Cell wall thickness spectrum of photosynthetic cells in herbaceous C3, C4, and crassulacean acid metabolism plants

Higher plants are divided into three major photosynthetic groups known as C-3, C-4, and crassulacean acid metabolism (CAM) plants. It is considered that cell wall thickness (T-CW) affects diffusion and leakiness of CO2 within leaves, but it is unclear whether T-CW of photosynthetic cells differs among these groups. This study investigated T-CW of photosynthetic cells in herbaceous C-3, C-4, and CAM species under an electron microscope. Among 75 species of monocots and eudicots grown in a growth chamber in the same environment, the T-CW of mesophyll cells (MCs) was much higher in CAM species than in C-3 and C-4 species. However, when T-CW was compared between C-3 and C-4 species of grasses and eudicots, T-CW of MCs tended to be lower in C-4 species than in C-3 species; the opposite trend was observed for T-CW of bundle sheath cells (BSCs). T-CW of MCs and BSCs almost did not differ among the C-4 decarboxylation types (NADP-ME, NAD-ME, and PCK). In plants grown outdoors (51 species), similar trends of T-CW were also found among photosynthetic groups, but their T-CW was generally higher than that of growth-chamber plants. This study provides the T-CW spectrum of photosynthetic cells in herbaceous C-3, C-4, and CAM species. The results obtained would be valuable for our understanding of the diffusion and leakage of CO2 in the leaves of different photosynthetic groups.