Effects of photorespiration, the cytochrome pathway, and the ...

30 - 6 ANGERT ET AL.: BIOLOGICAL EFFECTS ON THREE O 2 ISOTOPESThere was no significant difference between the l R valuesin the incubations of Lemna gibba and Orobanche aegyptica,nor between different experiments with the sameplants. The l R value determined for incubation withoutinhibitors of Lemna was 0.517 ± 0.001, and a value of 0.517± 0.002 was found for the natural soil. In the two diffusionexperiments we found l R value of 0.5228 ± 0.0002, with nosignificant difference between the experiments.[40] The 17 D, dO 2 /Ar, Ln 18 O and [CO 2 ] of the terrariumair in the course of the experiment are presented in Table 2and shown in Figure 4. In days 1–58, in which there was a24-hour illumination per day, the CO 2 concentrations in theterrarium were about 150 ppm. In days 127–178, when theillumination was of 10 h d 1 ,CO 2 concentration decreasedfrom 4500 ppm in the beginning of the light period to 150ppm, after about 3 hours. In days 184–235 and 330–365, inwhich there were 2–4 hours illumination, the CO 2 concentrationwere 10,000–40,000 ppm, and in days 249–252, inwhich there were 4 hours of illumination, CO 2 decreased toabout 150 ppm after 2–3 hours of illumination. The isotopicdiscrimination ( 18 e) that was calculated from the darkperiods (days 183–184 and 283–285) is 16.7%.Figure 3. Relative changes in the triple isotopic compositionof O 2 (plotted as Ln 17 O versus Ln 18 O in per mils) asresult of (a) uptake through the COX, (b) uptake through theAOX, and (c) removal by diffusion into N 2 . The slope of thefitted regression line is l, and the reported precisionaccuracy is standard error. The values of q are calculatedaccording to equation (17).are reported with respect to the atmospheric O 2 (HLAstandard).4. Results[39] The results of the dark incubation and the diffusionof O 2 in N 2 experiments, are illustrated in Figure 3, andsummarized in Table 1. The isotopic discrimination ( 18 e)was calculated by equation (15) from the results of the darkincubation experiments. As expected, higher fractionationfor each plant species was measured when the plants wereinhibited with NaCN, and lower fractionation when theplants were inhibited with SHAM. Considerably loweruptake rates were measured when the plant was inhibitedwith NaCN (data not shown). The value of l R in theseexperiments was determined from the Ln 17 O versus Ln 18 Oaccording to equation (4). The l R values that were found are0.518 ± 0.001 for the COX, 0.5179 ± 0.0003 for the AOX.5. Discussion5.1. Dark Incubation Experiments and Diffusion inLiquid Phase[41] The value of q was calculated from the measured l Rvalues by equation (17). The fractionations ( 17 e and 18 e)arestrongly controlled by the effects of slow diffusion to theconsumption site [Angert and Luz, 2001; Guy et al., 1989].However, the value of l R in our experiments can be shownto be independent of such effects. The overall fractionationof a system in which the oxygen supply is limited bydiffusion is given byx e ¼ x e diff þ ð x e x con e diff ÞC i =C a ; ð18Þtaken from Farquhar et al. [1982], where x e is the overallfractionation, x e con is the fractionation in consumption, x e diffTable 1. Summary of Dark Incubation Experiments and DiffusionExperiments a Inhibitor n l SE e SEOrobanche aegyptica SHAM 3 0.517 0.003 14.5 0.9Orobanche aegyptica SHAM 4 0.520 0.006 14.2 0.7Lemna gibba SHAM 7 0.518 0.003 20.7 0.1Lemna gibba SHAM 7 0.518 0.002 19.6 0.2Lemna (in water) SHAM 8 0.518 0.001 14.1 0.4COX 29 0.518 0.001Orobanche aegyptica NaCN 4 0.516 0.002 18.5 0.9Orobanche aegyptica NaCN 5 0.517 0.002 21.5 1.4Lemna gibba NaCN 6 0.5181 0.0003 27.9 0.1Lemna gibba NaCN 6 0.518 0.001 27.1 0.2AOX 21 0.5179 0.0003Lemna gibba - 4 0.517 0.001 20.1 0.3Soil - 4 0.517 0.002 16.6 0.3Diffusion in N 2 - 6 0.5226 0.0004 14.9 0.1Diffusion in N 2 - 6 0.5231 0.0003 - -Diffusion in N 2 12 0.5228 0.0002a Abbreviations: n, number of data points; SE, standard error.