Waterhemp is a major weed of field crops in the Midwestern United States. Its prevalence is at least partly due to its rapid evolution of resistance to many groups of herbicides over the last two decades. In light of its dioecy and anemophily, pollen movement in waterhemp is considered to be an important factor in the spread of herbicide resistance genes. Thus, the biology and dispersal profile of waterhemp pollen are critical determinants for understanding and predicting the spatial population dynamics of herbicide resistance in this species. In this study, pollen longevity was investigated with greenhouse experiments, and pollen dispersal and the effect of pollen competition were investigated in field plots. Pollen dispersal was determined by measuring the frequency of seeds produced on receptor plants positioned at various distances from a pollen source, which flowered in synchrony with the receptor plants. Results indicated that waterhemp pollen can remain viable up to 120 h, implying a low limitation of pollen dispersal by its longevity. Effective pollen dispersal declined exponentially with distance, with most pollen fertilizing recipient plants within 50 m of the pollen source. However, long distance pollen dispersal (800 m) was also observed. We also saw evidence for pollen swamping in this species. Under conditions of pollen competition among distinct genotypes, pollination success was inversely related to distance between pollen donors and receptors. However, relative pollen density may also play an important role in determining the rate of long distance gene flow. This study confirmed the potential of waterhemp pollen to effect long distance gene flow and provides supporting data for quantitative spatial modeling of waterhemp resistance dynamics.

Nomenclature: Common waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif AMATU.