Pollen morphology of three invasive Impatiens species in Europe under varying habitat conditions—a case study from Poland

The effect of the habitat conditions on the pollen features of invasive species has not been studied so far, and may affect the quality of their generative reproduction and contribute to the development of more effective methods of their control. Three species invasive in Europe and Poland were selected for the study - Impatiens parviflora DC., Impatiens glandulifera Royle and Impatiens capensis Meerb. The morphology and intraspecific variability of pollen grains in three Impatiens species growing under different habitat conditions were examined. Specimens were sampled from 198 sites throughout Poland, covering 10 ecologically distinct habitat types. In total, 5940 pollen grains were analysed in respect to the length of the polar axis (P), equatorial diameter (E), exine thickness (Exp), P/E, Exp/P ratios, and exine ornamentation and ectocolpi arrangement. Our research showed that the three studied species can be distinguished based on their palynomorphology. The most important traits were: exine ornamentation and ectocolpi arrangement, pollen size (P, E) and exine thickness (Exp). A relationship between the habitat conditions prevailing in the analysed habitats and the pollen grain characteristics was found, especially in I. glandulifera. In this species pollen size (P, E) increases in the optimal habitat conditions such as edges of reservoirs and watercourses, and decreases in the suboptimal habitat conditions such as anthropogenic habitats. A similar pattern is observed in I. parviflora, where optimal habitats such as mesic mixed coniferous forest favour larger pollen grains, whereas suboptimal habitats like swamp forest are associated with reduced pollen size. In I. capensis, optimal conditions also correspond to edges of watercourses, while suboptimal conditions include swamp forest. Additionally, exine thickness (Exp) may represent an adaptive trait, reflecting plant response to growth and development in unfavorable environments.