The role of pollen allelopathy in weed ecology

Pollen allelopathy results when pollen releases toxins that inhibit seed germination, seedling emergence, sporophytic growth, or sexual reproduction. Of the six known pollen-allelopathic species, two are crops (timothy and corn and four are weeds (orange hawkweed, parthenium, yellow hawkweed, and yellow-devil hawkweed). Allelopathic pollen in weeds could pose threats to crops, especially if both are wind pollinated. Even if it is the crop that is pollen-allelopathic, other crops could be threatened, or more likely, weeds might adapt to pollen allelopathy and pose a greater problem. Nonetheless, pollen allelopathy could be a useful approach to biological control because allelochemicals are packaged in a natural targeting system (pollen grains) and are biologically active at low doses (< 10 grains/mm(2) on stigmas). If it is to be an effective biological control agent, pollen allelopathy must be examined within the wider context of farming systems management and used as one method of varying selection pressures to prevent weeds from adapting to any one particular management technique or suite of techniques.