4.3 Interspecies comparison
Heavy metal accumulation and tolerance are two different quantitative traits that can vary in a continuous manner. Considering this, Goolsby & Mason (2015) proposed that the interaction between these two traits brings about four categories of plants: (i) high accumulation and high tolerance, (ii) high accumulation and low tolerance, (iii) low accumulation and high tolerance, and (iv) low accumulation and low tolerance. Given the distinct ecological characteristics of our two study species, i.e. obligate vs. facultative metallophyte, we predicted that they would follow different strategies to cope with heavy metals. Thus, S. cataractae would follow a stress-tolerator strategy (category i) due to its high affinity for heavy metals, whereas C. purpureus would follow a stress-avoidance strategy (category iii) due to its lack of specialization but seemingly innate capacity to tolerate these pollutants. The tolerance-accumulation patterns found for S. cataractae suggested that heavily polluted populations of this species, i.e. Sc2 and Sc3, have different and metal-dependent strategies relative to each other. Sc2 behaved as a category iii “low accumulator and high tolerator” for Cu (limited accumulation in leaves) and Cd (lower total concentrations), whereas Sc3 behaved as a category iii “low accumulator and high tolerator” for Cu (limited accumulation in leaves) and as a category i “high accumulator and high tolerator” for Cd (higher total concentrations). For C. purpureus , we speculate that Cp2.f, that hyperaccumulated Cd and showed greater tolerance relative to Cp1 and Cp2.m, behaved as category i “high accumulator and high tolerator” for Cd, while Cp1 and Cp2.m behaved as category ii “high accumulator and low tolerator” for Cd. Because this species did not hyperaccumulate Cu and all populations reached similar final concentrations of this metal, we cannot assign them to any of the categories above.
Finally, we avoided further between-species comparisons due to inevitable differences in the experimental design between our two study species (e.g. how the treatments were applied, the actual concentrations of Cd and Cu used, and the developmental stages assessed - protonema vs gametophore). Nevertheless, from a broad perspective, our results showed that the tolerance-accumulation patterns of bryophytes are varied, and are congruent with predictions of differences between specialist vs. generalist strategies.
In conclusion, our results provide novel insights into the patterns of intraspecific differentiation for heavy metal accumulation and tolerance in bryophytes and the mechanisms underlying such differentiation. Our results support the unparalleled capacity of bryophytes to respond to environmental challenge despite potentially low levels of genetic variation and lack of previous exposure to stress, as well as the idea that intraspecific phenotypic variation is important for the fate of natural plant populations in the current context of environmental change.