Evidence for long-range transport of viable maize pollen

Most experiments on the dispersal of maize pollen have been performed using ground-based systems with pollen traps located at various distances downwind from a source. Such measurements can only provide information on short-range dispersal in the surface boundary layer (i.e. within horizontal distances not exceeding a few hundred metres). In order to investigate whether longer-range transport may also occur, we study here the presence and viability of pollen throughout the atmospheric boundary layer (ABL). For this purpose a series of ten flights was performed with a light aircraft over a 4000 ha set of maize fields in South-West France, on several days with convective activity during the pollination period (July 2002 and 2003). Several flight legs were made at various altitudes between 150 m and 1800 m, i.e. within and above the ABL. A sampling device was built to collect pollen grains by impaction on Petri dishes located inside a plastic tube set up under the wing and facing forward. Pollen counts were made under a microscope after the flight was completed. Radio-soundings were collected in order to characterize the structure of the ABL. Pollen grains were found at all heights, with concentrations ranging from 0.2 (at the top) to 1.1 grains m^-3 (at the bottom). The vertical variation in concentration was typical of the structure of the ABL on a convective day. The viability showed a smooth decrease with height but remained significant throughout the boundary layer. These results have profound implications on the possibility of long-range gene dissemination, because they show that in such climatic conditions viable pollen grains may travel over kilometres before they settle.