OXYGEN SENSITIVITY OF PHOTOSYNTHESIS AND PHOTORESPIRATION IN DIFFERENT PHOTOSYNTHETIC TYPES IN THE GENUS FLAVERIA

Two major indicators were used to access the degree of photorespiratio n in various photosynthetic types of Flaveria species (C-3, C-3-C-4, C -4-like, and C-4): the O-2 inhibition of photosynthesis measured above the O-2 partial pressure which gives a maximum rate, and O-2- and lig ht-dependent whole-chain electron how measured at the CO2 compensation point (Gamma). The optimum level of O-2 for maximum photosynthetic ra tes under atmospheric levels of CO2 (34 Pa) was lower in C-3 and C-3-C -4 species (ca. 2 kPa) than in C-4-like and C-4 species (ca. 9 kPa). I ncreasing O-2 partial pressures from the optimum for photosynthesis up to normal atmospheric levels (ca. 20 kPa) caused an inhibition of pho tosynthesis which was more severe under lower CO2. This inhibition was calculated as the O-2 inhibition index (Theta(2), the percentage inhi bition of photosynthesis per kPa increase in O-2). From measurements o f 18 Flaveria species at atmospheric CO2, the Theta(A) values decrease d from C-3 (1.9-2.1) to C-3-C-4 (1.2-1.6), C-4-like (0.6-0.8) and C-4 species (0.3-0.4), indicating a progressive decrease in apparent photo respiration in this series. With increasing irradiance at Gamma under atmospheric levels of O-2, and increasing O-2 partial pressure at 300 mu mol quanta . m(-2). s(-1), there was a similar increase in the rate of O-2 evolution associated with whole-chain electron flow (JO(2), ca lculated from chlorophyll fluorescence analysis) in the C-3 and C-3-C- 4 species compared to a much lower rate in the C-4-like and C-4 specie s. The results indicate that there is substantial O-2-dependent electr on flow in C-3 and C-3-C-4 species, reflecting a high level of photore spiration compared to that in C-4-like and C-4, species. Consistent wi th these results, there was a significant decrease in Gamma from C-3 ( 6-6.2 Pa) to C-3-C-4 (1.0-3.0 Pa), to C-4-like and C-4 species (0.3-0. 8 Pa), indicating a progressive decrease in apparent photorespiration. However, C-3 and C-3-C-4 species examined had high intrinsic levels o f photorespiration with the latter maintaining low apparent rates of p hotorespiration and lower Gamma values, primarily by refixing photores pired CO2. The C-4-like and C-4 Flaveria species had low, but measurab le, levels of photorespiration via selective localization of ribulose- 1,5-bisphosphate carboxylase in bundle sheath cells and operation of a CO2 pump via the C-4 pathway.