Figure 3.
Representative cross‐sections of a typical C3 (a) and C4 (b) leaf. (a) C3 leaves generally have two types of mesophyll cells: palisade and spongy. Palisade mesophyll cells, located below the adaxial surface, are elongated cells containing many chloroplasts, which absorb a major portion of the light energy used for photosynthesis. Spongy mesophyll cells are close to the abaxial surface and composed of rounded cells with few chloroplasts. (b) C4 leaves typically exhibit Kranz anatomy, in which mesophyll cells surround bundle sheath cells and bundle sheath cells further surround vascular bundles. Phosphoenolpyruvate (PEP) carboxylase is localized in the mesophyll cells, but Rubisco and many other Calvin cycle enzymes are found in the bundle sheath cells. In both C3 and C4 leaves, O3 diffuses through stomata into the leaf intercellular airspaces and instantly forms a variety of reactive oxygen species (ROS), including hydroxyl radical (•OH), hydrogen peroxide (H2O2), superoxide radicals (O2 •−), singlet oxygen (1O2), and nitric oxide (NO). In the C3 leaf (a), ROS can directly damage both palisade and spongy mesophyll cells, leading to reductions in photosynthesis. In the C4 leaf (b), the surrounding mesophyll cells may protect bundle sheath cells from direct ROS damage and maintain photosynthetic capacity under O3 stress. P, palisade mesophyll cell; S, spongy mesophyll cell; M, mesophyll cell; BS, bundle sheath cell.