The degree of C
4 photosynthesis was assessed in four hybrids among C
4, C
4-like, and C
3-C
4 species in the genus
Flaveria using
14C labeling, CO
2 exchange,
13C discrimination, and C
4 enzyme activities. The hybrids incorporated from 57 to 88% of the
14C assimilated in a 10-s exposure into C
4 acids compared with 26% for the C
3-C
4 species
Flaveria linearis, 91% for the C
4 species
Flaveria trinervia, and 87% for the C
4-like
Flaveria brownii. Those plants with high percentages of
14C initially fixed into C
4 acids also metabolized the C
4 acids quickly, and the percentage of
14C in 3-phosphoglyceric acid plus sugar phosphates increased for at least a 30-s exposure to
12CO
2. This indicated a high degree of coordination between the carbon accumulation and reduction phases of the C
4 and C
3 cycles. Synthesis and metabolism of C
4 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 O
2 inhibition of photosynthesis was curvilinear, changing more rapidly at C
4-like values of
14C metabolism and
13C discrimination. Incorporation of initial
14C into C
4 acids showed a biphasic increase with increased activities of phosphoe
nolpyruvate carboxylase and NADP-malic enzyme (steep at low activities), but turnover of C
4 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 phosphoe
nolpyruvate carboxylase. The data indicate that the hybrids have variable degrees of C
4 photosynthesis but that the carbon accumulation and reduction portions of the C
4 and C
3 cycles are well coordinated.
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