Mechanism governing the upregulation of photosynthesis in stems
of invasive species after defoliation
As described above, the largest upregulation of stem photosynthesis was
found in M. micrantha among the four species. To determine the
regulatory mechanism governing stem photosynthesis of the invasive
species, morphological and physiological changes were further
investigated (Figure 5a-l). During the defoliation treatment, the
average internode lengths of the stems became shorter, and the average
diameter of the stems noticeably decreased (Figure 5g). On day 20 of
defoliation, anthocyanins were no longer detected in the stems (Figure
5h). We noted that leaves of M. micrantha had stomata on both the
adaxial and abaxial surfaces, and the stomatal density (270) on the
abaxial surface was 5 times that on the adaxial surface. The density of
stomata on the surface of stems was comparable to that on the adaxial
surface of the leaves (Figure 5i). However, the stomata on the stems
were evidently larger than those on the leaves. The stomatal density of
the stems was not influenced by the defoliation treatment, whereas the
stomatal aperture and stomatal conductance increased significantly
(Figure 5i-l). The width of the stomata on the stems of defoliated
plants was even larger than that on either the adaxial surface or
abaxial surface of the leaves of the control plants. However, the
stomatal conductance of stems of the defoliated plants was still
significantly lower than that of the leaves because the leaves had a
greater average stomatal density.
Under normal conditions, the total chlorophyll (Chl) content per unit
area (Chl a +b ) and the Chl a /b ratio in the
stems were equal to a quarter and a half of those in the leaves,
respectively (Figure 6a-b). In addition, the soluble sugar content in
the stems was also significantly lower than that in the leaves (Figure
6c). With the removal of leaves, both the total Chl content the Chla /b ratio gradually increased in the stems; however, the
soluble sugar level showed a decreasing trend. On day 10 of defoliation,
the differences in total Chl in the stems between the treatment group
and control group had reached a significant level. Though the total Chl
content continued to increase in the stems of defoliated plants during
the following 20 days, it was consistently lower than that in the leaves
of the control plants. By contrast, the Chl a/b ratio of the stems of
the defoliated plants was found to exceed that of leaves on day 20 of
defoliation. In fact, photosynthesis of the stems responded to
defoliation much faster than the chlorophyll and soluble sugar contents
did. On day 3 of defoliation, the gross photosynthesis rate of the stems
was greater than that of the control plants and continued to increase
during the next several days (Figure 6d-e). The ETR also significantly
increased in the stems 20 days after defoliation, reaching a level that
was equal to that of leaves (Figure 6f). In addition, the D1 protein
(PsbA) of PSII was dramatically upregulated in the stems, and its
concentration even exceeded that in the leaves, but the RbcL protein
content appeared to decrease (inserted in Figure 6f). Therefore, the
increase in stem photosynthesis of M. micrantha was due to
multiple factors that improved the gas exchange efficiency of the stems
and the ability of chloroplasts to absorb light energy.
Ultrastructural observations revealed that the chloroplasts in the stems
and leaves of M. micrantha were elliptical shaped and contained
1-2 starch grains (Figure 7a-f). The starch grains occupied more than
50% of the interior space of the leaf chloroplasts, but those that
accumulated in the stem chloroplasts were much smaller. Defoliation
significantly affected the structure of the stem chloroplasts. The
longitudinal section of the chloroplasts changed from being elliptical
shaped to spindle shaped. As the length increased by 72.5% and the
width decreased by 15.2%, the length/width ratio of the chloroplasts in
the defoliated plants was approximately 2-fold that of the control
plants (Figure 7g-i). Such changes might increase the light-reception
area of the chloroplasts. In addition to the change in shape, the number
and size of plastoglobuli in the stem chloroplasts of defoliated plants
were also larger than those in the intact plants, the diameter of the
former reached 0.44 μm.