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PubMed1974

Prechilling of Xanthium strumarium L. Reduces Net Photosynthesis and, Independently, Stomatal Conductance, While Sensitizing the Stomata to CO(2).

Greenhouse-grown plants of Xanthium strumarium L. were exposed in a growth cabinet to 10 C during days and 5 C during nights for periods of up to 120 hours. Subsequently, CO(2) exchange, transpiration, and leaf temperature were measured on attached leaves and in leaf sections at 25 or 30 C, 19 C dew point of the air, 61 milliwatts per square centimeter irradiance, and CO(2) concentrations between 0 and 1000 microliters per liter ambient air. Net photosynthesis and stomatal conductance decreased and dark respiration increased with increasing duration of prechilling. The reduction in net photosynthesis was not a consequence of decreased stomatal conductance because the intercellular CO(2) concentration in prechilled leaves was equal to or greater than that in greenhouse-grown controls. The intercellular CO(2) concentration at which one-half maximum net photosynthesis occurred remained the same in prechilled leaves and controls (175 to 190 microliters per liter). Stomata of the control plants responded to changes in the CO(2) concentration of the air only slightly. Prechilling for 24 hours or more sensitized stomata to CO(2); they responded to changes in CO(2) concentration in the range from 100 to 1000 microliters per liter.

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PubMed1974

Influence of assimilate demand on photosynthesis, diffusive resistances, translocation, and carbohydrate levels of soybean leaves.

Rates of net photosynthesis and translocation, CO(2) diffusive resistances, levels of carbohydrates, total protein, chlorophyll, and inorganic phosphate, and ribulose 1,5-diphosphate carboxylase activity were measured in soybean (Glycine max L. Merrill) leaves to ascertain the effect of altered assimilate demand. To increase assimilate demand, the pods, stems, and all but one leaf (the "source leaf") of potted plants were completely shaded for 6 or 8 days and the responses of the illuminated source leaf were monitored. Rate of net photosynthesis in the source leaf of the shaded plants was found to increase curvilinearly to a maximum on the 8th day. The source leaf of the control plants (no sink shading) maintained a constant photosynthetic rate during this period. Vapor-phase resistance to CO(2) diffusion did not vary with treatment, but mesophyll (liquid phase) resistance was significantly lower in the source leaf of the shaded plants.Starch concentration in the source leaf of shaded plants decreased more than 10-fold during the 8-day shading period. In this same period, sucrose concentration rose nearly 3-fold. Conversely, in the source leaf of the unshaded plants, starch concentration remained high (23% of leaf dry weight) and sucrose concentration remained very low (1.2%). When measured on the 8th day of treatment, translocation rate, ribulose 1,5-diphosphate carboxylase activity, and inorganic phosphate concentration were found to be significantly higher in the source leaf of the shaded plants than in the control source leaf.When shaded plants were again illuminated, all measured response trends in the source leaf were reversed. These data indicate that assimilate demand has a marked influence on source-leaf photosynthesis and carbohydrate formation and export.

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PubMed1975

Oxygen Isotope Fractionation during Photosynthesis in a Blue-Green and a Green Alga.

Oxygen isotope fractionation ((18)O/(16)O) at the natural abundance level has been measured during photosynthesis of a blue-green and a green alga. When sufficient attention is paid to removal of contaminating air O(2) before and during the experiments, then the photosynthetic O(2) evolved, as compared to the water O(2), had an average difference of -0.36% for a blue-green alga and -0.80% for a green alga. These experiments suggest that there is no reason to invoke an inverse isotope effect in photosynthesis as part of the explanation for the (18)O enrichment in atmospheric O(2) relative to O(2) in oceanic waters. In addition, in an indirect way, the experiments also support the argument that the bulk of O(2) evolved during photosynthesis comes from water. A 10% contribution of O(2) arising from CO(2) would have been detectable in the present work.

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PubMed1976

Analysis of steady state photosynthesis in alfalfa leaves.

A method for carrying out kinetic tracer studies of steady state photosynthesis in whole leaves has been developed. An apparatus that exposes whole leaves to (14)CO(2) under steady state conditions, while allowing individual leaf samples to be removed as a function of time, has been constructed. Labeling data on the incorporation of (14)C into Medicago sativa L. metabolite pools are reported. A carbon dioxide uptake rate of 79 micromoles (14)CO(2) per milligram chlorophyll per hour was observed at a CO(2) level slightly below that of air. Several actively turning over pools of early and intermediate metabolites, including 3-phosphoglyceric acid, glycerate, citrate, and uridine diphosphoglucose, showed label saturation after approximately 10 to 20 minutes of photosynthesis with (14)CO(2) under steady state conditions. Alanine labeling increased more rapidly at first, and then at a lower rate as saturation was approached. Sucrose was a major product of photosynthesis and label saturation of the sucrose pool was not observed. Labeled carbon appeared rapidly in secondary metabolites. The steady state apparatus used has numerous advantages, including leaf temperature control, protection against leaf dehydration, high illumination, known (14)CO(2) specific radioactivity, and provision for control and adjustment of (14)CO(2) concentration. The apparatus allows for experiments of long duration and for sufficient sample points to define clearly the metabolic steady state.

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PubMed1976

Photosynthesis, Dark Respiration, and Growth of Rumex patientia L. Exposed to Ultraviolet Irradiance (288 to 315 Nanometers) Simulating a Reduced Atmospheric Ozone Column.

Net photosynthesis, dark respiration, and growth of Rumex patientia L. exposed to a ultraviolet irradiance (288-315 nanometers) simulating a 0.18 atm.cm stratospheric ozone column were determined. The ultraviolet irradiance corresponding to this 38% ozone decrease from normal was shown to be an effective inhibitor of photosynthesis and leaf growth. The repressive action on photosynthesis accumulated through time whereas leaf growth was retarded only during the initial few days of exposure. Small increases in dark respiration rates occurred but did not continue to increase with longer exposure periods. A reduction in total plant dry weight and leaf area of approximately 50% occurred after 22 days of treatment, whereas chlorophyll concentrations remained unaltered.

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PubMed1978

Oxygen effects on photosynthesis and C metabolism in desert plants.

The effect of 1% and 21% O(2) upon (14)CO(2) assimilation by desert plants exposed for 10 to 90 seconds has been studied. The plants studied can be divided into three groups with respect to O(2). The C(3) plants display the usual Warburg effect. No changes could be observed in the intensity of photosynthesis as a function of O(2) content in another group of plants (showing signs of Crassulacean acid metabolism). In still another group of plants (C(4) plants) the stimulating effect of O(2) on photosynthesis could be detected. In C(3) plants, O(2) inhibits the processing of carbon through the Calvin cycle intermediates. The involvement of carbon in the glycolate pathway fails to explain completely the inhibiting effect of O(2) on photosynthesis. It is assumed that O(2) inhibits the enzymes of the Calvin cycle. In C(4) plants O(2) stimulates the incorporation of (14)C into malate and aspartate. The incorporation of (14)C into the intermediates of the Calvin cycle in C(4) plants is inhibited much like that in typical C(3) plants.

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PubMed1978

Effect of Glyoxylate on the Sensitivity of Net Photosynthesis to Oxygen (the Warburg Effect) in Tobacco.

The addition of glyoxylate to tobacco (Nicotiana tabacum) leaf discs inhibited glycolate synthesis and photorespiration and increased net photosynthetic (14)CO(2) fixation. This inhibition of photorespiration was investigated further by studying the effect of glyoxylate on the stimulation of photosynthesis that occurs when the atmospheric O(2) level was decreased from 21 to 3% (the Warburg effect). The Warburg effect is usually ascribed to the increased glycolate synthesis and metabolism that occurs at higher O(2) concentrations. Photosynthesis in control discs increased from 59.1 to 94.7 micromoles of CO(2) per gram fresh weight per hour (a 60% increase) when the O(2) level was lowered from 21 to 3%, while the rate for discs floated on 15 millimolar glyoxylate increased only from 82.0 to 99.7 micromoles of CO(2) per gram fresh weight per hour (a 22% increase). The decrease in the O(2) sensitivity of photosynthesis in the presence of glyoxylate was explained by changes in the rate of glycolate synthesis under the same conditions.The rate of metabolism of the added glyoxylate by tobacco leaf discs was about 1.35 micromoles per gram fresh weight per hour and was not dependent on the O(2) concentration in the atmosphere. This rate of metabolism is about 10% the amount of stimulation in the rate of CO(2) fixation caused by the glyoxylate treatment on a molar carbon basis. Glyoxylate (10 millimolar) had no effect on the carboxylase/oxygenase activity of isolated ribulose diphosphate carboxylase. Although the biochemical mechanism by which glyoxylate inhibits glycolate synthesis and photorespiration and thereby decreases the Warburg effect is still uncertain, these results show that cellular metabolites can regulate the extent of the Warburg effect.

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PubMed1979

Photosynthesis and photorespiration in whole plants of wheat.

Wheat was cultivated in a small phytotronic chamber. (18)O(2) was used to measure the O(2) uptake by the plant, which was recorded simultaneously with the O(2) evolution, net CO(2) uptake, and transpiration. At normal atmospheric CO(2) concentration, photorespiration, measured as O(2) uptake, was as important as the net photosynthesis. The level of true O(2) evolution was independent of CO(2) concentration and stayed nearly equal to the sum of net CO(2) photosynthesis and O(2) uptake. We conclude that at a given light intensity, O(2) and CO(2) compete for the reducing power produced at constant rate by the light reactions of photosynthesis.

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PubMed1979

Net Photosynthesis and Early Growth Trends of a Dominant White Oak (Quercus alba L.).

Examination of the relationship between photosynthesis and growth of a dominant white oak (Quercus alba L.) tree has shown that most growth processes were either completed or well underway before the establishment of significant positive rates of net photosynthesis. Growth was initiated first in the root system (March 3), followed by stem cambial growth (March 26) and later by flower, leaf, and branch growth (April 10). During the period of rapid leaf and branch growth, root and cambial growth ceased and then resumed as the leaves approached maturity. The rapid rate of leaf maturation, the early appearance of positive rates of net photosynthesis in leaves (15% of final size) and the CO(2)-refixing capability of elongating branch tissue reduced the period of time that this white oak tree was dependent on stored reserves. Lower temperature optima and compensation points in developing leaves and stems indicated that the growth-temperature response was optimized for the lower seasonal temperatures observed during the spring. This temperature adaptation further reduced the time that this tree was dependent on stored reserves.

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PubMed1980

Photosynthesis in Grass Species Differing in Carbon Dioxide Fixation Pathways: III. OXYGEN RESPONSE AND ENZYME ACTIVITIES OF SPECIES IN THE LAXA GROUP OF PANICUM.

Measurements of CO(2) exchange at varying O(2) concentrations in seven grass species of the Laxa group of Panicum and activities of five photosynthetic enzymes were compared to values obtained for these characters in a cool season C(3) grass, tall fescue (Festuca arundinacea Schreb.) and a C(4) grass, P. maximum Jacq. Plants were divided into three groups on the basis of the inhibition of apparent photosynthesis by 21% O(2.) Rates of apparent photosynthesis in P. prionitis Griseb. and P. maximum were virtually unaffected by changes in O(2) concentration. In another group consisting of P. hylaeicum Mez., P. rivulare Trin., P. laxum Sw., and tall fescue apparent photosynthesis was inhibited by 28.2 to 36.0% at 21% O(2.) An intermediate inhibition of 20.6 to 23.3% at 21% O(2) was exhibited by P. milioides Nees ex Trin., P. schenckii Hack., and P. decipiens Nees ex Trin. The CO(2) compensation concentration for P. prionitis and P. maximum was low (</=6 microliters per liter CO(2) at 21% O(2)) and affected little by O(2), whereas values for P. hylaeicum, P. rivulare, P. laxum, and tall fescue were much greater, and increased almost linearly from 2 to 48% O(2.) Values for P. milioides, P. schenckii, and P. decipiens were intermediate to the other two groups. The effect of O(2) on total leaf conductance to CO(2) was similar to the C(3) grasses and the intermediate Panicums. However, estimates of photorespiration in the intermediate species were low and changed little with O(2) in comparison to estimates for the C(3) species which were higher and increased greatly with increased O(2.)Activities of phosphoenolpyruvate carboxylase were greatest in P. maximum and P. prionitis and one-fourth or less in the remaining species. Activity of ribulose bisphosphate carboxylase was 548 micromoles per mg chlorophyll per hour in tall fescue; activity in the remaining species was approximately one-fourth or less of that in tall fescue, with the exception of P. rivulare, in which it was 440 micromoles per milligram chlorphyll per hour. High activities of two C(4) decarboxylating enzymes, phosphoenolpyruvate carboxykinase and NADP-malic enzyme, were observed in P. maximum (1,988 micromoles per milligram chlorophyll per hour) and P. prionitis (125 micromoles per milligram chlorophyll per hour), respectively. Only minimal activities of decarboxylating enzymes were detected in the remaining species.

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PubMed1980

Effects of Ultraviolet-B Irradiances on Soybean: II. INTERACTION BETWEEN ULTRAVIOLET-B AND PHOTOSYNTHETICALLY ACTIVE RADIATION ON NET PHOTOSYNTHESIS, DARK RESPIRATION, AND TRANSPIRATION.

Soybean plants (cv. Hardee) were grown from seed under four ultraviolet-B radiation flux densities and four photosynthetically active radiation levels in a factorial design. Net photosynthesis, dark respiration, and transpiration were measured after 2 and 6 weeks of exposure. Effects of ultraviolet-B radiation were dependent upon photosynthetically active radiation levels. Ultraviolet-B radiation adversely affected net photosynthesis at low photosynthetically active radiation levels, but had little consequence at levels normally saturating photosynthesis in the field. Ultraviolet-B radiation affected both stomatal and nonstomatal resistances to carbon dioxide under low levels of photosynthetically active radiation. The present study demonstrates interactions between ultraviolet-B and photosynthetically active radiation.

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PubMed1980

Photosynthesis in Isolated Chloroplasts of the Crassulacean Acid Metabolism Plant Sedum praealtum.

Intact chloroplasts were isolated from protoplasts of the Crassulacean acid metabolism plant Sedum praealtum D.C. Typical rates of CO(2) fixation or CO(2)-dependent O(2) evolution ranged from 20 to 30 micromoles per milligram chlorophyll per hour and could be stimulated 30 to 50% by several Calvin cycle intermediates. The pH optimum for CO(2) fixation was 7.0 to 7.6 with considerable activity as low as pH 6.4. Low concentrations of orthophosphate (Pi) (optimum 0.4 millimolar) stimulated photosynthesis while high concentrations (5 millimolar) caused some inhibition. Both CO(2) fixation and CO(2)-dependent O(2) evolution exhibited a relatively long lag phase (4 to 6 minutes) which remained constant between 0.4 to 5 millimolar Pi. The lag phase could be decreased by addition of dihydroxyacetone-phosphate or ribose 5-phosphate. Further results are presented which suggest these chloroplasts have a functional phosphate translocator.A long lag period at optimum Pi concentration, no effect of high Pi levels on the lag phase, lack of severe inhibition of photosynthesis by high Pi concentrations, and the relatively low pH optimum for photosynthesis are characteristics unlike those previously reported with chloroplasts of C(3) plants.

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PubMed1980

Kinetics of C Distribution during Photosynthesis by Chloroplast Preparations Isolated from the Siphonous Alga Caulerpa simpliciuscula.

The kinetics of (14)C-labeling of compounds produced during photosynthesis by chloroplast preparations isolated from the green alga Caulerpa simpliciuscula were studied. After 10 minutes photosynthesis sucrose contained more (14)C than any other product, and continued to accumulate radioactivity during the whole hour of incubation. Glucose-6-phosphate and alanine also behaved as end products and continued to accumulate label during the period. In these organelles, glucose-6-phosphate replaced triose phosphate as the main compound exported from the chloroplast during shorter periods of photosynthesis. When either glucose-6-phosphate or 3-phosphoglycerate was supplied to the isolated chloroplasts, they were metabolized, but were not converted to either sucrose or alanine. It is proposed that many of the differences in metabolism which distinguish these algal chloroplasts from those isolated from higher plants are due to their isolation in the form of cytoplasts, i.e. chloroplasts surrounded by a thin layer of extrachloroplastic material which is membrane-bound. The restriction of diffusion of intermediates from the chloroplast by this cytoplast membrane appears to be at least as important as the rather small amount of cytoplasm present in determining the properties observed.

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PubMed1980

Quantum Requirement for Photosynthesis in Sedum praealtum during Two Phases of Crassulacean Acid Metabolism.

The quantum requirement (QR) for photosynthesis in Sedum praealtum, a Crassulacean acid metabolism plant, was compared with that of wheat, a C(3) plant, and maize, a C(4) plant, at 30 C. During the deacidification phase in S. praealtum, approximately 16 moles quanta were absorbed per mole malate consumed. This is equivalent to 16 moles quanta per mole CO(2) fixed, assuming 1 mole CO(2) is assimilated per mole malate decarboxylated. This QR for Crassulacean acid metabolism is similar to that of the C(3) or C(4) plant under atmospheric conditions, even though there are considerable differences in the biochemistry of photosynthesis. During late-afternoon C(3)-like fixation of atmospheric CO(2) in S. praealtum, the QR was relatively high with values of 41 under 21% O(2) and 19 under 2% O(2). During the deacidification phase in S. praealtum, the relatively low QR can be accounted for by the repression of photorespiration and saturation of photosynthesis from the elevated CO(2) concentration in the leaves during malate decarboxylation.

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PubMed1981

Photosynthesis, Growth, and Ultraviolet Irradiance Absorbance of Cucurbita pepo L. Leaves Exposed to Ultraviolet-B Radiation (280-315 nm).

Net photosynthesis, growth, and ultraviolet (UV) radiation absorbance were determined for the first leaf of Cucurbita pepo L. exposed to two levels of UV-B irradiation and a UV-B radiation-free control treatment. Absorbance by extracted flavonoid pigments and other UV-B radiation-absorbing compounds from the first leaves increased with time and level of UV-B radiation impinging on leaf surfaces. Although absorbance of UV-B radiation by extracted pigments increased substantially, UV-B radiation attenuation apparently was insufficient to protect completely the photosynthetic apparatus or leaf growth processes. Leaf expansion was repressed by daily exposure to 1365 Joules per meter per day of biologically effective UV-B radiation but not by exposure to 660 Joules per meter per day. Photosynthesis measured through ontogenesis of the first leaf was depressed by both UV-B radiation treatments. Repression of photosynthesis by UV-B radiation was especially evident during the ontogenetic period of maximum photosynthetic activity.

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PubMed1982

Temperature Dependence of Photosynthesis in Agropyron smithii Rydb. : I. FACTORS AFFECTING NET CO(2) UPTAKE IN INTACT LEAVES AND CONTRIBUTION FROM RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE MEASURED IN VIVO AND IN VITRO.

As part of an extensive analysis of the factors regulating photosynthesis in Agropyron smithii Rydb., a C(3) grass, we have examined the response of leaf gas exchange and ribulose-1,5-bisphosphate (RuBP) carboxylase activity to temperature. Emphasis was placed on elucidating the specific processes which regulate the temperature response pattern. The inhibitory effects of above-optimal temperatures on net CO(2) uptake were fully reversible up to 40 degrees C. Below 40 degrees C, temperature inhibition was primarily due to O(2) inhibition of photosynthesis, which reached a maximum of 65% at 45 degrees C. The response of stomatal conductance to temperature did not appear to have a significant role in determining the overall temperature response of photosynthesis. The intracellular conductance to CO(2) increased over the entire experimental temperature range, having a Q(10) of 1.2 to 1.4. Increases in the apparent Michaelis constant (K(c)) for RuBP carboxylase were observed in both in vitro and in vivo assays. The Q(10) values for the maximum velocity (V(max)) of CO(2) fixation by RuBP carboxylase in vivo was lower (1.3-1.6) than those calculated from in vitro assays (1.8-2.2). The results suggest that temperature-dependent changes in enzyme capacity may have a role in above-optimum temperature limitations below 40 degrees C. At leaf temperatures above 40 degrees C, decreases in photosynthetic capacity were partially dependent on temperature-induced irreversible reductions in the quantum yield for CO(2) uptake.

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PubMed1982

Temperature Dependence of Photosynthesis in Agropyron smithii Rydb. : II. CONTRIBUTION FROM ELECTRON TRANSPORT AND PHOTOPHOSPHORYLATION.

As part of an analysis of the factors regulating photosynthesis in Agropyron smithii Rydb., a C(3) grass, the response of electron transport and photophosphorylation to temperature in isolated chloroplast thylakoids has been examined. The response of the light reactions to temperature was found to depend strongly on the preincubation time especially at temperatures above 35 degrees C. Using methyl viologen as a noncyclic electron acceptor, coupled electron transport was found to be stable to 38 degrees C; however, uncoupled electron transport was inhibited above 38 degrees C. Photophosphorylation became unstable at lower temperatures, becoming progressively inhibited from 35 to 42 degrees C. The coupling ratio, ATP/2e(-), decreased continuously with temperature above 35 degrees C. Likewise, photosystem I electron transport was stable up to 48 degrees C, while cyclic photophosphorylation became inhibited above 35 degrees C. Net proton uptake was found to decrease with temperatures above 35 degrees C supporting the hypothesis that high temperature produces thermal uncoupling in these chloroplast thylakoids. Previously determined limitations of net photosynthesis in whole leaves in the temperature region from 35 to 40 degrees C may be due to thermal uncoupling that limits ATP and/or changes the stromal environment required for photosynthetic carbon reduction. Previously determined limitations to photosynthesis in whole leaves above 40 degrees C correlate with inhibition of photosynthetic electron transport at photosystem II along with the cessation of photophosphorylation.

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PubMed1983

Reduced osmotic potential effects on photosynthesis : identification of stromal acidification as a mediating factor.

Addition of sorbitol, which facilitated reductions in reaction medium osmotic potential from standard (0.33 molar sorbitol, -10 bars) isotonic conditions to a stress level of 0.67 molar sorbitol (-20 bars), inhibited the photosynthetic capacity of isolated spinach (Spinacia oleracea) chloroplasts. This inhibition, which ranged from 64 to 74% under otherwise standard reaction conditions, was dependent on reaction medium inorganic phosphate concentration, with the phosphate optimum for photosynthesis reduced to 0.05 millimolar at the low osmotic potential stress treatment from a value of 0.25 millimolar under control conditions.Stromal alkalating agents such as NH(4)Cl (0.75 millimolar) and KCl (35 millimolar) were also found to affect the degree of low osmotic potential inhibition of photosynthesis. Both agents doubled the rate of NaHCO(3)-supported O(2) evolution under the stress treatment, while hardly affecting the control rate at optimal concentrations. These agents also reduced the length of the lag phase of photosynthetic O(2) evolution under the stress treatment to a much greater degree. The rate-enhancement effect of these agents under the stress treatment was reversed by sodium acetate, which is known to facilitate stromal acidification.The reaction medium pH optimum for photosynthesis under the stress treatment was higher than under control conditions. In the presence of optimal NH(4)Cl, this shift was no longer evident.Internal pH measurements indicated that the stress treatment caused a 0.43 and 0.24 unit reduction in the stromal and intrathylakoid pH, respectively, under illumination. This osmotically induced acidification was not evident in the dark. The presence of 0.75 millimolar NH(4)Cl partially reversed the osmotically induced reduction in the illuminated stromal pH. It was concluded that stromal acidification is a mediating mechanism of the most severe site of low osmotic potential inhibition of the photosynthetic process.

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