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.
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.
Elżbieta Kuźniak, Andrzej Kornas, Andrzej Kaźmierczak, Piotr Rozpądek, Michał Nosek, Maciej Kocurek, Günther Zellnig, Maria Müller, Zbigniew Miszalski, 2016, 'Photosynthesis-related characteristics of the midrib and the interveinal lamina in leaves of the C3–CAM intermediate plantMesembryanthemum crystallinum', Annals of Botany, vol. 117, no. 7, pp. 1141-1151 http://dx.doi.org/10.1093/aob/mcw049