As expected, the four Asteraceae AIP, particularly C. canadensis, E. annuus, and B. pilosa, form evident allelopathy on germination performance of L. sativa, especially on germination competitiveness, seed viability and germination uniformity, germination rate and vitality, germination responsiveness to the external environment, and seedling competitiveness for water and inorganic salt absorption in this study. Hence, the growth fitness of L. sativa can be remarkably attenuated under the allelopathy mediated by the four Asteraceae AIP. The most likely factor may be due to the created allelochemicals formed by AIP which can incur harmful influences, e.g., disrupting nutrient absorption efficiency and intensity on plant growth and development (Wang et al. 2020b; Gris et al. 2019; He et al. 2019; Lyytinen and Lindström 2019; Wei et al. 2020).
Further, there are noteworthy interspecific differences in the allelopathy of the four Asteraceae AIP on germination performance of L. sativa, especially on germination competitiveness, seed viability and germination uniformity, germination rate and vitality, germination responsiveness to the external environment, and seedling growth competitiveness, in this study. Further, the allelopathy of C. canadensis and E. annuus is noticeably superior to those of A. subulatus and B. pilosa in this study. Thus, the importance of allelopathy of C. canadensis and E. annuus is markedly greater than that of A. subulatus and B. pilosa. Interestingly, A. subulatus leaf extract does not display noteworthy allelopathy on germination performance of L. sativa in this study. Thus, the allelopathy of A. subulatus does not show a vital role in its successful colonization. Largely, the allelopathy of the four Asteraceae AIP on germination performance of L. sativa distinctly declines in the following order: E. annuus, C. canadensis, B. pilosa, and A. subulatus in this study. The key cause may be because of the diversification in the types of secondary substances, i.e., allelochemicals, and their corresponding relative content among the four Asteraceae AIP supposedly. The results confirm the first hypothesis.
Although the independent acid deposition does not markedly affect germination performance of L. sativa, the combined allelopathy of the four Asteraceae AIP (particularly B. pilosa) and acid deposition trigger a significant negative influence on germination performance of L. sativa, especially on germination competitiveness, seed viability and germination uniformity, germination rate and vitality, germination responsiveness to the external environment, and seedling competitiveness for water and inorganic salt absorption, in this study. Thus, the growth fitness of L. sativa will be significantly decreased under the condition when the plant invasion was polluted by acid deposition.
The acid deposition may be increasingly worse with the growing intensity and frequency of atmospheric activities in current periods and is estimated to upsurge in upcoming years. Hence, the allelopathy of AIP may be changed and even strengthened under the condition with the increasing level of acid deposition. Further, nitrogen, which is one of the main constituents of acid deposition, can influence and even expedite plant metabolic process (Throop and Lerdau 2004; Luo et al. 2008; Fallovo et al. 2011; Yang et al. 2014; Sun et al. 2020). Interestingly, the combined allelopathy of C. canadensis, E. annuus, and B. pilosa and acid deposition at pH 5.6 can promote germination performance (especially germination competitiveness, seed viability and germination uniformity, germination rate and vitality, germination responsiveness to the external environment, seedling competitiveness for sunlight capture, leaf photosynthetic area, and seedling growth competitiveness) of L. sativa compared with only leaf extracts in this study. Thus, acid deposition with a low level of acidity decreases the allelopathy of C. canadensis, E. annuus, and B. pilosa on germination performance of L. sativa. The foremost issue may be credited to the nutrient fertilization (especially nitrogen) mediated by the nutrition elements in an acid deposition with a low level of acidity. Further, the increased level of nutrition can lift the capability of plant species to resist hostile environments (Hassan et al. 2005, 2008; Xu et al. 2015; Xiong et al. 2018; Tariq et al. 2019). Inversely, the combined allelopathy of B. pilosa and acid deposition at pH 4.5 synergistically affect germination performance of L. sativa, especially on germination competitiveness, seed viability and germination uniformity, germination rate and vitality, and germination responsiveness to the external environment. Accordingly, acid deposition with a high level of acidity strengthens the allelopathy of B. pilosa on germination performance of L. sativa. The reason may be owed to the increased acidity under acid deposition with a high level of acidity which is poisonous to plant growth. Meanwhile, the high level of acidity recruited by acid deposition can increase the leaching process of acid-soluble substances (Zhang et al. 2007; Wang et al. 2016b; Pabian et al. 2012; Xu et al. 2015), such as phenolics (mainly polyphenols), which is one of the most abundant allelochemicals in AIP (Li et al. 2010; Zhang et al. 2011; Djurdjević et al. 2012; Gomaa et al. 2014; Harrison et al. 2017; Marksa et al. 2020). Earlier outcomes also identify that acid deposition can also increase the allelopathy of AIP on plant germination performance (Wang et al. 2012a, b, 2016b). Thus, the consequences confirm the second hypothesis partially.
In brief, the progressively growing level of acid deposition with high acidity in the environment can be good for the invasion process of B. pilosa via the enhanced allelopathy on plant germination performance.