Processes involved in the dispersal of maize pollen : some experimental results on the source behaviour and the settling speed variability

The understanding of pollen dispersal in the atmosphere and its modelling require to improve the quantification of elementary biophysical processes such as emission, sedimentation and deposition. Recent modelling efforts were made to quantify maize pollen movement in a maize canopy (Aylor, 2005) or downwind from the source (Jarosz et al, 2003, 2004). Our objective is to contribute some knowledge to the dispersal of maize pollen seen as a field source and a population of biotic particles. It is well known that pollen emission from male flowers follows a nycthemeral cycle but climatic factors may significantly modify the temporal pattern of pollen production under non limiting environmental conditions. Such experimental results should help in the formulation of a model devoted to the source term simulation in complex models of pollen dispersal at field or landscape scale. In physical models of pollen dispersal, a crucial parameter is the fall speed of pollen grains. Because this parameter varies in relation to water content, shape and size of the pollen grain, its frequency distribution must be characterized to feed the equations of motion. Experimental results are presented first on the source characterization at field or canopy scale and second on the settling velocity variability of pollen grains.

Following preliminary results (Jarosz et al, 2003) on maize pollen emission obtained in a large study of pollen dispersal downwind from a pollen source, field experiments were made in France with 2 or 3 sowing dates and 2 sites under various cultivar and microclimate conditions. During the full pollination period, mean concentration measurements were made every two hours at the male flower level using a Burkardt trap. Estimates of pollen deposition were obtained using containers placed within the pollen source above ground level. Pollen deposition sensors were handled manually in 2004 and mechanically in 2005 (both above and within the canopy). Relationships between pollen emission and microclimate are pointed out. Based on a new and simple method for measuring directly the settling velocity of particles (sequential pictures of particles falling under gravity in calm air), results obtained with inert spherical particles, Lycopodium spores and maize pollen illustrate the accuracy of the measuring technique and its adequacy for characterizing (bi-)Gaussian distributions. The variability observed in both emission and fall velocity should question our approaches of the interactions between physical and biological processes governing the success of maize pollination.

References

Aylor D.E. (2005). Quantifying maize pollen movement in a maize canopy. Agricultural and Forest Meteorology, 131, 247-256.

Jarosz, N., Loubet, B., Durand, B., McCartney, H.A. & Huber, L. (2003). Field measurements of airborne concentration and deposition rate of maize pollen. Agricultural and Forest Meteorology, 119, 37-51.

Jarosz, N., Loubet, B. & Huber, L. (2004). Modelling airborne concentration and deposition rate of maize pollen. Atmospheric Environment 38, 5555-5566.