Photosynthesis of F(1) Hybrids between C(4) and C(3)-C(4) Species of Flaveria

Photosynthetic characteristics were studied in several F(1) hybrids between C(4) and C(3)-C(4) species of Flaveria. Stable carbon isotope ratios, O(2) inhibition of apparent photosynthesis, and phosphoenolpyruvate carboxylase activities in the hybrids were similar to the means for the parents. Values of CO(2) compensation concentrations were nearer to those of the C(4) parent and apparent photosynthesis was below that of both parents, being only 60 and 74% of that of the lowest (C(3)-C(4)) parent in two experiments. Reductions of CO(2) compensation concentration and O(2) inhibition of apparent photosynthesis as well as increases in carbon isotope ratios and phosphoenolpyruvate carboxylase activities compared to values in C(3)-C(4) species suggest transfer of a limited degree of C(4) photosynthesis to the F(1) hybrids. However, the lower apparent photosynthesis of the hybrids suggests that transfer of C(4) characteristics to non-C(4) species is detrimental unless characteristics associated with C(4) photosynthesis are fully developed. There was a highly significant negative correlation (r = -0.90) between CO(2) compensation concentration and the logarithm of phosphoenolpyruvate carboxylase activity in the parents and hybrids, suggesting involvement of this enzyme in controlling the CO(2) compensation concentration. Although bundle-sheath cells were more developed in leaves of hybrids than in C(3)-C(4) parents, they appeared to contain lower quantities of organelles than those of the C(4) parent. Reduced quantities of organelles in bundle-sheath cells could indicate incomplete compartmentation of partial pathways of the C(4) cycle in the hybrids. This may mean that the reduction of CO(2) compensation and O(2) inhibition of apparent photosynthesis relative to the C(3)-C(4) parents is less dependent on fully developed Kranz anatomy than is increased apparent photosynthesis.