Degree of C(4) Photosynthesis in C(4) and C(3)-C(4)Flaveria Species and Their Hybrids : I. CO(2) Assimilation and Metabolism and Activities of Phosphoenolpyruvate Carboxylase and NADP-Malic Enzyme

The degree of C₄ photosynthesis was assessed in four hybrids among C₄, C₄-like, and C₃-C₄ species in the genus Flaveria using ¹⁴C labeling, CO₂ exchange, ¹³C discrimination, and C₄ enzyme activities. The hybrids incorporated from 57 to 88% of the ¹⁴C assimilated in a 10-s exposure into C₄ acids compared with 26% for the C₃-C₄ species Flaveria linearis, 91% for the C₄ species Flaveria trinervia, and 87% for the C₄-like Flaveria brownii. Those plants with high percentages of ¹⁴C initially fixed into C₄ acids also metabolized the C₄ acids quickly, and the percentage of ¹⁴C in 3-phosphoglyceric acid plus sugar phosphates increased for at least a 30-s exposure to ¹²CO₂. This indicated a high degree of coordination between the carbon accumulation and reduction phases of the C₄ and C₃ cycles. Synthesis and metabolism of C₄ acids by the species and their hybrids were highly and linearly correlated with discrimination against ¹³C. The relationship of ¹³C discrimination or ¹⁴C metabolism to O₂ inhibition of photosynthesis was curvilinear, changing more rapidly at C₄-like values of ¹⁴C metabolism and ¹³C discrimination. Incorporation of initial ¹⁴C into C₄ acids showed a biphasic increase with increased activities of phosphoenolpyruvate carboxylase and NADP-malic enzyme (steep at low activities), but turnover of C₄ acids was linearly related to NADP-malic enzyme activity. Several other traits were closely related to the in vitro activity of NADP-malic enzyme but not phosphoenolpyruvate carboxylase. The data indicate that the hybrids have variable degrees of C₄ photosynthesis but that the carbon accumulation and reduction portions of the C₄ and C₃ cycles are well coordinated.