Lecture 03 2010

Lecture 03 2010 - Weed Biology 2 PLS 176 Winter 2010 Weed...

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Unformatted text preview: Weed Biology 2 PLS 176 Winter 2010 Weed Seed Dispersal 1. From large areas to local redistribution Continental (between & within) • • • • Ship ballasts Imported crop seed Intentional: new crop or ornamental Accidental, human, livestock Within countries, states, habitat 2. Means of Dispersal Artificial dissemination • Disperse over the longest distances • Contaminated crop seed, grain feed, hay, exotic introductions. • Farm equipment Natural dispersal • Animals (incl. man) • Water • Wind, the most common • Diverse structures for dispersal Weed Seed Dispersal Structures 1 2 3 4 5 Avena fatua wild oat 6 7 1. Tribulis terrestris puncturevine 2. Xanthium strumarium cocklebur 3. Taraxacum officinale dandelion 4. Rumex crispus curly dock 5. Avena fatua wild oat 6. Cenchrus longispinus longspine sandbur 7. Bidens pilosa Spanish needle Infestation creeps as advancing front Parental-associated mortality New satellite colonies Aegilops cylindrica L Infested over 5 million acres and was spreading unchecked at a rate of 50,000 acres per year http://cru.cahe.wsu.edu/CEPublications/eb1931/EB1931.pdf Seed Dormancy temporary failure to germinate dispersal in time, perpetuation of seed producers adaptation to ensure favorable conditions “bred out” during crop domestication responsible for seed accumulation in soil seed bank Seed Dormancy • Enforced (Quiescence) deprived of requirements for germination buried, lack of O2 surface: lack of moisture, extreme temperatures waterlogged fields Will germinate when limitations removed Seed Dormancy • Enforced • Innate (Primary or Direct) No germination after shed from mother plant 1. Immature embryo 2. Thick impermeable seed coat (physical dormancy) 3. Physiological • Inhibitor in seed coat • Afterripening requirements Fluctuating temperatures Chilling often req. by spring-germinating weeds Winter annuals: a high temperature requirement often delays germination to fall Seed Dormancy • Enforced • Innate • Induced (Secondary or Indirect) Imposed after dispersal by: high Temp., dryness, high CO2/O2 Ex.: burial can impose afterripening or light requirement o Light can break it Phytochrome controlled Prevents germination under closed overstory One reason why tillage favors germination light Pr (no germ.) 660 nm (red) Pfr (germination) Pr (no germ.) Pfr (germ.) 730 nm (far red) Light under canopies is far-red enriched Common Ragweed Seed Dormancy Ambrosia artemisiifolia Bazzaz 1979 Lambsquarters (Chenopodium album) • 97% black, thick coat (60µ); stimuli required: nitrate, chill, then spring • 3% brown (16 µ), no requirements • Sp. will always find a favorable situation difficult to get wiped out Soil Seed-bank • All viable seeds (dormant and non-dormant) • Persistence of a species • Seeds are distributed vertically, but most weed seeds are at shallow depths (top 6 inches) • Tillage disturbance (annual cropping) promotes early colonizers (annuals): they produce lots of seed Often there is a larger seed bank in disturbed cropland • Smaller seedbank under stable vegetation (biennials, perennials, climax) Eventually seed that remains buried decays and senescences • Soil Seed Bank Dynamics In-situ seed rain immigration • Crop seed • Farm equipment • Animals Shallow seeds have shorter life Adapt. From Harper 1977 Weed Seed Longevities in Soil • Depend on species, environmental conditions, disturbances, biological interactions • In agricultural soils Grasses (thin coats) can disappear ~3-4 yr Legumes & non-grass usually have thicker coats and are longer lived: ~5-10 yr Buried seed can be very long-lived Weed management 1. prevent seeds from getting to soil control before produced (eliminate “escapes”) control immigration cover crops to prevent colonization 2. seed bank depletion 1. Promote weed emergence with irrigation 2. kill seed in first 2 cm with shallow tillage or use herbicide 3. avoid additions by burial with deep tillage discourages germination and favors dormancy: seed bank increases seeds are eventually brought back to surface The stale-seedbed concept (using glyphosate & zero tillage) Conventional rice seeding Stale seedbed + zero tillage: no weeds 1.0 Aerobic 0.8 Proportional germination 0.6 Echinochloa spp. L. fascicularis C. difformis 0.4 0.2 1.0 Anaerobic Ammannia spp. C. difformis 0.8 0.6 0.4 0.2 0.0 0 5 10 15 20 25 Days after imbibition in 2005 30 Weed management 1. Seeking seed bank depletion 2. Manage seedlings: Most plants die at this stage Eliminate early competition • Emerging Seedlings: adapted to crop safe sites Opportunity: crop rotation Anoxic seedbed Aerated seedbed WATER-SEEDED RICE 2004 - 2006 Cyperus difformis Sch. mucronatus DRY-SEEDED RICE Ammania spp. Heteranthera limosa Echinochloa spp. Leptochloa fascicularis ...
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This note was uploaded on 03/05/2010 for the course PLS 176 taught by Professor Fischer during the Winter '10 term at UC Davis.

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