The Rate of Photorespiration during Photosynthesis and the Relationship of the Substrate of Light Respiration to the Products of Photosynthesis in Sunflower Leaves

Abstract

Single attached leaves of sunflower (Helianthus annus L."Mennonite") were supplied "CO2 of constant specific radioactivity in gas mixtures containing various CO2 and 02 concentrations. The "CO2 and C02 fluxes were measured concurrently in an open system using an ionization chamber and infrared gas analyzer.The rate of photorespiration (5.7 + 0.3 mg CO2-dm 2 hr-') during photosynthesis in 21% 02 at 25 C and 3,500 footcandles was over three times the rate of dark respiration and was independent of C02 concentrations from 0 to 300 Al/l.The steady rate of C02 evolution into CO2-free air was about 30% lower. Low oxygen (1%) inhibited both "CO2 and C02 evolution, both during photosynthesis and in CO2-free air in the light.At 300 ul/l C02 apparent photosynthesis was inhibited 41% by 21% 02. Two-thirds of the inhibition was due to the inhibition of true photosynthesis by oxygen and one-third due to the stimulation of photorespiration. At 50 u1d/l C02, where the percentage inhibition of apparent photosynthesis by 21% oxygen was 92 %, photorespiration accounted for two-thirds of the total inhibition.The rate of 14CO2 uptake by the leaf decreased about 30 seconds after the introduction of 14CO2, indicating that "4CO2 was rapidly evolved from the leaf. The rate of "CO2 evolution increased rapidly with time, the kinetics depending on the C02 concentration. The high specific radioactivity of the "4CO2 evolved during photosynthesis or in the early period of flushing in C02-free air showed that the substrate for light respiration was an early product of photosynthesis. From the measurement of "CO2 and C02 evolution into C02-free air over a longer time period it was apparent that at least three compounds, each of deereased 1"C content, could supply the substrate for light respiration.Based on a consideration of the specific radioactivity of There is now much evidence to show that the rate of CO, evolution from green leaves in the light exceeds that of dark respiration and that the substrate and mechanism of CO2 evolution in the light are different from those of CO-evolution in the dark (24). All methods to date (23, 24, 38), however, have not measured the rate of photorespiration under conditions of steady state photosynthesis nor have they (18, 38) allowed a continuous measurement of the specific radioactivity of the 'CO. evolved from a leaf during or after a period of photosynthesis in 'CO2. We have recently described a system (26) which continuously measures the CO. or "CO2 fluxes from leaves. In this paper we present results on the rates of photorespiration duLring steady state photosynthesis and on the relationship of the "CO2 evolved to the products of "CO2 fixation. Brief accounts of this work have previously been presented (27,28). MATERIALS AND METHODSThe mnaterials and methods have been fully described in a preceding paper (26). Sunflower leaves (Helianthlls annulis L."Mennonite"), grown as previously described (26), were used for all experiments. RESULTSFrom the CO; and "CO2 uptake and the average specific ...

Regulation of Soybean Net Photosynthetic CO2 Fixation by the Interaction of CO2, O2, and Ribulose 1,5-Diphosphate Carboxylase

“…In the absence of 02, the scheme indicates that net photosynthesis will increase by about 43(>. This expected increase is consistent with experimental observations (12,19 rate of change of P-glycolate concentration was zero, and the rates of glycolate accumulation and P-glycolate hydrolysis were determined from the solution of two simultaneous equations (3). In these calculations, it was assumed that the rate of P-glycolate hydrolysis was proportional to the P-glycolate concentration (i.e.…”

“…C02) at 25 C. This scheme demonstrates that glycolate accumulation can equal 50%C of net carbon fixed in the presence of an inhibitor of glycolate oxidase, a-HPMS (a-hydroxypyridinemethanesulfonic acid), as observed by Zelitch (28), when RuDP carboxylase fixes one 02 molecule per four CO2 molecules. In the absence of a-HPMS, the scheme indicates that the rate of photorespiratory CO2 evolution equals about 14e1 the rate of photorespiratory CO2 evolution equals about 14%c the rate of net photosynthesis, in agreement with the direct measurements of Ludwig and Canvin (19). In the absence of 02, the scheme indicates that net photosynthesis will increase by about 43(>.…”

“…The total oxygen inhibition of soybean photosynthesis in air equals 40 to 45% of net photosynthesis (12,19). The stoichiometry presented in Figure 8 is consistent with this observation, and predicts that, in the absence of 02, net photosynthesis will increase from 3.5 to 5.0 units, an increase of 43%.…”

Natural Abundance Carbon Isotope Composition of Isoprene Reflects Incomplete Coupling between Isoprene Synthesis and Photosynthetic Carbon Flow

“…Such results provide a first approximation for the turnover rate of the alternative carbon source(s), i.e. on the order of 10 h. The partial labeling of isoprene emitted under CO 2 -free air is consistent with partial labeling of CO 2 respired into CO 2 -free air (Ludwig and Canvin, 1971). Although in principle, the unlabeled carbon source could result from refixation of the respired CO 2 , such refixation under CO 2 -free air is very small (Loreto et al, 1999).…”

“…But Hewitt et al (1990) showed that isoprene is not produced predominantly via photorespiration. It was previously suggested also that photorespiratory intermediates originate from short-term carbon storage, rapidly labeled by recent assimilation (Ludwig and Canvin, 1971;Loreto et al, 1999;Haupt-Herting et al, 2001). Such rapid labeling is inconsistent with the possibility that photorespiration is the source for unlabeled carbon in isoprene as observed here.…”