The high oxygen atmosphere toward the end-cretaceous; a possible contributing factor to the K/T boundary extinctions and to the emergence of C₄ species

Angiosperm plants were grown under either the present day 21 kPa O₂ atmosphere or 28 kPa, as estimated for the end-Cretaceous (100-65 MyBP). CO₂ was held at different levels, within the 24-60 Pa range, as also estimated for the same period. In C₃ Xanthium strumarium and Atriplex prostrata, leaf area and net photosynthesis per unit leaf area, were reduced by the high O₂, while the whole-plant respiration/photosynthesis ratio increased. The high O₂ effects were strongest under 24 Pa, but still significant under 60 Pa CO₂. Growth was reduced by high O₂ in these C₃ species, but not in Flaveria sp., whether C₃, C₄, or intermediary grown under light intensities <350 μmol m^-2 s^-1 PPF. Photosynthesis of C₃ Flaveria sp. was reduced by high O₂, but only at light intensities >350 μmol m^-2 s^-1 PPF. It is concluded that the high O₂ atmosphere at the end-Cretaceous would have reduced growth of at least some of the vegetation, thus adversely affecting dependent fauna. The weakened biota would have been predisposed to the consequences of volcanism and the K/T boundary bolide impact. Conversely, photosynthesis and growth of C₄ Zea mays and Atriplex halimus were little affected by high, 28 kPa, O₂. This suggests an environmental driver for the evolution of C₄ physiology.