Photosynthetic O₂ Exchange Kinetics in Isolated Soybean Cells

Abstract

Light-dependent 02 exchange was measured in intact, isolated soybean (Glycine max. var. Williams) cells using isotopicaHly labeled 02 and a mass spectrometer. The dependence of 02 exchange on 02 and C02 was investigated at high light in coupled and uncoupled cells. With coupled cells at high 02, 02 evolution followed similar kinetics at high and low C02. Steady-state rates of 02 uptake were insignificant at high C02, but progressively increased with decreasing C02. At low C02, steady-state rates of 02 uptake were 50% to 70% of the maximum C02-supported rates of 02 evolution. These high rates of 02 uptake exceeded the maximum rate of 02 reduction determined in uncoupled cells, suggesting the occurrence of another light-induced 02-uptake process (Le. photorespiration).Rates of 02 exchange in uncoupled cells were half-saturated at 7% to 8% 02-Initial rates (during induction) of 02 exchange in uninhibited cells were also half-saturated at 7% to 8% 02. In contrast, steady-state rates of 02 evolution and 02 uptake (at low C02) were half-saturated at 18% to 20% 02. 02 uptake was significantly suppressed in the presence of nitrate, suggesting that nitrate and/or nitrite can compete with 02 for photoreductant.These results suggest that two mechanisms (02 reduction and photorespiration) are responsible for the light-dependent 02 uptake observed in uninhibited cells under C02-limiting conditions. The relative contribution of each process to the rate of 02 uptake appears to be dependent on the 02 level. At high 02 concentrations (I-40%), photorespiration is the major 02-consuming process. At lower (ambient) 02 concentrations (c20%), 02 reduction accounts for a significant portion of the total light-dependent 02 uptake.It is generally recognized that one of the important factors limiting net photosynthesis in higher C3 plants under ambient conditions is the prevailing CO2 and 02 tensions (13,31,32). Decreasing CO2 or increasing 02 concentrations (from ambient) leads to significant decreases in the net rate of photosynthesis. Conversely, the net rate of photosynthesis can be increased by increasing the CO2 or decreasing the 02 concentrations. These observations have been interpreted as a reflection of the direct and/or indirect competition between CO2 and 02 for photosynthetic-reducing equivalents. Three principal 02-consuming represent a relatively minor component of the total 02 uptake (see, however, 9, 10) and most attention has focused on the lightdependent processes of 02 reduction and photorespiration. The extent to which each process occurs in vivo under different photosynthetic conditions is unresolved.Endogenous 02 reduction has been shown to occur in a variety of photosynthetic systems (2,8,11,17,21,29). This light-dependent process is eliminated by DCMU, is absent in PSI-deficient algal mutants (24), and is not directly dependent on reactions of the carbon reduction cycle. Inhibition of 02 reduction by CO2 presumably reflects the competition between 02 and NADP+ for photosynthetic reducing equivalents. 02 r...

Photoreduction of Oxygen in Mesophyll Chloroplasts of C4 Plants

“…At low light, less OAA was required to suppress 02 uptake than at high light and a greater suppression was observed. This is also analogous to data obtained with isolated thylakoids where more NADP+ is required to suppress 02 uptake at high light than at low light (1 1 even under conditions where photorespiration is inhibited (5,10).…”

“…However, in the presence of 3-PGA, 02 uptake was saturated by quite low 02 concentrations (Fig. 2), similar to C3 cells and chloroplasts in the presence of saturating bicarbonate (5,10). In the presence of OAA, the 02 uptake was even less sensitive to 02 tension (Fig.…”

“…Light-dependent 02 uptake in higher plants has previously been studied in a variety of systems: isolated thylakoids (11), intact chloroplasts and cells (5,9,10,16,21), and in the intact leaf (3,6,12,22,24 80 ,um) and the filtrate was centrifuged at 350g for 3 min. The pelletted protoplasts were gently resuspended in 20 ml of the solution previously described but with 0.6 M sucrose substituted for sorbitol.…”

“…Light-dependent 02 uptake in higher plants has previously been studied in a variety of systems: isolated thylakoids (11), intact chloroplasts and cells (5,9,10,16,21), and in the intact leaf (3,6,12,22,24 Approximately 10 g of leaves were harvested for each preparation. The leaves were sliced into approximately 1-mm strips with a razor blade and digested at 30°C for 2 to 2.5 h, with illumination (1 50-w flood lamp), in 50 ml of 0.5 M sorbitol, 0.2 mM CaCI2, I mM MgC92, 0.2 mM KH2PO4, 10 mM Mes, 0.2% BSA, 2% cellulase (Onozuka S-3), and 0.2% pectinase (Macerozyme R-10) at pH 5.5 with HCI.…”

Comparison of the Oxygen Exchange between Photosynthetic Cell Suspensions and Detached Leaves of Euphorbia characias L

“…An energy overflow would probably increase in nondividing cells with gross photosynthesis identical (Table I) (8) concluded that in leaves, other components than RuBP oxygenase contributed to less than 5% of the CO2 inhibition of light oxygen uptake. Taking into account these previous data, we consider that the main source of difference, in our results, between RuBP oxygenase activity and photorespiration measured as described resulted from the differences of the CO2 level in the CO2-limiting conditions for the various samples (check Table II (16), gross photosynthesis, and oxygen uptake activities (3,4,7).…”