Biology Meets Turbulence: Effects of Convective Eddies on Pollen Dispersion and Viability

Adoption of genetically modified (GM) crops has raised concerns that GM traits can accidentally cross into conventional crops or wild relatives through the transport of wind-borne pollen. In some cases, such as production of plant-made pharmaceuticals (PMPs), there is concern over even small probabilities of outcrossing. To assess this risk it is necessary not only to compute the transport and dispersion of pollen but also to recognize that pollen is a living organism that becomes non-viable as it loses moisture. The Lagrangian approach is well suited to this challenge because it allows diagnosis of environmental conditions that pollen grains experience as they travel. We take advantage of this capability by combining a high-resolution version of the WRF meteorological model with a Lagrangian particle dispersion model to predict maize pollen dispersion and viability. WRF is used to obtain fields of wind, turbulence kinetic energy, temperature, and humidity which are then used as input to the Lagrangian dispersion model. The dispersion model in turn predicts transport of tracer particles that represent a statistical sample of a pollen cloud. We diagnose vapor pressure deficit at each point along the path of each tracer particle (i.e., virtual pollen grain), from which changes in moisture content of the pollen grains and consequent loss of viability are calculated. Small amounts of pollen were predicted to be transported 5 km or more from the source. Previous estimates of maize pollen transport using only surface conditions have predicted transport only to a few tens of meters, owing to the large terminal velocity of maize pollen grains. Conversely, we found that turbulent motions in the convective boundary layer can lift maize pollen grains to heights of several hundred meters, so that they can be transported long distances before settling to the ground. We also found that pollen lifted into the upper part of the boundary layer remains viable longer than has been inferred from surface observations of temperature and humidity. This occurs because the daytime atmospheric boundary layer temperature decreases with height approximately as the dry adiabatic lapse rate while absolute humidity remains approximately constant, producing an environment of low vapor pressure deficit in the upper boundary layer which helps preserve the viability of pollen. Our results illustrate the complex interplay between transport and biology, and are consistent with recent studies using both piloted and remote-controlled aircraft that have shown the presence of viable maize pollen through the entire depth of the convective boundary layer.