Allelopathic pollen in Canadian Invasive species: Alliaria petiolata and Hesperis matronalis

We tested the in situ pollen transfers and allelopathic impacts of the putative pollen allelopathic spp. [Alliaria petiolata L. and Hesperis matronalis L. on the sympatric concurrently flowering spp. Alyssum alyssoides (L.), Cardamine diphylla (Michx.) Willd., Lepidium campestre (L.) R. Br. and Sisymbrium altissimum L]. Pearson product moment and Mann Whitney U tests indicated that regardless of whether Coulter Counters or microscope counts were used to count pollen, there was sufficient pollen transfer from the Alliaria petiolata and Hesperis matronalis to all spp. except Cardamine diphylla, where its pollinator is more fidelous. When there was a relatively low maximum of 10 pollen grains of either Alliaria petiolata or Hesperis matronalis on stigmas of Alyssum alyssoides, Lepidium campestre and Sisymbrium altissimum, the pollen tube generation declined by 80% and the difference compared to stigmas with no heterospecific pollen was significant (Pearson product moment tests, P < 0.001). While this is typical for pollen allelopathic impacts, our conclusion is more cautious, because after 5-days in a sterilized environment, fungi that already existed on pollen and stigmas began to grow. This is unlike the situation in our research group's previous studies, where there was no “contamination” from fungi or any other organism or chemical. As such, we conclude that while there is evidence of pollen allelopathic impacts, it is still possible that the declines in pollen tube generation in species receiving pollen from Alliaria petiolata or Hesperis matronalis may be due to immediate exudation from fungal spores carried on pollen or stigmas rather than pollen.