Photosynthesis of F1 Hybrids between C4 and C3-C4 Species of Flaveria

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Abstract
Photosynthetic characteristics were studied in several F1 hybrids between C4 and C3-C4 species of Flaveria. Stable carbon isotope ratios, O2 inhibition of apparent photosynthesis, and phosphoenolpyruvate carboxylase activities in the hybrids were similar to the means for the parents. Values of CO2 compensation concentrations were nearer to those of the C4 parent and apparent photosynthesis was below that of both parents, being only 60 and 74% of that of the lowest (C3-C4) parent in two experiments. Reductions of CO2 compensation concentration and O2 inhibition of apparent photosynthesis as well as increases in carbon isotope ratios and phosphoenolpyruvate carboxylase activities compared to values in C3-C4 species suggest transfer of a limited degree of C4 photosynthesis to the F1 hybrids. However, the lower apparent photosynthesis of the hybrids suggests that transfer of C4 characteristics to non-C4 species is detrimental unless characteristics associated with C4 photosynthesis are fully developed. There was a highly significant negative correlation (r = −0.90) between CO2 compensation concentration and the logarithm of phosphoenolpyruvate carboxylase activity in the parents and hybrids, suggesting involvement of this enzyme in controlling the CO2 compensation concentration. Although bundle-sheath cells were more developed in leaves of hybrids than in C3-C4 parents, they appeared to contain lower quantities of organelles than those of the C4 parent. Reduced quantities of organelles in bundle-sheath cells could indicate incomplete compartmentation of partial pathways of the C4 cycle in the hybrids. This may mean that the reduction of CO2 compensation and O2 inhibition of apparent photosynthesis relative to the C3-C4 parents is less dependent on fully developed Kranz anatomy than is increased apparent photosynthesis.