Diurnal variation in settling velocity of pollen released from corn and consequences for atmospheric dispersion

Quantifying the atmospheric dispersion of corn pollen by turbulent wind is crucial in predicting cross pollination and assessing risks associated with gene flow from genetically modified (GM) crops. Corn pollen dispersion is strongly influenced by atmospheric turbulence and the physical properties of pollen itself. Of these properties, settling velocity is the most relevant. The water content of corn pollen has a strong effect on its settling velocity because it determines the pollen's size, shape and density. The goal of this work was to determine the settling velocity probability density function of freshly shed corn pollen at different times of the day, and study its consequences on pollen atmospheric dispersion.

The probability density function of settling velocity was experimentally measured in a field study. A settling velocity chamber was used, in which long exposure digital pictures were taken and the size of the streaks and exposure time were used to determine the settling velocity of individual pollen grains. A strongly bi-modal distribution was observed indicating that pollen grains were either “wet” or “dry”, with settling velocities of 0.29m/s and 0.20m/s respectively. The results indicated that the wet and dry pollen fractions varied throughout the morning, shifting from mostly wet in the early morning, to mostly dry around noon. Simultaneous measurements of meteorological variables suggest that this effect was related to reductions in the ambient relative humidity and increases in vapor pressure deficit.

An analytical two-dimensional dispersion model was used to characterize the effects of the settling velocity distributions on corn pollen dispersion. On average, the model indicated that dry pollen grains travel considerably farther from the source than wet ones. This suggests that pollen grains released later in the morning are more effective in cross-pollinating corn fields than pollen released earlier in the day (assuming they remain viable).