Variation in Quantum Yield for CO(2) Uptake among C(3) and C(4) Plants

The quantum yield for CO₂ uptake was measured on a number of C₃ and C₄ monocot and dicot species. Under normal atmospheric conditions (330 microliters per liter CO₂, 21% O₂) and a leaf temperature of 30°C, the average quantum yields (moles CO₂ per einstein) were as follows: 0.052 for C₃ dicots, 0.053 for C₃ grasses, 0.053 for NAD-malic enzyme type C₄ dicots, 0.060 for NAD-malic enzyme type C₄ grasses, 0.064 for phosphoenolpyruvate carboxykinase type C₄ grasses, 0.061 for NADP-malic enzyme C₄ dicots, and 0.065 for NADP-malic enzyme type C₄ grasses. The quantum yield under normal atmospheric conditions was temperature dependent in C₃ species, but apparently not in C₄ species. Light and temperature conditions during growth appeared not to influence quantum yield. The significance of variation in the quantum yields of C₄ plants was discussed in terms of CO₂ leakage from the bundle sheath cells and suberization of apoplastic regions of the bundle sheath cells.     

The quantum yield for CO2 uptake in C3 and C4 grasses

The quantum yield for CO₂ uptake was measured in C₃ and C₄ monocot species from several different grassland habitats. When the quantum yield was measured in the presence of 21% O₂ and 340 cm³ m^-3 CO₂, values were very similar in C₃ monocots, C₃ dicots, and C₄ monocots (0.045–0.056 mole CO₂ · mole^-1 quanta absorbed). In the presence of 2% O₂ and 800 cm³ m^-3 CO₂, enhancements of the quantum yield values occurred for the C₃ plants (both monocots and dicots), but not for C₄ monocots. A dependence of the quantum yield on leaf temperature was observed in the C₃ grass, Agropyron smithii, but not in the C₄ grass, Bouteloua gracilis, in 21% O₂ and 340 cm³ m^-3 CO₂. At leaf temperatures between 22–25°C the quantum yield values were approximately equal in the two species.     

Effect of the Long-Term Elevation of CO(2) Concentration in the Field on the Quantum Yield of Photosynthesis of the C(3) Sedge, Scirpus olneyi

CO₂ concentration was elevated throughout 3 years around stands of the C₃ sedge Scirpus olneyi on a tidal marsh of the Chesapeake Bay. The hypothesis that tissues developed in an elevated CO₂ atmosphere will show an acclimatory decrease in photosynthetic capacity under light-limiting conditions was examined. The absorbed light quantum yield of CO₂ uptake (ø_abs and the efficiency of photosystem II photochemistry were determined for plants which had developed in open top chambers with CO₂ concentrations in air of 680 micromoles per mole, and of 351 micromoles per mole as controls. An Ulbricht sphere cuvette incorporated into an open gas exchange system was used to determine ø_abs and a portable chlorophyll fluorimeter was used to estimate the photochemical efficiency of photosystem II. When measured in an atmosphere with 10 millimoles per mole O₂ to suppress photorespiration, shoots showed a ø_abs of 0.093 ± 0.003, with no statistically significant difference between shoots grown in elevated or control CO₂ concentrations. Efficiency of photosystem II photochemistry was also unchanged by development in an elevated CO₂ atmosphere. Shoots grown and measured in 680 micromoles per mole of CO₂ in air showed a ø_abs of 0.078 ± 0.004 compared with 0.065 ± 0.003 for leaves grown and measured in 351 micromoles per mole CO₂ in air; a highly significant increase. In accordance with the change in ø_abs, the light compensation point of photosynthesis decreased from 51 ± 3 to 31 ± 3 micro-moles per square meter per second for stems grown and measured in 351 and 680 micromoles per mole of CO₂ in air, respectively. The results suggest that even after 3 years of growth in elevated CO₂, there is no evidence of acclimation in capacity for photosynthesis under light-limited conditions which would counteract the stimulation of photosynthetic CO₂ uptake otherwise expected through decreased photorespiration.     

Maximum Quantum Yields of O2 Evolution in C4 Plants under High CO2

The quantum yields of photosynthetic O₂ evolution were measuredin 15 species of C₄ plants belonging to three different decarboxylationtypes (NADP-ME type, NAD-ME type and PEP-CK type) and 5 speciesof C₃ plants and evaluated relative to the maximum theoreticalvalue of 0.125 mol oxygen quanta^-1. At 25°C and 1% CO₂,the quantum yield in C₄ plants averaged 0.079 (differences betweensubgroups not significant) which was significantly lower thanthe quantum yield in C₃ plants (average of 0.105 for 5 species).This lower quantum yield in C₄ plants is thought to reflectthe requirement of energy in the C₄ cycle. For the C₄ NADP-MEtype plant Z. mays and NAD-ME type plant P. miliaceum, quantumyields were also measured over a range of CO₂ levels between1 and 20%. In both species maximum quantum yields were obtainedunder 10% CO₂ (0.105 O₂ quanta_-1 in Z. mays and 0.097 O₂ quanta_-1in P. miliaceum) indicating that at this CO₂ concentration thequantum yields are similar to those obtained in C₃ plants underCO₂ saturation. The high quantum yield values in C₄ plants undervery high CO₂ may be accomplished by direct diffusion of atmosphericCO₂ to bundle sheath cells, its fixation in the C₃ pathway,and feedback inhibition of the C₄ cycle by inorganic carbon. (Received June 6, 1995; Accepted August 15, 1995)     

Effects of O(2) and CO(2) Concentrations on Quantum Yields of Photosystems I and II in Tobacco Leaf Tissue

The interactive effects of irradiance and O₂ and CO₂ levels on the quantum yields of photosystems I and II have been studied under steady-state conditions at 25°C in leaf tissue of tobacco (Nicotiana tabacum). Assessment of radiant energy utilization in photosystem II was based on changes in chlorophyll fluorescence yield excited by a weak measuring beam of modulated red light. Independent estimates of photosystem I quantum yield were based on the light-dark in vivo absorbance change at 830 nanometers, the absorption band of P700⁺. Normal (i.e. 20.5%, v/v) levels of O₂ generally enhanced photosystem II quantum yield relative to that measured under 1.6% O₂ as the irradiance approached saturation. Photorespiration is suspected to mediate such positive effects of O₂ through increases in the availability of CO₂ and recycling of orthophosphate. Conversely, at low intercellular CO₂ concentrations, 41.2% O₂ was associated with lower photosystem II quantum yield compared with that observed at 20.5% O₂. Inhibitory effects of 41.2% O₂ may occur in response to negative feedback on photosystem II arising from a build-up in the thylakoid proton gradient during electron transport to O₂. Covariation between quantum yields of photosystems I and II was not affected by concentrations of either O₂ or CO₂. The dependence of quantum yield of electron transport to CO₂ measured by gas exchange upon photosystem II quantum yield as determined by fluorescence was unaffected by CO₂ concentration.     

Oxygen inhibition of photosynthesis

The effect of leaf temperature, O₂ and calculated O₂/CO₂ solubility ratio in the leaf on the quantum yield of photosynthesis was studied for the C₄ species, Zea mays L., and the C₃ species, Triticum aestivum L. Over a range of leaf temperatures of 16 to 35° C, the quantum yield of Z. mays was relatively constant and was similar under 1.5 and 21% O₂, being ca. 0.059 mol CO₂ mol^-1 quanta absorbed. Under 1.5% O₂ and atmospheric levels of CO₂, the quantum yield of T. aestivum was relatively constant (0.083 mol CO₂ mol^-1 quanta absorbed) at leaf temperatures from 15 to 35° C. Atmospheric levels of O₂ (21%) reduced the quantum yield of photosynthesis in T. aestivum and as leaf temperature increased, the quantum yield decreased from 0.062 at 15°C to 0.046 mol CO₂ mol^-1 quanta absorbed at 35°C. Increasing temperature decreases the solubility of CO₂ relatively more than the solubility of O₂, resulting in an increased solubility ratio of O₂/CO₂. Experimentally manipulating the atmospheric levels of O₂ or CO₂ to maintain a near-constant solubility ratio of O₂/CO₂ at varying leaf temperatures largely prevented the temperature-dependent decrease in quantum yield in t. aestivum. Thus, the decrease in quantum yield with increasing leaf temperature in C₃ species may be largely caused by a temperaturedependent change in the solubility ratio of O₂/CO₂.J and II=Ku and Edwards, 1977a, b     

Quantum Yields of CAM Plants Measured by Photosynthetic O(2) Exchange

The quantum yield of photosynthetic O₂ exchange was measured in eight species of leaf succulents representative of both malic enzyme type and phosphoenolpyruvate carboxykinase type CAM plants. Measurements were made at 25°C and CO₂ saturation using a leaf disc O₂ electrode system, either during or after deacidification. The mean quantum yield was 0.095 ± 0.012 (sd) moles O₂ per mole quanta, which compared with 0.094 ± 0.006 (sd) moles O₂ per mole quanta for spinach leaf discs measured under the same conditions. There were no consistent differences in quantum yield between decarboxylation types or during different phases of CAM metabolism. On the basis of current notions of compartmentation of CAM biochemistry, our observations are interpreted to indicate that CO₂ refixation is energetically independent of gluconeogenesis during deacidification.     

C(3)-C(4) Intermediate Species in Alternanthera (Amaranthaceae) : Leaf Anatomy, CO(2) Compensation Point, Net CO(2) Exchange and Activities of Photosynthetic Enzymes

Two naturally occurring species of the genus Alternanthera, namely A. ficoides and A. tenella, were identified as C₃-C₄ intermediates based on leaf anatomy, photosynthetic CO₂ compensation point (Γ), O₂ response of г, light intensity response of г, and the activities of key enzymes of photosynthesis. A. ficoides and A. tenella exhibited a less distinct Kranz-like leaf anatomy with substantial accumulation of starch both in mesophyll and bundle sheath cells. Photosynthetic CO₂ compensation points of these two intermediate species at 29°C were much lower than in C₃ plants and ranged from 18 to 22 microliters per liter. Although A. ficoides and A. tenella exhibited similar intermediacy in г, the apparent photorespiratory component of O₂ inhibition in A. ficoides is lower than in A. tenella. The г progressively decreases from 35 microliters per liter at lowest light intensity to 18 microliters per liter at highest light intensity in A. tenella. It was, however, constant in A. ficoides at 20 to 25 microliters per liter between light intensities measured. The rates of net photosynthesis at 21% O₂ and 29°C by A. ficoides and A. tenella were 25 to 28 milligrams CO₂ per square decimeter per hour which are intermediate between values obtained for Tridax procumbens and A. pungens, C₃ and C₄ species, respectively. The activities of key enzymes of C₄ photosynthesis, phosphoenolpyruvate carboxylase, pyruvate Pi dikinase, NAD malic enzyme, NADP malic enzyme and phosphoenolpyruvate carboxykinase in the two intermediates, A. ficoides and A. tenella are very low or insignificant. Results indicated that the relatively low apparent photorespiratory component in these two species is presumably the basis for the C₃-C₄ intermediate photosynthesis.     

Effects of Temperature and CO(2) Enrichment on Carbon Translocation of Plants of the C(4) Grass Species Echinochloa crus-galli (L.) Beauv. from Cool and Warm Environments

Plants of Echinochloa crus-galli from Québec and Mississippi were grown under two thermoperiods (28°C/22°C, 21°C/15°C) and two atmospheric CO₂ concentrations (350 and 675 microliters per liter) to examine possible differential responses of northern and southern populations of this C₄ grass species. Translocation was monitored using radioactive tracing with short-lived ¹¹C. CO₂ enrichment induced a decrease in the size of the export pool in plants of both populations. Other parameters did not strongly respond to elevated CO₂. Low temperature reduced translocation drastically for plants from Mississippi in normal CO₂ concentration, but this reduction was ameliorated at high CO₂. Overall, plants from Québec had a higher ¹¹C activity in leaf phloem and a higher percentage of ¹¹C exported, whereas these northern plants had lower turnover time and smaller pool size than plants from the southern population.     

Photosynthetic Characteristics of C(3)-C(4) Intermediate Flaveria Species : I. Leaf Anatomy, Photosynthetic Responses to O(2) and CO(2), and Activities of Key Enzymes in the C(3) and C(4) Pathways

Four species of the genus Flaveria, namely F. anomala, F. linearis, F. pubescens, and F. ramosissima, were identified as intermediate C₃-C₄ plants based on leaf anatomy, photosynthetic CO₂ compensation point, O₂ inhibition of photosynthesis, and activities of C₄ enzymes. F. anomala and F. ramosissima exhibit a distinct Kranz-like leaf anatomy, similar to that of the C₄ species F. trinervia, while the other C₃-C₄ intermediate Flaveria species possess a less differentiated Kranz-like leaf anatomy. Photosynthetic CO₂ compensation points of these intermediates at 30°C were very low relative to those of C₃ plants, ranging from 7 to 14 microliters per liter. In contrast to C₃ plants, net photosynthesis by the intermediates was not sensitive to O₂ concentrations below 5% and decreased relatively slowly with increasing O₂ concentration. Under similar conditions, the percentage inhibition of photosynthesis by 21% O₂ varied from 20% to 25% in the intermediates compared with 28% in Lycopersicon esculentum, a typical C₃ species. The inhibition of carboxylation efficiency by 21% O₂ varied from 17% for F. ramosissima to 46% for F. anomala and were intermediate between the C₄ (2% for F. trinervia) and C₃ (53% for L. esculentum) values. The intermediate Flaveria species, especially F. ramosissima, have substantial activities of the C₄ enzymes, phosphoenolpyruvate carboxylase, pyruvate, orthophosphate dikinase, NADP-malic enzyme, and NADP-malate dehydrogenase, indicating potential for C₄ photosynthesis. It appears that these Flaveria species may be true biochemical C₃-C₄ intermediates.     

Effect of Temperature, CO(2) Concentration, and Light Intensity on Oxygen Inhibition of Photosynthesis in Wheat Leaves

The effect of 21% O₂ and 3% O₂ on the CO₂ exchange of detached wheat leaves was measured in a closed system with an infrared carbon dioxide analyzer. Temperature was varied between 2° and 43°, CO₂ concentration between 0.000% and 0.050% and light intensity between 40 ft-c and 1000 ft-c. In most conditions, the apparent rate of photosynthesis was inhibited in 21% O₂ compared to 3% O₂. The degree of inhibition increased with increasing temperature and decreasing CO₂ concentration. Light intensity did not alter the effect of O₂ except at light intensities or CO₂ concentrations near the compensation point. At high CO₂ concentrations and low temperature, O₂ inhibition of apparent photosynthesis was absent. At 3% O₂, wheat resembled tropical grasses in possessing a high rate of photosynthesis, a temperature optimum for photosynthesis above 30°, and a CO₂ compensation point of less than 0.0005% CO₂. The effect of O₂ on apparent photosynthesis could be ascribed to a combination of stimulation of CO₂ production during photosynthesis, and inhibition of photosynthesis itself.     

Photosynthesis in Flaveria brownii A.M. Powell : A C(4)-Like C(3)-C(4) Intermediate

Leaves of Flaveria brownii exhibited slightly higher amounts of oxygen inhibition of photosynthesis than the C₄ species, Flaveria trinervia, but considerably less than the C₃ species, Flaveria cronquistii. The photosynthetic responses to intercellular CO₂, light and leaf temperature were much more C₄-like than C₃-like, although 21% oxygen inhibited the photosynthetic rate, depending on conditions, up to 17% of the photosynthesis rate observed in 2% O₂. The quantum yield for CO₂ uptake in F. brownii was slightly higher than that for the C₄ species F. trinervia in 2% O₂, but not significantly different in 21% O₂. The quantum yield was inhibited 10% in the presence of 21% O₂ in F. brownii, yet no significant inhibition was observed in F. trinervia. An inhibition of 27% was observed for the quantum yield of F. cronquistii in the presence of 21% O₂. The photosynthetic response to very low intercellular CO₂ partial pressures exhibited a unique pattern in F. brownii, with a break in the linear slope observed at intercellular CO₂ partial pressure values between 15 and 20 μbar when analyzed in 21% O₂. No significant break was observed when analyzed in 2% O₂. When taken collectively, the gas-exchange results reported here are consistent with previous biochemical studies that report incomplete intercellular compartmentation of the C₃ and C₄ enzymes in this species, and suggest that F. brownii is an advanced, C₄-like C₃-C₄ intermediate.     

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.     

Reduced Apparent Photorespiration by the C(3)-C(4) Intermediate Species, Moricandia arvensis and Panicum milioides

The CO₂/O₂ specificity factor of sucrose gradient purified ribulose 1,5-bisphosphate carboxylase/oxygenase from the C₃-C₄ intermediate plants Moricandia arvensis (79 ± 1) and Panicum milioides (89 ± 2) was similar to the respective values of the enzyme from the closely related C₃ species, Moricandia foetida (80 ± 5) and Panicum laxum (86 ± 2). Thus, the kinetic properties of this bifunctional enzyme do not explain the reduced rates of photorespiration exhibited by either of these intermediate species.     

Co-function of C3-and C4-photosynthetic pathways in C3, C4 and C3-C4 intermediate Flaveria species

The potential for C₄ photosynthesis was investigated in five C₃-C₄ intermediate species, one C₃ species, and one C₄ species in the genus Flaveria, using ¹⁴CO₂ pulse-¹²CO₂ chase techniques and quantum-yield measurements. All five intermediate species were capable of incorporating ¹⁴CO₂ into the C₄ acids malate and aspartate, following an 8-s pulse. The proportion of ¹⁴C label in these C₄ products ranged from 50–55% to 20–26% in the C₃-C₄ intermediates F. floridana Johnston and F. linearis Lag. respectively. All of the intermediate species incorporated as much, or more, ¹⁴CO₂ into aspartate as into malate. Generally, about 5–15% of the initial label in these species appeared as other organic acids. There was variation in the capacity for C₄ photosynthesis among the intermediate species based on the apparent rate of conversion of ¹⁴C label from the C₄ cycle to the C₃ cycle. In intermediate species such as F. pubescens Rydb., F. ramosissima Klatt., and F. floridana we observed a substantial decrease in label of C₄-cycle products and an increase in percentage label in C₃-cycle products during chase periods with ¹²CO₂, although the rate of change was slower than in the C₄ species, F. palmeri. In these C₃-C₄ intermediates both sucrose and fumarate were predominant products after a 20-min chase period. In the C₃-C₄ intermediates, F. anomala Robinson and f. linearis we observed no significant decrease in the label of C₄-cycle products during a 3-min chase period and a slow turnover during a 20-min chase, indicating a lower level of functional integration between the C₄ and C₃ cycles in these species, relative to the other intermediates. Although F. cronquistii Powell was previously identified as a C₃ species, 7–18% of the initial label was in malate+aspartate. However, only 40–50% of this label was in the C-4 position, indicating C₄-acid formation as secondary products of photosynthesis in F. cronquistii. In 21% O₂, the absorbed quantum yields for CO₂ uptake (in mol CO₂·[mol quanta]^-1) averaged 0.053 in F. cronquistii (C₃), 0.051 in F. trinervia (Spreng.) Mohr (C₄), 0.052 in F. ramosissima (C₃-C₄), 0.051 in F. anomala (C₃-C₄), 0.050 in F. linearis (C₃-C₄), 0.046 in F. floridana (C₃-C₄), and 0.044 in F. pubescens (C₃-C₄). In 2% O₂ an enhancement of the quantum yield was observed in all of the C₃-C₄ intermediate species, ranging from 21% in F. ramosissima to 43% in F. pubescens. In all intermediates the quantum yields in 2% O₂ were intermediate in value to the C₃ and C₄ species, indicating a co-function of the C₃ and C₄ cycles in CO₂ assimilation. The low quantum-yield values for F. pubescens and F. floridana in 21% O₂ presumably reflect an ineffcient transfer of carbon from the C₄ to the C₃ cycle. The response of the quantum yield to four increasing O₂ concentrations (2–35%) showed lower levels of O₂ inhibition in the C₃-C₄ intermediate F. ramosissima, relative to the C₃ species. This indicates that the co-function of the C₃ and C₄ cycles in this intermediate species leads to an increased CO₂ concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase and a concomitant decrease in the competitive inhibition by O₂.Abbreviations PEP phosphoenolpyruvate - PGA 3-phosphoglycerate - RuBP ribulose-1,5-bisphosphate     

Effects of Irradiance on the in Vivo CO(2):O(2) Specificity Factor in Tobacco Using Simultaneous Gas Exchange and Fluorescence Techniques

The effects of gas phase O₂ concentration (1%, 20.5%, and 42.0%, v/v) on the quantum yield of net CO₂ fixation and fluorescence yield of chlorophyll a are examined in leaf tissue from Nicotiana tabacum at normal levels of CO₂ and 25 to 30°C. Detectable decreases in nonphotochemical quenching of absorbed excitation occurred at the higher O₂ levels relative to 1% O₂ when irradiance was nearly or fully saturating for photosynthesis. Photochemical quenching was increased by high O₂ levels only at saturating irradiance. Simultaneous measurements of CO₂ and H₂O exchange and fluorescence yield permit estimation of partitioning of linear photosynthetic electron transport between net CO₂ fixation and O₂-dependent, dissipative processes such as photorespiration as a function of leaf internal CO₂ concentration. Changes in the in vivo CO₂:O₂ `specificity factor' (K_sp) with increasing irradiance are examined. The magnitude K_sp was found to decline from a value of 85 at moderate irradiance to 68 at very low light, and to 72 at saturating photon flux rates. The results are discussed in terms of the applicability of the ribulose bisphosphate carboxylase/oxygenase enzyme model to photosynthesis in vivo.     

An Apparatus to Produce Gas Mixtures with Controlled CO(2), O(2), and Water Vapor Concentrations

An apparatus to produce continuous gas mixtures for use in measurements of plant gas exchange is described. A wide range of CO₂ and water vapor concentrations can be provided and O₂ concentration can be varied from 0 to 21%. Changes in the concentrations of the components are accomplished conveniently, rapidly, and independently. With occasional adjustments, CO₂ and O₂ concentrations can be maintained to within ± 1 μl/l and ± 0.1%, respectively. Dew point of the gas mixture can be maintained to within ± 0.05 C.     

Photorespiration and Internal CO(2) Accumulation in Chara corallina as Inferred from the Influence of DIC and O(2) on Photosynthesis

An O₂ electrode system with a specially designed chamber for `whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O₂ concentrations on photosynthetic O₂ evolution. At pH = 5.5 and 20% O₂, cells grown in HCO₃⁻ medium (low CO₂, pH ≥ 9.0) exhibited a higher affinity for external CO₂ (K_½(CO₂) = 40 ± 6 micromolar) than the cells grown for at least 24 hours in high-CO₂ medium (pH = 6.5), (K_½(CO₂) = 94 ± 16 micromolar). With O₂ ≤ 2% in contrast, both types of cells showed a high apparent affinity (K_½(CO₂) = 50 − 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO₂ medium (pH = 6.5). The high-pH, HCO₃⁻-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO₂ which exceeded the CO₂ externally supplied, and the reverse situation has been observed in high-CO₂-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO₃⁻ (K_½[HCO₃⁻]) was 0.6 ± 0.2 millimolar, at 20% O₂. But under low O₂ concentrations (≤2%), surprisingly, an inhibition of net O₂ evolution was elicited, which was maximal at low HCO₃⁻ concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO₂ medium, (b) Chara cells are able to accumulate CO₂ internally by means of a process apparently independent of the plasmalemma HCO₃⁻ transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO₃⁻: pseudocyclic electron flow, sustained by O₂ photoreduction, may produce the additional ATP needed for the HCO₃⁻ transport.     

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.     

The Effect of Temperature on the Occurrence of O(2) and CO(2) Insensitive Photosynthesis in Field Grown Plants

The sensitivity of photosynthesis to O₂ and CO₂ was measured in leaves from field grown plants of six species (Phaseolus vulgaris, Capsicum annuum, Lycopersicon esculentum, Scrophularia desertorum, Cardaria draba, and Populus fremontii) from 5°C to 35°C using gas-exchange techniques. In all species but Phaseolus, photosynthesis was insensitive to O₂ in normal air below a species dependent temperature. CO₂ insensitivity occurred under the same conditions that resulted in O₂ insensitivity. A complete loss of O₂ sensitivity occurred up to 22°C in Lycopersicon but only up to 6°C in Scrophularia. In Lycopersicon and Populus, O₂ and CO₂ insensitivity occurred under conditions regularly encountered during the cooler portions of the day. Because O₂ insensitivity is an indicator of feedback limited photosynthesis, these results indicate that feedback limitations can play a role in determining the diurnal carbon gain in the field. At higher partial pressures of CO₂ the temperature at which O₂ insensitivity occurred was higher, indicating that feedback limitations in the field will become more important as the CO₂ concentration in the atmosphere increases.