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).