Allelopathy 1
Allelopathy By the end of this lesson, you should be able to define allelopathy explain the difference between allelopathy and competition identify the key interactions in allelopathy provide three examples of allelopathic plants in weed science describe how allelopathy can be used in weed management. 2
Allelopathy Defined Allelopathy is the negative effect of one plant on another plant by the release of chemicals from plant parts. Source: Ferguson, J.J. and B. Rathinasabapathi. 2003. Allelopathy: How Plants Suppress Other Plants. University of FL Extension Publication HS944. 3
Allelopathic Compounds Allelopathic chemicals can be present in any part of the plant They can be found in the soil around the plant. Species may be affected by these chemicals in different ways inhibit shoot and root growth inhibit nutrient uptake destroy the plant's ability to use a nutrient 4
Allelopathic Compounds Allelopathy is associated strongly with environmental stresses including: insects and disease temperatures radiation herbicides Stress enhances allelochemical production and increases the potential for allelopathic interference. 5
A Classic Example of Allelopathy Black walnut (Juglans nigra) has long been recognized as a plant with allelopathic properties. The chemical responsible for toxicity in black walnut is juglone and it acts as a respiration inhibitor. Solanaceous plants, such as tomato, pepper, and eggplant, are especially susceptible to juglone. plants exhibit symptoms such as wilting, chlorosis, and death. Juglone is present in all parts of the black walnut but concentrated in the buds, nut hulls, and roots. 6
Allelopathy and Competition Allelopathy and competition are both mechanisms of plant interference. Competition is for limited resources. By definition, allelopathy is not competition. However, allelopathy might explain some of the harmful effects that one plant has on another when competition is not involved. 7
Allelopathy is Difficult to Study It is hard to separate the effects of allelopathy from competition. An extensive and fibrous root system allows a weed to successfully compete for water and nutrients, but also provides a large potential for release of allelopathic compounds. Screening for allelopathic compounds requires that the experimental design eliminate resource competition as a factor in the experiment. This is extremely difficult to do. 8
Allelopathy is Difficult to Study To convincingly demonstrate allelopathy, the following are needed: laboratory, GH, and field studies must illustrate the effect of the released allelochemical the allelochemical must be isolated, identified, and characterized a correlation between the allelochemical and grown inhibition must be established the genes associated with allelopathy must be mapped competitive cultivars with allelopathy must be released. Some, if not most, allelopathic studies never get past step one. 9
Relationships in Allelopathy Crop on weed Crop on crop (when it s the same crop = autotoxicity = alfalfa) Weed on crop Weed on weed Weed against natural enemy (usually insects) The role of microbes (assist with decomposition & release) 10
Allelopathy and Weed Ecology Natural successions of plants occur in nature. Allelopathy is thought to play a part in weed composition and succession. Perennial weeds are often found in pure stands. The density and biomass of other species around perennial stands are greatly reduced. The explanation for this pattern has mostly been attributed to competition. 11
Allelopathy and Weed Ecology Evidence is accumulating that suggests that allelopathy plays an important role in succession. The fact that dense colonies of some perennials frequently occur essentially as pure stands may suggest allelopathy. 12
Allelopathy and Weed Ecology Hoary cress perennial noxious weed found in all parts of the continental USA except the southeast invaded over 100,000 ha in Oregon forms dense, monotypic stands Invasiveness attributed to a creeping root system that depletes the soil of moisture and nutrients. Allelopathy has been suspected as an interference mechanism. Hoary cress photo courtesy of the Kansas Department of Agriculture. 13
Allelopathy of Hoary Cress Roots of hoary cress plants were collected in the field. An extract of 5 g of ground roots + 100 ml water was prepared. Root length was measured for five species over time as influenced by exposure to hoary cress root extract. Root length after 2 d exposure to hoary cress extract was less for all species when compared with growth in distilled water. Source: Kiemnec, G.L. and M.L. McInnis. 2002. Hoary Cress (Cardaria draba) Root extract Reduces Germination and Root Growth of Five Plant Species. Weed Technol. 16:231-234.
Allelopathy of Wild Radish Wild radish winter annual broadleaf germinates in the fall and winter matures from March through June in the southeastern USA common in wheat fields. Reduced growth of crops and weeds is often reported following the addition of Brassica residues to soil or following the planting of Brassica spp. Wild radish photo courtesy of the Department of Water Resources, California. 16
Above-ground biomass of wild radish in water reduces germination of crops & weeds Source: Norsworthy, J.K. 2003. Allelopathic potential of wild radish (Rhaphanus raphanistrum). Weed Technol. 17:307-313.
Brassicaceae crops Mustard Rapeseed Canola Broccoli Cauliflower Kale Morra, 2007; Univ. of Idaho
Brassica Species Have been shown to have allelopathic effects Produce glucosinolates which break down to secondary metabolites Similar to some soil fumigants 19
Brassica seed meal Yaya carrot, common lambsquarters, Baronet lettuce, and Cabernet spring wheat seeded in rows at 20 seeds/flat Seed meal applied 3 days after seeding for PRE and at 2 leaf stage for POST treatments Biomass collected 18 DAT for PRE and POST treatments 2 lf stage for POST treatments 20
Carrot biomass by treatment as a 120 percent of control Biomass, % of average control 100 80 60 40 20 0 Dry seed meal pre - emergence Dry Seed meal post - emergence Extract alone Extract plus organic surfactant Extract plus synthetic surfactant 0 0.5 1 2 3 4 Dose of yellow mustard seed meal, mt/ha 21
Common lambsquarters biomass by treatment as a percent of control Biomass, % of average control 120 100 80 60 40 20 0 Dry seed meal pre - emergence Dry Seed meal post - emergence Extract alone Extract plus organic surfactant Extract plus synthetic surfactant 0 0.5 1 2 3 4 Dose of yellow mustard seed meal, mt/ha
Results Carrot Common lambsquarters Lettuce Spring wheat Treatment ---------------- GR50, mt/ha --------------- Dry Seed Meal (PRE) 15.5 0.7 16 4.4 Dry Seed Meal (POST) 8.5 2 4 12.4 Extract 7.5 1.5 3.5 6.4 Extract plus NIS 6 0.3 1.2 1.8 Extract plus OS 2.5 0.5 1.4 2.1 23
0.5 1.0 2.0 3.0 4.0 24
Rice Allelopathy and Weed Management Allelopathy in rice (Oryza sativa L.) dates back to the 1970s Rice has tremendous variability in allelopathic activity. Activity varies with the origin, grain type, height of plants, development stages and maturity of rice. Rice photo courtesy of OSU. 25
Rice Allelopathy and Weed Management Rice cultivars with strong allelopathic activity inhibit both grass and broadleaf weed species. The direct use of rice residues and genetic control of rice allelopathy to reduce weed infestations via breeding programs will signify a major breakthrough in weed management. 26
Allelopathy Milestones in Rice More than 12,000 rice cultivars have been screened for allelopathic potential. Allelopathic characteristics in rice involve more than one gene. Potential allelochemicals have been identified. may be more than one allelochemical present. The fate and modes of action of these allelochemicals is not well understood. Despite locating genes related to allelopathy, the introduction of these genes into improved cultivars has not yet been achieved. 27
Allelopathy and Weed Management Cultivars Cover crops Crop rotation Intercropping Residues as mulch (both crop & weed mulches) Extracts as foliar sprays (both crop & weed extracts) 28
STAGES OF DEVELOPMENT Propagule Germination Growth and development Maturation and reproduction CHARACTERISTICS OF ALLELOCHEMICALS They are present in all plant parts: flowers, fruit, leaves, stems and roots. They are produced by a variety of X metabolic processes. = The quantities produced increase under stress. They enter the environment in several ways; volatilization, leaching, exudation, and decomposition. They affect many internal growth processes: enzyme activity, cell division, growth regulator activity, membrane permeability, and other processes. WEED-CROP RESPONSE Persistence of some weeds. Reduced crop yields Shifts in weed composition 29
Allelopathy This lesson provided information that defined allelopathy, explained the difference between allelopathy and competition, identified the key interactions in allelopathy, provided three examples of allelopathic plants in weed science, and described how allelopathy can be used in weed management. 30