Plant adaptation to metal polluted environments-Physiological, morphological, and evolutionary insights from Biscutella laevigata

Polluted soils promote rapid plant adaptation to high concentrations of trace metal elements. Biscutella laevigata is a little investigated pseudometallophyte that appears promising to study these evolutionary processes and advance understanding beyond existing model species, but its metal tolerance is insufficiently understood. We determined the zinc tolerance level and various plant responses to environmentally relevant zinc concentrations in ten metallicolous and non-metallicolous B. laevigata populations from southern Poland. In a two-phase hydroponic experiment, we scored multiple morphological and physiological traits (e.g. biomass, visible stress symptoms, element content in foliage) and assessed phenotypic variability within plant families. The structure of these quantitative traits was compared to that of neutral molecular markers to test, whether natural selection caused population differentiation in zinc tolerance. While zinc tolerance was species-wide (i.e. enhanced in all genotypes compared to a zinc sensitive reference species), we found congruent trends toward higher tolerance in metallicolous compared to non-metallicolous plants. The most indicative parameters for these differences were dry shoot biomass, dry root biomass, and particularly effective photosystem II yield. We found that enhanced zinc tolerance in metallicolous populations is driven by divergent selection in response to metal contamination. In addition, analyses of genotype x environment interaction indicated that this differentiation is heritable. These findings promote diploid accessions of B. laevigata as zinc tolerant but non-hyperaccumulating organisms to study plant adaptation to contaminated environments. Remarkably, tolerance differences between edaphic types emerged already at an environmentally relevant zinc concentration of 150 μM. This opens an unusual perspective on plant adaptation that should be tested in other non-hyperaccumulating species.