Heavy metals in soil affect reproductive processes more than morphological characters in Viola tricolor

Viola tricolor (Violaceae), a species very differentiated morphologically and showing intra- and interpopulation variability, occupies metalliferous (Zn, Pb, Cd, Cu) and nonmetalliferous sites through its geographic range. Here we analyzed morphological and anatomical features and also sexual reproduction in metallicolous and nonmetallicolous populations to determine whether and how they differ.Two-dimensional correspondence analysis based on selected morphological characters of vegetative and generative organs showed that plants from metalliferous soils did not form a compact group separated from those growing on nonmetalliferous soils. SEM of leaf and stipule anatomy showed differences in leaf hair ornamentation. There were significant differences in embryological processes in ovules and anthers: disturbed microsporogenesis (metallicolous 33% vs. nonmetallicolous 18%), lower pollen stainability (75% and 78% vs. 84% and 93%, depending on test), and higher frequency of degeneration in ovules (23% vs. 4%). These ultimately did not impede sexual reproduction of metallicolous populations but they do provide evidence that reproductive processes are sensitive to elevated heavy metals in soil and therefore can be viewed as a cost of metal tolerance.

► Metalliferous and nonmetalliferous populations of Viola tricolor are not morphologically distinguished.
► Minor influence of environmental conditions on sexual reproduction.
► Costs of tolerance low as compared with other species colonizing metal-rich soils.