This study explores the below-ground ecology of plants adapted to grow on serpentine soils. Serpentine soils naturally contain elevated levels of toxic elements such as Ni and Mg, and low levels of nutrient elements such as P and Ca. Some serpentine species (Ni-hyperaccumulators) absorb Ni from soil and store it in shoot tissue, where it may provide defense against herbivores and pathogens. In this experiment, we designed "root foraging arenas" that permit single plants to grow roots simultaneously in four different edaphic environments. Arenas consist of bottomless upper pots suspended above four conical pots containing soil treatments. Single seedlings are transplanted into sand in the upper pot, allowing their roots to explore the four soil treatments below. A greenhouse experiment examined whether roots of serpentine plants show a preference for soils containing varying Ni concentrations (0 ppm, 250 ppm) and Ca/Mg ratios (0.2, 2.0). We specifically wished to determine whether root foraging responses differ between an endemic serpentine Ni-hyperaccumulator (Alyssum corsicum) and a congeneric Ni-tolerant non-accumulator (Alyssum montanum) that naturally grows on serpentine and non-serpentine soils. Root preferences were quantified by measuring the proportion of total root biomass in each of four soil treatments: (1) low Ni & high Ca/Mg, (2) high Ni & high Ca/Mg (3) low Ni & low Ca/Mg, and (4) high Ni & low Ca/Mg.
Results/Conclusions
Our results showed a difference in root foraging behavior of the Ni-hyperaccumulator and the non-accumulator. The endemic Ni-hyperaccumulator showed a significant root preference for soil treatments with high Ni (Pearson Chi Square = 10.55; p = 0.014) but no preference for high or low Ca/Mg. The non-accumulator did not show a preference for any soil treatment. These results suggest that the Ni-hyperaccumulator (A. corsicum) is a more “precise” forager than the non-accumulator (A. montanum). No differences were observed in the total biomass of plants with different root preferences, but significant differences were observed in leaf chlorophyll content. Serpentine plant adaptation is a fascinating example of evolution in response to variation in edaphic factors. A study of another congeneric Ni-hyperaccumulator by Quintela-Sabarís et al. (2017) found that genes for uptake of Ni, P, Ca and Mg are key differences in the genomes of plants from serpentine or non-serpentine populations. Results of our study highlight the urgent need for understanding root ecology for conservation of serpentine plant communities where many endemic species face threats due to mining, development and climate change.