Oxygen sensitivity of photosynthesis and photorespiration in different photosynthetic types in the genus Flaveria

Abstract

Two major indicators were used to access the degree of photorespiration in various photosynthetic types of Flaveria species (C₃, C₃-C₄, C₄-like, and C₄): the O₂ inhibition of photosynthesis measured above the O₂ partial pressure which gives a maximum rate, and O₂- and light-dependent whole-chain electron flow measured at the CO₂ compensation point (Γ). The optimum level of O₂ for maximum photosynthetic rates under atmospheric levels of CO₂ (34 Pa) was lower in C₃ and C₃-C₄ species (ca. 2 kPa) than in C₄-like and C₄ species (ca. 9 kPa). Increasing O₂ partial pressures from the optimum for photosynthesis up to normal atmospheric levels (ca. 20 kPa) caused an inhibition of photosynthesis which was more severe under lower CO₂. This inhibition was calculated as the O₂ inhibition index (Θ_A, the percentage inhibition of photosynthesis per kPa increase in O₂). From measurements of 18 Flaveria species at atmospheric CO₂, the Θ_A values decreased from C₃ (1.9–2.1) to C₃-C₄ (1.2–1.6), C₄-like (0.6–0.8) and C₄ species (0.3–0.4), indicating a progressive decrease in apparent photorespiration in this series. With increasing irradiance at Γ under atmospheric levels of O₂, and increasing O₂ partial pressure at 300 μmol quanta·m⁻²·s⁻¹, there was a similar increase in the rate of O₂ evolution associated with whole-chain electron flow (J_{o 2}, calculated from chlorophyll fluorescence analysis) in the C₃ and C₃-C₄ species compared to a much lower rate in the C₄-like and C₄ species. The results indicate that there is substantial O₂-dependent electron flow in C₃ and C₃-C₄ species, reflecting a high level of photorespiration compared to that in C₄-like and C₄ species. Consistent with these results, there was a significant decrease in Γ from C₃ (6–6.2 Pa) to C₃-C₄ (1.0–3.0 Pa), to C₄-like and C₄ species (0.3–0.8 Pa), indicating a progressive decrease in apparent photorespiration. However, C₃ and C₃-C₄ species examined had high intrinsic levels of photorespiration with the latter maintaining low apparent rates of photorespiration and lower Γ values, primarily by refixing photorespired CO₂. The C₄-like and C₄ Flaveria species had low, but measurable, levels of photorespiration via selective localization of ribulose-1,5-bisphosphate carboxylase in bundle sheath cells and operation of a CO₂ pump via the C₄ pathway.