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.