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.