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.