Effects of Elevated Atmospheric CO2 Concentration on Leaf Anatomy and Morphology in Panicum Species Representing Different Photosynthetic Modes.

Panicum tricanthum Nees, Panicum antidotale Retz., and Panicum decipiens Nees ex Trin. were selected to represent C3, C4, and C3/C4 intermediate perennial species of Panicum, respectively. Plants grown from seed with 900 ppm [CO2] under natural sunlight and controlled temperatures (30 degrees /22 degrees C) were compared with plants grown with ambient [CO2]. The anatomy of the last fully expanded leaf of the main tiller was studied by light microscopy with computerized graphic image analysis and by transmission electron microscopy. Leaf anatomy did not change qualitatively in response to elevated [CO2], but there were changes in leaf thickness and in the proportions of total transsectional area occupied by mesophyll, bundle sheath cells, vascular elements, and sclerenchyma, according to species. The abaxial stomatal frequency decreased by 22% for P. tricanthum but increased by ca. 30% for the other two species. With 900 ppm CO2, all three species showed a considerable increase in leaf starch content (to >30% of dry matter). Starch granules accumulated in chloroplasts of the mesophyll and bundle sheath cells. Increased leaf glaucousness in response to elevated [CO2] was the result of increased or modified deposition of epicuticular wax on both leaf surfaces, a response to elevated [CO2] that is unusual and one that has not been previously recorded for monocotyledons. The wax patterns were studied by scanning electron microscopy. Panicum decipiens did not respond to elevated [CO2] in a truly intermediate fashion; its responses resembled those of either the C3 or the C4 species. C3/C4 intermediates may thus be interpreted as developmental chimeras and not as species in transition between C3 and C4 modes in an evolutionary sense.