Discussion:
Analysis of the embryological traits of Biscutella laevigata , a
facultative metallophyte, from heap and mountain populations clearly
indicates that the costs of tolerance, measured as the success of sexual
reproduction, are not high. Plants colonizing the Bolesław calamine heap
develop fruits and seeds to an amount allowing the population to be
maintained. Plants accumulated metals in their generative organs
(flowers), as was demonstrated by histochemical tests, but they
developed defense mechanisms, including the biosynthesis of
high-esterified homogalacturonans in the cell wall of embryos. The
fraction of pectins is probably involved in the reactions of resistance
to metals that retain noxious ions in the apoplast of the new generation
(embryo). The cell wall, apoplastic compartment, is a well-known
extracellular site involved in the detoxification of metal ions harmful
to the cell (le Gall et al. 2015; Krzesłowska 2011).
Disturbances in female and male lineages and degeneration processes
occurred with higher frequency in plants growing in the calamine
population than in the mountain population. Are these changes only a
result of the heavy metal presence, or are they also influenced by other
factors? On post-industrial areas, secondary enriched with heavy metals
by industrial and mining activities, other factors might influence plant
physiological processes and reproduction, including: very low nutrient
content, skeletal soil structure with low water retention, strong
insolation and eolian erosion resulting from strong winds in open dump
sites. To survive these harsh environmental conditions, plants must
overcome heavy metal, drought, low/high temperatures, nutrient and salt
stresses. Species spontaneously colonizing contaminated sites undergo
micro-evolutionary processes, which enable the survival of individuals
adapted to local conditions and the beneficial traits / genes to be
passed on to their offspring (Wierzbicka & Rostański 2002; Wierzbicka
et al. 2017).
The frequency of disturbances and degenerations in male and female
lineages are good indicators of the costs of tolerance/adaptation
(Izmaiłow et al. 2015). The flowering plant life cycle is divided into
haploid and diploid generations. The haploid phase, extremely reduced
and consisting of only a few cells, is dependent on the sporophytic
generation for nutrition and protection (Ge et al. 2010). All processes
of sexual reproduction (male and female meiosis, gametophyte
development, double fertilization and embryo development) are not only
dependent on genotype or species, but also on external conditions. The
taxon in an early stage of colonization is poorly adapted to
environmental stress and disturbances, with degenerations in particular
being much more frequent than in metallophytes, which are better adapted
to harsh conditions. However, several traits related to the biology of
reproduction are conservative, regardless of the environmental factors,
e.g. type of cytokinesis, pattern of embryo sac development,
embryogenesis model or type of seeds (Izmaiłow et al. 2015; Siwek 2007).
From the research conducted on B. laevigata, it follows that the
simultaneous type of cytokinesis in anthers, three-celled pollen grains,
a monosporic embryo sac developing according to the Polygonum type, the
Onagrad type of embryo development, nucellar endosperm, and additional
embryo sacs in an ovule, were conservative and the same in plants from
the mountain and calamine populations. Necrosis and degenerations found
in male and female lineages of B. laevigata from the calamine
population could be a result of the same evolutionary strategies that
were observed in Armeria maritima s.l., another metallophyte of
calamine flora (author’s observations, unpublished). These processes
reduced the pollen viability and seed set. This was also reflected in
the frequency of one-seed siliculas. Selective abortion of seeds is a
well-known phenomenon in flowering plants. Developing embryos compete
and the weakest are aborted. Moreover, the mother plant may set the
level of abortion. The abortion of viable embryos leads to higher
offspring quality (de Jong & Klinghamer 2005). We observed in B.
laevigata from the calamine population that some gametophytes (female
or male) were also excluded from further development and this could lead
to better quality of offspring. The presence of unfertilized mature and
properly developed female gametophytes at a stage of aging indicates
that the reduced fertility of B. laevigata , a self-incompatible
species (Leducq et al. 2010; Young et al. 2012), might also result from
a lack or insufficient number of pollinators.
Quantitative analysis of metals in specimens from metallicolous
populations indicates that they are present in generative organs
(flowers and fruits), although metals are generally at a much lower
level than in vegetative tissues (e.g. Dhiman et al. 2017; Godzik 1993;
Mesjasz-Przybyłowicz et al. 2001). In plants from metallicolous sites,
metal uptake, distribution and deposition depend on the strategy that
plants have evolved. Specimens could accumulate (even hyperaccumulate)
metals in the aerial part or exclude them. Metals are quickly detoxified
by binding with many molecules (e.g. phytochelatines), localized in
vacuoles, sequestered in cell walls, deposited in aging leaves or in
trichomes, for the protection of organelles and biochemical reactions
against free toxic heavy metal ions in
cytosol (Antonovics
1971; Ernst et al. 1992; Prasad 2004).
The reproductive processes depend on how the plant fights ionic stress
and other extreme conditions, how much energy and resources it devotes
to it and how much energy remains to produce flowers, fruits and seeds.
Metal ions, along with water, mineral salts and assimilates, are
transported to generative structures, where they might directly affect
the haploid cells of gametophyte phase or embryos, even when they are
present in small doses (Kranner and Colville 2011; Sperotto et al. 2014;
Walker & Waters 2011). The gametophyte phase of ontogenesis is more
sensitive to abiotic stress (metal, salt, hot/cold temperature) than the
sporophyte, therefore adaptations develop first on the sporophyte level,
and then in the cells of the germlines (Hedhly 2011; Kazan and Lyons
2016; Kwiatkowska & Izmaiłow 2014; Słomka et al. 2012; Sun et al. 2004;
Zinn et al. 2010).