The genus Flaveria shows evidence of evolution in the mechanism of photosynthesis as its 21 species include C3, C3-C4, C4-like, and C4 plants. In this study, several physiological and biochemical parameters of photosynthesis and photorespiration were measured in 18 Flaveria species representing all the photosynthetic types. The 10 species classified as C3-C4 intermediates showed an inverse continuum in level of photorespiration and development of the C, syndrome. This ranges from F. sonorensis with relatively high apparent photorespiration and lacking C4 photosynthesis to F. Among the intermediates, the photosynthetic CO2 compensation points at 30-degrees-C and 1150 micromoles quanta per square meter per second varied from 9 to 29 microbars. The values for the three C4-like species varied from 3 to 6 microbars, similar to those measured for the C4 species. The activities of the photorespiratory enzymes glycolate oxidase, hydroxypyruvate reductase, and serine hydroxymethyltransferase decreased progressively from C3 to C3-C4 to C4-like and C4 species. On the other hand, most intermediates had higher levels of phosphenolpyruvate carboxylase and NADP-malic enzyme than C3 species, but generally lower activities compared to C4-like and C4 species. The levels of these C4 enzymes are correlated with the degree of C4 photosynthesis, based on the initial products of photosynthesis. Another indication of development of the C4 syndrome in C3-C4 Flaveria species was their intermediate chlorophyll a/b ratios. The chlorophyll a/b ratios of the various Flaveria species are highly correlated with the degree Of C4 photosynthesis suggesting that the photochemical machinery is progressively altered during evolution in order to meet the specific energy requirements for operating the C4 pathway. In the progression from C3 to C4 species in Flaveria, the CO2 compensation point decreased more rapidly than did the decrease in O2 inhibition of photosynthesis or the increase in the degree of C4 photosynthesis. These results suggest that the reduction in photorespiration during evolution occurred initially by refixation of photorespired CO2 and prior to substantive reduction in O2 inhibition and development of the C4 syndrome. However, further reduction in O2 inhibition in some intermediates and C4-like species is considered primarily due to the development of the C4 syndrome. Thus, the evolution of C3-C4 intermediate photosynthesis likely occurred in response to environmental conditions which limit the intercellular CO2 concentration first via refixation of photorespired CO2, followed by development of the C4 syndrome.
