2.1 Strategies of adaptation to the environment
A large number of toxic weeds are long-lived perennial species with
self-incompatible mating systems and therefore generally have high
quantities of genetic variation, which facilitates adaptive evolution to
various environmental conditions and contributes to their wide
geographic distribution (Ghalambor et al. 2007; Zhang et
al. 2015b; Bruijning et al. 2019). For example, Stellera
chamaejasme inhabits a wide range of altitudes from 130 to 4200 m,
including the North China Plain, the Inner Mongolia Plateau and the
Tibetan Plateau, as well as a wide area from southern Russia to
southwest China and the western Himalayas, which is suggestive of high
adaptability (Liu, Long & Yao 2004; Wang 2004; Wang & Gilbert 2007;
Zhang, Volis & Sun 2010; Zhao et al. 2010) (Fig. 1). The various
morphological and physiological traits of toxic weeds promote increases
in the fitness to harsh environments, such as drought, cold or barren
soils (Wong et al. 2004; Kraft et al. 2015; Wang et
al. 2016). As shown in Fig. 2,
leaves
of these weeds are often lanceolate with thick waxy layers that tolerate
prolonged drought conditions (Dou, Feng & Hou 2013). Moreover, many
toxic weeds can capture water and nutrients from deeper soil profiles
via their long and deeply distributed roots (Sun et al. 2009).
Additionally, rhizobacteria has been found to stimulate the growth of
these weeds by optimizing nutrient
supplies and promoting plant
metabolism and systemic resistance under unsuitable growth conditions
(Lugtenberg & Kamilova 2009; Lehmann et al. 2011; Cui et
al. 2015; Hui et al. 2018). Endophytic bacteria also make toxic
weeds more tolerant to abiotic stress (Sieber 2007; Hyde & Soytong
2008; Jin et al. 2014) .
Toxic weeds follow the optimal partitioning rule wherein plants
partition photosynthate among their various organs to maximise growth
rate in different habitats (Chapin et al. 1987). For example,
some toxic weeds have been observed to allocate more biomass to
hydrotropic roots under drought stress (Sun et al. 2014). In
addition, plant body size decreases at higher elevations to reduce
nutritional needs in less resource-rich environments; however, more
photosynthetic products are allocated to flowers at higher elevations to
enhance reproductive success (Zhang et al. 2013). High altitudes
make toxic weeds produce fewer, but larger, flowers with colour
polymorphisms to attract pollinators in adverse environments (Zhanget al. 2013; Zhang et al. 2015a) where low temperatures
and strong winds discourage insect activity (Zhang, Zhang & Sun 2011).
On the other hand, the number of branches on toxic weeds is reduced and
plant height is increased in north-facing compared with south-facing
slopes, suggesting that toxic weeds allocate more photosynthate to
vertical growth than to horizontal growth in response to competition for
light (Hou et al. 2014). The physiological responses of toxic
weeds also show signatures of adaptation to resource-constrained
conditions. For example, toxic weeds have higher proline concentrations
and rates of water use in south-facing slopes with arid environments
(Liu & Ma 2010; Hou, Liu & Sun 2017). However, toxic weeds in
north-facing slopes with weaker light intensities have higher
chlorophyll contents and photosynthetic efficiencies (Liu et al.2017).