Oxygen Exchange in Leaves in the Light 1

Photosynthetic 02 production and photorespiratory 02 uptake were measured using isotopic techniques, in the C3 species Hirschfeldia incana Lowe., Helianthus annuus L., and Phaseolus vulgaris L. At high CO2 and normal 02,02 production increased linearly with light intensity. At low 02 or low C02, 02 production was suppressed, indicating that increased concentrations of both 02 and CO2 can stimulate 02 production. At the CO2 compensation point, 02 uptake equaled 02 production over a wide range of 02 concentrations. 02 uptake increased with light intensity and 02 concentration. At low light intensities, 02 uptake was suppressed by increased CO2 concentrations so that 02 uptake at 1,000 microliters per liter CO2 was 28 to 35% of the uptake at the CO2 compensation point. At high light intensities, 02 uptake was stimulated by low concentrations of CO2 and suppressed by higher concentrations of C02. 02 uptake at high light intensity and 1000 microliters per liter CO2 was 75% or more of the rate of 02 uptake at the compensation point. The response of 02 uptake to light intensity extrapolated to zero in darkness, suggesting that 02 uptake via dark respiration may be suppressed in the light. The response of 02 uptake to 02 concentration saturated at about 30% 02 in high light and at a lower 02 concentration in low light. 02 uptake was also observed with the C4 plant Amaranthus edulis, the rate of uptake at the CO2 compensation point was 20% of that observed at the same light intensity with the C3 species, and this rate was not influenced by the CO2 concentration. The results are discussed and interpreted in terms of the ribulose1,5-bisphosphate oxygenase reaction, the associated metabolism of the photorespiratory pathway, and direct photosynthetic reduction of 02. Both 02 evolution and 02 uptake take place in leaves of C3 and C4 plants in the light (4, 8, 17, 21, 23, 24, 27, 28). 02 evolution is derived entirely from the water-splitting reaction of PSII, but three principal 02 uptake processes are presently recognized. These are: the oxygenase reaction of ribulose bisP carboxylase-oxygenase and the associated metabolism of P-glycolate (2, 4, 5, 18, 20); the Mehler reaction (22), which results in the direct photoreduction of 02 and may support ATP synthesis via pseudocyclic photophosphorylation (9, 13, 16); and the possibility that 02 uptake associated with mitochondrial respiration continues in the light (18). Volk and Jackson and their colleagues (17, 23, 24, 27, 28) have made substantial contributions to the study of 02 exchange in intact leaves, but only a limited range of conditions were emThis paper is Carnegie Institute of Washington publication No. 686. 2 Permanent address: Department of Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada. N Permanent address: Department of Plant Biology, Carnegie Institution of Washington, 290 Panama Street, Stanford, Calif. 94305. 4 Permanent address: Fachbereich Biologie, Universitat Kaiserslautern, Postfach 3049, D-6750 Kaiserslautern, Federal Republic of Germany. ployed. In this paper, we describe experiments over a wide range ofCO2 and 02 concentrations in which net CO2 uptake, 02 uptake, and 02 evolution were measured. These show that, in C3 plants but not in C4 plants, 02 uptake is depressed by high CO2 and permits partial resolution of the alternative pathways of02 uptake in intact leaves. MATERIALS AND METHODS Leaves of Indian mustard (Hirschfeldia incana Lowe. syn. Brassica genniculata Desf.) were harvested from plants growing under natural conditions. Leaves of sunflower (Helianthus annuus L. var. Bronze Hybrid), Amaranthus edulis L. and bean (Phaseolus vulgaris L.) were harvested from plants growing in soil in a glasshouse. Experiments were conducted with the closed gas exchange system of Berry et al. (4). The system had been modified by Berry and Badger (unpublished) who added a capillary arrangement to supply CO2 to the system. Leaves of the above species were detached under water and placed in the plant chamber. After equilibration of the leaves in air at 400 AE m-2 s-' illumination, the air in the system was replaced by flushing the system with argon. The flow of argon was stopped and, with the two-way valve open, the required amount of 802 (99% 1O, Norsk Hydro, Oslo, Norway) was injected into the system. The system was closed and the gas was circulated over the leaf with a metal bellows pump. Mass 32, mass 36, and mass 40 were monitored continuously with a GD 150/4 mass spectrometer. 02 uptake and 02 evolution were calculated using the methods previously described (25, 27). CO2 concentration was measured with an IRGA analyzer (UNOR-2, Maihak, Hamburg, Germany) included in the gas circuit, and CO2 concentration during illumination could be controlled by varying the pressure of CO2 on a capillary that bled pure CO2 into the closed system. CO2 uptake, at constant CO2 concentration in the system, was calculated from the rate of CO2 addition. Each measurement was averaged over an 8to 10-min period of gas exchange after the rate of CO2 uptake had reached a steady rate at each CO2 concentration. The total gas pressure in the small system increased due to 02 production and was equilibrated to atmospheric pressure between measurements. In this closed system, water vapor was condensed in a trap held at 5 C below the leaf temperature but it was not possible to measure stomatal responses, so that all CO2 concentrations cited in the text refer to ambient, not intercellular, CO2 concentration.