The present study demonstrated that the PPFD of
300 mol m−2 s−1 caused the hightest Pn. From the above,
we observed that a tendency of leaf thickness, palisade cells,
stomatal frequency and stomatal pore area per unit leaf area
was similar to the activity of Pn. According to Evans (1989) and
Goncalves et al. (2008), the greater photosynthetic capacity is
often related to higher leaf thickness and palisade cells, which
is enhanced by ambient irradiance. In addition, higher stomatal
frequency could facilitate CO2 uptake and thus maintain a high
photosynthetic activity (Chartzoulakis et al., 2000). Therefore, we
presume the enhancement of Pn due to 300 mol m−2 s−1 seems
to be concerned with well-developed structure of mesophyll tissue
cells, higher stomatal frequency and stomatal area per unit leaf
area in leaves.