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Plant and Cell Physiology, 1978, Vol. 19, No. 4 665-675
© 1978

Article

Photosynthetic efficiency of Panicum hians and Panicum milioides in relation to C3 and C4 plants1

S. B. Ku and G. E. Edwards

Department of Horticulture, University of Wisconsin Madison, Wisconsin 53706, USA

Panicum hians and Panicum milioides were found to have characteristics intermediate to those of C3 and C4 species with respect to CO2 compensation point, percentage inhibition of photosynthesis by O2 at various O2/CO2 solubility ratios, and water use efficiency. C4 species have a higher carboxylation efficiency than either the intermediate or C3 species. During photosynthesis, even under 2.5% O2, C4 species have a higher affinity for intercellular CO2 (Km 1.6 µM) apparently due to the initial carboxylation through PEP carboxylase. Under low O2 the intermediate and C3 species had a similar affinity for intercellular CO2 during photosynthesis (Km 5–7 µM) consistent with carboxylation of atmospheric CO2 through RuDP carboxylase. There were considerable variation in photosynthesis/unit leaf area at saturating CO2 levels in the species examined which in part is due to differences in RuDP carboxylase /unit leaf area. The highest rates of photosynthesis/unit leaf area under CO2-saturating conditions were with the C3 species which had a correspondingly high level of RuDP carboxylase/unit leaf area. Possibilities for the greater efficiency of P. hians and P. milioides in comparison to C3 species in utilizing low levels of CO2 in the presence of atmospheric O2 are discussed.

1 This research was supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison; and the University of Wisconsin Research Committee with funds from the Wisconsin Alumni Research Foundation.

(Received June 25, 1977; )
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