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.     

CO(2) and O(2) Exchange in Two Mosses, Hypnum cupressiforme and Dicranum scoparium

Photosynthetic CO₂ and O₂ exchange was studied in two moss species, Hypnum cupressiforme Hedw. and Dicranum scoparium Hedw. Most experiments were made during steady state of photosynthesis, using ¹⁸O₂ to trace O₂ uptake. In standard experimental conditions (photoperiod 12 h, 135 micromoles photons per square meter per second, 18°C, 330 microliters per liter CO₂, 21% O₂) the net photosynthetic rate was around 40 micromoles CO₂ per gram dry weight per hour in H. cupressiforme and 50 micromoles CO₂ per gram dry weight per hour in D. scoparium. The CO₂ compensation point lay between 45 and 55 microliters per liter CO₂ and the enhancement of net photosynthesis by 3% O₂versus 21% O₂ was 40 to 45%. The ratio of O₂ uptake to net photosynthesis was 0.8 to 0.9 irrespective of the light intensity. The response of net photosynthesis to CO₂ showed a high apparent K_m (CO₂) even in nonsaturating light. On the other hand, O₂ uptake in standard conditions was not far from saturation. It could be enhanced by only 25% by increasing the O₂ concentration (saturating level as low as 30% O₂), and by 65% by decreasing the CO₂ concentration to the compensation point. Although O₂ is a competitive inhibitor of CO₂ uptake it could not replace CO₂ completely as an electron acceptor, and electron flow, expressed as gross O₂ production, was inhibited by both high O₂ and low CO₂ levels. At high CO₂, O₂ uptake was 70% lower than the maximum at the CO₂ compensation point. The remaining activity (30%) can be attributed to dark respiration and the Mehler reaction.     

Active CO(2) Transport by the Green Alga Chlamydomonas reinhardtii

Mass spectrometric measurements of dissolved free ¹³CO₂ were used to monitor CO₂ uptake by air grown (low CO₂) cells and protoplasts from the green alga Chlamydomonas reinhardtii. In the presence of 50 micromolar dissolved inorganic carbon and light, protoplasts which had been washed free of external carbonic anhydrase reduced the ¹³CO₂ concentration in the medium to close to zero. Similar results were obtained with low CO₂ cells treated with 50 micromolar acetazolamide. Addition of carbonic anhydrase to protoplasts after the period of rapid CO₂ uptake revealed that the removal of CO₂ from the medium in the light was due to selective and active CO₂ transport rather than uptake of total dissolved inorganic carbon. In the light, low CO₂ cells and protoplasts incubated with carbonic anhydrase took up CO₂ at an apparently low rate which reflected the uptake of total dissolved inorganic carbon. No net CO₂ uptake occurred in the dark. Measurement of chlorophyll a fluorescence yield with low CO₂ cells and washed protoplasts showed that variable fluorescence was mainly influenced by energy quenching which was reciprocally related to photosynthetic activity with its highest value at the CO₂ compensation point. During the linear uptake of CO₂, low CO₂ cells and protoplasts incubated with carbonic anhydrase showed similar rates of net O₂ evolution (102 and 108 micromoles per milligram of chlorophyll per hour, respectively). The rate of net O₂ evolution (83 micromoles per milligram of chlorophyll per hour) with washed protoplasts was 20 to 30% lower during the period of rapid CO₂ uptake and decreased to a still lower value of 46 micromoles per milligram of chlorophyll per hour when most of the free CO₂ had been removed from the medium. The addition of carbonic anhydrase at this point resulted in more than a doubling of the rate of O₂ evolution. These results show low CO₂ cells of Chlamydomonas are able to transport both CO₂ and HCO₃⁻ but CO₂ is preferentially removed from the medium. The external carbonic anhydrase is important in the supply to the cells of free CO₂ from the dehydration of HCO₃⁻.     

Effects of “Reduced” and “Business-As-Usual” CO2 Emission Scenarios on the Algal Territories of the Damselfish Pomacentrus wardi (Pomacentridae)

Turf algae are a very important component of coral reefs, featuring high growth and turnover rates, whilst covering large areas of substrate. As food for many organisms, turf algae have an important role in the ecosystem. Farming damselfish can modify the species composition and productivity of such algal assemblages, while defending them against intruders. Like all organisms however, turf algae and damselfishes have the potential to be affected by future changes in seawater (SW) temperature and pCO₂. In this study, algal assemblages, in the presence and absence of farming Pomacentrus wardi were exposed to two combinations of SW temperature and pCO₂ levels projected for the austral spring of 2100 (the B1 “reduced” and the A1FI “business-as-usual” CO₂ emission scenarios) at Heron Island (GBR, Australia). These assemblages were dominated by the presence of red algae and non-epiphytic cyanobacteria, i.e. cyanobacteria that grow attached to the substrate rather than on filamentous algae. The endpoint algal composition was mostly controlled by the presence/absence of farming damselfish, despite a large variability found between the algal assemblages of individual fish. Different scenarios appeared to be responsible for a mild, species specific change in community composition, observable in some brown and green algae, but only in the absence of farming fish. Farming fish appeared unaffected by the conditions to which they were exposed. Algal biomass reductions were found under “reduced” CO₂ emission, but not “business-as-usual” scenarios. This suggests that action taken to limit CO₂ emissions may, if the majority of algae behave similarly across all seasons, reduce the potential for phase shifts that lead to algal dominated communities. At the same time the availability of food resources to damselfish and other herbivores would be smaller under “reduced” emission scenarios.     

Histochemical Compartmentation of Photosynthesis in the Crassulacean Acid Metabolism Plant Crassula falcata

The succulent leaf of the obligate Crassulacean acid metabolism plant Crassula falcata comprises two distinct types of parenchyma. The peripheral tissue is dark green, whereas the central tissue is relatively colorless. We have investigated whether the conventional interpretation of Crassulacean acid metabolism—simply, temporal separation of light and dark CO₂ fixation within individual cells—is sufficient. Ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) and chlorophyll, indicating the photosynthetic-carbon-reduction pathway, were concentrated in peripheral tissue. Specific activities of P-enolpyruvate carboxylase (4.1.1.31) and of NAD⁺-malic enzyme (1.1.1.39), indicating capacity for dark CO₂ fixation and release, respectively, were high in both types of parenchyma. Measured directly as malic acid decline at the beginning of the photoperiod, CO₂ “storage” occurred in both tissues. These data indicate that there is a spatial component to Crassulacean acid metabolism in C. falcata.     

Light-Dependent Oxygen Uptake, Glycolate, and Ammonia Release in l-Methionine Sulfoximine-Treated Chlamydomonas

Glycolate and ammonia excretion plus oxygen exchanges were measured in the light in l-methionine-dl-sulfoximine treated air-grown Chlamydomonas reinhardii. At saturating CO₂ (between 600 and 700 microliters per liter CO₂) neither glycolate nor ammonia were excreted, whereas at the CO₂ compensation concentration (<10 microliters per liter CO₂) treated algae excreted both glycolate and ammonia at rates of 37 and 59 nanomoles per minute per milligram chlorophyll, respectively. From the excretion values we calculate the amount of O₂ consumed through the glycolate pathway. The calculated value was not significantly different from the component of O₂ uptake sensitive to CO₂ obtained from the difference between O₂ uptake of the CO₂ compensation point and at saturating CO₂. This component was about 40% of stationary O₂ uptake measured at the CO₂ compensation point. From these data we conclude that glyoxylate decarboxylation in air-grown Chlamydomonas represents a minor pathway of metabolism even in conditions where amino donors are deficient and that processes other than glycolate pathway are responsible for the O₂ uptake insensitive to CO₂.     

Oxygen Uptake and Photosynthesis of the Red Macroalga, Chondrus crispus, in Seawater: Effects of Light and CO(2) Concentration

With an experimental system using mass spectrometry techniques and infra-red gas analysis of CO₂ developed for aquatic plants, we studied the responses to various light intensities and CO₂ concentrations of photosynthesis and O₂ uptake of the red macroalga Chondrus crispus S. The CO₂ exchange resistance at air-water interface which could limit the photosynthesis was experimentally measured. It allowed the calculation of the free dissolved CO₂ concentration. The response to light showed a small O₂ uptake (37% of net photosynthesis in standard conditions) compared to C₃ plants; it was always higher than dark respiration and probably included a photoindependent part. The response to CO₂ showed: (a) an O₂ uptake relatively insensitive to CO₂ concentration and not completely inhibited with high CO₂, (b) a general inhibition of gas exchanges below 130 microliters CO₂ per liter (gas phase), (c) an absence of an inverse relationship between O₂ and CO₂ uptakes, and (d) a low apparent K_m of photosynthesis for free CO₂ (1 micromolar). These results suggest that O₂ uptake in the light is the sum of different oxidation processes such as the glycolate pathway, the Mehler reaction, and mitochondrial respiration. The high affinity for CO₂ is discussed in relation to the use of HCO₃⁻ and/or the internal CO₂ accumulation.     

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

Single attached leaves of sunflower (Helianthus annus L. “Mennonite”) were supplied ¹⁴CO₂ of constant specific radioactivity in gas mixtures containing various CO₂ and O₂ concentrations. The ¹⁴CO₂ and CO₂ fluxes were measured concurrently in an open system using an ionization chamber and infrared gas analyzer.     

Photosynthetic Units

Leaf tissues of aurea mutants of tobacco and Lespedeza have been shown to have higher photosynthetic capacity per molecule of chlorophyll, a higher saturation intensity, a simpler lamellar structure, and the same quantum yield as their dark green parents. Here we report on the values of photosynthetic units for both types of plants and some algae. The unit has been assumed to be about as uniform and steady in the plant world as the quantum efficiency. The number on which all theoretical discussions have been based so far is 2400 per O₂ evolved or CO₂ reduced. With dark green plants and algae our determinations of units by means of 40 µsec flashes superimposed on a steady rate of background photosynthesis at 900 ergs cm^-2 sec^-1 of red light yielded mostly numbers between 2000 and 2700. However, the photosynthetic unit turned out to be very variable, even in these objects. In aurea mutants the unit was distinctly smaller, averaging 600 chl/CO₂. By choosing the right combination of colors for flash and background light, units as low as 300 chl/CO₂ or 40 chl/e^- could be measured consistently. We found five well-defined groups of units composed of multiples of its smallest member. These new findings are discussed in terms of structural entities that double or divide under the influence of far-red light.     

Maintenance of High Photosynthetic Rates in Mesophyll Cells Isolated from Papaver somniferum

The establishment and maintenance of high rates of photosynthetic CO₂ incorporation in mesophyll cells of Papaver somniferum (opium poppy) depend on a regime of dark and light periods immediately following isolation, as well as carefully adjusted conditions of isolation. Analysis of the incorporation pattern of ¹⁴CO₂ by the isolated cells indicates an initial “stress-response” period of approximately 20 hours characterized by increased respiratory-type metabolism and diminished photosynthesis. Under the favorable regime, this period is followed by rapid recovery and the reinstatement of a metabolic state strikingly similar to that of intact leaves in which the initial rate of CO₂ incorporation is between 110 and 175 μmoles CO₂ fixed per mg chlorophyll per hour. The photosynthetic viability of these cells can be maintained for up to 80 hours.     

Two Systems for Concentrating CO(2) and Bicarbonate during Photosynthesis by Scenedesmus

Scenedesmus cells grown on high CO₂, when adapted to air levels of CO₂ for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO₂ from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O₂ evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO₂ pump with K_0.5 for O₂ evolution of less than 5 micromolar CO₂ was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO₃⁻ concentrating process between pH 8 to 10 lowered the K_0.5 (DIC) from 7300 micromolar HCO₃⁻ in high CO₂ grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO₃⁻ pump was inhibited by vanadate (K_i of 150 micromolar), as if it involved an ATPase linked HCO₃⁻ transporter. The CO₂ pump was formed on low CO₂ by high-CO₂ grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO₃⁻ pump was partially activated on low CO₂ within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO₃⁻ pump at pH 9 had requirements similar to the activation of the CO₂ pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO₂-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO₂ pump and in some algae an additional vanadate-sensitive plasmalemma HCO₃⁻ transporter for a HCO₃⁻ pump.     

Simultaneous Measurements of Steady State Chlorophyll a Fluorescence and CO(2) Assimilation in Leaves: The Relationship between Fluorescence and Photosynthesis in C(3) and C(4) Plants

Rates of CO₂ assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO₂ in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25°C. Electron transport activity for CO₂ assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O₂, and between fluorescence and rates of CO₂ assimilation at 19 millibars but not 200 millibars O₂. The values of fluorescence measured at about 220 microbars intercellular CO₂ were not greatly affected by increasing O₂ from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO₂ partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO₂ partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO₂ partial pressures which reversed to a positive correlation at higher values of CO₂ partial pressures. Similar results were observed in the C₃ species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C₄ species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO₂ assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (q_E) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed.     

Isolation of Guard Cell Protoplasts from Mechanically Prepared Epidermis of Vicia faba Leaves

A method for isolating guard cell protoplasts (GCP) from mechanically prepared epidermis of Vicia faba is described. Epidermis was prepared by homogenizing leaves in a Waring blender in a solution of 10% Ficoll, 5 millimolar CaCl₂, and 0.1% polyvinylpyrrolidone 40 (PVP). Attached mesophyll and epidermal cells were removed by shaking epidermis in a solution of Cellulysin, mannitol, CaCl₂, PVP, and pepstatin A. Cleaned epidermis was transferred to a solution of mannitol, CaCl₂, PVP, pepstatin A, cellulase “Onozuka” RS, and pectolyase Y-23 for the isolation of GCP. Preparations made by this method included both adaxial and abaxial GCP and contained ≤0.017% mesophyll protoplasts, ≤0.6% mesophyll fragments, and no epidermal cell contaminants. Yields averaged 9 × 10⁴ protoplasts/leaflet and 98 to 100% of the GCP excluded trypan blue, concentrated neutral red, and hydrolyzed fluorescein diacetate. Isolated GCP increased in diameter by 2.2 micrometers after incubation in darkness in 10 micromolar fusicoccin, 0.4 molar mannitol, 5 millimolar KCl, and 1 millimolar CaCl₂. Illumination of GCP with 800 micromoles per square meter per second of red light resulted in alkalinization of their suspension medium. When 10 micromolar per square meter per second of blue light was superimposed onto the red light background, the medium acidified. Measurements of chlorophyll a fast fluorescence transients from isolated GCP indicated that GCP were capable of electron transport, and slow transients contained the “M” peak usually associated with a functional photosynthetic carbon reduction pathway.     

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.     

Utilization of Exogenous Inorganic Carbon Species in Photosynthesis by Chlorella pyrenoidosa

The nature of the inorganic carbon utilized during photosynthesis by Chlorella pyrenoidosa was investigated using three experimental techniques (open gas analysis system with “artificial leaf” or “aqueous” chambers and O₂ electrode system) to measure carbon assimilation. Photosynthesis was studied as a function of pH and CO₂ concentration. The CO₂ concentration was inadequate to meet the requirements of photosynthesis only when HCO₃⁻ was added at high pH. Under all other conditions, the low and constant K_m (CO₂), in contrast to the highly variable K_m (HCO₃⁻), suggested that CO₂ was the major species utilized.     

Analysis of changes in minimal and maximal fluorescence yields with irradiance and o(2) level in tobacco leaf tissue

The responses of minimal and maximal fluorescence yields of chlorophyll a to irradiance of actinic white light were determined by pulse modulated fluorimetry in leaf discs from tobacco, Nicotiana tabacum, at 1.6, 20.5, and 42.0% (v/v) O₂. Steady-state maximal fluorescence yield (F_m′, measured during a saturating light pulse) declined with increasing irradiance at all O₂ levels. In contrast, the steady-state minimal fluorescence yield (F_o′, measured during a brief dark interval) increased with irradiance relative to that recorded for the fully dark-adapted leaf (F_o) or that observed after 5 minutes of darkness (F_o^*). The relative magnitude of this increase was somewhat greater and extended to higher irradiances at the elevated O₂ levels compared with 1.6% O₂. Suppression of F_o′ was only observed consistently at saturating irradiance. The results are interpreted in terms of the occurrence of photosystem II units possessing exceedingly slow turnover times (i.e. “inactive” units). Inactive units play an important role, along with thermal deactivation of excited chlorophyll, in determining the response of in vivo fluorescence yield to changes in irradiance. Also, a significant interactive effect of O₂ concentration and the presence or absence of far red light on oxidation of photosystem II acceptors in the dark was noted.     

Adaptation to CO(2) Level and Changes in the Phosphorylation of Thylakoid Proteins during the Cell Cycle of Chlamydomonas reinhardtii

The photosynthetic performance of synchronously grown Chlamydomonas reinhardtii alternated rhythmically during the cell cycle. The activity of the “CO₂ concentrating mechanism” including the ability to accumulate CO₂ internally and the activity of carbonic anhydrase peaked after 6 to 9 hours of light and reached minimum after 6 to 9 hours of dark. Consequently, the apparent photosynthetic affinity to extracellular CO₂ alternated rhythmically. At the end of the dark period the cells behaved as if they were adapted to high CO₂ even though they were continuously aerated with air. Results from experiments in which the light or dark periods were extended bear on the interaction between the internal (cell cycle or biological clock) and the external (light) signal. The observed rhythmical alterations in photosynthetic V_max may result from changes in PSII activity. The latter may be partly explained by the capacity for phosphorylation of thylakoid proteins, which reached maximum after 9 hours of light and decreased toward the dark period.     

Evidence for Light-Stimulated Fatty Acid Synthesis in Soybean Fruit

In leaves, the light reactions of photosynthesis support fatty acid synthesis but disagreement exists as to whether this occurs in green oilseeds. To address this question, simultaneous measurements of the rates of CO₂ and O₂ exchange (CER and OER, respectively) were made in soybean (Glycine max L.) fruits. The imbalance between CER and OER was used to estimate the diverted reductant utilization rate (DRUR) in the equation: DRUR = 4 × (OER + CER). This yielded a quantitative measure of the rate of synthesis of biomass that is more reduced per unit carbon than glucose (in photosynthesizing tissues) or than the substrates of metabolism (in respiring tissues). The DRUR increased by about 2.2-fold when fruits were illuminated due to a greater increase in OER than decrease in CER. This characteristic was shown to be a property of the seed (not the pod wall), to be present in fruits at all developmental stages, and to reach a maximal response at relatively low light. When seeds were provided with ¹³CO₂, light reduced ¹²CO₂ production but had little effect on ¹³CO₂ fixation. When they were provided with ¹⁸O₂, light stimulated ¹⁶O₂ production but had no effect on ¹⁸O₂ uptake. Together, these findings indicate that light stimulates fatty acid synthesis in photosynthetic oilseeds, probably by providing both ATP and carbon skeletons.     

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.     

Relationship between Respiration and Photosynthesis in Guard Cell and Mesophyll Cell Protoplasts of Commelina communis L

A mass spectrometric method combining ¹⁶O/¹⁸O and ¹²C/¹³C isotopes was used to quantify the unidirectional fluxes of O₂ and CO₂ during a dark to light transition for guard cell protoplasts and mesophyll cell protoplasts of Commelina communis L. In darkness, O₂ uptake and CO₂ evolution were similar on a protein basis. Under light, guard cell protoplasts evolved O₂ (61 micromoles of O₂ per milligram of chlorophyll per hour) almost at the same rate as mesophyll cell protoplasts (73 micromoles of O₂ per milligram of chlorophyll per hour). However, carbon assimilation was totally different. In contrast with mesophyll cell protoplasts, guard cell protoplasts were able to fix CO₂ in darkness at a rate of 27 micromoles of CO₂ per milligram of chlorophyll per hour, which was increased by 50% in light. At the onset of light, a delay observed for guard cell protoplasts between O₂ evolution and CO₂ fixation and a time lag before the rate of saturation suggested a carbon metabolism based on phosphoenolpyruvate carboxylase activity. Under light, CO₂ evolution by guard cell protoplasts was sharply decreased (37%), while O₂ uptake was slowly inhibited (14%). A control of mitochondrial activity by guard cell chloroplasts under light via redox equivalents and ATP transfer in the cytosol is discussed. From this study on protoplasts, we conclude that the energy produced at the chloroplast level under light is not totally used for CO₂ assimilation and may be dissipated for other purposes such as ion uptake.