Comparative studies of C(3) and C(4) Atriplex hybrids in the genomics era: physiological assessments

We crossed the C(3) species Atriplex prostrata with the C(4) species Atriplex rosea to produce F(1) and F(2) hybrids. All hybrids exhibited C(3)-like δ(13)C values, and had reduced rates of net CO(2) assimilation compared with A. prostrata. The activities of the major C(4) cycle enzymes PEP carboxylase, NAD-malic enzyme, and pyruvate-P(i) dikinase in the hybrids were at most 36% of the C(4) values. These results demonstrate the C(4) metabolic cycle was disrupted in the hybrids. Photosynthetic CO(2) compensation points (Г) of the hybrids were generally midway between the C(3) and C(4) values, and in most hybrids were accompanied by low, C(3)-like activities in one or more of the major C(4) cycle enzymes. This supports the possibility that most hybrids use a photorespiratory glycine shuttle to concentrate CO(2) into the bundle sheath cells. One hybrid exhibited a C(4)-like Г of 4 µmol mol(–1), indicating engagement of a C(4) metabolic cycle. Consistently, this hybrid had elevated activities of all measured C(4) cycle enzymes relative to the C(3) parent; however, C(3)-like carbon isotope ratios indicate the low Г is mainly due to a photorespiratory glycine shuttle. The anatomy of the hybrids resembled that of C(3)-C(4) intermediate species using a glycine shuttle to concentrate CO(2) in the bundle sheath, and is further evidence that this physiology is the predominant, default condition of the F(2) hybrids. Progeny of these hybrids should further segregate C(3) and C(4) traits and in doing so assist in the discovery of C(4) genes using high-throughput methods of the genomics era.