Photosynthesis in midrib and interveinal lamina

Except in C₄ plants, photosynthetic activity is attributed mainly to leaf mesophyll cells, and photosynthesis in veins of C₃ and CAM (Crassulacean acid metabolism) plants is usually neglected. Kuźniak et al. studied the photosynthesis-related properties of the midrib and the interveinal lamina cells in leaves of Mesembryanthemum crystallinum, a C₃-CAM intermediate plant.

Visualization of H2O2 by DAB staining in detached leaves of C3 (A) and CAM (B) plants, and APX (C) and CAT (D) activities in leaf laminae and midribs. — Visualization of H₂O₂ by DAB staining in detached leaves of C₃ (A) and CAM (B) plants, and APX (C) and CAT (D) activities in leaf laminae and midribs. The inset in (B) shows details of the DAB staining effects. (A, B) Representative examples of three experiments with similar outcomes. A minimum of three C₃ and CAM plants were used per experiment. (C, D) The APX and CAT data are means ± s.d. from three experiments with two replicates (n = 6). Image by Kuźniak *et al*. (2016).

The vascular system of C₃– and CAM-performing plants contained photosynthetically-competent parenchyma cells but with limited photosynthetic activity. The midrib chloroplasts resembled those in the bundle sheath of C₄ plants. The authors propose different metabolic roles for midrib chloroplasts in C₃ and CAM plants of M. crystallinum.