuals

–biennials–perennials

• simple • creeping

Weed Reproduction

• Two means–sexual

• seed or spores–asexual

• vegetative reproduction

Weed Reproduction

• Sexual–many weeds cross-pollination or outcrossing not necessary• weeds are self-compatible

–important weed characteristic

Weed Reproduction

• Sexual–weeds colonize wide variety disturbed habitats• breeding system in relation to ability to colonize varied habitats is of interest–adaptive value of uniparental reproduction with occasional outcrossing - Baker’s Rule (1974)

Weed Reproduction

• Sexual–many weeds use inbreeding• produce stable duplicate individuals adapted to local environment

• occasional outcrossing for recombination to occupy new or changing microenvironments

Weed Reproduction

• Asexual/vegetative reproduction–common weed characteristic

• most common to perennials –not restricted to perennials

• Definitions– ramet is a single unit of

clonal growth–genet genetically distinct

individuals

Weed Reproduction

• Types vegetative reproduction:–stolons

• above ground shoots grow along surface

– produce adventitious roots at nodes

• bermudagrass - perennial• large crabgrass - annual

Weed Reproduction

• Types vegetative reproduction:– rhizomes

• underground shoots– produce adventitious roots

usually at nodes• johnsongrass - perennial• quackgrass - perennial• yellow nutsedge - perennial

Weed Reproduction

• Types vegetative reproduction:– tubers

• enlarged terminal portions of rhizomes

– extensive storage tissue & axillary buds

• yellow nutsedge - perennial• Jerusalem artichoke - perennial

Weed Reproduction

• Types vegetative reproduction:–bulbs

• underground modified buds – shoot & fleshy scale leaves– nutrient storage in leaves

• wild garlic - perennial

Bulbousbluegrass

bulbs

Bulbous bluegrass bulbils;replace seed formation;

longevity is unknown

Weed Reproduction

• Types vegetative reproduction:–corms

• enlarged vertical underground shoot covered with one or more layers of leaf bases

– nutrient storage• bulbous buttercup - perennial

Weed Reproduction

• Types vegetative reproduction:– roots

• usually, long, extensive • horizontal sections produce

adventitious shoots & new roots– often form buds

• Canada thistle - perennial• Leafy spurge - perennial

Weed Reproduction

• Types vegetative reproduction:–shoots or stems

• some spp produce adventitious roots & new shoots near branch tips

– tamarisk species• sprout from base or stump

– dandelion– locoweed

Tamarix parviflora shoots from brokenFragment of stem washed downstream

Weed Reproduction

• Types vegetative reproduction:– fragmentation

• spread/establishment from excised plant part

– most underground tissues capable– also from leaves or foliar shoots

» purslane - annual» field bindweed - perennial

Weed Dispersal

• Dissemination/dispersal–scattering of propagules

• re-infest site or transport new site• seeds most important mechanism

– movement of vegetative propagules also occurs, but less important, particularly long distance dispersal

» management difficulty increased» human activities often involved

Weed Seed Dispersal

• Dissemination/dispersal–seeds dispersed in space and time

Weed Dispersal• Seeds dispersed in space

–adaptations to move away from parents• most fall close to parent• probability seedling mortality

(competition) decreases w/distance– optimum distance seed/seedling

numbers population establishment– creeping infestation occurs

Weed Dispersal

• Seeds dispersed in space–wind, water, wildlife, humans

• main dispersal vehicles or forces• weeds evolved appendages,

coatings, palatability aid in dispersal

– fruits or seeds


–wildlife & humans• barbs, hooks

– adhere hooves, hair, clothing• palatable fruits/seeds

– eaten & excreted• harvested with crops

– seed or hay crops• autos & other vehicles


–wind• dust

– spores of Equisetum• winged

– toadflaxes• plumed

– pappus of many Asteraceae• tumbling

– diffuse knapweed, Russian thistle, kochia

Dalmatian toadflax seeds


–water• irrigation water

– 1920, CO, 81 weed spp., 156 ditches – 1980, NE, 77 weed spp

» 2-5X more in canals than North Platte– 1975, WA

» 125,000,000/ha non-irrigated» 500,000,000/ha irrigated 5 yr

• rivers, streams, creeks– invasive weeds

Weed Seed Dispersal

• Seeds dispersed in time• dormancy

–arrest in seed embryo development under suitable germination conditions

–dormancy is dispersal in time

Weed Seed Dispersal• Seeds dispersed in time

–Classification of dormancy• several schemes

–seasonal dormancy• predictable, e.g. based on daylength

–opportunistic dormancy• take advantage of favorable conditions in unpredictable environment

Weed Reproduction

• Seeds dispersed in time–Classification of dormancy

• seasonal v opportunistic–not mutually exclusive–more mechanistic description desirable

Weed Seed Dispersal

• Seeds dispersed in time–Harper 1977

• some seeds ‘born dormant’ (innate)

• some ‘achieve dormancy’ (induced)

• some ‘have it thrust upon them’ (enforced)

Weed Seed Dispersal

• Seeds dispersed in time–Classification of dormancy–primary dormancy:

• innate dormancy is an arrested state of embryo at dispersal

– incomplete embryo development» “afteripening”

– chemical inhibitors fruit or seed coat

Weed Seed Dispersal• Seeds dispersed in time

–Classification of dormancy–primary dormancy:

• innate dormancy is an arrested state of embryo at dispersal–somatic polymorphisms good e.g.

» production of seed from same parent but with different morphologies and/or behaviors

» e.g. 2 cocklebur seeds/fruit

Weed Seed Dispersal

• Seeds dispersed in time–Classification of dormancy–secondary dormancy

• enforced dormancy is the arrested state of embryo maintained in a limited environment

– e.g. inadequate O2, temp., pH, moisture

– seed will germinate when conditions corrected

Weed Seed Dispersal

• Seeds dispersed in time–Classification of dormancy– secondary dormancy

• induced dormancy – seed shed from parent not

dormant;– but limiting environment causes

arrested embryo state that will persist even after germination conditions optimal

» internal changes within embryo

Germination & Establishment

• Germination – from seed to seedling or bud

to ramet most critical phase in plant’s life• environmental factors influencing transition not well understood

– control weeds with preemergence techniques or postemergence

– do not prevent or break dormancy under field conditions


• Germination –difficult to separate

environmental factors that cause dormancy break from simple germination:• light• temperature• pH• aeration (O2, CO2)


• Germination – light requirement

• long known seeds need light to germinate

• light flash mechanism may allow/assist weeds or pioneer plants to exploit disturbed environments



• most weed seeds abundant shallow soil depths

– germinate best shallow soil• deep burial major cause weed seedling mortality

– although also causes increased seed survival

» enforced or induced dormancy


• Germination –light requirement

• burial eliminates exposure to red light–excavation causes exposure then germination

–phytochrome give “clue” to potential as to position in or on soil

» or presence or absence of plant community (canopy); competition



• phytochrome system involved–two forms

» Pr & Pfr

» forms regulated by ratio of red (660 nm) to far red (730 nm) light



• Pr inactive form–absorbs red light and converted to Pfr, which is the active form

–Pfr then stimulates germination process

–seed must be imbibed

• in absence of light Pfr gradually returns to Pr



• unfiltered light lots of red wavelength

– bare soil or open canopy promotes germination

• leaf canopy filters red light (chlorophyll)

– far red passes thru depressing germination

• freshly harvested seed no light requirement

– light requirement enforced dormancy


• Propagule dispersal in space and time–important mechanisms for establishment and survival of weeds

Weed Seed Dynamics

• Soil seed reserve• major component of weed populations–previous primarily & recent

dispersals• locally produced & imports

–previous & recent• short seed survival can be managed

http://www.ipm.msu.edu/new-ag/issues07/5-23Soil.htmJohn Masiunas; University of Illinois

http://www.ipm.msu.edu/new-ag/issues07/5-23Soil.htm

http://www.ipm.msu.edu/new-ag/issues07/5-23Soil.htm

Weed Seed Dynamics• Soil seed reserve

• Harper - “deposits & withdrawals”

– deposits from reproduction– withdrawals from germination,

senescence & death, and predation

• storage occurs with vertical distribution thru soil profile

– weeds mostly at shallow depths

Loamy sand

Silty loam

Menallad & Schonbeck 2011http://www.extension.org/pages/18527/manage-the-weed-seed-bank-minimize-deposits-and-maximize-withdrawals

http://www.extension.org/pages/18527/manage-the-weed-

http://www.extension.org/pages/18527/manage-the-weed-


• Safe site concept:–derived from observations that all of seed in seed bank do not germinate at once• small % per unit of time• most that germinate do not survive–kochia


• Safe site concept:– safe site provides (Harper, 1977)

• stimuli for dormancy break• suitable germination conditions• available resources for growth • absence of hazards

– placement of seed in safe site critical for survival• although typically a chance event


• Create safe site for crops:–weeds adapt to safe site

• or develop mechanisms to avoid death

– light requirement – somatic polymorphisms

» seed w/different morphologies/behaviors– nitrophiles (tolerance high N)

» enzyme induction or signal


• Create safe site for crops:–weeds adapt to safe site

• or develop mechanisms to avoid death

– awns allow seed to twist into soil with alternate wetting & drying cycles

– pappus can be hygroscopic– mucilaginous coats that are

hygroscopic

Wild oatcaryopses; awns


• Safe site:–weeds (plants) can evolve

adaptations to improve finding or creating safe site• but largely a chance event• capability of any species to occupy

a site in abundance function of predecessors leaving behind well adapted progeny


• Risk of death:–seedling is weak link

• knowing biology helps select mgmt tools to exploit this weakness

–probability of mortality• critical risk of death

– determines initial weed density in field


• Critical risk of death:–each species specific

requirements dormancy break & germination• some spp favored, some

discouraged by modification of safe site (mgmt)

• eventually all safe sites filled– further germ unlikely, high risk of

death– environmental signals keep

“withdrawals” to minimum– adopt cultural practices that affect

safe sites is good mgmt practice

Treatments

Mowing Litter & seed Herbicide

None Leave None

None Leave Plateau 8 fl oz

2 inches tall Leave None

2 inches tall Leave Plateau 8 fl oz

2 inches tall Collect None

2 inches tall Collect Plateau 8 fl oz

6 inches tall Leave None

6 inches tall Leave Plateau 8 fl oz

6 inches tall Collect None

6 inches tall Collect Plateau 8 fl oz

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• Critical risk of death:–11 yr study wild oats illustrates

• 7 yr fall tillage increased WO density

– 400/m2

• followed 4 yr spring tillage & barley– 1st yr decreased WO to 85/m2

– stabilized succeeding yr at 5/m2


• Adopting cultural practices to influence safe site:–wild oats germinate fall and

spring• different ecotypes• fall tillage killed fall germinating

wild oats but favored spring germinators

• spring tillage decreased spring germinating populations


• Adopting cultural practices to influence safe site:–perennial weeds different story

• changing from fall to spring tillage may control seedlings better

• but will transplant rapidly growing perennials

– need different cultivation intervals to exhaust root nutrient stores

» generally multiple per season


• Predicting weed infestations:–value in knowing soil seed

reserve & knowing factors that affect probable mortality for predicting weed infestations• several demographic models

– most success with annuals

4 intro inv wds weed biology reproduction dispersal germination establishment

Documents