Results
Differences in the plasticity of stem photosynthesis and relevant traits in response to defoliation between M. micrantha and native species
To explore the plasticity of stem photosynthesis in M. micrantha , the leaves of M. micrantha and three native species (i.e.,Pharbitis nil , Paederia scandens , and Pueraria lobata ) were removed. In the control group, no significant differences in net photosynthesis (P n) and gross photosynthesis (P g) were found between the invasive species and the three native species, although the dark respiration rate (R d) of the invasive species was lower than those of the three native species (Figure 1a-c). After defoliation, both P n andP g of the invasive species and native species increased significantly, but the R d remained relatively constant. The increases in P n andP g in the invasive species were significantly greater than those in the native species. On day 20 of defoliation, the increase in P g (△P g) in the invasive species reached 2.4 μmol m–2s–1, while that of the three native species reached only 0.5 μmol m–2 s–1 (Figure 1d), indicating that photosynthesis of the stems of the invasive species is more responsive than that of the stems of the native species.
According to the cross-sections, the stem epidermis of M. micrantha , P. nil , and P. scandens evidently contained anthocyanin pigments, and these pigments disappeared with the removal of leaves (Figure 2a). Though P. lobata did not contain anthocyanins, it had more epidermal pubescence than the other species did, and the epidermal pubescence was not affected by defoliation. In addition to the disappearance of anthocyanin pigments, the 3-4 layers of cells below the stem epidermis of M. micrantha became more apparent (indicated by the red arrow). These layers of cells became larger and contained more chlorophyll, which formed a conspicuous green ring zone. However, no similar structural change was found in the native species.
The results of the chlorophyll and chlorophyll a fluorescence measurements showed that there were no significant differences in Chla , Chl b , the Chl a /b ratio and Fv/Fm between the invasive species and the native species (Figure 2b-e). However, the invasive species had a greater ETR than the three native species did, especially when the PAR was greater than 400 μmol m–2 s–1 (Figure 2f-d). With the removal of leaves, the Chl a content sharply increased in the stems of M. micrantha , but the Chl b content remained at a relatively constant state. By contrast, both Chl a and Chlb contents showed little change in the native species, except that there was a slight increase in Chl b in P. nil . The marked increase in Chl a in the stems of M. micrantharesulted in their Chl a /b ratio being higher than that of the native species. Accompanied by the increases in the Chl acontent and the Chl a /b ratio, the ETR also increased in the invasive species. Thus, the morphology, photosynthetic pigments and photochemistry of the stems in the invasive species were more dynamic than were those of the native species.