Occurrence of C 3 and C 4 Photosynthetic Pathways in North American Grasses

Abstract

A literature survey was made for the occurrence of C, and C, photosynthetic pathways in the United States Gramineae. Distinctive characteristics of the two photosynthetic pathways are discussed. Leaf anatomy, CO, compensation point, net enhancement of photosynthesis in oxygen-deficient atmosphere, QC discrimination, and initial product labeling were criteria selected to evaluate data for 6 subfamilies including 25 tribes, 138 genera, and 632 species. The Arundinoideae, Bambusoideae, Oryzoideae, and Pooideae (Festucoideae) are composed of species with C, pathways. All tribes within the Eragrostoideae have C, pathways with the exception of Unioleae. Within the Panicoideae, the Andropogoneae and all of the Paniceae, excepting the genera Sacciolepus, Isachne, Oplismenus, Amphicatpum, and Panicum, have C, pathways. The subgenus Dichanthelium within Panicum is C, while the Bupanicum subgenus contains plants with both C, and C, photosynthetic pathways. Plant productivity is dependent on several environmental and biological factors. The most important single factor is photosynthesis. A pathway for carbon dioxide (CO;?) fixation was described by Calvin and Bassham (1962) in which CO2 was incorporated into a 6-carbon compound and rapidly converted to a 3-carbon compound, 3-phosphoglyceric acid (3PGA). Previous to discoveries of Kortschalk et al. (1965) and Hatch and Slack (1966), the Calvin cycle (C,, reductive pentose pathway) was considered the major photosynthetic mechanism for carbon (C) fixation. However, Hatch and Slack (1966) described CO;! fixation in which labeled CO;! was first incorporated in 4-carbon compounds (malic, aspartic , or oxaloacetic acid) prior to transfer to sugars by way of 3-phosphoglycerate. The proposed mechanism involved the operation of two interconnected metabolic cycles. Downton (1970) described carbon fixation into Cd-dicarboxylic acids in mesophyll cells and subsequent incorporation into the Calvin cycle located in the bundle sheath cells. Plants (Cd plants) possessing the 4-carbon pathway (also called Cd, dicarboxylic acid, Kranz type, low CO;! compensation, tropical, Hatch and Slack, or p carboxylation pathway) were of tropical ongm and more efficient. They produced two- to threefold more dry matter than plants possessing the 3-carbon pathway (C, plants), especially in relatively sunny, warm, dry climates (Black 197 1). Distinctive characteristics associated with the Ca pathway prompted intensive research in photosynthetic processes of flowering plants. The most important photosynthetic pathways