Photosynthetic/photorespiratory characteristics of C3?C4 intermediate species

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Abstract
The extent of photorespiration, the inhibition of apparent photosynthesis (APS) by 21% O2, and the leaf anatomical and ultrastructural features of the naturally occurring C3−C4 intermediate species in the diverse Panicum, Moricandia, and Flaveria genera are between those features of representative C3 and C4 plants. The greatest differences between the photosynthetic/photorespiratory CO2 exchange characteristics of the C3−C4 intermediates and C3 plants occur for the parameters which are measured at low pCO2 (i.e., the CO2 compensation concentration and rates of CO2 evolution into CO2-free air in the light). The rates of APS by the intermediate species at atmospheric pCO2 are similar to those of C3 plants. The mechanisms which are responsible for reducing photorespiration in the C3−C4 intermediate species are poorly understood, but two proposals have been advanced. One emphasizes the importance of limited C4 photosynthesis which reduces O2 fixation by ribulose 1,5-bisphosphate carboxylase/oxygenase, and, thus, reduces photorespiration by a CO2-concentrating mechanism, while the other emphasizes the importance of the internal recycling of photorespiratory CO2 evolved from the chloroplast/mitochondrion-containing bundle-sheath cells. There is no evidence from recent studies that limited C4 photosynthesis is responsible for reducing photorespiration in the intermediate Panicum and Moricandia species. However, preliminary results suggest that some, but not all, of the intermediate Flaveria species may possess a limited C4 cycle. The importance of a chlorophyllous bundle-sheath layer in the leaves of intermediate Panicum and Moricandia species in a mechanism based on the recycling of photorespiratory CO2 is uncertain. Therefore, although they have yet to be clearly delineated, different strategies appear to exist in the C3−C4 intermediate group to reduce photorespiration. Of major importance is the finding that some mechanism(s) other than Crassulacean acid metabolism or C4 photosynthesis has (have) evolved in at least the majority of these terrestrial intermediate species to reduce the seemingly wasteful metabolic process of photorespiration.

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