Estimation of Photorespiration Based on the Initial Rate of Postillumination CO(2) Release: II. Effects of O(2), CO(2), and Temperature

An open system associated with an infrared gas analyzer was employed to study transients in CO₂ exchange generated upon darkening preilluminated leaf discs of tobacco (Nicotiana tabacum vars John Williams Broadleaf and Havana Seed). An empirical formula presented previously enabled prediction of the analyzer response under nonsteady state conditions as a function of time and of the leaf CO₂ exchange rate. A computer was used to evaluate parameters of the leaf CO₂ release rate to provide an estimate of the initial rate of postillumination CO₂ evolution and to produce maximal agreement between predicted and observed analyzer responses. In 21% O₂, the decline in rate of CO₂ evolution upon darkening followed first order kinetics. Initial rates of CO₂ evolution following darkening were relatively independent of the prior ambient CO₂ concentrations. However, rates of photorespiration expressed as a fraction of net photosynthesis declined rapidly with increasing external CO₂ concentration at 21% O₂. Under normal atmospheric conditions, photorespiration was 45 to 50% of the net CO₂ fixation rate at 32°C and high irradiance. The rapid initial CO₂ evolution observed upon darkening at 21% O₂ was absent in 3% O₂. Rates of photorespiration under normal atmospheric concentrations of CO₂ and O₂ as measured by the postillumination burst were highly dependent upon temperature (observed activation energy = 30.1 kilocalories per mole). The results are discussed with respect to previously published estimates of photorespiration in C₃ leaf tissue.     

Photorespiration in Air and High CO(2)-Grown Chlorella pyrenoidosa

Oxygen inhibition of photosynthesis and CO₂ evolution during photorespiration were compared in high CO₂-grown and air-grown Chlorella pyrenoidosa, using the artificial leaf technique at pH 5.0. High CO₂ cells, in contrast to air-grown cells, exhibited a marked inhibition of photosynthesis by O₂, which appeared to be competitive and similar in magnitude to that in higher C₃ plants. With increasing time after transfer to air, the photosynthetic rate in high CO₂ cells increased while the O₂ effect declined. Photorespiration, measured as the difference between ¹⁴CO₂ and ¹²CO₂ uptake, was much greater and sensitive to O₂ in high CO₂ cells. Some CO₂ evolution was also present in air-grown algae; however, it did not appear to be sensitive to O₂. True photosynthesis was not affected by O₂ in either case. The data indicate that the difference between high CO₂ and air-grown algae could be attributed to the magnitude of CO₂ evolution. This conclusion is discussed with reference to the oxygenase reaction and the control of photorespiration in algae.     

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.     

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.

     

Quantitation of the O(2)-Dependent, CO(2)-Reversible Component of the Postillumination CO(2) Exchange Transient in Tobacco and Maize Leaves

The postillumination transient of CO₂ exchange and its relation to photorespiration has been examined in leaf discs from tobacco (Nicotiana tabacum) and maize (Zea mays). Studies of the transients observed by infrared gas analysis at 1, 21, and 43% O₂ in an open system were extended using the nonsteady state model described previously (Peterson and Ferrandino 1984 Plant Physiol 76: 976-978). Cumulative CO₂ exchange equivalents (i.e. nanomoles CO₂) versus time were derived from the analyzer responses of individual transients. In tobacco (C₃), subtraction of the time course of cumulative CO₂ exchange under photorespiratory conditions (21 or 43% O₂) from that obtained under nonphotorespiratory conditions (1% O₂) revealed the presence of an O₂-dependent and CO₂-reversible component within the first 60 seconds following darkening. This component was absent in maize (C₄) and at low external O₂:CO₂ ratios (i.e. <100) in tobacco. The size of the component in tobacco increased with net photosynthesis as irradiance was increased and was positively associated with inhibition of net photosynthesis by O₂. This relatively simple and rapid method of analysis of the transient is introduced to eliminate some uncertainties associated with estimation of photorespiration based on the maximal rate of postillumination CO₂ evolution. This method also provides a useful and complementary tool for detecting variation in photorespiration.     

Photosynthetic o(2) exchange kinetics in isolated soybean cells

Light-dependent O₂ exchange was measured in intact, isolated soybean (Glycine max. var. Williams) cells using isotopically labeled O₂ and a mass spectrometer. The dependence of O₂ exchange on O₂ and CO₂ was investigated at high light in coupled and uncoupled cells. With coupled cells at high O₂, O₂ evolution followed similar kinetics at high and low CO₂. Steady-state rates of O₂ uptake were insignificant at high CO₂, but progressively increased with decreasing CO₂. At low CO₂, steady-state rates of O₂ uptake were 50% to 70% of the maximum CO₂-supported rates of O₂ evolution. These high rates of O₂ uptake exceeded the maximum rate of O₂ reduction determined in uncoupled cells, suggesting the occurrence of another light-induced O₂-uptake process (i.e. photorespiration).

Rates of O₂ exchange in uncoupled cells were half-saturated at 7% to 8% O₂. Initial rates (during induction) of O₂ exchange in uninhibited cells were also half-saturated at 7% to 8% O₂. In contrast, steady-state rates of O₂ evolution and O₂ uptake (at low CO₂) were half-saturated at 18% to 20% O₂. O₂ uptake was significantly suppressed in the presence of nitrate, suggesting that nitrate and/or nitrite can compete with O₂ for photoreductant.

These results suggest that two mechanisms (O₂ reduction and photorespiration) are responsible for the light-dependent O₂ uptake observed in uninhibited cells under CO₂-limiting conditions. The relative contribution of each process to the rate of O₂ uptake appears to be dependent on the O₂ level. At high O₂ concentrations (≥40%), photorespiration is the major O₂-consuming process. At lower (ambient) O₂ concentrations (≤20%), O₂ reduction accounts for a significant portion of the total light-dependent O₂ uptake.

     

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.

     

Nitrate photo-assimilation in tomato leaves under short-term exposure to elevated carbon dioxide and low oxygen

The role of photorespiration in the foliar assimilation of nitrate (NO₃^–) and carbon dioxide (CO₂) was investigated by measuring net CO₂ assimilation, net oxygen (O₂) evolution, and chlorophyll fluorescence in tomato leaves (Lycopersicon esculentum). The plants were grown under ambient CO₂ with ammonium nitrate (NH₄NO₃) as the nitrogen source, and then exposed to a CO₂ concentration of either 360 or 700 µmol mol^?1, an O₂ concentration of 21 or 2%, and either NO₃^– or NH₄⁺ as the sole nitrogen source. The elevated CO₂ concentration stimulated net CO₂ assimilation under 21% O₂ for both nitrogen treatments, but not under 2% O₂. Under ambient CO₂ and O₂ conditions (i.e. 360 µmol mol^?1 CO₂, 21% O₂), plants that received NO₃^– had 11–13% higher rates of net O₂ evolution and electron transport rate (estimated from chlorophyll fluorescence) than plants that received NH₄⁺. Differences in net O₂ evolution and electron transport rate due to the nitrogen source were not observed at the elevated CO₂ concentration for the 21% O₂ treatment or at either CO₂ level for the 2% O₂ treatment. The assimilatory quotient (AQ) from gas exchange, the ratio of net CO₂ assimilation to net O₂ evolution, indicated more NO₃^– assimilation under ambient CO₂ and O₂ conditions than under the other treatments. When the AQ was derived from gross O₂ evolution rates estimated from chlorophyll fluorescence, no differences could be detected between the nitrogen treatments. The results suggest that short‐term exposure to elevated atmospheric CO₂ decreases NO₃^– assimilation in tomato, and that photorespiration may help to support NO₃^– assimilation.     

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.     

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.     

Photosynthetic Characteristics of the C(3)-C(4) Intermediate Parthenium hysterophorus

The weedy species Parthenium hysterophorus (Asteraceae) possesses a Kranz-like leaf anatomy. The bundle sheath cells are thick-walled and contain numerous granal chloroplasts, prominent mitochondria, and peroxisomes, all largely arranged in a centripetal position. Both mesophyll and bundle sheath chloroplasts accumulate starch. P. hysterophorus exhibits reduced photorespiration as indicated by a moderately low CO₂ compensation concentration (20-25 microliters per liter at 30°C and 21% O₂) and by a reduced sensitivity of net photosynthesis to 21% O₂. In contrast, the related C₃ species P. incanum and P. argentatum (guayule) lack Kranz anatomy, have higher CO₂ compensation concentrations (about 55 microliters per liter), and show a greater inhibition of photosynthesis by 21% O₂. Furthermore, in P. hysterophorus the CO₂ compensation concentration is relatively less sensitive to changes in O₂ concentrations and shows a biphasic response to changing O₂, with a transition point at about 11% O₂. Based on these results, P. hysterophorus is classified as a C₃-C₄ intermediate. The activities of diagnostic enzymes of C₄ photosynthesis in P. hysterophorus were very low, comparable to those observed in the C₃ species P. incanum (e.g. phosphoenolpyruvate carboxylase activity of 10-29 micromoles per milligram of chlorophyll per hour). Exposures of leaves of each species to ¹⁴CO₂ (for 8 seconds) in the light resulted in 3-phosphoglycerate and sugar phosphates being the predominant initial ¹⁴C products (77-84%), with ≤4% of the ¹⁴C-label in malate plus aspartate. These results indicate that in the C₃-C₄ intermediate P. hysterophorus, the reduction in leaf photorespiration cannot be attributed to C₄ photosynthesis.     

Salicylhydroxamic Acid (SHAM) Inhibition of the Dissolved Inorganic Carbon Concentrating Process in Unicellular Green Algae

Rates of photosynthetic O₂ evolution, for measuring K_0.5(CO₂ + HCO₃⁻) at pH 7, upon addition of 50 micromolar HCO₃⁻ to air-adapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent K₁(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO₂ uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O₂ evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO₃), and the rate of ¹⁴CO₂ fixation with 100 micromolar [¹⁴C] HCO₃⁻. Salicylhydroxamic acid inhibition of O₂ evolution and ¹⁴CO₂-fixation was reversed by higher levels of NaHCO₃. Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO₂ accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related.     

Investigation on photorespiration with a sensitive C-assay

A leaf disk assay for photorespiration has been developed based on the rate of release of recently fixed ¹⁴CO₂ in light in a rapid stream of CO₂-free air at 30° to 35°. In tobacco leaves (Havana Seed) photorespiration with this assay is 3 to 5 times greater than the ¹⁴CO₂ output in the dark. In maize, photorespiration is only 2% of that in tobacco.

The importance of open leaf stomata, rapid flow rates of CO₂-free air, elevated temperatures, and oxygen in the atmosphere in order to obtain release into the air of a larger portion of the ¹⁴CO₂ evolved within the tissue in the light was established in tobacco. Photorespiration, but not dark respiration, was inhibited by α-hydroxy-2-pyridinemethanesulfonic acid, an inhibitor of glycolate oxidase, and by 3-(4-chlorophenyl)-1,1-dimethylurea (CMU), an inhibitor of photosynthetic electron transport, under conditions which did not affect the stomata. These experiments show that the substrates of photorespiration and dark respiration differ and also provide additional support for the role of glycolate as a major substrate of photorespiration. It was also shown that at 35° the quantity of ¹⁴CO₂ released in the assay may represent only 33% of the gross ¹⁴CO₂ evolved in the light, the remainder being recycled within the tissue.

It was concluded that maize does not evolve appreciable quantities of CO₂ in the light and that this largely accounts for the greater efficiency of net photosynthesis exhibited by maize. Hence low rates of photorespiration may be expected to be correlated with a high rate of CO₂ uptake at the normal concentrations of CO₂ found in air and at higher light intensities.

     

Is Transfer of CO2 to C3-Plants Purely by Diffusion?

The dependence of the CO₂ compensation concentration on O₂ partial pressure and the dependence of differential uptake of ¹⁴CO₂ and ¹²CO₂ on CO₂ and O₂ partial pressures are analyzed in illuminated white clover (Trifolium repens L.) leaves. The data show a deviation of the photosynthetic gas exchange from ribulose bisphosphate carboxylase oxygenase kinetics at 10°C but not at 30°C. This deviation is due to an effect of CO₂ partial pressure on the ratio of photosynthesis to photorespiration which can be explained if active inorganic carbon transport is assumed.     

Effect of vacuum infiltration on photosynthetic gas exchange in leaf tissue

Using a manometric method, photosynthetic oxygen evolution and ¹⁴CO₂ fixation have been determined for leaf tissue of Triticum aestivum L., Hordeum vulgare L., Phaseolus vulgaris L., and Lemna minor L. Approximately similar values in the range 0.2 to 0.4 millimoles grams fresh weight⁻¹ hour⁻¹ were obtained for both gases. In tissue subjected to vacuum infiltration, O₂ evolution and ¹⁴CO₂ fixation were barely measurable. It is considered that the elimination of photosynthetic gas exchange results from a decreased supply of CO₂ to the chloroplasts. Chopping wheat laminae also leads to a reduction in photosynthetic gas exchange, slices 1 millimeter or less giving only 10 to 20% of the value for whole tissue. Respiration is unaffected by either treatment. Carbonic anhydrase did not improve photosynthetic gas exchange in infiltrated tissue. The use of sliced or vacuum-infiltrated leaf tissue in photosynthetic studies is discussed.     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways: IV. ANALYSIS OF REDUCED OXYGEN RESPONSE IN PANICUM MILIOIDES AND PANICUM SCHENCKII

Reduced photorespiration has been reported in Panicum milioides on the basis of lower CO₂ compensation concentrations than in C₃ species, lower CO₂ evolution in the light, and less response of apparent photosynthesis to O₂ concentration. The lower response to O₂ in P. milioides could be due to reduced O₂ competition with CO₂ for reaction with ribulose 1,5-bisphosphate, to a reduced loss of CO₂, or to an initial fixation of CO₂ by phosphoenolpyruvate carboxylase. Experiments were carried out with Panicum maximum Jacq., a C₄ species having no apparent photorespiration; tall fescue (Festuca arundinacea Schreb.), a C₃ species; P. milioides Nees ex Trin.; and Panicum schenckii Hack. The latter two species are closely related and have low photorespiration rates. CO₂ exchange was measured at five CO₂ concentrations ranging from 0 to 260 microliters per liter at both 2 and 21% O₂. Mesophyll conductance or carboxylation efficiency was estimated by plotting substomatal CO₂ concentrations against apparent photosynthesis. In the C₄ species P. maximum, mesophyll conductance was 0.96 centimeters per second and was unaffected by O₂ concentration. At 21% O₂ mesophyll conductance of tall fescue was decreased 32% below the value at 2% O₂. Decreases in mesophyll conductance at 21% O₂ for P. milioides and P. schenckii were similar to that for tall fescue. On the other hand, loss of CO₂ in CO₂-free air, estimated by extrapolating the CO₂ response curve to zero CO₂, was increased from 1.8 to 6.5 milligrams per square decimeter per hour in tall fescue as O₂ was raised from 2-21%. Loss of CO₂ was less than 1 milligram per square decimeter per hour for P. milioides and P. schenckii and was unaffected by O₂. The results suggest that the reduced O₂ response in P. milioides and P. schenckii is due to a lower loss of CO₂ in the light rather than less inhibition of carboxylation by O₂, since the decrease in carboxylation efficiency at 21% O₂ was similar for P. milioides, P. schenckii, and tall fescue. The inhibition of apparent photosynthesis by 21% O₂ in these three species at low light intensities was similar at 31 to 36% which also indicates similar O₂ effects on carboxylation. Apparent photosynthesis at high light intensity was inhibited less by 21% O₂ in P. milioides (16.8%) and P. schenckii (23.8%) than in tall fescue (28.4%). This lower inhibition in the Panicum species may have been due to a higher degree of recycling of photorespired CO₂ in these species than in tall fescue.     

Further studies on o(2)-resistant photosynthesis and photorespiration in a tobacco mutant with enhanced catalase activity

The increase in net photosynthesis in M₄ progeny of an O₂-resistant tobacco (Nicotiana tabacum) mutant relative to wild-type plants at 21 and 42% O₂ has been confirmed and further investigated. Self-pollination of an M₃ mutant produced M₄ progeny segregating high catalase phenotypes (average 40% greater than wild type) at a frequency of about 60%. The high catalase phenotype cosegregated precisely with O₂-resistant photosynthesis. About 25% of the F₁ progeny of reciprocal crosses between the same M₃ mutant and wild type had high catalase activity, whether the mutant was used as the maternal or paternal parent, indicating nuclear inheritance. In high-catalase mutants the activity of NADH-hydroxypyruvate reductase, another peroxisomal enzyme, was the same as wild type. The mutants released 15% less photorespiratory CO₂ as a percent of net photosynthesis in CO₂-free 21% O₂ and 36% less in CO₂-free 42% O₂ compared with wild type. The mutant leaf tissue also released less ¹⁴CO₂ per [1-¹⁴C]glycolate metabolized than wild type in normal air, consistent with less photorespiration in the mutant. The O₂-resistant photosynthesis appears to be caused by a decrease in photorespiration especially under conditions of high O₂ where the stoichiometry of CO₂ release per glycolate metabolized is expected to be enhanced. The higher catalase activity in the mutant may decrease the nonenzymatic peroxidation of keto-acids such as hydroxypyruvate and glyoxylate by photorespiratory H₂O₂.     

The effect on net photosynthesis of pedigree selection for low and high rates of photorespiration in tobacco

A normal appearing plant with a low rate of photorespiration (ratio of ¹⁴CO₂ released light/dark = 1.6) was found in an unselected tobacco (Nicotiana tabacum) cultivar. The plant was self-pollinated, and further selections were made on several successive generations. Excised leaves from the progeny of the selections were examined for photorespiration and net CO₂ assimilation in normal air during photosynthesis. Similar measurements were made of plants derived from selfed parents with high rates of photorespiration (ratio of ¹⁴CO₂ released light/dark = 3.0 or greater). Efficient photosynthetic plants (greater than 22.0 mg of CO₂ dm⁻² hr⁻¹) with low rates of photorespiration produced a larger proportion of efficient progeny (about 25%) than did selfing inefficient plants (about 6%), but this proportion did not increase in successive generations.     

Growth,photosynthesis and photorespiration of Lemna gibba: response to variations in CO2 and O2 concentrations and photon flux density

Dry weight and Relative Growth Rate of Lemna gibba were significantly increased by CO₂ enrichment up to 6000 l CO₂ l^–1. This high CO₂ optimum for growth is probably due to the presence of nonfunctional stomata. The response to high CO₂ was less or absent following four days growth in 2% O₂. The Leaf Area Ratio decreased in response to CO₂ enrichment as a result of an increase in dry weight per frond. Photosynthetic rate was increased by CO₂ enrichment up to 1500 l CO₂ l^–1 during measurement, showing only small increases with further CO₂ enrichment up to 5000 l CO₂ l^–1 at a photon flux density of 210 mol m^–2 s^–1 and small decreases at 2000 mol m^–1 s^–1. The actual rate of photosynthesis of those plants cultivated at high CO₂ levels, however, was less than the air grown plants. The response of photosynthesis to O₂ indicated that the enhancement of growth and photosynthesis by CO₂ enrichment was a result of decreased photorespiration. Plants cultivated in low O₂ produced abnormal morphological features and after a short time showed a reduction in growth.     

Selection and characterization of tobacco plants with novel o(2)-resistant photosynthesis

Plants were obtained with novel O₂-resistant photosynthetic characteristics. At low CO₂ (250-350 μL CO₂ L⁻¹) and 30°C when O₂ was increased from 1% to 21% to 42%, the ratio of net CO₂ uptake in O₂-resistant whole plants or leaf discs compared to wild type increased progressively, and this was not related to stomatal opening. Dihaploid plantlets regenerated from anther culture were initially screened and selected for O₂-resistant growth in 42% O₂/160 μL CO₂ L⁻¹ and 0.18% of the plantlets showed O₂-resistant photosynthesis. About 30% of the progeny (6 of 19 plants) of the first selfing of a fertile plant derived from a resistant dihaploid plant had O₂-resistant photosynthesis, and after a second selfing this increased to 50% (6 of 12 plants). In 21% O₂ and low CO₂, net photosynthesis of the resistant plants was about 15% greater on a leaf area basis than wild type. Net photosynthesis was compared in leaf discs at 30 and 38°C in 21% O₂, and at the higher temperature O₂-resistant plants showed still greater photosynthesis than wild type. The results suggest that the O₂-resistant photosynthesis described here is associated with a decreased stoichiometry of CO₂ release under conditions of rapid photorespiration. This view was supported by the finding that leaves of O₂-resistant plants averaged 40% greater catalase activity than wild type.