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.