of Mikania micrantha
Understanding the ecophysiological mechanisms underlying species
invasion is required to perform effective management. Morphological and
physiological plasticity associated with the broad expansion of various
invasive species has been verified by a large number of empirical
studies (Richards et al. , 2006; Riis et al. , 2010; Geng et
al. , 2016). For invasive vines, morphological plasticity of
internode length and internode number of the stems may allow these
plants to actively occupy favorable microhabitats, ultimately affecting
plant fitness (Schweitzer & Larson, 1999). However, little is known
about the physiological plasticity of stems in invasive vine species.
In addition to leaves, nonfoliar organs such as stems, bark, and fruits
of many higher plants can also carry out photosynthesis (Xu et al.,
1997; Tarvainen et al., 2018). Although the photosynthesis ability of
nonfoliar organs is often significantly less than that of leaves, it
still plays an important physiological role in the growth, development
and stress resistance of plants. For example, stem photosynthesis can
increase stem growth (Cernusak & Hutley, 2011; Steppe et al., 2015),
improve the carbon economy of whole plants (Nilsen, 1995) and improve
drought resistance (Vandegehuchte et al., 2015). In this study, we used
a gas exchange analysis technique to confirm that the stems of M.
micrantha and native species were able to perform photosynthesis
because dramatically more CO2 was released from the
stems in the dark than in the light (Figure S1). Our results also
revealed that the stem cortex of the vine species was permeable to
CO2. This is unlike that in woody species, as the
cambium in the stems of these species is gas impermeable and blocks
radial CO2 diffusion (Steppe et al., 2007). As a result,
stem photosynthesis in woody species can be used only to refix respired
CO2, but in vine species, stem photosynthesis can fix
CO2 from both cellular respiration and ambient air. In
evolutionary terms, stem photosynthesis in vine species is much more
similar to leaf photosynthesis than that in woody species.
Stem photosynthesis ability and its impact on fitness vary from species
to species (Berveiller et al., 2007). In the current study, compared
with the three native species, M. micrantha manifested a greater
plasticity of stem photosynthesis, as reflected by changes in the net
photosynthesis rate, total photosynthesis rate, chlorophyll a content,
and ETR during defoliation (Figure 1-2). Related to these physiological
characteristics, compared with the three native species, M.
micrantha demonstrated a higher stem elongation rate during the leaf
defoliation treatment (Figure 3). Moreover, the survival rate of the
defoliated M. micrantha plants reached 100%, while the rates of
the native species ranged from 10%-90%, suggesting that physiological
plasticity of the stems plays an important role in maintaining the
survival and growth of M. micrantha under defoliation. Similarly,
under the conditions of isolated culture, M. micrantha showed
higher stem photosynthesis than did the native species, as indicated by
the ETR (Figure 4). As a result, the survival rate of M.
micrantha stem segments was 100%, which was higher than that of two of
the three native species. We noted that the stem segment-regeneration
strategy adopted by M. micrantha was different from that adopted
by the native species (Figure 4). M. micrantha preferentially
rooted first, whereas the native species P. lobata and P.
scandens tended to grow leaves first; moreover, P. nil segments
could hardly regenerate. This may also be associated with stem
photosynthesis performance. Compared with those of the native species,
the stem segments of M. micrantha, which a higher photosynthesis
rate, produced more carbohydrates, setting the stage for the isolated
stems of M. micrantha to root first. The growth of roots allows
the regenerated plants to rapidly uptake water and nutrients, thereby
increasing the probability of survival. In contrast, the photosynthesis
ability of the stems of the native species was lower, so prioritizing
the growth of only new leaves as photosynthetic organs can guarantee
supplies of carbohydrates for further growth of the regenerated plants.
In summary, high plasticity of stem photosynthesis improves the survival
and fitness of M. micrantha compared with native species under
harsh conditions and allows the plants to rapidly recover from
defoliation injuries. Many invasive alien plants can be controlled by
releasing enemy insects to attack their leaves (Clewley et al., 2012;
Havens et al., 2019). This technique may not be effective to manageM. micrantha , as this invasive species can enhance its stem
photosynthesis to compensate for the decrease in leaf photosynthesis to
some extent. Our results show that physiological phenotypic plasticity
promotes the invasion success of alien vine plant invaders.