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.