CO2 Concentrating Mechanism of C4 Photosynthesis

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Abstract
Diffusion of inorganic carbon into isolated bundle sheath cells from a variety of C4 species was characterized by coupling inward diffusion of CO2 to photosynthetic carbon assimilation. The average permeability coefficient for CO2 (PCO2) for five representatives from the three decarboxylation types was approximately 20 micromoles per minute per milligram chlorophyll per millimolar, on a leaf chlorophyll basis. The average value for the NAD-ME species Panicum miliaceum (10 determinations) was 26 with a standard deviation of 6 micromoles per minute per milligram chlorophyll per millimolar, on a leaf chlorophyll basis. A PCO2 of at least 500 micromoles per minute per milligram chlorophyll per millimolar was determined for cells isolated from the C3 plant Xanthium strumarium. It is concluded that bundle sheath cells are one to two orders of magnitude less permeable to CO2 than C3 photosynthetic cells. These data also suggest that CO2 diffusion in bundle sheath cells may be made up of two components, one involving an apoplastic path and the other a symplastic (plasmodesmatal) path, each contributing approximately equally.