Pollen flow out of greenhouses for wind-pollinated species

18 august 2009: Incidental formation of hybrids between cultivated plants and their wild relatives is well documented for areas where they co-occur. While crop-wild hybridization has most likely been going on for ages, the effect of gene escape from cultivated plants into wild plant populations has become cause for public concern, connected to the construction of transgenic crops. Transgenes built into the crop through genetic modification might become introgressed into the genomes of nearby wild relatives. If these newly introduced genes confer a competitive advantage of hybrids over wild relatives, the gene may spread and in the extreme case this might result in invasiveness that can not be easily reversed. This has resulted in increased attention for methods and policies that would circumvent the initial formation of hybrids, and legislation to regulate the use of transgenic plants.
One potential route of gene flow from crops to wild relatives is by outcrossing after pollen escape from a greenhouse. In order to prevent this from happening, the Dutch government has implemented the “inhullingsplicht” for transgenic wind pollinated species with compatible relatives (crops or wild species) in the Netherlands: such plants may only be grown if inflorescences are covered by a fine, pollen-proof mesh, a time-consuming and expensive measure. The use of insect-proof screens is a common practice in greenhouses in many countries. These screens act as barriers that prevent insects entering the greenhouse, thus avoiding crop damage by herbivory and/or pathogen transfer. Potentially, these screens could also serve as a barrier restraining pollen escape from the greenhouse, although the mesh size used is often much larger than pollen size. Reduced wind speeds near the netting could provide a barrier for at least part of the pollen, and also air flows and turbulence in the greenhouse might be affected. As yet the empirical data on the actual pollen flow from greenhouses is very sparse. This instigated the COGEM to commission a study aiming to gather relevant data on pollen escape from such greenhouses. Researchers from the Institute for Biodiversity and Ecosystem Dynamics and the University of Amsterdam investigated how the presence of insect netting affects pollen escaping from greenhouses.

Conclusions:

1. The flow of pollen from greenhouses is relatively high. The high ratios of the concentration outside to concentration within greenhouse at 2.5m height indicate that a significant proportion of airborne pollen leaves a greenhouse during periods that the greenhouse is vented through opening the roof windows.

2. Insect netting is not effective in reducing the pollen flow out of a greenhouse. The addition of insect netting in our experiments did not lower thepollen concentrations measured outside of the greenhouse. Further research would be needed to see if alternative (cheap) pollen barriers are more effective.

3. The amount of pollen escape from a greenhouse is not relevant during periods where there are no flowering, compatible recipients present in the vicinity of the greenhouse. Although this issue is not specifically addressed here, quantitative risk assessment of the escape phase is only relevant if the other steps in the formation of crop-wild (or crop-crop) hybrids are feasible. If there are no wild relatives flowering in nature crop-to-wild gene flow is not possible.

4. The amount of turbulence in the greenhouse is a decisive factor for pollen escape.
More (artificial) turbulence increased the concentration of airborne pollen inside the greenhouse and hence also outside. Variation in airflows among greenhouses with different ventilation regimes are expected to lead to different rates of pollen escaping, also depending on pollen sizes. The escape of particles from greenhouses could be further studied using artificial pollen grains, like Lycopodium spores (Ø 30 μm), allowing better standardized releases and independence of plant flowering behaviour.

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