Pollen allelopathy is phenomenon when pollen chemicals (e.g., phenols, terpenoids, sesquiterpene lactones, etc.) inhibit sexual reproduction in heterospecific individuals due to influencing of fertilization (Murphy 1992). The phenomenon includes excretion of signaling compounds from the donor cell (pollens, pistil stigma), recognition of a specific signal, transmission of information (pollen), and the development of a characteristic response in the acceptor cell. The possible mechanism of the effect was described in Roshchina (2001).
Pollen allelopathy can find utilization in field cultivations that could contain pollen of allelopathic crops or weeds. Pollen allelopathy could be an effective method for annual weed control that reproduce, at least in part, via wind pollination and flower concurrently with the allelopathic species. The effects of allelopathy should result in the loss of genetic variation and so in reduction of reproductive ability, but some plants are probably able to detoxify the pollen allelochemicals (Murphy and Aarssen 1995a, b). Murphy and Aarssen (1989) suggested possible delaying of weed flowering at later, less favorable times of the season or diurnal period, so decrease in weed pressure. However, infestation by perennial weeds can worsen due to compensation of pollen allelopathy through increase in the formation of rhizomes.
Pollen allelopathy is not common (Murphy 2000). To date, two allelopathic crops were found. Maize (Zea mays var. chalquinoco'nico) was pollen-allelopathic against barnyard grass (E. crus-galli (L.) P. Beauv.), hairy beggar-ticks (Bidens pilosa L.), curly dock (Rumex crispus L.), Cassia jalapensis (Britton) Lundell and Amaranthus leucocarpus S. Wats. (Ortega et al. 1988). The second crop, timothy grass (Phleum pratense L.), demonstrated pollen-allelopathic effects on Solidago rugosa Mill., Melilotus alba Medic., Euphrasia officinalis, Cirsium arvense L. Scop, Agrostis lateriflora Michx., Aster ericoides L., Aster dumosus L., Ambrosia artemisifolia L., Elymus repens (L.) Gould, Bromus inermis Leyss., Danthonia compressa Austin, and Poa compressa L. (Murphy and Aarssen 1989).
The most promising allelopathic weeds are hawkweeds (Hieracium spp.). They are effective in reducing other asteraceous weeds (Murphy and Aarssen 1995b) and yellow hawkweed (H. pratense Tausch.) inhibited Canada thistle (Cirsium arvense (L.) Scop.), perennial sow-thistle (Sonchus arvensis L.), yarrow (Achillea millefolium L.), and annual sow thistle (Sonchus oleraceus L.); however, the long-term effect on perennial species is unclear (Murphy 2001).
Pollen grains of another species, parthenium (Parthenium hysterophorus L.), contained growth inhibitors which inhibited fruit set in many test species (Sukhada and Jayachandra 1980). However this plant is a noxious weed and its pollen is strong, therefore use of parthenium in agriculture is unlikely.
The main advantage of pollen allelopathy is that allelochemicals occur in a natural form, i.e., pollen grains and are biologically active at low doses (10 grains mm-2 on stigmas) (Murphy 2001). In case of pollen allelopathy, autotoxicity was not established (Murphy and Aarssen 1995a).
For pollen-allelopathic breeding, plants with tall growth habit and relatively large quantity of pollen should be selected (Murphy and Aarssen 1989). Pollen of these species could be artificially dusted on the stigmatic surface of other plants. This phenomenon is yet to be studied and field tested.
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