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)     

Quantum Yields for CO(2) Uptake in C(3) and C(4) Plants: Dependence on Temperature, CO(2), and O(2) Concentration

The quantum yields of C₃ and C₄ plants from a number of genera and families as well as from ecologically diverse habitats were measured in normal air of 21% O₂ and in 2% O₂. At 30 C, the quantum yields of C₃ plants averaged 0.0524 ± 0.0014 mol CO₂/absorbed einstein and 0.0733 ± 0.0008 mol CO₂/absorbed einstein under 21 and 2% O₂. At 30 C, the quantum yields of C₄ plants averaged 0.0534 ± 0.0009 mol CO₂/absorbed einstein and 0.0538 ± 0.0011 mol CO₂/absorbed einstein under 21 and 2% O₂. At 21% O₂, the quantum yield of a C₃ plant is shown to be strongly dependent on both the intercellular CO₂ concentration and leaf temperature. The quantum yield of a C₄ plant, which is independent of the intercellular CO₂ concentration, is shown to be independent of leaf temperature over the ranges measured. The changes in the quantum yields of C₃ plants are due to changes in the O₂ inhibition. The evolutionary significance of the CO₂ dependence of the quantum yield in C₃ plants and the ecological significance of the temperature effects on the quantum yields of C₃ and C₄ plants are discussed.     

Environmental Regulation of CO2 Exchange Pattern in Facultative CAM Plants

Three facultative CAM plants, Sedum spectabile, S. aizoon and Mesembryanthemum cordifolium, could take up CO2 throughout the night and daytime, and no phase Ill was observed during cloudy weather. The CO2 exchange patterns during cloudy day differed obviously from that during sunny day. But in the obligate CAM plants, Kalanchoe daigremontiana, Orostachys fimbriatus and Bryophyllum pinnatum, there were phase Ⅲ during cloudy day. These results showed that the COs exchange patterns with uptake of CO2 throughout the night and daytime were universal in facultative CAM plants during cloudy day, but not in obligate CAM plants, of which the CO2 exchange patterns were very stable. In the three facultative CAM plants, the difference of exchange patterns between cloudy and sunny days depended mainly on temperature change. The effect of the temperature on CO2 exchange patterns was mediated by the decarboxylation rate. At high temperature, the decarboxylation rate could be enhanced. It was found that the accumulation of malic acid at night in the three obligate CAM plants was much more than that in the three facultative C AM plants. So during cloudy day, the decarboxytion rate in the three obligate CAM plants was also higher. This might be an important cause that obligate CAM plants need not to take up CO2 during the daytime.     

Partitioning of Noncyclic Photosynthetic Electron Transport to O(2)-Dependent Dissipative Processes as Probed by Fluorescence and CO(2) Exchange

The partitioning of noncyclic photosynthetic electron transport between net fixation of CO₂ and collective O₂-dependent, dissipative processes such as photorespiration has been examined in intact leaf tissue from Nicotiana tabacum. The method involves simultaneous application of CO₂ exchange and pulse modulated fluorescence measurements. As either irradiance or CO₂ concentration is varied at 1% O₂ (i.e. absence of significant O₂-dependent electron flow), the quantum efficiency of PSII electron transport (_se) with CO₂ as the terminal acceptor is a linear function of the ratio of photochemical:nonphotochemical fluorescence quenching coefficients (i.e. q_Q:q_NP). When the ambient O₂ concentration is raised to 20.5% or 42% the q_Q:q_NP is assumed to predict the quantum efficiency of total noncyclic electron transport (′_se). A factor which represents the proportion of electron flow diverted to the aforementioned dissipative processes is calculated as (′_se − _se)/′_se where _se is now the observed quantum efficiency of electron transport in support of net fixation of CO₂. Examination of changes in electron allocation with CO₂ and O₂ concentration and irradiance at 25°C provides a test of the applicability of the Rubisco model to photosynthesis in vivo.     

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 and Hydrogen Isotope Ratios of Water from Photosynthetic Tissues of CAM and C(3) Plants

Water samples from photosynthetic tissues of C₃ and Crassulacean acid metabolism (CAM) plants that grew together in the field were extracted and the stable oxygen and hydrogen isotope ratios determined. During the day, ¹⁸O/¹⁶O and deuterium/hydrogen (D/H) ratios of water from CAM plants were lower than those observed in water from C₃ plants. The patterns of diurnal variation (or lack thereof) in isotope ratios of plant water are consistent with the gross anatomical and physiological characteristics of the plants studied here. Our observations support the previously advanced hypothesis that high D/H ratios in cellulose nitrate prepared from CAM plants relative to those for C₃ plants are not caused by greater deuterium enrichment in the water in CAM plants, but rather by isotopic fractionations associated with different biochemical reactions in the two types of plants.     

Photosynthetic Pigments and Gas Exchange of in vitro Grown Tobacco Plants as Affected by CO2 Supply

Contents and functioning of photosynthetic pigments and gas exchange of Nicotiana tabacum L. leaves were studied in platlets cultivated in vitro under different CO₂ supply. The plantlets were grown for six weeks either in glass vessels tightly closed with aluminium foil (G-plants) or in polycarbonate Magenta GA-7 vessels covered with closures with microporous vents (M-plants). M-plants (better supplied with CO₂) had higher contents of chlorophyll (Chl) a. Chl b. and β-carotene, higher photochemical activities of photosystem 2 and whole electron transport chain, and lower contents of xanthophyll cycle pigments. Differences in Chl a fluorescence kinetic parameters between G-plants and M-plants were not statistically significant. M-plants had higher net photosynthetic rate, and lower transpiration rate and stomatal conductance than G-plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of CO(2) and O(2) on the Photosynthetic O(2) Evolution of Spirodela polyrrhiza Turions

Net photosynthetic rates of Spirodela polyrrhiza turions, at low O₂ levels, were 6.2 and 38.8 micromoles O₂ per gram fresh weight per hour at 1 millimolar HCO₃⁻ and CO₂ saturation, respectively, and much lower in a regular low-pH growth solution. Air equilibration O₂ concentrations decreased rates considerably, except at CO₂ saturation. The surfacing rate of turions in various inorganic carbon surroundings correlated positively with their photosynthetic rates, but were the same at high and low O₂ levels. The relevance of these findings in relation to environmental conditions conductive to germination of autotrophically growing turions is discussed.     

The Identification of Photosynthetic Subpathways of Some C4 and CAM Plants

The photosynthetic subpathways of five C4 plants and one CAM plant were distinguished according to their chemical, physiological and cytological characteristics. Based on C4 acid decarboxylation enzymes, four C4 plants of Setaria glauca, Sporobolus indicus, Zoysia tenuifolia and Leptochloa chinensis all exhibited the functional high activities of PEP carboxykinase and aspartate aminotransferase as seen in the known PEP-CK subtype. The δ13C value of –12.43% in leaves of L. chinensis was also consistent with that range among PEP-CK subtype. So, these species were classified into PEP-CK subtype. However, their chloroplasts in bundle sheath cells were evenly distributed, not as that displayed centrifugally or centripetally in three typical subtypes. The even arrangement of chloroplasts in bundle sheath cells was likely to be an evolutional intermediate from centripetal (NAD ME type) to centrifugal types (NADP-ME and most PEP-CK types). The high activities of NAD-malic enzyme and aspartate aminotransferase, accompanied with the centripetally located chloroplasts, 0.057 of quantum yield and tile δ13C value of –15.3% in leaves of C4 dicot Euphobia hirta indicated characteristics of NAD-ME subtype. Moreover, CAM plant Aloe vera clearly fell into PEP-CK sybtype because of its high activity of PEP-CK both in whole leaf and green tissue.     

Photosynthetic Gas Exchange and Discrimination against 13CO2 and C18O16O in Tobacco Plants Modified by an Antisense Construct to Have Low Chloroplastic Carbonic Anhydrase

The physiological role of chloroplastic carbonic anhydrase (CA) was examined by antisense suppression of chloroplastic CA (on average 8% of wild type) in Nicotiana tabacum. Photosynthetic gas-exchange characteristics of low-CA and wild-type plants were measured concurrently with short-term, on-line stable isotope discrimination at varying vapor pressure deficit (VPD) and light intensity. Low-CA and wild-type plants were indistinguishable in the responses of assimilation, transpiration, stomatal conductance, and intercellular CO2 concentration to changing VPD or light intensity. At saturating light intensity, low-CA plants had lower discrimination against 13CO2 than wild-type plants by 1.2 to 1.8[per mille (thousand) sign]. Consequently, tissue of the low-CA plants was higher in 13C than the control plants. It was calculated that low-CA plants had chloroplast CO2 concentrations 13 to 22 [mu]mol mol-1 lower than wild-type plants. Discrimination against C18O16O in low-CA plants was 20% of that of the wild type, confirming a role of chloroplastic CA in the mechanism of discrimination against C18O16O ([delta]C18O16O). As VPD increased, stomatal closure caused a reduction in chloroplastic C02 concentration, and since VPD and chloroplastic CO2 concentration act in opposing directions on [delta]C18O16O, no effect of VPD was seen on [delta]C18O16O.     

Correlation of alpha-Linolenate to Photosynthetic O(2) Production in Chlorella

Photosynthetic oxygen evolution per milligram of chlorophyll in Chlorella vulgaris varies with the age of the culture. The rate of oxygen evolution is low in the starting cells, it rises to a maximum after 24 hours of growth and then declines to the initial low value after 72 to 90 hours. These changes in photosynthetic competence of chlorophyll in Chlorella are paralleled by changes in α-linolenate per milligram of chlorophyll. In general the magnitude of the photosynthetic competence of chlorophyll is directly proportional to the magnitude of the ratio of α-linolenate to chlorophyll, regardless of whether high ratios are due to high α-linolenates or low chlorophyll values. This relationship holds when the cultures are grown either under continuous or intermittent illumination.     

Involvement of Photosynthetic Carbon Reduction Cycle Intermediates in CO(2) Fixation and O(2) Evolution by Isolated Chloroplasts

The photosynthetic carbon reduction cycle intermediates can be divided into three classes according to their effects on the rate of photosynthetic CO₂ evolution by whole spinach (Spinacia oleracea) chloroplasts and on their ability to affect reversal of certain inhibitors (nigericin, arsenate, arsenite, iodoacetate, antimycin A) of photosynthesis: class I (maximal): fructose 1, 6-diphosphate, dihydroxyacetone phosphate, glyceraldehyde-3-phosphate, ribose-5-phosphate; class 2 (slight): glucose 6-phosphate, fructose 6-phosphate, ribulose-1, 5-diphosphate; class 3 (variable): glycerate 3-phosphate. While class 1 compounds influence the photosynthetic rate, they do not lower the Michaelis constant of the chloroplast for bicarbonate or affect strongly other photosynthetic properties such as the isotopic distribution pattern. It was concluded that the class 1 compounds influence the chloroplast by not only supplying components to the carbon cycle but also by activating or stabilizing a structural component of the chloroplast.     

Photosynthetic utilization of radiant energy by CAM Dendrobium flowers

14CO2 fixation was observed in orchid Dendrobium flowers; its rate decreased with the flower development. Chlorophyll (Chl) fluorescence in different developmental stages of flowers was compared to other green plant parts (leaf, inflorescence stalk, and fruit capsule). The photochemical efficiency of photosystem 2 (PS2) (Fv/Fm) of a leaf was 14-21 % higher than that of a mature flower perianth (sepal, petal, and labellum) which had a much lower total Chl content and Chl a/b ratio. A higher quantum yield of PS2 (ΦPS2) than in the mature flowers was observed in all green parts. Flower sepals had higher Chl content, Chl a/b ratio, and Fv/Fm values than the petal and labellum. During flower development the Chl content, Chl a/b ratio, Fv/Fm, and qN decreased while ΦPS2 and qP remained constant. An exposure of developing flowers to irradiances above 50 µmol m-2 s-1 resulted in a very drastic drop of ΦPS2 and qP, and a coherent increase of qN as compared to other green plant organs. A low saturation irradiance (PFD of 100 µmol m-2 s-1) and the increase in qN in the flower indicate that irradiation stress may occur since there is no further protection when the flower is exposed to irradiances above 100 µmol m-2 s-1. A low Chl/carotenoid ratio in mature flower perianth as a consequence of Chl content reduction in the course of flower development suggests a relief of irradiation stress via this mean.     

Exchange of (18)O in the reaction of peroxynitrite with CO(2)

We have observed an exchange of (18)O in the reactions of CO(2) with peroxynitrite using membrane-inlet mass spectrometry and HPLC negative electrospray ionization mass spectrometry. The exchange appeared on addition of peroxynitrite to a solution containing (18)O-labeled CO(2) in equilibrium with bicarbonate. It was observed as a temporarily enhanced rate of depletion of (18)O from CO(2), a rate that was greater than the rate of (18)O depletion caused by the hydration/dehydration cycle of CO(2). In addition, we detected the appearance of mass peaks attributed to (18)O in product NO(3)(-).As a further measure of the (18)O exchange, there was a redistribution of (18)O such that the ratio of doubly to singly labeled CO(2) could not be described by the binomial expansion. This is not due to the hydration/dehydration cycle of CO(2) but most likely to recycling of CO(2) in the reaction with peroxynitrite. This (18)O exchange associated with the reactions of CO(2) and peroxynitrite may open a new methodology for studying this significant process.     

H2O2-Induced Inhibition of Photosynthetic O2 Evolution by Anabaena variabilis Cells

Hydrogen peroxide inhibits photosynthetic O2 evolution. It has been shown that H2O2 destroys the function of the oxygen-evolving complex (OEC) in some chloroplast and Photosystem (PS) II preparations causing release of manganese from the OEC. In other preparations, H2O2 did not cause or caused only insignificant release of manganese. In this work, we tested the effect of H2O2 on the photosynthetic electron transfer and the state of OEC manganese in a native system (intact cells of the cyanobacterium Anabaena variabilis). According to EPR spectroscopy data, H2O2 caused an increase in the level of photooxidation of P700, the reaction centers of PS I, and decreased the rate of their subsequent reduction in the dark by a factor larger than four. Combined effect of H2O2, CN-, and EDTA caused more than eight- to ninefold suppression of the dark reduction of P700+. EPR spectroscopy revealed that the content of free (or loosely bound) Mn2+ in washed cyanobacterial cells was ~20% of the total manganese pool. This content remained unchanged upon the addition of CN- and increased to 25-30% after addition of H2O2. The content of the total manganese decreased to 35% after the treatment of the cells with EDTA. The level of the H2O2-induced release of manganese increased after the treatment of the cells with EDTA. Incubation of cells with H2O2 for 2 h had no effect on the absorption spectra of the photosynthetic pigments. More prolonged incubation with H2O2 (20 h) brought about degradation of phycobilins and chlorophyll a and lysis of cells. Thus, H2O2 causes extraction of manganese from cyanobacterial cells, inhibits the OEC activity and photosynthetic electron transfer, and leads to the destruction of the photosynthetic apparatus. H2O2 is unable to serve as a physiological electron donor in photosynthesis.     

Photosynthetic utilization of radiant energy by CAM Dendrobium flowers

14CO2 fixation was observed in orchid Dendrobium flowers; its rate decreased with the flower development. Chlorophyll (Chl) fluorescence in different developmental stages of flowers was compared to other green plant parts (leaf, inflorescence stalk, and fruit capsule). The photochemical efficiency of photosystem 2 (PS2) (Fv/Fm) of a leaf was 14-21 % higher than that of a mature flower perianth (sepal, petal, and labellum) which had a much lower total Chl content and Chl a/b ratio. A higher quantum yield of PS2 (PS2) than in the mature flowers was observed in all green parts. Flower sepals had higher Chl content, Chl a/b ratio, and Fv/Fm values than the petal and labellum. During flower development the Chl content, Chl a/b ratio, Fv/Fm, and qN decreased while PS2 and qP remained constant. An exposure of developing flowers to irradiances above 50 µmol m-2 s-1 resulted in a very drastic drop of PS2 and qP, and a coherent increase of qN as compared to other green plant organs. A low saturation irradiance (PFD of 100 µmol m-2 s-1) and the increase in qN in the flower indicate that irradiation stress may occur since there is no further protection when the flower is exposed to irradiances above 100 µmol m-2 s-1. A low Chl/carotenoid ratio in mature flower perianth as a consequence of Chl content reduction in the course of flower development suggests a relief of irradiation stress via this mean.     

Diffusional Contribution to Carbon Isotope Fractionation during Dark CO(2) Fixation in CAM Plants

A mathematical model is developed which can be used to predict in vivo carbon isotope fractionations associated with carbon fixation in plants in terms of diffusion, CO₂ hydration, and carboxylation components. This model also permits calculation of internal CO₂ concentration for comparison with results of gas-exchange experiments. The isotope fractionations associated with carbon fixation in Kalanchoë daigremontiana and Bryophyllum tubiflorum have been measured by isolation of malic acid following dark fixation and enzymic determination of the isotopic composition of carbon-4 of this material. Corrections are made for residual malic acid, fumarase activity, and respiration. Comparison of these data with calculations from the model indicates that the rate of carbon fixation is limited principally by diffusion, rather than by carboxylation. Processes subsequent to the initial carboxylation also contribute to the over-all isotopic composition of the plant.     

Electron Transport-Dependent Chlorophyll-a Fluorescence Quenching by O(2) in Various Algae and Higher Plants

A comparison of chlorophyll-a fluorescence in brown algae (Macrocystis integrifolia, Fucus vesiculosis), green algae (Scenedesmus obliquus, Ulva sp.) and higher plants (bean, corn) show differences in the relative fluorescence intensities and induction time courses which characterize each type of plant. These differences are not reflected in either the maximum fluorescence emission in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (F_max) or the nonvariable fluorescence (F_o). Constancy of F_o and F_max suggests functional similarities of photosystem II and associated antennae pigments in the various classes of plants. The time course differences are observed only in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and appear, therefore, to be electron transport dependent. During induction, the peak in fluorescence (F_p) is much lower in all of the algae studied than in the higher plants. Exogenous O₂ strongly quenches F_p in all plants studied and our data indicate that the low F_p in the algae can be partially accounted for by endogenous O₂ quenching.     

Joint Action of O(3) and SO(2) in Modifying Plant Gas Exchange

The joint action of O₃ and SO₂ stress on plants was investigated by determining the quantitative relationship between air pollutant fluxes and effects on stomatal conductance. Gas exchange measurements of O₃, SO₂, and H₂O vapor were made for Pisum sativum L. (garden pea). Plants were grown under controlled environments, and O₃, SO₂, and H₂O vapor fluxes were evaluated with a whole-plant gas exchange chamber using the mass-balance approach. Maximum O₃ and SO₂ fluxes per unit area (2 sided) into leaves averaged 8 nanomoles per square meter per second with exposure to either O₃ or SO₂ at 0.1 microliters per liter. Internal fluxes of either O₃ or SO₂ were reduced by up to 50% during exposure to combined versus individual pollutants; the greatest reduction occurred with simultaneous versus sequential combinations of the pollutants. Stomatal conductance to H₂O was substantially altered by the pollutant exposures, with O₃ molecules twice as effective as SO₂ molecules in inducing stomatal closure. Stomatal conductance was related to the integrated dose of pollutants. The regression equations relating integrated dose to stomatal conductance were similar with O₃ alone, O₃ plus added SO₂, and O₃ plus SO₂ simultaneously; i.e. a dose of 100 micromoles per square meter produced a 39 to 45% reduction in conductance over nonexposed plants. With SO₂ alone, or SO₂ plus added O₃, a dose of 100 micromoles per square meter produced a 20 to 25% reduction in conductance. When O₃ was present at the start of the exposure, then stomatal response resembled that for O₃ more than the response for SO₂. This study indicated that stomatal responses with combinations of O₃ and SO₂ are not dependent solely on the integrated dose of pollutants, but suggests that a metabolic synergistic effect exists.