ABSTRACT. In recent years there has been interest in the dispersal of maize (Zea mays) pollen from crops, particularly in relation to gene flow and seed quality. We report the results of experiments that measured maize pollen dispersal from a 20 m × 20 m experimental crop. The experiments were done in a commercial farm in France during the summer of 2000. Pollen production was estimated to range from 10 4 to 2×10 6 grains per day per plant. Pollen concentrations and deposition rates decreased rapidly with distance from the crop: concentrations decreased by about a factor of 3 between 3 m and 10 m downwind of the source; deposition rates at 30 m were less than 10% of those at 1 m. Horizontal flux of pollen were estimated from pollen concentration and wind speed profiles using a mass balance approach, and ranged from 5 to 560 grains m -1 s -1 at 3 m from the source. Comparison of deposition rates estimated with the mass balance and direct measurement suggests that only a small proportion of the pollen released from the crop would have been still airborne at distances greater than 30 m downwind. Deposition velocity determined as the ratio of the deposition rate to the airborne concentration at 3 m from the source averaged 0.6 m s -1 , which is twice as large as the settling velocity for maize pollen.

To make quantitative predictions about the pollen dispersal of a plant species under different environmental conditions, it is necessary to determine its individual pollen dispersal function, i.e., the two‐dimensional density function describing the probability that a pollen grain emitted in (0, 0) fertilizes an ovule in (x, y). This function will depend on biological and climate parameters. We present models for the individual dispersal function of corn. These models are based on Brownian motion with drift and integrate biological (difference of height between male and female flowers) and aerodynamic (settling velocity, wind speed, air turbulence) parameters. The models presented differ in the importance of vegetation in stopping the paths of pollen grains. The models were fitted to data from two large field experiments of corn using the color of kernels as a phenotypic marker for pollen dispersal. The resulting estimations for the parameters of the models and comparisons between models indicate that (1) these models can provide good predictions of the observed data, (2) vegetation is not the major obstacle that stops pollen paths, and (3) there is a benefit in considering the difference in height between male and female flowers. Furthermore, values of the parameters estimated from dispersal data appear consistent with meteorological and biological data acquired independently. Corresponding Editor: S. T. Jackson.

ABSTRACT. The co-existence of genetically modified (GM) crops with conventional crops has become a subject of debate and inquiry. Maize (Zea mays L.) is one of the most cultivated crop plants in the world and there is a need to assess the risks of cross-pollination. Concentration and deposition rate downwind from different-sized maize crops were measured during three flowering seasons, together with micrometeorological conditions in the surrounding environment. Pollen release started once the air vapour pressure deficit (VPD) increases above 0.2 to 0.5 kPa. Moreover, the dynamics of release was correlated with the dynamics of VPD surrounding the tassels. Horizontal deposition appeared to follow a power law over short distance downwind from the source, and the dispersal distance increased with the source canopy height, and the roughness length of the downwind canopy. This work also provides a data set containing both pollen measurements and contrasting weather conditions to validate dispersal models and further investigate maize pollen dispersal processes.

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