Photorespiration: metabolic pathways and their role in stress protection.
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 (ΘA, the percentage inhibition of photosynthesis per kPa increase in O2). From measurements of 18 Flaveria species at atmospheric CO2, the ΘA 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 (Jo 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.