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G E Edwards

Publications and source records attributed to G E Edwards.

At least 91 records · Page 5Linked to original sources
PubMed1978

Photosynthesis by isolated protoplasts, protoplast extracts, and chloroplasts of wheat: influence of orthophosphate, pyrophosphate, and adenylates.

Protoplasts, protoplast extracts (intact chloroplasts plus extrachloroplastic material), and chloroplasts isolated from protoplasts of wheat (Triticum aestivum) have rates of photosynthesis as measured by light-dependent O(2) evolution of about 100 to 150 micromoles of O(2) per milligram of chlorophyll per hour at 20 C and saturating bicarbonate. The assay conditions sufficient for this activity were 0.4 molar sorbitol, 50 millimolar N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid KOH (pH 7.6), and 10 millimolar NaHCO(3) with protoplast, plus a requirement of 1 to 10 millimolar ethylenediaminetetraacetate (EDTA) and 0.2 to 0.5 millimolar inorganic orthophosphate (Pi) with protoplast extracts and chloroplasts. Protoplast extracts evolved approximately 6 micromoles of O(2) per milligram of chlorophyll before photosynthesis became largely dependent on exogenous Pi while photosynthesis by chloroplasts had a much stronger dependence on exogenous Pi from the outset.Photosynthesis by chloroplasts from 6-day-old wheat plants under optimum levels of Pi was similar to that with the addition of 5 millimolar inorganic pyrophosphate (PPi) plus 0.2 millimolar adenosine-5'-diphosphate (ADP). Either PPi or ADP added separately inhibited photosynthesis. When chloroplasts were incubated in the dark for 2 to 6 minutes, photosynthesis was strongly inhibited by 5 millimolar PPi and this inhibiting was relieved by including adenosine-5'-triphosphate (ATP) or ADP (0.2 to 0.6 millimolar). Chloroplasts from 9-day-old wheat leaves were slightly less sensitive to inhibition by PPi and showed little or no inhibition by ADP.Chloroplasts isolated from protoplasts and assayed with 0.3 millimolar Pi added before illumination have an induction time from less than 1 minute up to 16 minutes depending on the time of the assay after isolation and the components of the medium. In order to obtain maximum rates of photosynthesis and minimum induction time, NaHCO(3) and chelating agents, EDTA or PPi (+ATP), are required in the chloroplast isolation, resuspension and assay medium. With these inclusions in the isolation and resuspension medium the induction time decreased rapidly during the first 20 to 30 minutes storage of chloroplasts on ice. Requirements for isolating intact and photosynthetically functional chloroplasts from wheat protoplasts are discussed.

Journal Article↗
PubMed1977

Transport in C4 mesophyll chloroplasts: evidence for an exchange of inorganic phosphate and phosphoenolpyruvate.

1. Mesophyll chloroplasts of the C4 plant Digitaria sanguinalis contain endogenous phosphoenolpyruvate which appears to distribute across the envelope according to the existing pH gradient. The phosphoenolpyruvate remaining in the stroma can be rapidly released by external inorganic phosphate or 3-phosphoglycerate while external pyruvate did not affect the distribution. 2. Phosphoenolpyruvate (PEP) was a competitive inhibitor (Ki (PEP) = 450 micrometer) of 32Pi uptake (Km(Pi)=200 micrometer) by chloroplasts in the dark and also reduced the steady-state internal concentration of 32Pi, which is consistent with phosphate and phosphoenolpyruvate sharing a common carrier. 3. Phosphoenolpyruvate formation by chloroplasts in the light in the presence of pyruvate but in the absence of inorganic phosphate was slow and the concentration ratio of phosphoenolpyruvate (internal/external) was high. Addition of 0.1 mM phosphate induced a high rate of phosphoenolpyruvate formation and the concentration ratio (internal/external) decreased 15-fold. It is proposed that external phosphate is required both for phosphoenolpyruvate formation and efflux from the chloroplast.

Binding, Competitive↗
PubMed1977

Effects of Light, Carbon Dioxide, and Temperature on Photosynthesis, Oxygen Inhibition of Photosynthesis, and Transpiration in Solanum tuberosum.

Individual leaves of potato (Solanum tuberosum L. W729R), a C(3) plant, were subjected to various irradiances (400-700 nm), CO(2) levels, and temperatures in a controlled-environment chamber. As irradiance increased, stomatal and mesophyll resistance exerted a strong and some-what paralleled regulation of photosynthesis as both showed a similar decrease reaching a minimum at about 85 neinsteins.cm(-2).sec(-1) (about (1/2) of full sunlight). Also, there was a proportional hyperbolic increase in transpiration and photosynthesis with increasing irradiance up to 85 neinsteins.cm(-2).sec(-1). These results contrast with many C(3) plants that have a near full opening of stomata at much less light than is required for saturation of photosynthesis.Inhibition of photosynthesis by 21% O(2) was nearly overcome by a 2-fold increase in atmospheric levels of CO(2) (about 1,200 ng.cm(-3)). Photosynthesis at 25 C, high irradiance, 2.5% O(2) and atmospheric levels of CO(2) was about 80% of the CO(2)-saturated rate, suggesting that CO(2) can be rate-limiting even without O(2) inhibition of photosynthesis. With increasing CO(2) concentration, mesophyll resistance decreased slightly while stomatal resistance increased markedly above 550 ng.cm(-3) which resulted in a significant reduction in transpiration.Although potato is a very productive C(3) crop, there is substantial O(2) inhibition of photosynthesis. The level of O(2) inhibition was maximum around 25 C but the percentage inhibition of photosynthesis by O(2) increased steadily from 38% at 16 C to 56% at 36 C. Photosynthesis and transpiration showed broad temperature optima (16-25 C). At higher temperatures, both the increased percentage inhibition of photosynthesis by O(2) and the increased stomatal resistance limit photosynthesis, while increased stomatal resistance limits transpiration. Water use efficiency, when considered at a constant vapor pressure gradient, increased with increasing irradiance, CO(2) concentration, and temperature.

Journal Article↗
PubMed1977

Oxygen Inhibition of Photosynthesis: I. Temperature Dependence and Relation to O(2)/CO(2) Solubility Ratio.

The magnitude of the percentage inhibition of photosynthesis by atmospheric levels of O(2) in the C(3) species Solanum tuberosum L., Medicago sativa L., Phaseolus vulgaris L., Glycine max L., and Triticum aestivum L. increases in a similar manner with an increase in the apparent solubility ratio of O(2)/CO(2) in the leaf over a range of solubility ratios from 25 to 45. The solubility ratio is based on calculated levels of O(2) and CO(2) in the intercellular spaces of leaves as derived from whole leaf measurements of photosynthesis and transpiration. The solubility ratio of O(2)/CO(2) can be increased by increased leaf temperature under constant atmospheric levels of O(2) and CO(2) (since O(2) is relatively more soluble than CO(2) with increasing temperature); by increasing the relative levels of O(2)/CO(2) in the atmosphere at a given leaf temperature, or by increased stomatal resistance. If the solubility ratio of O(2)/CO(2) is kept constant, as leaf temperature is increased, by varying the levels of O(2) or CO(2) in the atmosphere, then the percentage inhibition of photosynthesis by O(2) is similar. The decreased solubility of CO(2) relative to O(2) (decreased CO(2)/O(2) ratio) may be partly responsible for the increased percentage inhibition of photosynthesis by O(2) under atmospheric conditions with increasing temperature.

Journal Article↗
PubMed1977

Oxygen Inhibition of Photosynthesis: II. Kinetic Characteristics as Affected by Temperature.

The response of whole leaf photosynthesis of wheat (Triticum aestivum L.) in relation to soluble CO(2) available to the mesophyll cells, under low (1.5%) O(2) at 25, 30, and 35 C, followed Michaelis-Menten kinetics up to saturating CO(2) but deviated at high CO(2) levels where the experimental V(max) is considerably less than the calculated V(max). The affinity of the leaves for CO(2) during photosynthesis was similar from 25 to 35 C with Km (CO(2)) values of approximately 3.5 to 5 muM.In considering the effect of O(2) on photosynthesis at 25, 30, and 35 C where O(2) and CO(2) are expressed on a solubility basis: (a) the effect of O(2) on carboxylation efficiency was similar at the three temperature; (b) increasing temperature caused only a slight increase in kinetic constants Ki(O(2)) and Km(CO(2)), while the ratio of Ki(O(2))/Km(CO(2)) was similar at the three temperatures; and (c) the reciprocal plots of apparent rate of photosynthesis versus (CO(2) - Gamma) at various O(2) levels showed O(2) to be a competitive inhibitor of photosynthesis.A model for separating O(2) inhibition of photosynthesis into two components, direct competitive inhibition and inhibition due to photorespiration, was presented from both simulated and experimental data of photosynthetic response curves to varying CO(2) concentrations at low O(2)versus 21% O(2). The photorespiratory part of O(2) inhibition is considered as a major component at Gamma and increases with increasing temperature and with increase in O(2)/CO(2) solubility ratio. The competitive component of O(2) inhibition is considered as a major component of O(2) inhibition under atmospheric CO(2) levels and is relatively independent of temperature at a given O(2)/CO(2) ratio.

Journal Article↗
PubMed1977

Measurement of ozone injury by determination of leaf chlorophyll concentration.

A simple, rapid procedure is described for evaluating ozone injury to leaves of Phaseolus vulgaris L. cv. Pinto. Leaf chlorophyll is extracted with ethanol and analyzed spectrophotometrically; the concentration is expressed on the basis of leaf dry weight.The per cent chlorophyll reduction of ozone-injured leaves was highly correlated with the per cent visible necrosis and chlorosis (r = 0.96). The variability in injury estimates with chlorophyll analysis was slightly less than with visual evaluation. An evaluation of chlorophyll a and b concentrations separately showed that the chlorophyll a/b ratio decreased with increasing amounts of injury. Chlorophyll determinations made for leaves harvested at intervals after an ozone treatment indicated that maximum chlorophyll reduction had occurred by 4 days.This procedure for measuring ozone injury should be useful in eliminating the human bias associated with visible estimates of injury.

Journal Article↗
PubMed1976

Studies on the pathway of cyclic electron flow in mesophyll chloroplasts of a C4 plant.

1. Cyclic photophosphorylation driven by white light, as followed by 14CO2 fixation by mesophyll chloroplast preparations of the C4 plant Digitaria sanguinalis, was specifically inhibited by disalicylidenepropanediamine (DSPD), antimycin A, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 1-ethyl-3(3-dimethyl-aminopropyl)-carbodiimide (EDAC), and KCN suggesting that ferredoxin, cytochrome b563, plastoquinone, cytochrome f, and plastocyanin are obligatory intermediates of cyclic electron flow. It was found that 0.2 muM DCMU and 40 muM o-phenanthroline blocked noncyclic electron flow, stimulated cyclic photophosphorylation, and caused a partial reversal (40-100%) of the inhibition by DBMIB and antimycin A, but not DSPD. 2. Cyclic photophosphorylation could also be activated using only far-red illumination. Under this condition, however, cyclic photophosphorylation was much less sensitive to the inhibitors DBMIB, EDAC and antimycin A, but remained completely sensitive to DSPD and KCN. Inhibition in far-red light was not increased by preincubating the chloroplasts with the various inhibitors for several minutes in white light. 3. The striking correspondence between the effects of photosystem II inhibitors, DCMU and o-phenanthroline, on cyclic photophosphorylation under white light and cyclic photophosphorylation under far-red light (in the absence of photosystem II inhibitors) suggests that electrons flowing from photosystem II may regulate the pathway of cyclic electron flow.

Carbodiimides↗
PubMed1976

A highactivity ATP translocator in mesophyll chloroplasts of Digitaria sanguinalis, a plant having the C-4 dicarboxylic acid pathway of photosynthesis.

The effect of exogenous adenine nucleotides on CO2 fixation and oxygen evolution was studied with mesophyll protoplast extracts of the C4 plant Digitaria sanguinalis. Exogenous ATP was found to stimulate the rate of pyruvate and pyruvate+oxalacetate induced CO2 fixation, as well as reverse the inhibition of CO2 fixation of carbonyl cyanide m-chlorophenyl hydrazone and several electron transport inhibitors. The ATP-dependent stimulation of CO2 fixation varied from 40 to 70 mumol CO2 fixed/mg chlorophyll per h, suggesting that ATP was crossing the chloroplast membranes at rates of 80-140 mumol/mg chlorophyll per h, since 2 ATP are required for each CO2 fixed. Fixation of CO2 could also be induced in the dark by exogenous ATP, in which case ADP accumulated outside the chloroplasts...

Adenosine Diphosphate↗
PubMed1976

Differential Localization of Fraction I Protein between Chloroplast Types.

The soluble proteins of C(3) and C(4) mesophyll chloroplasts and C(4) bundle sheath extracts have been analyzed by gel electrophoresis for fraction I protein. Gel scans of soluble protein from C(4) bundle sheath extracts and C(3) mesophyll chloroplasts showed typical fraction I protein peaks that could be identified by ribulose diphosphate carboxylase activity. No such peak was observed for C(4) mesophyll chloroplasts, which also lacked both large and small subunits of ribulose diphosphate carboxylase on sodium dodecyl sulfate gels. The absence of fraction I protein in these chloroplasts was reflected in the soluble protein to chlorophyll ratios, which were roughly 3-fold lower than the ratio obtained for C(3) chloroplasts. The carboxylating enzyme in C(4) mesophyll cells, phosphoenolpyruvate carboxylase, was found to be a major protein in the cytoplasm of C(4) mesophyll protoplasts, and had higher mobility than fraction I protein.

Journal Article↗
PubMed1976

Distribution of Nitrate-assimilating Enzymes between Mesophyll Protoplasts and Bundle Sheath Cells in Leaves of Three Groups of C(4) Plants.

Intercellular distribution of enzymes involved in amino nitrogen synthesis was studied in leaves of species representing three C(4) groups, i.e. Sorghum bicolor, Zea mays, Digitaria sanguinalis (NADP malic enzyme type); Panicum miliaceum (NAD malic enzyme type); and Panicum maximum (phosphoenolpyruvate carboxykinase type). Nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate synthase were predominantly localized in mesophyll cells of all the species, except in P. maximum where nitrite reductase had similar activity on a chlorophyll basis, in both mesophyll and bundle sheath cells. NADH-glutamate dehydrogenase was concentrated in the bundle sheath cells, while NADPH-glutamate dehydrogenase was localized in both mesophyll and bundle sheath cells. The activities of nitrate-assimilating enzymes, except for nitrate reductase, were high enough to account for the proposed in vivo rates of nitrate assimilation.Based on the differential centrifugation of cell homogenates of P. miliaceum, mesophyll chloroplasts appear to be the major site of nitrate assimilation since nitrite reductase, glutamine synthetase, glutamate synthase, and NADPH-glutamate dehydrogenase were primarily localized in the chloroplast fraction. Both the glutamine synthetase-glutamate synthase and glutamate dehydrogenase pathways were considered as alternative routes of amino nitrogen synthesis.

Journal Article↗
PubMed1975

C(4) Photosynthesis: Light-dependent CO(2) Fixation by Mesophyll Cells, Protoplasts, and Protoplast Extracts of Digitaria sanguinalis.

Mesophyll cells, protoplasts, and protoplast extracts of Digitaria sanguinalis were used for comparative studies of light-dependent CO(2) fixation. CO(2) fixation was low without the addition of organic substrates. Pyruvate, oxaloacetate, and 3-phosphoglycerate induced relatively low rates (10 to 90 mumoles/mg chlorophyll.hr) of CO(2) fixation when added separately. However, a highly synergistic relationship was found between pyruvate + oxaloacetate and pyruvate + 3-phosphoglycerate for inducing light-dependent CO(2) fixation in the mesophyll preparations. Highest rates of CO(2) fixation were obtained with protoplast extracts. Pyruvate, in combination with oxaloacetate or 3-phosphoglycerate induced light-dependent rates from 150 to 380 mumoles of CO(2) fixed/mg chlorophyll.hr which are equivalent to or exceed reported rates of whole leaf photosynthesis in C(4) species. Concentrations of various substrates required to give half-maximum velocities of CO(2) fixation were determined, with the protoplast extracts generally saturating at the lowest substrate concentrations. Chloroplasts separated from protoplast extracts showed little capacity for CO(2) fixation. The results suggest that CO(2) fixation in C(4) mesophyll cells is dependent on chloroplasts and extrachloroplastic phosphoenolpyruvate carboxylase.The stimulation of pyruvate-induced CO(2) fixation by oxaloacetate and 3-phosphoglycerate is thought to be due to induction of noncyclic electron transport which generates ATP for the conversion of pyruvate to phosphoenolpyruvate by pyruvate Pi dikinase. The primary products of the substrate-induced CO(2) fixation were oxaloacetate and malate, which provides further evidence for carbon fixation through the beta-carboxylation pathway. High rates of light-dependent CO(2) fixation with a significant percentage of (14)C fixed into malate suggest an efficient operation of both photosystems I and II.The substrate inductions are discussed with respect to the proposed role of the mesophyll cell in C(4) photosynthesis, and schemes suggesting the stoichiometry of energy requirements for photosynthetic carbon metabolism in C(4) mesophyll cells are presented.

Journal Article↗
PubMed1975

Regulation of oxaloacetate, aspartate, and malate formation in mesophyll protoplast extracts of three types of c(4) plants.

The use of mesophyll protoplast extracts from various C(4) species has provided an effective method for studying light-and substrate-dependent formation of oxaloacetate, malate, and asparate at rates equivalent to whole leaf C(4) photosynthesis. Conditions regulating the formation of the C(4) acids were studied with protoplast extracts from Digitaria sanguinalis, an NADP-malic enzyme C(4) species, Eleusineindica, an NAD-malic enzyme C(4) species, and Urochloa panicoides, a phosphoenolpyruvate (PEP) carboxykinase C(4) species. Light-dependent induction of CO(2) fixation by the mesophyll extracts of all three species was relatively low without addition of exogenous substrates. Pyruvate, alanine and alpha-ketoglutarate, or 3-phosphoglycerate induced high rates of CO(2) fixation in the mesophyll extracts with oxaloacetate, malate, and aspartate being the primary products. In all three species, it appears that pyruvate, alanine, or 3-phosphoglycerate may serve as effective precursors to the formation of PEP for carboxylation through PEP-carboxylase in C(4) mesophyll cells. Induction by pyruvate or alanine and alpha-ketoglutarate was light-dependent, whereas 3-phosphoglycerate-induced CO(2) fixation was not.Several differences between these species representing the three C(4) groups were observed. Substrate induction of CO(2) fixation in mesophyll protoplast extracts of D. sanguinalis gave malate as a major product; only by an apparent exchange reaction with cold aspartate did substantial label appear in aspartate (up to 53% of labeled products). In contrast, aspartate was a major product when alanine and alpha-ketoglutarate served as inducing substrates with E. indica (up to 57%) and U. panicoides (up to 86%). With induction by pyruvate or 3-phosphoglycerate, mesophyll preparations of U. panicoides and E. indica were less effective in forming malate (up to 31% of products) than D. sanguinalis (up to 87% of products). After 2 seconds of whole leaf (14)CO(2) fixation, malate was the major labeled product (57%) with D. sanguinalis, whereas with E. indica and U. panicoides aspartate was the predominant product (73% and 76%, respectively).With mesophyll protoplast extracts of D. sanguinalis, aspartate inhibited CO(2) fixation (about 50% at 0.6 mm), while malate was relatively uninhibitory at comparable concentrations. CO(2) fixation by mesophyll protoplast extracts of E. indica was inhibited by malate (about 50% at 0.6 mm), while aspartate was relatively uninhibitory. With mesophyll preparations of U. panicoides, malate or aspartate (2 mm) caused only slight inhibition of CO(2) fixation. The regulation of aspartate and malate synthesis in C(4) mesophyll cells is discussed relative to initial products of photosynthesis in C(4) species in vivo and species differences in the mechanisms of C(4) photosynthesis.

Journal Article↗