Introduction
Alien species invasion has been recognized as one of the most severe global ecological issues and environmental threats (Vitousek et al. , 1996; Mack et al., 2000). Alien species invasion also has substantial negative effects on community structure and function and has contributed to global loss of biodiversity (Courchamp et al., 2017). Determining the factors that contribute to the spread of plants in areas outside their native ranges is currently an important issue of research in invasion biology.
Phenotypic plasticity allows plants to cope with complex heterogeneous environments and is thus often cited as an important mechanism of plant invasion (Richards et al., 2006; Zenni et al., 2014). Phenotypic plasticity occurs when a single genotype alters its morphological, physiological and life-history traits in response to changing environmental conditions (Pigliucci, 2001). These adaptive changes can improve plant growth and fitness (Davidson et al. , 2011), particularly for clonal plants with low levels of genetic variation (Keser et al. , 2014; Geng et al. , 2016). According to an analysis of 133 invasive plant species, the invasion mechanisms in approximately 50% of them were related to phenotypic plasticity (Ren & Zhang, 2009).
Plastic changes can occur in various organs (e.g., roots, stems and leaves). Because the roots and leaves function in resource acquisition, the magnitude of phenotypic plasticity in these organs is critical for plant ecological adaptability (Violle et al. , 2009). Because stems are not the major organs that capture resources from the environment, few studies have focused on the effects of stem plasticity on the viability of plant species.
Stems are connected to roots and leaves and function in supporting the leaves, flowers and fruits; transporting water, mineral elements and organic nutrients; and even carrying out photosynthesis, storing materials and functioning as vegetative propagules (Pate & Jeschke, 1995; Nivot et al. , 2008). Nearly all green parts of stems can conduct photosynthesis, particularly those of young stems or twigs. Although stem photosynthesis is often lower than leaf photosynthesis, with maximum rates of up to 75% of those of leaf photosynthesis (Pfanz et al., 2002; Ávila et al., 2014), it has important functions in maintaining whole-plant carbon balance (Kocurek et al., 2020), especially in the case of defoliation events caused by insect attacks, leaf fungal pathogens, etc. (Pfanz & Aschan, 2001), or under stress conditions when leaf photosynthesis is limited due to stomatal closure (Bossard & Rejmanek, 1992; Cernusak & Cheesman, 2015). Stem photosynthesis can refix 60%-90% of the CO2 respired from local tissues (Pfanz et al., 2002; Kocurek et al., 2020) and increase the growth of stems by 10%-30% (Cernusak & Hutley, 2011; Bloemen et al., 2016). The carbohydrates produced by stem photosynthesis are involved in maintaining hydraulic function (Bloemen et al., 2016), refilling xylem vessels after embolism (Schmitz et al., 2012) and alleviating xylem vulnerability to cavitation (De Baerdemaeker et al., 2017). Furthermore, the O2 released from stem photosynthesis is important for preventing low-oxygen limitations of mitochondrial respiration in metabolically active stem tissues (Wittmann & Pfanz, 2014). Stem photosynthesis is also associated with drought tolerance (Cernusak & Cheesman, 2015; Ávila-Lovera & Tezara, 2018) and is involved in maintaining sap flow flux (Gao et al., 2016). Based on the results of these previous studies, it can be predicted that if a species is capable of dynamically adjusting stem photosynthesis, its viability is greatly enhanced.
Mikania micrantha Kunth, commonly known as the ‘mile-a-minute’ weed, is a perennial herbaceous creeping vine belonging to the Asteraceae family and is native to Central and South America (Holm et al., 1977). It has caused substantial economic and ecological losses in plantation crops and commercial and secondary forests within its range of introduction, which includes tropical Asia, Pacific islands, Indian Ocean islands, and Florida in the US (Waterhouse, 1994; Manrique et al., 2011; Zhang et al., 2004; Day et al., 2016). In China, M. micrantha was introduced into Hong Kong in the late 1800s and has since spread throughout southern China (Wang et al., 2003). M. micrantha has been listed as one of the 10 worst weeds and one of the 100 worst invasive species in the world (Lowe et al., 2001). It grows extremely fast (up to 20 cm in a 24-h period) (Li et al., 2012), can climb to the top of plant canopies, forms dense thickets, outcompetes existing vegetation by blocking sunlight and releasing allelochemicals (Zhang et al., 2004), and ultimately leads to a loss of species diversity. Moreover, it can alter the soil microbial community structure and soil nutrient cycling in invaded areas (Li et al., 2006; Chen et al. , 2009).
The stems of M. micrantha are photosynthetic (Liu et al. , 2020), and once this species becomes established, it can rapidly expand via stems that creep along the ground or over other plants, forming clonal ramets. In this study, we hypothesized that the stems of M. micrantha are more plastic than those of the native species that co-occur in southern China. To test this hypothesis, we perform pot experiments to investigate whether there is a difference in photosynthesis performance and growth in response to defoliation treatment between M. micrantha and native species and then further revealed the mechanism through which stem photosynthesis is modified in M. micrantha by evaluating stomatal aperture, chlorophyll content, chlorophyll fluorescence, photosynthesis-related proteins and chloroplast ultrastructure. We also used isolated culture experiments to analyze the regeneration patterns and photosynthesis potential in stem cuttings of both M. micrantha and the native species.