Biomedical subjects

G E Edwards

Publications and source records attributed to G E Edwards.

At least 37 records · Page 2Linked to original sources

PubMed1992

Control of Photosynthesis and Stomatal Conductance in Ricinus communis L. (Castor Bean) by Leaf to Air Vapor Pressure Deficit.

Castor bean (Ricinus communis L.) has a high photosynthetic capacity under high humidity and a pronounced sensitivity of photosynthesis to high water vapor pressure deficit (VPD). The sensitivity of photosynthesis to varying VPD was analyzed by measuring CO(2) assimilation, stomatal conductance (g(s)), quantum yield of photosystem II (phi(II)), and nonphotochemical quenching of chlorophyll fluorescence (q(N)) under different VPD. Under both medium (1000) and high (1800 micromoles quanta per square meter per second) light intensities, CO(2) assimilation decreased as the VPD between the leaf and the air around the leaf increased. The g(s) initially dropped rapidly with increasing VPD and then showed a slower decrease above a VPD of 10 to 20 millibars. Over a temperature range from 20 to 40 degrees C, CO(2) assimilation and g(s) were inhibited by high VPD (20 millibars). However, the rate of transpiration increased with increasing temperature at either low or high VPD due to an increase in g(s). The relative inhibition of photosynthesis under photorespiring (atmospheric levels of CO(2) and O(2)) versus nonphotorespiring (700 microbars CO(2) and 2% O(2)) conditions was greater under high VPD (30 millibars) than under low VPD (3 millibars). Also, with increasing light intensity the relative inhibition of photosynthesis by O(2) increased under high VPD, but decreased under low VPD. The effect of high VPD on photosynthesis under various conditions could not be totally accounted for by the decrease in the intercellular CO(2) in the leaf (C(i)) where C(i) was estimated from gas exchange measurements. However, estimates of C(i) from measurements of phi(II) and q(N) suggest that the decrease in photosynthesis and increase in photorespiration under high VPD can be totally accounted for by stomatal closure and a decrease in C(i). The results also suggest that nonuniform closure of stomata may occur in well-watered plants under high VPD, causing overestimates in the calculation of C(i) from gas exchange measurements. Under low VPD, 30 degrees C, high light, and saturating CO(2), castor bean (C(3) tropical shrub) has a rate of photosynthesis (61 micromoles CO(2) per square meter per second) that is about 50% higher than that of tobacco (C(3)) or maize (C(4)) under the same conditions. The chlorophyll content, total soluble protein, and ribulose-1,5-bisphosphate carboxylase/oxygenase level on a leaf area basis were much higher in castor bean than in maize or tobacco, which accounts for its high rates of photosynthesis under low VPD.

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PubMed1991

Oxalate as a potent and selective inhibitor of spinach (Spinacia oleracea) leaf NADPH-dependent hydroxypyruvate reductase.

Purified spinach (Spinacia oleracea) NADPH-preferring hydroxypyruvate reductase (HPR-2) was potently and selectively inhibited by oxalate, an end product of metabolism in plants. Both hydroxypyruvate- and glyoxylate-dependent rates of the HPR-2 enzyme were affected. Oxalate acted as an uncompetitive inhibitor of the enzyme, with Ki values of 7 and 36 microM for the NADPH/hydroxypyruvate and NADPH/glyoxylate pairs of reactants respectively. Oxalate, at millimolar levels, caused less than 10% inhibition of purified spinach NADH-preferring HPR (HPR-1) and had no effect on purified spinach NADPH-preferring glyoxylate-specific reductase (GR-1). The inhibition of spinach HPR-2 by oxalate is by far the strongest for any known inhibitor of leaf HPR and GR activities. In photosynthetic tissues, oxalate could potentially act as a primary regulator of extraperoxisomal metabolism of hydroxypyruvate and glyoxylate.

Alcohol Oxidoreductases↗

PubMed1991

Photosynthetic and photorespiratory characteristics of flaveria species.

The genus Flaveria shows evidence of evolution in the mechanism of photosynthesis as its 21 species include C(3), C(3)-C(4), C(4)-like, and C(4) plants. In this study, several physiological and biochemical parameters of photosynthesis and photorespiration were measured in 18 Flaveria species representing all the photosynthetic types. The 10 species classified as C(3)-C(4) intermediates showed an inverse continuum in level of photorespiration and development of the C(4) syndrome. This ranges from F. sonorensis with relatively high apparent photorespiration and lacking C(4) photosynthesis to F. Among the intermediates, the photosynthetic CO(2) compensation points at 30 degrees C and 1150 micromoles quanta per square meter per second varied from 9 to 29 microbars. The values for the three C(4)-like species varied from 3 to 6 microbars, similar to those measured for the C(4) species. The activities of the photorespiratory enzymes glycolate oxidase, hydroxypyruvate reductase, and serine hydroxymethyltransferase decreased progressively from C(3) to C(3)-C(4) to C(4)-like and C(4) species. On the other hand, most intermediates had higher levels of phosphenolpyruvate carboxylase and NADP-malic enzyme than C(3) species, but generally lower activities compared to C(4)-like and C(4) species. The levels of these C(4) enzymes are correlated with the degree of C(4) photosynthesis, based on the initial products of photosynthesis. Another indication of development of the C(4) syndrome in C(3)-C(4)Flaveria species was their intermediate chlorophyll a/b ratios. The chlorophyll a/b ratios of the various Flaveria species are highly correlated with the degree of C(4) photosynthesis suggesting that the photochemical machinery is progressively altered during evolution in order to meet the specific energy requirements for operating the C(4) pathway. In the progression from C(3) to C(4) species in Flaveria, the CO(2) compensation point decreased more rapidly than did the decrease in O(2) inhibition of photosynthesis or the increase in the degree of C(4) photosynthesis. These results suggest that the reduction in photorespiration during evolution occurred initially by refixation of photorespired CO(2) and prior to substantive reduction in O(2) inhibition and development of the C(4) syndrome. However, further reduction in O(2) inhibition in some intermediates and C(4)-like species is considered primarily due to the development of the C(4) syndrome. Thus, the evolution of C(3)-C(4) intermediate photosynthesis likely occurred in response to environmental conditions which limit the intercellular CO(2) concentration first via refixation of photorespired CO(2), followed by development of the C(4) syndrome.

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PubMed1990

Induction of Crassulacean Acid Metabolism in the Facultative Halophyte Mesembryanthemum crystallinum by Abscisic Acid.

The facultative halophyte, Mesembryanthemum crystallinum, shifts its mode of carbon assimilation from the C(3) pathway to Crassulacean acid metabolism (CAM) in response to water stress. In this study, exogenously applied abscisic acid (ABA), at micromolar concentrations, could partially substitute for water stress in induction of CAM in this species. ABA at concentrations of 5 to 10 micromolar, when applied to leaves or to the roots in hydroponic culture or in soil, induced the expression of CAM within days (as indicated by the nocturnal accumulation of total titratable acidity and malate). After applying ABA there was also an increase in phosphoenolpyruvate carboxylase and NADP-malic enzyme activities. The degree and time course of induction by ABA were comparable to those induced by salt and water stress. Electrophoretic analyses of leaf soluble protein indicate that the increases in phosphoenolpyruvate carboxylase activity during the induction by ABA, salt, and water stress are due to an increase in the quantity of the enzyme protein. ABA may be a factor in the stress-induced expression of CAM in M. crystallinum, serving as a functional link between stress and biochemical adaptation.

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PubMed1990

Enzymology of the reduction of hydroxypyruvate and glyoxylate in a mutant of barley lacking peroxisomal hydroxypyruvate reductase.

The use of LaPr 88/29 mutant of barley (Hordeum vulgare), which lacks NADH-preferring hydroxypyruvate reductase (HPR-1), allowed for an unequivocal demonstration of at least two related NADPH-preferring reductases in this species: HPR-2, reactive with both hydroxypyruvate and glyoxylate, and the glyoxylate specific reductase (GR-1). Antibodies against spinach HPR-1 recognized barley HPR-1 and partially reacted with barley HPR-2, but not GR-1, as demonstrated by Western immunoblotting and immunoprecipitation of proteins from crude leaf extracts. The mutant was deficient in HPR-1 protein. In partially purified preparations, the activities of HPR-1, HPR-2, and GR-1 could be differentiated by substrate kinetics and/or inhibition studies. Apparent K(m) values of HPR-2 for hydroxypyruvate and glyoxylate were 0.7 and 1.1 millimolar, respectively, while the K(m) of GR-1 for glyoxylate was 0.07 millimolar. The K(m) values of HPR-1, measured in wild type, for hydroxypyruvate and glyoxylate were 0.12 and 20 millimolar, respectively. Tartronate and P-hydroxypyruvate acted as selective uncompetitive inhibitors of HPR-2 (K(i) values of 0.3 and 0.4 millimolar, respectively), while acetohydroxamate selectively inhibited GR-1 activity. Nonspecific contributions of HPR-1 reactions in assays of HPR-2 and GR-1 activities were quantified by a direct comparison of rates in preparations from wild-type and LaPr 88/29 plants. The data are evaluated with respect to previous reports on plant HPR and GR activities and with respect to optimal assay procedures for individual HPR-1, HPR-2, and GR-1 rates in leaf preparations.

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PubMed1989

Rotatory atlantoaxial subluxation following pharyngoplasty.

Atlantoaxial rotatory subluxation often presents in childhood as a stiff, painful neck with associated torticollis. It can occur spontaneously following minor trauma or an inflammatory process in the cervical tissues. Failure to recognise the condition may result in a fixed rotatory subluxation which can be difficult to treat. The authors report on two cases of atlantoaxial rotatory subluxation which occurred following pharyngoplasty and discuss the clinical presentation and management of these cases. The literature on this subject is briefly reviewed.

Atlanto-Axial Joint↗

PubMed1989

Protection of Pyruvate,Pi Dikinase from Maize against Cold Lability by Compatible Solutes.

Most C(4) species are chilling sensitive and certain enzymes like pyruvate,Pi dikinase of the C(4) pathway are also cold labile. The ability of cations and compatible solutes to protect maize (Zea mays) dikinase against cold lability was examined. The enzyme in desalted extracts at pH 8 from preilluminated leaves could be protected against cold lability (at 0 degrees C) by the divalent cations Mn(2+), Mg(2+), and Ca(2+). There was substantial protection by sulfate based salts but little protection by chloride based salts of potassium or ammonium (concentration 250 millimolar). The degree of protection against cold lability under limiting MgCl(2) (5 millimolar) was pH sensitive (maximum protection at pH 8), but independent of ionic strength (up to 250 millimolar by addition of KCl). In catalysis Mg(2+) is required and Mn(2+) could not substitute as a cofactor. Several compatible solutes reduced or prevented the cold inactivation of dikinase (in desalted extracts and the partially purified enzyme), including glycerol, proline, glycinebetaine and trimethylamine-N-oxide (TMAO). TMAO and Mg(2+) had an additive effect in protecting dikinase against cold inactivation. TMAO could largely substitute for the divalent cation and addition of TMAO during cold treatment prevented further inactivation. Cold inactivation was partially reversed by incubation at room temperature; with addition of TMAO reversal was complete. The temperature dependence of inactivation at pH 8 and 3 millimolar MgCl(2) was evaluated by incubation at 2 to 17 degrees C for 45 minutes, followed by assay at room temperature. At preincubation temperatures below 11 degrees C there was a progressive inactivation which could be prevented by TMAO (450 millimolar). The results are discussed relative to possible effects of the solutes on the quaternary structure of this enzyme, which is known to dissociate at low temperatures.

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PubMed1989

Photosynthetic Plasticity in Flaveria brownii: Growth Irradiance and the Expression of C(4) Photosynthesis.

Photosynthesis was examined in leaves of Flaveria brownii A. M. Powell, grown under either 14% or 100% full sunlight. In leaves of high light grown plants, the CO(2) compensation point and the inhibition of photosynthesis by 21% O(2) were significantly lower, while activities of ribulose 1,5-bisphosphate carboxylase/oxygenase and various C(4) cycle enzymes were considerably higher than those in leaves grown in low light. Both the CO(2) compensation point and the degree of O(2) inhibition of apparent photosynthesis were relatively insensitive to the light intensity used during measurements with plants from either growth conditions. Partitioning of atmospheric CO(2) between Rubisco of the C(3) pathway and phosphoenolpyruvate carboxylase of the C(4) cycle was determined by exposing leaves to (14)CO(2) for 3 to 16 seconds, and extrapolating the labeling curves of initial products to zero time. Results indicated that approximately 94% of the CO(2) was fixed by the C(4) cycle in high light grown plants, versus approximately 78% in low light grown plants. Thus, growth of F. brownii in high light increased the expressed level of C(4) photosynthesis. Consistent with the carbon partitioning patterns, photosynthetic enzyme activities (on a chlorophyll basis) in protoplasts from leaves of high light grown plants showed a more C(4)-like pattern of compartmentation. Pyruvate, Pi dikinase and phosphoenolpyruvate carboxylase were more enriched in the mesophyll cells, while NADP-malic enzyme and ribulose 1,5-bisphosphate carboxylase/oxygenase were relatively more abundant in the bundle sheath cells of high light than of low light grown plants. Thus, these results indicate that F. brownii has plasticity in its utilization of different pathways of carbon assimilation, depending on the light conditions during growth.

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PubMed1989

Identification of hydroxypyruvate and glyoxylate reductases in maize leaves.

At least two hydroxypyruvate reductases (HPRs), differing in specificity for NAD(P)H and (presumably) utilizing glyoxylate as a secondary substrate, were identified by fractionation of crude maize leaf extracts with ammonium sulfate. The NADH-preferring enzyme, which most probably represented peroxisomal HPR, was precipitated by 30 to 45% saturated ammonium sulfate, while most of the NADPH-dependent activity was found in a 45 to 60% precipitate. The HPRs had similar low K(m)s for hydroxypyruvate (about 0.1 millimolar), regardless of cofactor, while affinities of glyoxylate reductase (GR) reactions for glyoxylate varied widely (K(m)s of 0.4-12 millimolar) depending on cofactor. At high hydroxypyruvate concentrations, the NADPH-HPR from the 30 to 45% precipitate showed negative cooperativity with respect to this reactant, having a second K(m) of 6 millimolar. In contrast, NADPH-HPR from the 45 to 60% precipitate was inhibited at high hydroxypyruvate concentrations (K(1) of 3 millimolar) and, together with NADPH-GR, had only few, if any, common antigenic determinants with NADH-HPR from the 30 to 45% fraction. Both NADPH-HPR and NADPH-GR activities from the 45 to 60% precipitate were probably carried out by the same enzyme(s), as found by kinetic studies. Following preincubation with NADPH, there was a marked increase (up to sixfold) in activity of NADPH-HPR from either crude or fractionated extracts. Most of this increase could be attributed to an artefact resulting from an interference by endogeneous NADPH-phosphatase, which hydrolyzed NADPH to NADH, the latter being utilized by the NADH-dependent HPR. However, in the presence of 15 millimolar fluoride (phosphatase inhibitor), preincubation with NADPH still resulted in over 60% activation of NADPH-HPR. The NADPH treatment stimulated the V(max) of the reductase but had no effect on its K(m) for hydroxypyruvate. Enzyme distribution studies revealed that both NADH and NADPH-dependent HPR and GR activities were predominantly localized in the bundle sheath compartment. Rates of NADPH-HPR and NADPH-GR in this tissue (over 100 micromoles per hour per milligram of chlorophyll each) are in the upper range of values reported for leaves of C(3) species.

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PubMed1989

Effects of Polyploidy on Photosynthetic Rates, Photosynthetic Enzymes, Contents of DNA, Chlorophyll, and Sizes and Numbers of Photosynthetic Cells in the C(4) Dicot Atriplex confertifolia.

Photosynthetic rates, chlorophyll content, and activities of several photosynthetic enzymes were determined per cell, per unit DNA, and per unit leaf area in five ploidal levels of the C(4) dicot Atriplex confertifolia. Volumes of bundle sheath and mesophyll protoplasts were measured in enzymatic digestions of leaf tissue. Photosynthetic rates per cell, contents of DNA per cell, and activities of the bundle sheath enzymes ribulose 1,5-bisphosphate carboxylase (RuBPC) and NAD-malic enzyme per cell were correlated with ploidal level at 99% or 95% confidence levels, and the results suggested a near proportional relationship between gene dosage and gene products. There was also a high correlation between volume of mesophyll and bundle sheath cells and the ploidal level. Contents of DNA per cell, activity of RuBPC per cell, and volumes of cells were correlated with photosynthetic rate per cell at the 95% confidence level. The mesophyll cells did not respond to changes in ploidy like the bundle sheath cells. In the mesophyll cells the chlorophyll content per cell was constant at different ploidal levels, there was less increase in cell volume than in bundle sheath cells with an increase in ploidy, and there was not a significant correlation (at 95% level) of phosphoenolpyruvate carboxylase activity or content and pyruvate,Pi dikinase activity with increase in ploidy. The number of photosynthetic cells per unit leaf area progressively decreased with increasing ploidy from diploid to hexaploid, but thereafter remained constant in octaploid and decaploid plants. Numbers of cells per leaf area were not correlated with cell volumes. The mean photosynthetic rates per unit leaf area were lowest in the diploid, similar in 4x, 6x, and 8x, and highest in the decaploid. The photosynthetic rate per leaf area was highly correlated with the DNA content per leaf area.

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PubMed1988

CO(2) Assimilation and Activities of Photosynthetic Enzymes in High Chlorophyll Fluorescence Mutants of Maize Having Low Levels of Ribulose 1,5-Bisphosphate Carboxylase.

Photosynthetic properties were examined in several hcf (high chlorophyll fluorescence 11, 21, 42 and 45) nuclear recessive mutants of maize which were previously found to have normal photochemistry and low CO(2) fixation. Mutants usually either died after depletion of seed reserves (about 18 days after planting), or survived with slow growth up to 7 or 8 weeks. Both the activity and quantity of ribulose 1,5-bisphosphate carboxylase (Rubisco) were low in the mutants (5-25% of the normal siblings on a leaf area basis) and the loss of Rubisco tended to parallel the reduction in photosynthetic capacity. The Rubisco content in the mutants was often marginal for photosynthetic carbon gain, with some leaves and positions along a leaf having no net photosynthesis, while other leaves had a low carbon gain. Conversely, the activities of C(4) cycle enzymes, phosphoenolpyruvate carboxylase, pyruvate, Pi dikinase, NADP-malate dehydrogenase, and NADP-malic enzyme, were the same or only slightly reduced compared to the normal siblings. The mutants had about half as much chlorophyll content per leaf area as the normal green plants. However, the Rubisco activity in the mutants was low on both a leaf area and chlorophyll basis. Low Rubisco activity and lower chlorophyll content may both contribute to the low rates of photosynthesis in the mutants on a leaf area basis.

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PubMed1988

Specific Labeling of the Phosphate Translocator in C(3) and C(4) Mesophyll Chloroplasts by Tritiated Dihydro-DIDS (1,2-Ditritio-1,2-[2,2' -Disulfo-4,4' -Diisothiocyano] Diphenylethane).

The phosphate translocator protein of C(3) and C(4) mesophyll chloroplast envelopes was specifically labeled using the anion exchange inhibitor, 1,2-ditritio-1,2-(2,2' -disulfo-4,4' -diisothiocyano) diphenylethane ([(3)H](2)-DIDS). Intact mesophyll chloroplasts were isolated from the C(3) plants, Spinacia oleracea L. (spinach) and Pisum sativum L. (pea), and the C(4) plant, Zea mays L. (corn). Chloroplasts were incubated with 5 to 50 mum [(3)H](2)-DIDS and, in addition, pea chloroplasts were also incubated with pyridoxal phosphate/tritiated sodium borohydride. The chloroplasts were washed, the envelopes isolated and solubilized. Following sodium dodecyl sulfate polyacrylamide gel electrophoresis, label from bound [(3)H](2)-DIDS was detected only in the 28- to 30-kilodalton protein (proposed C(3) phosphate translocator) for both C(3) and C(4) chloroplasts, as demonstrated by fluorography. In contrast, when pyridoxal phosphate/tritiated sodium borohydride was used to label pea chloroplasts, radioactivity was detected in several other bands in addition to the 29-kilodalton polypeptide. These findings suggest that DIDS is a much more specific inhibitor than reagents previously employed to study the phosphate translocator and could be used to isolate and characterize the differences in the C(3) and C(4) phosphate translocator protein(s).

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PubMed1988

Photosynthesis in Flaveria brownii, a C(4)-Like Species: Leaf Anatomy, Characteristics of CO(2) Exchange, Compartmentation of Photosynthetic Enzymes, and Metabolism of CO(2).

Light microscopic examination of leaf cross-sections showed that Flaveria brownii A. M. Powell exhibits Kranz anatomy, in which distinct, chloroplast-containing bundle sheath cells are surrounded by two types of mesophyll cells. Smaller mesophyll cells containing many chloroplasts are arranged around the bundle sheath cells. Larger, spongy mesophyll cells, having fewer chloroplasts, are located between the smaller mesophyll cells and the epidermis. F. brownii has very low CO(2) compensation points at different O(2) levels, which is typical of C(4) plants, yet it does show about 4% inhibition of net photosynthesis by 21% O(2) at 30 degrees C. Protoplasts of the three photosynthetic leaf cell types were isolated according to relative differences in their buoyant densities. On a chlorophyll basis, the activities of phosphoenolpyruvate carboxylase and pyruvate, Pi dikinase (carboxylation phase of C(4) pathway) were highest in the larger mesophyll protoplasts, intermediate in the smaller mesophyll protoplasts, and lowest, but still present, in the bundle sheath protoplasts. In contrast, activities of ribulose 1,5-bisphosphate carboxylase, other C(3) cycle enzymes, and NADP-malic enzyme showed a reverse gradation, although there were significant activities of these enzymes in mesophyll cells. As indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the banding pattern of certain polypeptides of the total soluble proteins from the three cell types also supported the distribution pattern obtained by activity assays of these enzymes. Analysis of initial (14)C products in whole leaves and extrapolation of pulse-labeling curves to zero time indicated that about 80% of the CO(2) is fixed into C(4) acids (malate and aspartate), whereas about 20% of the CO(2) directly enters the C(3) cycle. This is consistent with the high activity of enzymes for CO(2) fixation by the C(4) pathway and the substantial activity of enzymes of the C(3) cycle in the mesophyll cells. Therefore, F. brownii appears to have some capacity for C(3) photosynthesis in the mesophyll cells and should be considered a C(4)-like species.

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PubMed1988

The Influence of Leaf Age on C(4) Photosynthesis and the Accumulation of Inorganic Carbon in Flaveria trinervia, a C(4) Dicot.

Characteristics of C(4) photosynthesis were examined in young, mid-age, and mature leaves of Flaveria trinervia (an NADP-malic enzyme-type C(4) dicot). The turnover of [4-(14)C] (malate plus aspartate) following a pulse with (14)CO(2) was similar in leaves of different ages (apparent half-time of 18-25 seconds). However, the rate of (14)CO(2) incorporation in mid-age leaves was about 1.5-fold higher than in young leaves, and about 2.5-fold higher than in mature leaves. The rate of (14)CO(2) fixation was proportional to the total active pool of malate plus aspartate but was not correlated with the total photosynthetically derived inorganic carbon pool. The leaf's ability to concentrate inorganic carbon photosynthetically declined during leaf expansion, from 29 down to 7 nanomoles per milligram chlorophyll. Similarly, the active aspartate pool also declined during leaf expansion, from about 123 down to 20 nanomoles per milligram chlorophyll. Enhanced metabolism of aspartate to CO(2) and pyruvate in young leaves is suggested to facilitate the maintenance of high CO(2) levels in bundle sheath cells which are thought to have a higher conductance to CO(2).

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PubMed1987

Influences of leaf temperature on photosynthetic carbon metabolism in wheat.

Net photosynthetic assimilation rate (A), extractable activities of three photosynthetic enzymes, and the concentrations of six metabolites were determined for wheat (Tricum aestivum L.) leaves as leaf temperature was varied under photorespiring (350 microliters per liter CO(2) and 21% O(2)) and under nonphotorespiring conditions (800 microliters per liter CO(2) and 2% O(2)). The extractable activity of ribulose-1,5-bisphosphate carboxylase (Rubisco) and fructose-1,6-bisphosphatase declined with increasing leaf temperature from 15 to 45 degrees C. Leaf concentrations of ribulose-1,5-bisphosphate (RuBP) declined slightly between 15 and 25 degrees C but increased to a level which is 4 to 5 times the binding site concentration of Rubisco at leaf temperatures of 35 and 45 degrees C. Leaf concentrations of 3-phosphoglycerate, fructose-6-phosphate, and glucose-6-phosphate all declined with increasing leaf temperature. Outside of the limitations imposed by photorespiration, it is proposed that under high light and at suboptimal temperatures, A is limited by rate of utilization of triose phosphate; at optimal temperatures, by the availability of substrate (CO(2) and RuBP) under photorespiring conditions or utilization of triose phosphate under nonphotorespiring conditions; and at supraoptimal temperatures, by the activation state of Rubisco.

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PubMed1987

Photosynthesis, Leaf Anatomy, and Cellular Constituents in the Polyploid C(4) Grass Panicum virgatum.

Photosynthetic gas exchange, activities of six key C(4) cycle enzymes, amounts of soluble protein, chlorophyll, and DNA, and various leaf anatomical and structural features were measured in naturally occurring tetraploid and octaploid plants of the NAD-malic enzyme type C(4) grass Panicum virgatum L. On a leaf area basis, the photosynthetic rate and concentrations of DNA, soluble protein, and chlorophyll were 40 to 50% higher, and enzyme activities 20 to 70% higher in the octaploid than in the tetraploid. Photosynthetic cells in the octaploid were only 17 to 19% larger in volume, yet contained 33 to 38% more chloroplasts than cells in the tetraploid. On a per cell basis the contents of DNA, soluble protein, and chlorophyll, activities of carboxylating photosynthetic enzymes, and carbon assimilation rate were all doubled in octaploid compared with tetraploid cells. Since cellular volume did not double with genome doubling, cellular constituents were more concentrated in the cells of the octaploid. The influences of polyploidy were balanced between mesophyll and bundle sheath cells since the changes in physical and biochemical parameters with ploidy level were similar in both cell types. We conclude that photosynthetic activity in these two polyploid genotypes of P. virgatum is determined by enzyme activities and concentrations of biochemical constituents, and that selection for smaller cell volume has led to higher photosynthetic rates per unit leaf area in the octaploid. The ratio of DNA content to cellular volume is a major factor determining the concentrations of gene products in cells. The number of chloroplasts, however, is controlled more by cellular volume than by the number of nuclear chromosomes.

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PubMed1987

Influence of leaf age on photosynthesis, enzyme activity, and metabolite levels in wheat.

The rate of photosynthesis under high light (1000 micromole quanta per square meter per second) and at 25 degrees C was measured during development of the third leaf on wheat plants and compared with the activity of several photosynthetic enzymes and the level of metabolites. The rate of photosynthesis reached a maximum the 7th day after leaf emergence and declined thereafter. There was a high and significant correlation between the rate of photosynthesis per leaf area and the activities of the enzymes ribulose 5-phosphate kinase (r = 0.91), ribulose 1,5-bisphosphate (RuBP) carboxylase (r = 0.94), 3-phosphoglycerate (PGA) kinase (r = 0.82), and fructose 1,6-bisphosphatase (r = 0.80) per leaf area. There was not a significant correlation of photosynthesis rate with chlorophyll content. The rate of photosynthesis was strongly correlated with the level of PGA (r = 0.85) and inversely correlated with the level of triose phosphate (dihydroxyacetone phosphate and glyceraldehyde 3-phosphate) (r = 0.92). RuBP levels did not change much during leaf development; therefore photosynthesis rate was not correlated with the level of RuBP. The rate of photosynthesis was at a maximum when the ratio of PGA/triose phosphate was high, and when the ratio of RuBP/PGA was low. Although several enzymes change in parallel with leaf development, the metabolite changes suggest the greatest degree of control may be through RuBP carboxylase. The sucrose content of the leaf was highest under high rates of photosynthesis. There was no evidence that later in leaf development, photosynthesis (measured under high light and at 25 degrees C) was limited by utilization of photosynthate.

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PubMed1987

Hill Reaction, Hydrogen Peroxide Scavenging, and Ascorbate Peroxidase Activity of Mesophyll and Bundle Sheath Chloroplasts of NADP-Malic Enzyme Type C(4) Species.

Intact mesophyll and bundle sheath chloroplasts wee isolated from the NADP-malic enzyme type C(4) plants maize, sorghum (monocots), and Flaveria trinervia (dicot) using enzymic digestion and mechanical isolation techniques. Bundle sheath chloroplasts of this C(4) subgroup tend to be agranal and were previously reported to be deficient in photosystem II activity. However, following injection of intact bundle sheath chloroplasts into hypotonic medium, thylakoids had high Hill reaction activity, similar to that of mesophyll chloroplasts with the Hill oxidants dichlorophenolindophenol, p-benzoquinone, and ferricyanide (approximately 200 to 300 micromoles O(2) evolved per mg chlorophyll per hour). In comparison to that of mesophyll chloroplasts, the Hill reaction activity of bundle sheath chloroplasts of maize and sorghum was labile and lost activity during assay. Bundle sheath chloroplasts of maize also exhibited some capacity for 3-phosphoglycerate dependent O(2) evolution (29 to 58 micromoles O(2) evolved per milligram chlorophyll per hour). Both the mesophyll and bundle sheath chloroplasts were equally effective in light dependent scavenging of hydrogen peroxide. The results suggest that both chloroplast types have noncyclic electron transport and the enzymology to reduce hydrogen peroxide to water. The activities of ascorbate peroxidase from these chloroplast types was consistent with their capacity to scavenge hydrogen peroxide.

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