Weeds are known to cause enormous losses due to their interference in agroecosystems. Because of environmental and human health concerns, worldwide efforts are being made to reduce the heavy reliance on synthetic herbicides that are used to control weeds. In this regard the phenomenon of allelopathy, which is expressed through the release of chemicals by a plant, has been suggested to be one of the possible alternatives for achieving sustainable weed management. The use of allelopathy for controlling weeds could be either through directly utilizing natural allelopathic interactions, particularly of crop plants, or by using allelochemicals as natural herbicides. In the former case, a number of crop plants with allelopathic potential can be used as cover, smother, and green manure crops for managing weeds by making desired manipulations in the cultural practices and cropping patterns. These can be suitably rotated or intercropped with main crops to manage the target weeds (including parasitic ones) selectively. Even the crop mulch/residues can also give desirable benefits. Not only the terrestrial weeds, even allelopathy can be suitably manipulated for the management of aquatic weeds. The allelochemicals present in the higher plants as well as in the microbes can be directly used for weed management on the pattern of herbicides. Their bioefficacy can be enhanced by structural changes or the synthesis of chemical analogues based on them. Further, in order to enhance the potential of allelopathic crops, several improvements can be made with the use of biotechnology or genomics and proteomics. In this context either the production of allelochemicals can be enhanced or the transgenics with foreign genes encoding for a particular weed-suppressing allelochemical could be produced. In the former, both conventional breeding and molecular genetical techniques are useful. However, with conventional breeding being slow and difficult, more emphasis is laid on the use of modern techniques such as molecular markers and the selection aided by them. Although the progress in this regard is slow, nevertheless some promising results are coming and more are expected in future. This review attempts to discuss all these aspects of allelopathy for the sustainable management of weeds.

Allelopathic Interactions and Allelochemicals: New Possibilities for Sustainable Weed Management

Preface.- List of contributors.- 1. Introduction to allelopathy Chou,C-H.- 2. Basic pathways for the origin of allelopathic compounds Seigler, D.S.- Methodological aspects. 3. Clues in the search of new herbicides Dayan, F.E., Duke, S.O.- 4. Distinguishing allelopathy from resource competition: the role of density Weidenhamer, J.D.- 5. Toxicity in allelopathy: in silico approach Lo Piparo, E. et al.- Physiological aspects of allelopathy. 6. Allelochemicals and photosynthesis Zhou, Y.H., Yu, J.Q.- 7. Cell cycle analyses for understanding growth inhibition Sanchez-Moreiras, A.M. et al. 8. Detoxification of allelochemicals. The case of bezoxazolin-2(3H)-one (BOA) Schulz, M. et al.- 9. Allelopathy and abiotic stress Pedrol, M.N. et al.- 10. Allelopathy and biotic stresses Gawronska, H., Golisz, A.- 11. Plant secondary metabolites. Targets and mechanisms of allelopathy Lotina-Hennsen, B. et al.- 12. Mitochondria as a site of allelochemical action Ishii-Iwamoto, E.L. et al.- Ecophysiology and allelopathy. 13. Weed germination, seedling growth and their lesson for allelopathy in agriculture Aliotta, A. et al.- 14. Allelopathy: A soil system perspective Blum, U.- 15. Microorganisms and allelopathy: A one-sided approach Vokou, D. et al.- Ecological aspects of allelopathy. 16. Ecological relationships and allelopathy Sinkkonen, A.- 17. Resistance and susceptibility of plant communities to invasion:revisiting rabotnov's ideas about community homeostasis Callaway, R.M., Hierro, J.L.- Allelopathy in different environments. 18. Allelopathy in marine ecosystems Graneli, E., Pavia, H.- 19. Allelopathy in aquatic environments Erhard, D.- 20. Forest ecosystems and allelopathy Reigosa, M.J., Gonzalez, L.- 21. Allelopathic interactions in agroecosystems Kohli, R.K. et al.- Applied aspects of allelopathy. 22. Playing with chemistry: studies on Orobanche spp. germination stimulants Macias, F.A. et al.- 23. Modes of action of phytotoxins from plants Duke, S.O., Dayan, F.E.- 24. Allelopathy in ecological sustainable agriculture Narwal, S.S.- 25. Parasitic weeds and allelopathy: from the hypothesis to the proof Qasem, J.R.-

Allelopathy : a physiological process with ecological implications

Humans the world over have depended on wild-growing plants in their diets for hundreds of thousands of years, and many people continue to rely on these species to meet at least part of their daily nutritional needs. Wild harvested plant foods include: roots and other underground parts; shoots and leafy greens; berries and other fleshy fruits; grains, nuts and seeds; and mushrooms, lichens, algae and other species. Use of any of these species requires special cultural knowledge regarding harvesting, preparation, cooking and other forms of processing. Many were, and are, prepared and served in mixtures or combinations. In most cases, too, the species are managed, tended or manipulated in some way to increase their productivity and availability. Many of the most widely used species are categorized as weeds—species that grow and reproduce readily in disturbed or cleared land, and are common around human settlements and agricultural areas. This paper presents case examples of edible wild plant use and the roles of these species in agroecosystems from different parts of the world and discusses similarities and differences in use across different cultures and segments of society.

http://www.andreapieroni.eu/Turner%20et%20al.,%202011.pdf

Edible and Tended Wild Plants, Traditional Ecological Knowledge and Agroecology

Annual crop fields typically are simple habitats dominated by a few plant species where pesticides play a major role in managing weed and insect infestations. Recently, there has been significant interest in the potential to reduce reliance on pesticides by manipulating plant species and communities to benefit natural enemies of insects and weeds. Such efforts aim to enhance natural enemy impact by providing appropriate food, shelter, and hosts, and efforts typically are accomplished by manipulation of plant species, populations, or communities. Habitat management is generally viewed as an important factor in maintaining stable insect and natural enemy populations in agricultural systems and may have a similar function in increasing weed seed predation. Crop and noncrop habitats provide resources to natural enemies either directly through floral nectar and pollen, indirectly by increased host or prey availability, or through emergent properties of the habitat such as by moderating the microclimate. These critical resources for natural enemies can be provided in agricultural ecosystems at several scales: within fields, at field margins, or as a component of the larger landscape. Because individual natural enemy species may require quite specific resources at different times and spatial scales, not all attempts to manipulate habitat diversity are equally effective. We review the role of plant resources, including weeds, in supporting natural enemy communities and provide case studies of how varying plant diversity at different spatial scales can influence the effectiveness of biological control in agricultural landscapes.

Manipulating plant resources to enhance beneficial arthropods in agricultural landscapes

To improve understanding of relationships between crop diversity, weed management practices, and weed population dynamics, we used data from a field experiment and matrix models to examine how contrasting crop rotations affect velvetleaf. We compared a 2-yr rotation system (corn–soybean) managed with conventional rates of herbicides with a 4-yr rotation (corn–soybean–triticale + alfalfa–alfalfa) that received 82% less herbicide. In November 2002, a pulse of velvetleaf seeds (500 seeds m−2) was added to 7- by 7-m areas within replicate plots of each crop phase–rotation system combination. Velvetleaf seed, seedling, and reproductive adult population densities, seed production, and seed losses to predators were measured during the next year. Velvetleaf seed production was greater in the 4-yr rotation than in the 2-yr rotation (460 vs. 16 seeds m−2). Averaged over 12 sampling periods from late May to mid-November 2003, loss of velvetleaf seeds to predators also was greater in the 4-yr rotation than i...

Are many little hammers effective? Velvetleaf (Abutilon theophrasti) population dynamics in two- and four-year crop rotation systems

Weeds from an ecological perspective Weeds share certain ecological characteristics that distinguish them from other plants. Specifically, weeds are plants that are especially successful at colonizing disturbed, but potentially productive, sites and at maintaining their abundance under conditions of repeated disturbance . That is, weeds are the plants that thrive where soil and climate are favorable to plant growth, but disturbance frequently reduces competition among plants to low levels. Unlike previous conceptions of weediness (Baker, 1965; Harlan & de Wet, 1965; Buchholtz et al. , 1967), this ecologically based definition lacks reference to humans and human disturbance. The species people refer to as weeds mostly existed prior to human disturbance, and the repertoire of behaviors that makes them invasive and persistent in human-dominated habitats largely evolved independently of human society. Nevertheless, as discussed in Chapter 10, human activities selectively modify weed characteristics such that weeds are becoming better adapted to human disturbance regimes. The subcategory of weeds dealt with in this book consists of the weeds of agriculture – specifically, the plants that colonize and increase in the disturbances created by farming. These are sometimes termed agrestal weeds, as distinguished from the ruderal weeds of roadsides, waste piles, and other non-agricultural disturbances (Baker, 1965). Agricultural weeds share certain life-history characteristics that adapt them for life on farms (Table 2.1). The thesis of this chapter is that understanding life-history characteristics provides insights into how weed management practices work and how they can be improved.

Charles L. Mohler

Papers

Ecological Management of Agricultural Weeds: Weed life history: identifying vulnerabilities