4 Intro Inv Wds Weed Biology Reproduction Dispersal Germination Establishment
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Transcript of 4 Intro Inv Wds Weed Biology Reproduction Dispersal Germination Establishment
Weed Classifications
• Life cycle–annuals
• summer annuals• winter annuals
–biennials–perennials
• simple • creeping
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
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
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 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
Weed Dispersal• Seeds dispersed in space
–wildlife & humans• barbs, hooks
– adhere hooves, hair, clothing• palatable fruits/seeds
– eaten & excreted• harvested with crops
– seed or hay crops• autos & other vehicles
Weed Dispersal• Seeds dispersed in space
–wind• dust
– spores of Equisetum• winged
– toadflaxes• plumed
– pappus of many Asteraceae• tumbling
– diffuse knapweed, Russian thistle, kochia
Weed Dispersal• Seeds dispersed in space
–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 & Establishment
• Germination –difficult to separate
environmental factors that cause dormancy break from simple germination:• light• temperature• pH• aeration (O2, CO2)
Germination & Establishment
• Germination – light requirement
• long known seeds need light to germinate
• light flash mechanism may allow/assist weeds or pioneer plants to exploit disturbed environments
Germination & Establishment
• Germination – light requirement
• 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 & Establishment
• 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
Germination & Establishment
• Germination –light requirement
• phytochrome system involved–two forms
» Pr & Pfr
» forms regulated by ratio of red (660 nm) to far red (730 nm) light
Germination & Establishment
• Germination –light requirement
• 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
Germination & Establishment
• Germination – light requirement
• 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
Germination & Establishment
• 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
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
Germination & Establishment
• 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
Germination & Establishment
• 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
Germination & Establishment
• 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
Germination & Establishment
• 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
Germination & Establishment
• 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
Germination & Establishment
• 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
Germination & Establishment
• 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
Plate
au +
Mow
6, L
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Untre
ated
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ow 2
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ve
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rbicide
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ow 2
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Plate
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bicide
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ow 6
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ve
No Her
bicide
+ M
ow 6
, Lea
ve
0
5
10
15
20
25
30
October 2010 % Cover (1YAT)
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Plate
au +
Mow
6, L
eave
Untre
ated
No he
rbicide
+ M
ow 2
, Lea
ve
No he
rbicide
+ M
ow 2
, Collect
Plate
au +
Mow
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ollect
Plate
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No Her
bicide
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ow 6
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Plate
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Mow
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No Her
bicide
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ow 6
, Lea
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0
5
10
15
20
25
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35
40
45
November 2010 % Cover (1YAT)
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Germination & Establishment
• 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
Germination & Establishment
• 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
Germination & Establishment
• 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
Germination & Establishment
• 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