Effect of dissolved inorganic carbon on oxygen evolution and uptake by Chlamydomonas reinhardtii suspensions adapted to ambient and CO2-enriched air

Mass spectrometric measurements of ¹⁶O₂ and ¹⁸O₂ isotopes were used to compare the rates of gross O₂ evolution (E₀), O₂ uptake (U₀) and net O₂ evolution (NET) in relation to different concentrations of dissolved inorganic carbon (DIC) by Chlamydomonas reinhardtii cells grown in air (air-grown), in air enriched with 5% CO₂ (CO₂-grown) and by cells grown in 5% CO₂ and then adapted to air for 6h (air-adapted).At a photon fluence rate (PFR) saturating for photosynthesis (700 mol photons m^-2 s^-1), pH=7.0 and 28°C, U₀ equalled E₀ at the DIC compensation point which was 10M DIC for CO₂-grown and zero for air-grown cells. Both E₀ and U₀ were strongly dependent on DIC and reached DIC saturation at 480 M and 70 M for CO₂-grown and air-grown algae respectively. U₀ increased from DIC compensation to DIC saturation. The U₀ values were about 40 (CO₂-grown), 165 (air-adapted) and 60 mol O₂ mg Chl^-1 h^-1 (air-grown). Above DIC compensation the U₀/E₀ ratios of air-adapted and air-grown algae were always higher than those of CO₂-grown cells. These differences in O₂ exchange between CO₂- and air-grown algae seem to be inducable since air-adapted algae respond similarly to air-grown cells.For all algae, the rates of dark respiratory O₂ uptake measured 5 min after darkening were considerably lower than the rates of O₂ uptake just before darkening. The contribution of dark respiration, photorespiration and the Mehler reaction to U₀ is discussed and the energy requirement of the inducable CO₂/HCO₃ ^- concentrating mechanism present in air-adapted and air-grown C. reinhardtii cells is considered.Abbreviations DIC dissolved inorganic carbon - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - E₀ rate of photosynthetic gross O₂ evolution - PCO photosynthetic carbon oxidation - PFR photon fluence rate - PS I photosystem I - PS II photosystem II - U₀ rate of O₂ uptake in the light - MS mass spectrometer     

Photosynthetic characteristics of coccoid marine cyanobacteria

Two strains of marine Synechococcus possessed a much greater potential for photorespiration than other marine algae we have studied. This conclusion was based on the following physiological and biochemical characteristics: a) a light-dependent O₂ inhibition of photosynthetic CO₂ assimilation at atmospheric O₂ concentrations. The degree of inhibition was dependent on the relative concentrations of dissolved O₂ and CO₂, being greatest at 100% O₂ with no extra bicarbonate added to the medium; b) actively photosynthesizing cells had high levels of ribulose-1,5-bisphosphate carboxylase compared with phosphoenolpyruvate carboxylase; ribulose-1,5-bisphosphate oxygenase activities were three times greater than ribulose-1,5-bisphosphate carboxylase activities; c) cells photosynthesizing in 21% O₂, showed significant ¹⁴C-labelling of phosphoglycolate and glycolate and the percentage of total carbon-14 incorporated into these two compounds increased when the O₂ concentration was 100%; d) at 100% O₂, there was a post-illumination enhanced rate of O₂ consumption, which was three times greater than dark respiration, and the rate declined with increasing bicarbonate concentrations. The inhibitory effect of O₂ on photosynthesis did not appear to be solely due to photorespiration, since O₂ inhibition of photosynthetic O₂ evolution was much greater than that of photosynthetic CO₂ assimilation. Also, O₂ inhibition of photosynthetic O₂ evolution declined only slightly with decreasing light intensities, while the inhibition of CO₂ assimilation declined rapidly with decreasing light intensity.     

Effect of Photon Fluence Rate on Oxygen Evolution and Uptake by Chlamydomonas reinhardtii Suspensions Grown in Ambient and CO(2)-Enriched Air

A closed system consisting of an assimilation chamber furnished with a membrane inlet from the liquid phase connected to a mass spectrometer was used to measure O₂ evolution and uptake by Chlamydomonas reinhardtii cells grown in ambient (0.034% CO₂) or CO₂-enriched (5% CO₂) air. At pH = 6.9, 28°C and concentrations of dissolved inorganic carbon (DIC) saturating for photosynthesis, O₂ uptake in the light (U_o) equaled O₂ production (E_o) at the light compensation point (15 micromoles photons per square meter per second). E_o and U_o increased with increasing photon fluence rate (PFR) but were not rate saturated at 600 micromoles photons per square meter per second, while net O₂ exchange reached a saturation level near 500 micromoles photons per square meter per second which was nearly the same for both, CO₂-grown and air-grown cells. Comparison of the U_o/E_o ratios between air-grown and CO₂-grown C. reinhardtii showed higher values for air-grown cells at light intensities higher than light compensation. For both, air-grown and CO₂-grown algae the rates of mitochondrial O₂ uptake in the dark measured immediately before and 5 minutes after illumination were much lower than U_o at PFR saturating for net photosynthesis. We conclude that noncyclic electron flow from water to NADP⁺ and pseudocyclic electron flow via photosystem I to O₂ both significantly contribute to O₂ exchange in the light. In contrast, mitochondrial respiration and photosynthetic carbon oxidation cycle are regarded as minor O₂ consuming reactions in the light in both, air-grown and CO₂-grown cells. It is suggested that the “extra” O₂ uptake by air-grown algae provides ATP required for the energy dependent CO₂/HCO₃⁻ concentrating mechanism known to be present in these cells.     

Photosynthesis and photorespiration in algae

The CO₂ exchange of several species of fresh water and marine algae was measured in the laboratory to determine whether photorespiration occurs in these organisms. The algae were positioned as thin layers on filter paper and the CO₂ exchange determined in an open gas exchange system. In either 21 or 1% O₂ there was little difference between ¹⁴CO₂ and ¹²CO₂ uptake. Apparent photosynthesis was the same in 2, 21, or 50% O₂. The compensation points of all algae were less than 10 μl 1⁻¹. CO₂ or ¹⁴CO₂ evolution into CO₂-free air in the light was always less than the corresponding evolution in darkness. These observations are inconsistent with the proposal that photorespiration exists in these algae.     

Two photosynthetic mechanisms mediating the low photorespiratory state in submersed aquatic angiosperms

The submersed angiosperms Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royal exhibited different photosynthetic pulse-chase labeling patterns. In Hydrilla, over 50% of the ¹⁴C was initially in malate and aspartate, but the fate of the malate depended upon the photorespiratory state of the plant. In low photorespiration Hydrilla, malate label decreased rapidly during an unlabeled chase, whereas labeling of sucrose and starch increased. In contrast, for high photorespiration Hydrilla, malate labeling continued to increase during a 2-hour chase. Thus, malate formation occurs in both photorespiratory states, but reduced photorespiration results when this malate is utilized in the light. Unlike Hydrilla, in low photorespiration Myriophyllum, ¹⁴C incorporation was via the Calvin cycle, and less than 10% was in C₄ acids.

Ethoxyzolamide, a carbonic anhydrase inhibitor and a repressor of the low photorespiratory state, increased the label in glycolate, glycine, and serine of Myriophyllum. Isonicotinic acid hydrazide increased glycine labeling of low photorespiration Myriophyllum from 14 to 25%, and from 12 to 48% with high photorespiration plants. Similar trends were observed with Hydrilla. Increasing O₂ increased the per cent [¹⁴C]glycine and the O₂ inhibition of photosynthesis in Myriophyllum. In low photorespiration Myriophyllum, glycine labeling and O₂ inhibition of photosynthesis were independent of the CO₂ level, but in high photorespiration plants the O₂ inhibition was competitively decreased by CO₂. Thus, in low but not high photorespiration plants, glycine labeling and O₂ inhibition appeared to be uncoupled from the external [O₂]/[CO₂] ratio.

These data indicate that the low photorespiratory states of Hydrilla and Myriophyllum are mediated by different mechanisms, the former being C₄-like, while the latter resembles that of low CO₂-grown algae. Both may require carbonic anhydrase to enhance the use of inorganic carbon for reducing photorespiration.

     

Effect of CO2, CO2 and Diamox on photosynthesis and photorespiration in Chlamydomonas reinhardtii (green alga) and Anacystis nidulans (Cyanobacterium, blue-green alga)

Photorespiration by Chlamydomonas reinhardtii and Anacystis nidulans was measured as the oxygen inhibition of CO₂ uptake and the CO₂ compensation points. Net photosynthesis was oxygen dependent in Chlamydomonas grown in 5% CO₂, but CO₂ insensitive in cultures bubbled with air. Anacystis, even when cultured in 5% CO₂, exhibited an CO₂ insensitive net photosynthesis. The CO₂ compensation point of Chlamydomonas grown in cultures bubbled with air and Anacystis grown in 5% CO₂ enriched air, were reached shortly after the measurement was begun and the values were very low, less than 10 μl CO₂ 1^?1; while Chlamydomonas grown in 5% CO₂ enriched air for 4 days showed a high, but temporary CO₂ compensation point (60 μl CO₂ 1^?1). After a two hour adaptation in low CO₂, a stable, low CO₂ compensation point was reached. It seems that photorespiration can only be detected by the methods used in this study when the algae are cultured in high CO₂, but a mechanism exists which blocks photorespiration when the green algae are adapted to low CO₂ concentrations. When Chlamydomonas was treated with Diamox, an inhibitor of carbonic anhydrase, after cultivation in low CO₂ (air), the cells behaved as if they had been grown in high CO₂. They showed an oxygen sensitive net photosynthesis and a high CO₂ compensation point. This indicates that carbonic anhydrase plays an important role in the regulation of a measurable photorespiration in Chlamydomonas. The results are discussed in relation to previous observations of photorespiration measured by enzyme assay, metabolic products and gas exchange properties.     

Oxygen inhibition of photosynthesis and stimulation of photorespiration in soybean leaf cells

The occurrence of photorespiration in soybean (Glycine max [L.] Merr.) leaf cells was demonstrated by the presence of an O₂-dependent CO₂ compensation concentration, a nonlinear time course for photosynthetic ¹⁴CO₂ uptake at low CO₂ and high O₂ concentrations, and an O₂ stimulation of glycine and serine synthesis which was reversed by high CO₂ concentration. The compensation concentration was a linear function of O₂ concentration and increased as temperature increased. At atmospheric CO₂ concentration, 21% O₂ inhibited photosynthesis at 25 C by 27%. Oxygen inhibition of photosynthesis was competitive with respect to CO₂ and increased with increasing temperature. The Km (CO₂) of photosynthesis was also temperature-dependent, increasing from 12 μm CO₂ at 15 C to 38 μm at 35 C. In contrast, the Ki (O₂) was similar at all temperatures. Oxygen inhibition of photosynthesis was independent of irradiance except at 10 mm bicarbonate and 100% O₂, where inhibition decreased with increasing irradiance up to the point of light saturation of photosynthesis. Concomitant with increasing O₂ inhibition of photosynthesis was an increased incorporation of carbon into glycine and serine, intermediates of the photorespiratory pathway, and a decreased incorporation into starch. The effects of CO₂ and O₂ concentration and temperature on soybean cell photosynthesis and photorespiration provide further evidence that these processes are regulated by the kinetic properties of ribulose-1,5-diphosphate carboxylase with respect to CO₂ and O₂.     

Carbon Oxysulfide Inhibition of the CO(2)-Concentrating Process of Unicellular Green Algae

Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, O₂ evolution, dissolved inorganic carbon accumulation, and photosynthetic CO₂ fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a CO₂-concentrating process. This COS inhibition can be attributed to inhibition of external HCO₃⁻ conversion to CO₂ and OH⁻ by the carbonic anhydrase component of an active CO₂ pump. At a low pH of 5 to 6, COS stimulated O₂ evolution during photosynthesis by algae with low CO₂ in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO₂. Although COS had less effect on HCO₃⁻ accumulation at pH 9 by a HCO₃⁻ pump in Scenedesmus, COS reduced O₂ evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO₂ and H₂S, its inhibition of the CO₂ pump activity and photosynthesis is not accurate, when measured by O₂ evolution, by NaH¹⁴CO₃ accumulation, or by ¹⁴CO₂ fixation.     

Photosynthetic and Photorespiratory Characteristics of Mutants of Hordeum vulgare L

The relationship between photosynthesis and photorespiration was determined in normal and 26 mutants of barley (Hordeum vulgare L. var. Himalaya). The rate of apparent photosynthesis ranged from 1 to 30 milligrams of CO₂ per square decimeter per hour. The variation in rate of photosynthesis was due, in some cases, to differences in chlorophyll content, in others to stomatal resistance, and in still others to unknown factors; but no single factor accounted for the variation. Photorespiratory activity, as determined by the ¹⁴CO₂/¹²CO₂ technique, CO₂ evolution into CO₂-free air, and the response of photosynthesis to low and high O₂ concentrations, was positively and significantly correlated with photosynthesis. This supports the idea that the two processes are integrally and tightly coupled. There appears to be no competition between photosynthesis and photorespiration, and the probability of finding plants with high rates of photosynthesis and low rates of photorespiration measured under natural conditions, appears to be very low.     

Inorganic Carbon Uptake by Chlamydomonas reinhardtii

The rates of CO₂-dependent O₂ evolution by Chlamydomonas reinhardtii, grown with either air levels of CO₂ or air with 5% CO₂, were measured at varying external pH. Over a pH range of 4.5 to 8.5, the external concentration of CO₂ required for half-maximal rates of photosynthesis was constant, averaging 25 micromolar for cells grown with 5% CO₂. This is consistent with the hypothesis that these cells take up CO₂ but not HCO₃⁻ from the medium and that their CO₂ requirement for photosynthesis reflects the K_m(CO₂) of ribulose bisphosphate carboxylase. Over a pH range of 4.5 to 9.5, cells grown with air required an external CO₂ concentration of only 0.4 to 3 micromolar for half-maximal rates of photosynthesis, consistent with a mechanism to accumulate external inorganic carbon in these cells. Air-grown cells can utilize external inorganic carbon efficiently even at pH 4.5 where the HCO₃⁻ concentration is very low (40 nanomolar). However, at high external pH, where HCO₃⁻ predominates, these cells cannot accumulate inorganic carbon as efficiently and require higher concentrations of NaHCO₃ to maintain their photosynthetic activity. These results imply that, at the plasma membrane, CO₂ is the permeant inorganic carbon species in air-grown cells as well as in cells grown on 5% CO₂. If active HCO₃⁻ accumulation is a step in CO₂ concentration by air-grown Chlamydomonas, it probably takes place in internal compartments of the cell and not at the plasmalemma.     

Dependence of the Post-Illumination Burst of CO2 on Temperature, Light, CO2, and O2 Concentration in Wheat (Triticum aestivum)

The effect of environmental factors on the post-illumination burst of CO₂ (PIB) and O₂ inhibition of apparent photosynthesis (APS) in wheat (Triticum aestivum L.) was studied in an open gas exchange system utilizing the mathematics of non-steady-state systems. Two components of inhibition by O₂ are suggested: one is caused by photorespiration as measured from the maximum rate of the PIB, and the second is direct inhibition as taken as APS_2%O2— (APS_x%O2+ PIB_x%O2) where X is the oxygen concentration. A primary PIB which occurred from 16–28 s after the darkening of the foliage was attributed to photorespiration. No primary PIB was observed at 2% O₂. At a CO₂ concentration of 100 μ/1 in the atmosphere (about 2.5 μM based on leaf intercellular concentration) and at 30°C and 145 nE/cm² nE/cm²·s, APS decreased curve-linearly with increasing O₂ and reached an O₂ compensation point of 560 μM (48% by volume), above which there was a net loss of CO₂ in the light. The PIB increased with increasing O₂ and became saturated at about 500 μM O₂ but decreased above 900 μM O₂. Direct inhibition of photosynthesis by O₂ increased with increasing O₂ concentration. Decreasing CO₂ concentration had an effect on the magnitude of the PIB similar to that of increasing O₂. At 30°C and 21% O₂, the PIB increased with decreasing CO₂ down to the CO₂ compensation point (I) of 1.4 μM (47 μM/l). Below Γ, both PIB and CO₂ evolution into the air in the light (at 21% O₂) increased and then decreased at CO₂ below 0.8 μM. The ratio of the PIB to APS_{2% o} O₂ increased linearly with increasing O₂/CO₂ ratio where O₂ was held constant at 21% and CO₂ was varied from 1.4 to 8.5 μM, while direct inhibition of photosynthesis expressed as a proportion of APS_2%O2 remained constant over this range. At low CO₂ concentration photorespiration as estimated by the PIB is the major part of O₂ photosynthesis, while at atmospheric CO₂ levels, direct inhibition is the major component. The PIB and APS at 2% and 21% O₂ increased hyperbolically with increasing irradiance and all became light-saturated at about 65 nE/cm₂ s. The percentage total O₂ inhibition of photosynthesis remained constant with increasing irradiance as did the relative contribution of direct O₂ inhibition or photorespiration (PIB) to total O₂ inhibition. The PIB and APS at 21% O₂ had similar temperature optima of 30°C when experimental conditions were adjusted to provide a constant internal O₂/CO₂ solubility ratio at varying temperatures. However, with a constant external CO₂ concentration, the temperature optimum for the PIB shifted upward to 35°C while that for APS at 21% O₂ remained at 30°C, which may be due to an increased O₂/CO₂ concentration in the leaf with increasing temperature.     

Photorespiration and Oxygen Inhibition of Photosynthesis in Chlorella pyrenoidosa

The inhibition of photosynthesis by O₂ in air-grown Chlorella pyrenoidosa was investigated using three experimental techniques (artificial leaf, aqueous method, and O₂ electrode) to measure carbon assimilation. CO₂ response curves were determined under different O₂, pH, and temperature conditions. Regardless of the experimental technique and condition, O₂ inhibition was not evident until a concentration of 50% was reached; V_max values were reduced whereas K_m (CO₂) values were unaffected by the increasing O₂ concentration. The response of photosynthesis to O₂ was independent of CO₂ and HCO₃⁻ concentrations as well as temperature. Relative rates of photosynthesis showed a 4 to 5% stimulation in 2% O₂, a 12% inhibition in 50% O₂, and a 24% inhibition in 100% O₂. The inhibition by 50% O₂ was still reversible after 20 minutes exposure whereas 100% O₂ caused irreversible inhibition after only 4 minutes.     

Chemical inhibition of the glycolate pathway in soybean leaf cells

Isolated soybean (Glycine max [L.] Merr.) leaf cells were treated with three inhibitors of the glycolate pathway in order to evaluate the potential of such inhibitors for increasing photosynthetic efficiency. Preincubation of cells under acid conditions in α-hydroxypyridinemethanesulfonic acid increased ¹⁴CO₂ incorporation into glycolate, but severely inhibited photosynthesis. Isonicotinic acid hydrazide (INH) increased the incorporation of ¹⁴CO₂ into glycine and reduced label in serine, glycerate, and starch. Butyl 2-hydroxy-3-butynoate (BHB) completely and irreversibly inhibited glycolate oxidase and increased the accumulation of ¹⁴C into glycolate. Concomitant with glycolate accumulation was the reduction of label in serine, glycerate, and starch, and the elimination of label in glycine. The inhibitors INH and BHB did not eliminate serine synthesis, suggesting that some serine is synthesized by an alternate pathway. The per cent incorporation of ¹⁴CO₂ into glycolate by BHB-treated cells or glycine by INH-treated cells was determined by the O₂/CO₂ ratio present during assay. Photosynthesis rate was not affected by INH or BHB in the absence of O₂, but these compounds increased the O₂ inhibition of photosynthesis. This finding suggests that the function of the photorespiratory pathway is to recycle glycolate carbon back into the Calvin cycle, so if glycolate metabolism is inhibited, Calvin cycle intermediates become depleted and photosynthesis is decreased. Thus, chemicals which inhibit glycolate metabolism do not reduce photorespiration and increase photosynthetic efficiency, but rather exacerbate the problem of photorespiration.     

Membrane-Associated Polypeptides Induced in Chlamydomonas by Limiting CO(2) Concentrations

Chlamydomonas reinhardtii and other unicellular green algae have a high apparent affinity for CO₂, little O₂ inhibition of photosynthesis, and reduced photorespiration. These characteristics result from operation of a CO₂-concentrating system. The CO₂-concentrating system involves active inorganic carbon transport and is under environmental control. Cells grown at limiting CO₂ concentrations have inorganic carbon transport activity, but cells grown at 5% CO₂ do not. Four membrane-associated polypeptides (M_r 19, 21, 35, and 36 kilodaltons) have been identified which either appear or increase in abundance during adaptation to limiting CO₂ concentrations. The appearance of two of the polypeptides occurs over roughly the same time course as the appearance of the CO₂-concentrating system activity in response to CO₂ limitation.     

Intraspecific measurements of photorespiration

The relative magnitudes of (a) CO₂ compensation concentration, (b) zero CO₂ intercept of the CO₂ response curve, (c) O₂ suppression of net photosynthesis, (d) differential ¹²CO₂ and ¹⁴CO₂ uptake, and (e) ¹⁴CO₂ efflux into CO₂-free air were determined in the dry bean (Phaseolus vulgaris L.) varieties Michelite-62 (M-62) and Red Kidney (RK). In comparing the two varieties for each of the above processes, there were three categories of response, M-62 > RK, M-62 = RK, and M-62 < RK. Since these processes did not give the same relative difference for the two varieties being studied, it was concluded that these phenomena cannot validly be used to estimate the magnitude of photorespiration, although they do identify its presence. The results suggest that photorespiration is but one component of O₂ inhibition of net photosynthesis and that photorespiration itself has two or more component metabolic pathways.     

Evaluation of the light/dark C assay of photorespiration: tobacco leaf disk studies with glycidate and glyoxylate

Preincubation of illuminated tobacco (Nicotiana tabacum L.) leaf disks in glycidate (2,3-epoxypropionate) or glyoxylate inhibited photorespiration by about 40% as determined by the ratio of ¹⁴CO₂ evolved into CO₂-free air in light and in darkness. However, under identical preincubation conditions used for the light/dark ¹⁴C assays, the compounds failed to reduce photorespiration or stimulate net photosynthesis in tobacco leaf disks based on other CO₂ exchange parameters, including the CO₂ compensation concentration in 21% O₂, the inhibitory effect of 21% O₂ on net photosynthesis in 360 microliters per liter of CO₂ and the rate of net photosynthetic ¹⁴CO₂ uptake in air.

The effects of both glycidate and glyoxylate on the ¹⁴C assay are inconsistent with other measures of photorespiratory CO₂ exchange in tobacco leaf disks, and thus these data question the validity of the light to dark ratio of ¹⁴CO₂ efflux as an assay for relative rates of photorespiration (Zelitch 1968, Plant Physiol 43: 1829-1837). The results of this study specifically indicate that neither glycidate nor glyoxylate reduces photorespiration or stimulates net photosynthesis by tobacco leaf disks under physiological conditions of pO₂ and pCO₂, contrary to previous reports.

     

Metabolism of <Superscript>14</Superscript>C-glycine as a substrate for photorespiration of the leaf at different developmental stages of ephemeroides

The metabolism of ¹⁴C-glycine (a substrate for photorespiration) was studied in the light and in darkness under natural CO₂ concentration (0.03%) in the leaves of ephemeroides Scilla sibirica Haw. and Ficaria verna Huds. at different developmental stages. Using one and the same sample, potential photosynthesis (at 1% CO₂), true photosynthesis (at 0.03% CO₂), and leaf respiratory capacity were measured by the radiometric and manometric methods, respectively. All measurements were performed at 15°C, an average temperature during ephemer growth. It was found that, in the white zone of the Scilla leaf, the rate of CO₂ evolution resulting from metabolization of exogenous ¹⁴C-glycine was similar in the light and in darkness. In the green zone of the Scilla leaf and in the green leaf of Ficaria, both ¹⁴C-glycine absorption and ¹⁴CO₂ evolution were lower in the light as compared with darkness, which is explained by CO₂ reassimilation. In all treatments of both plant species, a specific inhibitor of glycine decarboxylase complex (GDC), aminoacetonitrile (5 mM) suppressed CO₂ evolution by 20–40%. It was concluded that in ephemeroides mitochondrial GDC, responsible for CO₂ evolution in photorespiration, is formed at the earliest stage of leaf development. This indicates that photorespiration can occur simultaneously with the development of the leaf photosynthetic activity. On the basis of the assumption that carbon losses in the form of CO₂ evolved during photorespiration comprise 25% of true photosynthesis, it was calculated that, in ephemer leaves, the highest rates of photorespiration and photosynthesis were attained during flowering when the leaf area was the largest and the rate of dark respiration was reduced by 1.5–2.0 times. The highest rates of dark respiration were observed in the beginning of growth. In senescing leaves by the end of the plant vegetation, potential photosynthesis and true photosynthesis were reduced, whereas dark respiration remained essentially unchanged. It is concluded that the high rates of potential and true photosynthesis are characteristic of ephemeroides when they complete their short developmental program in early spring (at 15°C); theoretically, photorespiration also occurs at a high rate during this period, when this process provides for a defense against the threat of photoinhibition at low temperature and high insolation.     

Photosynthesis and photorespiration in whole plants of wheat

Wheat was cultivated in a small phytotronic chamber. ¹⁸O₂ was used to measure the O₂ uptake by the plant, which was recorded simultaneously with the O₂ evolution, net CO₂ uptake, and transpiration. At normal atmospheric CO₂ concentration, photorespiration, measured as O₂ uptake, was as important as the net photosynthesis. The level of true O₂ evolution was independent of CO₂ concentration and stayed nearly equal to the sum of net CO₂ photosynthesis and O₂ uptake. We conclude that at a given light intensity, O₂ and CO₂ compete for the reducing power produced at constant rate by the light reactions of photosynthesis.     

Oxygen sensitivity of C4 photosynthesis: evidence from gas exchange and chlorophyll fluorescence analyses with different C4 subtypes

Because photosynthetic rates in C₄ plants are the same at normal levels of O₂ (c, 20 kPa) and at c, 2 kPa O₂ (a conventional test for evaluating photorespiration in C₃ plants) it has been thought that C₄ photosynthesis is O₂ insensitive. However, we have found a dual effect of O₂ on the net rate of CO₂ assimilation among species representing all three C₄ subtypes from both monocots and dicots. The optimum O₂ partial pressure for C₄ photosynthesis at 30 °C, atmospheric CO₂ level, and half full sunlight (1000 μmol quanta m^?2 s^?1) was about 5–10 kPa. Photosynthesis was inhibited by O₂ below or above the optimum partial pressure. Decreasing CO₂ levels from ambient levels (32.6 Pa) to 9.3 Pa caused a substantial increase in the degree of inhibition of photosynthesis by supra-optimum levels of O₂ and a large decrease in the ratio of quantum yield of CO₂ fixation/quantum yield of photosystem II (PSII) measured by chlorophyll a fluorescence. Photosystem II activity, measured from chlorophyll a fluorescence analysis, was not inhibited at levels of O₂ that were above the optimum for CO₂ assimilation, which is consistent with a compensating, alternative electron How as net CO₂ assimilation is inhibited. At suboptimum levels of O₂, however, the inhibition of photosynthesis was paralleled by an inhibition of PSII quantum yield, increased state of reduction of quinone A, and decreased efficiency of open PSII centres. These results with different C₄ types suggest that inhibition of net CO₂ assimilation with increasing O₂ partial pressure above the optimum is associated with photorespiration, and that inhibition below the optimum O₂ may be caused by a reduced supply of ATP to the C₄ cycle as a result of inhibition of its production photochemically.