Planta (1988) 174:145-151 Planta 9 Springer-Verlag 1988 Carbon-isotope discrimination by leaves of Flaveria species exhibiting different amounts of C3- and C4-cycle co-function R.K. Monson 1 *, J.A. Teeri 2.*, M.S.B. Ku 3, J. Gurevitch 4, L.J. Mets 2 and S. Dudley a 1 Department of Environmental, Population and Organismic Biology, Campus Box 334, University of Colorado, Boulder, CO 80309, 2 Barnes Laboratory, University of Chicago, 5630 South Ingleside Avenue, Chicago, IL 60637, 3 Department of Botany, Washington State University, Pullman, WA 99164, and 4 Department of Ecology and Evolution, State University of New York, Stony Brook, NY 11794, USA Abstract. Carbon-isotope ratios were examined as c513C values in several C3, C~, and C3-C4 Flaveria species, and compared to predicted ~3C values generated from theoretical models. The measured cI13C values were within 4%~ of those predicted from the models. The models were used to identify factors that contribute to C3-1ike 5t3C values in C3-C4 species that exhibit considerable CA-cycle activity. Two of the factors contributing to C3-1ike fi13C values are high CO2 leakiness from the C4 pathway and pi/pa values that were higher than C4 congeners. A marked break occurred in the re- lationship between the percentage of atmospheric CO2 assimilated through the C4 cycle and the c5 13 C value. Below 50% C4-cycle assimilation there was no significant relationship between the variables, but above 50% the 613C values became less nega- tive. These results demonstrate that the level of C4-cycle expression can increase from 0 to 50% with little integration of carbon transfer from the C4 to the C3 cycle. As expression increases above 50%, however, increased integration of C3- and C4-cycle co-function occurs. Key words: C3-C4 intermediate plants - Carbon isotope discrimination (ratio, theory) - Flaveria - Photosynthesis (C3, C4, C3 C4). Introduction Species that exhibit characteristics of both the C3 and C4 photosynthetic pathways have been the * To whom correspondence should be addressed ** Present address: Biological Station, University of Michigan, Natural Science Building, Ann Arbor, MI 48109 Abbreviations and symbols." RuBP carboxylase=ribulose-1,5- bisphosphate carboxylase (EC4.1.1.39); PEP carboxylase= phosphoenolpyruvate carboxylase (EC 4.1.1.31); pa = atmo- spheric CO2 partial pressure; pi = intercellular CO2 partial pres- sure; fi = isotope ratio ; ~b = quantum yield for CO2 uptake subject of considerable recent research (see reviews by Monson et al. 1984; Holaday and Chollet 1984; Edwards and Ku 1988). Such species have impor- tance in applied disciplines, in that they might con- tribute knowledge towards breeding efforts; to in- troduce C4 traits into otherwise C3 plants, and more basic disciplines, in that they might contrib- ute knowledge to the paths taken during the evolu- tion of C4 photosynthesis. In addition to C3 and C4 species, the genus Flaveria (Asteraceae) con- tains many species that exhibit anatomical and physiological traits characteristic of both the C3 and C4 syndromes (Ku et al. 1983; Holaday et al. 1984; Edwards and Ku 1988). Several of these C3- C4 species assimilate atmospheric CO2 through both the C3 and C4 photosynthetic pathways (Rumpho et al. 1984; Bassiiner et al. 1984; Mon- son et al. 1986). In a number of these species, C4 photosynthesis can be responsible for up to 50% of the atmospheric CO2 assimilation. Despite the biochemical evidence for consider- able C4-cycle function in many of the Flaverih spe- cies, previous measurements of carbon-isotope values are suggestive of little contribution of C4 photosynthesis to growth (Smith and Turner 1975; Powell 1978; Smith and Powell 1984). The pres- ence of C3-1ike carbon-isotope ratios in these oth- erwise intermediate plants has resulted in a para- dox, since differences in the levels of atmospheric CO2 assimilation through the C3 or C4 pathways should be detectable as differences in the level of discrimination against 13CO2. The assimilation of atmospheric CO2 through the C3 cycle :will result in greater discrimination against t3C and a more negative 513C value, relative to CO2 assimilation through the C4 cycle (O'Leary 1981). Thus, in C3 plants, c5 ~3C values between -25 and --30%0 are typically measured, whereas in C4 plants the values are typically between -10 and -16%o. In plants that exhibit a balance of Ca- and C4-cycle co-func-