DEGREE OF C-4 PHOTOSYNTHESIS IN C-4 AND C-3-C-4 FLAVERIA SPECIES AND THEIR HYBRIDS .1. CO2 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 C-14 labeling, CO2 exchange, C-13 discrimination, and C4 enzyme activities. The hybrids incorporated from 57 to 88% of the C-14 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 C-14 initially fixed into C4 acids also metabolized the C4 acids quickly, and the percentage of C-14 in 3-phosphoglyceric acid plus sugar phosphates increased for at least a 30-s exposure to (CO2)-C-12. 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 C-13. The relationship of C-13 discrimination or C-14 metabolism to O2 inhibition of photosynthesis was curvilinear, changing more rapidly at C4-like values of C-14 metabolism and C-13 discrimination. Incorporation of initial C-14 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.