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

Abstract

Higher plants are divided into three major photosynthetic groups known as C₃, C₄, 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 C₃, C₄, 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 C₃ and C₄ species. However, when T_CW was compared between C₃ and C₄ species of grasses and eudicots, T_CW of MCs tended to be lower in C₄ species than in C₃ 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 C₄ 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 C₃, C₄, 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.