The effect of Cl- upon the sensitivity of starch-containing and starch-deficient stomata and guard cell protoplasts towards potassium ions,fusicoccin and abscisic acid

Chloride ions are necessary to compensate for the positively charged potassium ions imported into guard cells of Allium cepa L. during stomatal opening. Therefore an external Cl^- supply of intact Allium plants is important. But high levels of chloride have been found to reduce the sensitivity of the starch-lacking stomata and isolated guard cell protoplasts (GCPs) from Allium to potassium ions, fusicoccin and abscisic acid. Furthermore, with high levels of chloride, malate anions disappear from the guard cells of Allium, a finding which contrasts with situation in Vicia where the stomatal sensitivity to K⁺ ions, fusicoccin and ABA is not influenced by Cl^- ions and malate levels are unaffected. It is suggested that the absence of malate as a proton yielding primer inhibits the mechanism of H⁺/K⁺ exchange in Allium.Abbreviations ABA abscisic acid - FC fusicoccin - GCPs guard cell protoplasts     

CO2 and malate metabolism in starch-containing and starch-lacking guard-cell protoplasts

Isolated, purified mesophyll and guard-cell protoplasts of Vicia faba L. and Allium cepa L. were exposed to ¹⁴CO₂ in the light and in the dark. The guard-cell protoplasts of Vicia and Allium did not show any labeling in phosphorylated products of the Calvin cycle, thus appearing to lack the ability to reduce CO₂ photosynthetically. In Vicia, high amounts of radioactivity (35%) appeared in starch after 60-s pulses of ¹⁴CO₂ both in the light and in the dark. Presumably, the ¹⁴CO₂ is fixed into the malate via PEP carboxylase and then metabolized into starch as the final product of gluconeogenesis. This is supported by the fact that guard-cell protoplasts exposed to malic acid uniformly labeled with ¹⁴CO₂ showed high amounts of labeled starch after the incubation, whereas cells labeled with [4-¹⁴C]malate had minimal amounts of labeled starch (1/120).In contrast, the starch-deficient Allium, guard-cell protoplasts did not show any significant ¹⁴CO₂ fixation. However, adding PEP to an homogenate stimulated ¹⁴CO₂ uptake, thus supporting the interpretation that the presence of starch as a source of PEP is necessary for incorporating CO₂ and delivering malate. With starch-containing Vicia guard-cell protoplasts, the correlation between changes in volume and the interconversion of malate and starch was demonstrated. It was shown that the rapid gluconeogenic conversion of malate into starch prevents an increase of the volume of the protoplasts, whereas the degradation of starch to malate is accompanied by a swelling of the protoplasts.Abbreviations GCPs guard-cell protoplasts - MCPs mesophyll cell protoplasts - PEP phosphoenolpyruvate - DTT dithiothreitol - 3-PGA 3-phosphoglyceric acid - RiBP ribulose 1,5 bisphosphate - MDH malate dehydrogenase - MES 2-(N-morpholino)ethane sulfonic acid - CAM crassulacean acid metabolism     

The mechanism of stomatal movement in Allium cepa L.

In the guard cells of Allium cepa leaves, no starch was found either when the stomata were open or closed. The lack of other soluble polysaccharides that could be hydrolyzed during the opening reaction of the stomata (Schnabl, Planta 1977, in press) leads to the question, how is the osmotic effect, which is the basis of the stomatal movement, achieved in Allium? It is shown in this paper, by histochemical and microprobe analyses, that in Allium — as in other plant species—the K⁺ concentration of the guard cells increases during stomatal opening. The charges of the K⁺ ions in the guard cells seem to be fully compensated by imported Cl^- ions. This could mean that if starch is present in the guard cells, as in the majority of plant species, its major role in the mechanism of stomatal movement is to deliver the cuunteranions for the imported K⁺ ions.     

Levels of short-chain fatty acids and of abscisic acid in water-stressed and non-stressed leaves and their effects on stomata in epidermal strips and excised leaves

Straight-chain saturated fatty acids (C6-C11) and abscisic acid (ABA) accumulate in the leaves of Phaseolus vulgaris L. and Hordeum vulgare L. under water stress. ABA and certain of the fatty acids, particularly decanoic and undecanoic acid, can inhibit stomatal opening and cause stomatal closure in epidermal strips of Commelina communis L. depending on the incubating medium used. 10^-4 M (±)-ABA inhibits opening in media containing either high or relatively low concentrations of KCl but causes closure only in the latter medium. The fatty acids (at 10^-4 M) prevent opening in both media while significant closure of open stomata was caused only by undecanoic acid in both media and, additionally, by decanoic acid in the low-KCl medium. 10^-4 M formic acid also caused stomatal closure and prevented opening to significant extents in the low-KCl medium (it was not tested in the high-KCl medium). The efficacy of undecanoic acid in causing 50% inhibition of opening is about three orders of magnitude lower than that of ABA. At a concentration of 10^-3 M, nonanoic, decanoic and particularly undecanoic acid and all-trans-farnesol cause increased cell leakage in Beta vulgaris L. root tissue. Undecanoic acid (10^-4 M) also causes some loss of guard cell integrity in C. communis within 1.5 h of treatment. ABA (10^-4 M) reduces transpiration rates in barley and C. communis leaves when applied via the transpiration stream but decanoic and undecanoic acids did not have this effect. Transpiration was not affected when ABA or the fatty acids were applied to the leaf surfaces.Abbreviations ABA abscisic acid - RWC relative water content - SCFA short-chain fatty acids Deceased May 1977     

The relationship of biomass,polysaccharide and H2 formation in the wild-type and nifA/nifB mutants of Rhodobacter capsulatus

The production of biomass, polysaccharide storage material and H₂ from malate was studied in the wild-type and mutants RdcI, RdcII and RdcI/cII of Rhodobacter capsulatus. The mutants are defective in either copy I, copy II or both copies of the nitrogenase genes nifA and nifB. Stationary phase levels of biomass, polysaccharide and H₂ were determined in phototrophic batch cultures grown with 30 mM of d,l-malate and either 2, 5, or 8 mM of ammonium or 7 mM of glutamate. Calculation of the amounts of malate converted into the three products revealed that, at 8 mM of ammonium and 7 mM of glutamate, malate consumption and product formation were balanced. But with decreasing ammonium concentrations malate not converted into biomass was utilized with decreasing efficiency in polysaccharide and H₂ formation. This suggests formation of unknown products at the lower ammonium concentrations. Under conditions of optimal N supply, 80% of the malate not used for biomass production was converted by the wild-type and strain RdcII to H₂ and CO₂. Mutant RdcI exhibited slightly decreased H₂ production. The double mutant did not evolve H₂ but accumulated increased amounts of polysaccharide. However, the amounts of polysaccharide were lower than should be expected if all of the spare malate, not utilized by the double mutant for H₂ production, was converted into storage material. This and incomplete conversion of malate into known products at low ammonium supplies suggests that polysaccharide accumulation does not compete with the process of H₂ formation for malate.     

Uptake and metabolism of carbohydrates by epidermal tissue

Isolated epidermis of Commelina communis L. and Tulipa gesneriana L. assimilated ¹⁴CO₂ into malic acid and its metabolites but not into sugars or their phosphates; epidermis could not reduce CO₂ by photosynthesis and therefore must be heterotrophic (Raschke and Dittrich, 1977). If, however, isolated epidermis of Commelina communis was placed on prelabelled mesophyll (obtained by an exposure to ¹⁴CO₂ for 10 min), radioactive sugars appeared in the epidermis, most likely by transfer from the mesophyll. Of the radioactivity in the epidermis, 60% was in sucrose, glucose, fructose, 3-phosphoglyceric acid and sugar phosphates. During a 10-min exposure to ¹⁴CO₂, epidermis in situ incorporated 16 times more radioactivity than isolated epidermal strips. Isolated epidermis of Commelina communis and Tulipa gesneriana took up ¹⁴C-labelled glucose-1-phosphate (without dephosphorylation), glucose, sucrose and maltose. These substances were transformed into other sugars and, simultaneously, into malic acid. Carbons-1 through-3 of malic acid in guard cells can thus be derived from sugars. Radioactivity appeared also in the hydrolysate of the ethanol-insoluble residue and in compounds of the tricarboxylic-acid cycle, including their transamination products. The hydrolysate contained glucose as the only radioactive compound. Radioactivity in the hydrolysate was therefore considered an indication of starch. Starch formation in the epidermis began within 5 min of exposure to glucose-1-phosphate. Autoradiograms of epidermal sections were blackened above the guard cells. Formation of starch from radioactive sugars therefore occurred predominantly in these cells. Epidermis of tulip consistently incorporated more ¹⁴C into malic and aspartic acids than that of Commelina communis (e.g. after a 4-h exposure to [¹⁴C]glucose in the dark, epidermis, with open stomata, of tulip contained 31% of its radioactivity in malate and aspartate, that of Commelina communis only 2%). The results of our experiments allow a merger of the old observations on the involvement of starch metabolism in stomatal movement with the more recent recognition of ion transfer and acid metabolism as causes of stomatal opening and closing.Abbreviation G-1-P glucose-1-phosphate     

Potassium Chloride as Stomatal Osmoticum in Allium cepa L., a Species Devoid of Starch in Guard Cells

K⁺ and Cl⁻ contents of guard cells and of ordinary epidermal cells were determined in epidermal samples of Allium cepa L. by electron probe microanalysis; malate contents of the same samples were determined by enzymic oxidation. KCl was, in general, the major osmoticum in guard cells, irrespective of whether stomata had opened on leaves or in epidermal strips floating on solutions. The solute requirement varied between 50 and 110 femtomoles KCl per micrometer increase in aperture per pair of guard cells. Stomata did not open on solutions of K iminodiacetate, presumably because its anion could not be taken up. Stomata opened if KCl or KBr was provided. Taken together, the results indicate that the absence of starch from guard cells deprived them of the ability to produce malate in amounts of osmotic consequence and that the presence of absorbable Cl⁻ (or Br⁻) was necessary for stomatal opening.     

Malate metabolism in isolated epidermis of Commelina communis L. in relation to stomatal functioning

Epidermal strips with closed stomata were exposed to malic acid labelled with ¹⁴C either uniformly or in 4-C only. During incubation with [U-¹⁴C]malate, radioactivity appeared in products of the tricarboxylic-acid cycle and in transamination products within 10 min, in sugars after 2 h. Hardly any radioactivity was found in sugars if [4-¹⁴C]malate had been offered. This difference in the degree of labelling of sugars indicates that gluconeogenesis can occur in epidermal tissue, involving the decarboxylation of malate. Epidermis incubated with labelled malate was hydrolyzed after extraction with aqueous ethanol. The hydrolysate contained glucose as the only radioactive product, indicating that starch had been formed from malate. Microautoradiograms were black above stomatal complexes, showing that the latter were sites of starch formation. In order to follow the fate of malate during stomatal closure, malate was labelled in guard cells by exposing epidermes with open stomata to ¹⁴CO₂ and then initiating stomatal closure. Of the radioactive fixation products of CO₂ only malate was released into the water on which the epidermal samples floated; the epidermal strips retained some of the malate and all of its metabolites. In the case of rapid stomatal closure initiated by abscisic acid and completed within 5 min, 63% of the radioactivity was in the malate released, 22% in the malate retained, the remainder in aspartate, glutamate, and citrate. We conclude that during stomatal closing guard cells can dispose of malate by release, gluconeogenesis, and consumption in the tricarboxylic-acid cycle.Abbreviations ABA abscisic acid - NAD nicotinamide adenine dinucleotide - NADP nicotinamide adenine dinucleotide phosphate - PEP phosphoenolpyruvate     

C4 photosynthesis in Spartina townsendii at low and high temperatures

The metabolism of ¹⁴CO₂ in the cool temperate saltmarsh grass Spartina townsendii was investigated in plants grown in their natural habitats at two temperatures. Both in the spring at 10°C and in the late summer at 25°C radioactivity was initially incorporated into the organic acids malate and aspartate and then transferred to 3-phosphoglycerate in the manner characteristic of the C₄ pathway of photosynthesis. Metabolism was not disrupted at the lower temperature as in some C₄ plants. Radioactivity was transferred more slowly from malate into alanine, glycine and serine at 10°C, but sugars were labelled equally at both temperatures.     

The roles of malate and aspartate in C4 photosynthetic metabolism of Flaveria bidentis (L.)

In C₄ grasses belonging to the NADP-malic enzyme-type subgroup, malate is considered to be the predominant C₄ acid metabolized during C₄ photosynthesis, and the bundle sheath cell chloroplasts contain very little photosystem-II (PSII) activity. The present studies showed that Flaveria bidentis (L.), an NADP-malic enzyme-type C₄ dicotyledon, had substantial PSII activity in bundle sheath cells and that malate and aspartate apparently contributed about equally to the transfer of CO₂ to bundle sheath cells. Preparations of bundle sheath cells and chloroplasts isolated from these cells evolved O₂ at rates between 1.5 and 2 mol · min^–1 · mg^–1 chlorophyll (Chl) in the light in response to adding either 3-phosphoglycerate plus HCO ₃ ^– or aspartate plus 2-oxoglutarate. Rates of more than 2 mol O₂ · min^–1 · mg^–1 Chl were recorded for cells provided with both sets of these substrates. With bundle sheath cell preparations the maximum rates of light-dependent CO₂ fixation and malate decarboxylation to pyruvate recorded were about 1.7 mol · min^–1 · mg^–1 Chl. Compared with NADP-malic enzyme-type grass species, F. bidentis bundle sheath cells contained much higher activities of NADP-malate dehydrogenase and of aspartate and alanine aminotransferases. Time-course and pulse-chase studies following the kinetics of radiolabelling of the C-4 carboxyl of C₄ acids from ¹⁴CO₂ indicated that the photosynthetically active pool of malate was about twice the size of the aspartate pool. However, there was strong evidence for a rapid flux of carbon through both these pools. Possible routes of aspartate metabolism and the relationship between this metabolism and PSII activity in bundle sheath cells are considered.Abbreviations DHAP dihydroxyacetone phosphate - NADP-ME(-type) NADP-malic enzyme (type) - NADP-MDH NADP-malate dehydrogenase - OAA oxaloacetic acid - 2-OG 2-oxoglutarate - PEP phosphoenolpyruvate - PGA 3-phosphoglycerate - Pi orthophosphate - Ru5P ribulose 5-phosphate     

Malate-sensitive anion channels enable guard cells to sense changes in the ambient CO2 concentration

Malate is a characteristic metabolite in the photosynthesis of C4 and CAM plants. Furthermore, changes in the intracellular concentration of this organic acid provide part of the osmotic motor for guard cells. Since alterations in the malate concentration influence the photosynthetic capacity on one side and stomatal action on the other, it was studied whether the extracellular malate level represents an indicator of changes in the ambient CO₂ concentration and a key regulator of ion transport in guard cells. Here it is demonstrated that alterations in the ambient CO₂ level modify the extracellular malate concentration of Vicia faba leaves. Elevated external malate caused stomatal closure in a concentration-dependent manner (K_m^mal = 0.3 mM). Slight variations in the external malate concentration strongly regulate the voltage-dependent properties of GCAC1, an anion-release channel in the plasma membrane of guard cells. Superfusion of guard cell protoplasts with malate levels in the physiological range (K_m^mal = 0.4 mM) caused the voltage gate to shift towards the resting potential of the cell-activating GCAC1. Single-channel conductance was dependent on the extracellular chloride concentration (K_m^Cl = 3 mM). In the absence of extracellular chloride the plasma membrane lacked anion conductance until the addition of malate induced channel opening. Isophthalate was a powerful agonist in both malate-induced processes, channel regulation and stomatal closure, indicating that modulation of GCAC1 is a key step in stomatal action. It was thus concluded that feedback regulation of volume and turgor with respect to the ambient CO₂ concentration via malate-sensitive anion channels may provide a CO₂ sensor to guard cells.     

Oxidation of organic compounds to CO2 with sulfur or thiosulfate as electron acceptor in the anaerobic hyperthermophilic archaea Thermoproteus tenax and Pyrobaculum islandicum proceeds via the citric acid cycle

The oxidation of organic compounds with elemental sulfur or thiosulfate as electron acceptor was studied in the anaerobic hyperthermophilic archaea Thermoproteus tenax and Pyrobaculum islandicum. T. tenax was grown on either glucose or casamino acids and sulfur; P. islandicum on peptone and either elemental sulfur or thiosulfate as electron acceptor. During exponential growth only CO₂ and H₂S rather than acetate, alanine, lactate, and succinate were detected as fermentation products of both organisms; the ratio of CO₂/H₂S formed was 1:2 with elemental sulfur and 1:1 with thiosulfate as electron acceptor. Cell extracts of T. tenax and P. islandicum contained all enzymes of the citric acid cycle in catabolic activities: citrate synthase, aconitase, isocitrate dehydrogenase (NADP⁺-reducing), oxoglutarate: benzylviologen oxidoreductase, succinyl-CoA synthetase, succinate dehydrogenase, fumarase and malate dehydrogenase (NAD⁺-reducing). Carbon monoxide dehydrogenase activity was not detected. We conclude that in T. tenax and P. islandicum organic compounds are completely oxidized to CO₂ with sulfur or thiosulfate as electron acceptor and that acetyl-CoA oxidation to CO₂ proceeds via the citric acid cycle.     

Stomatal responses to carbon dioxide of isolated epidermis from a C3 plant,the Argenteum mutant of Pisum sativum L., and a crassulacean-acid-metabolism plant Kalanchoë daigremontiana Hamet et Perr

The response of stomata in isolated epidermis to the concentration of CO₂ in the gaseous phase was examined in a C₃ species, the Argenteum mutant of Pisum sativum, and a crassulacean-acid-metabolism (CAM) species, Kalanchoë daigremontiana. Epidermis from leaves of both species was incubated on buffer solutions in the presence of air containing various volume fractions of CO₂ (0 to 10000·10^–6). In both species and in the light and in darkness, the effect of CO₂ was to inhibit stomatal opening, the maximum inhibition of opening occurring in the range 0 to 360·10^–6. The inhibition of opening per unit change in concentration was greatest between volume fractions of 0 and 240·10^–6. There was little further closure above the volume fraction of 360·10^–6, i.e. approximately ambient concentration of CO₂. Thus, although leaves of CAM species may experience much higher internal concentrations of CO₂ in the light than those of C₃ plants, this does not affect the sensitivity of their stomata to CO₂ concentration or the range over which they respond. Stomatal responses to CO₂ were similar in both the light and the dark, indicating that effects of CO₂ on stomata occur via mechanisms which are independent of light. The responses of stomata to CO₂ in the gaseous phase took place without the treatments changing the pH of the buffered solutions. Thus it is unlikely that CO₂ elicited stomatal movement by changing either the pH or the HCO ₃ ^– /CO ₃ ^2- equilibria. It is suggested that the concentration of dissolved unhydrated CO₂ may be the effector of stomatal movement and that its activity is related to its reactivity with amines.     

Some properties of carbohydrate and C4-dicarboxylic acid utilization negative mutants ofRhizobium leguminosarum biovarphaseoli strain P121

After NTG treatment of the very effective wild type strain P121 ofRhizobium leguminosarum biovarphaseoli, mutants defective in the utilization of sugars or organic acids were obtained. All the mutants nodulated the cultivar Goldie ofPhaseolus vulgaris. The arabinose, fructose, glucose and pyruvate utilization mutants formed nodules similar in shape and size to the nodules formed by the wild type strain. These mutants exhibited an acetylene reduction activity significantly lower than the activity observed with the wild type strain. All the C₄-dicarboxylic acid utilization mutatns, formed ineffective nodules that did not show a significant acetylene reduction activity. The C₄-dicarboxylic acids uptake system is apparently inducible in the free-living bacteria of strain P121. When P121 cells were grown on glucose in the presence of 2.5 mM malate, the rate of glucose-dependent O₂ consumption significantly decreased suggesting the presence of a catabolite repression-like phenomenon. Isolated bacteroids of strain P121, under the experimental conditions used, were able to oxidize succinate, fumarate or malate but did not oxidize pyruvate, glucose, fructose or sucrose.     

Isolation and characterization of an extracellular hydroxyproline-rich glycoprotein and a mannose-rich polysaccharide from Eudorina californica (Shaw)

Mucilage and colony walls of E. californica were separated from the cells by homogenization, filtration, and differential centrifugation. The chief components of the mucilage were a high-molecular-weight (MW) hydroxyproline-rich glycoprotein and a very high-MW polysaccharide in the proportions 47% and 34%, respectively. The glycoprotein consisted of galactose, arabinose, xylose and an unidentified neutral sugar; and the amino acids cysteine, aspartic acid, glutamic acid, arginine, lysine, glycine, serine, methionine, histidine, alanine, proline, hydroxyproline, tyrosine, threonine, valine, phenylalanine, isoleucine and leucine. The principal sugar of the polysaccharide was mannose. The chemical composition of the colony walls was essentially the same as that of the glycoprotein in the mucilage except that there was almost twice as much hydroxyproline. Also the protein content of the colony walls was 34% while that of the glycoprotein in the mucilage was 22%. No glucose, sugar acids or nucleic acids were found in the extracellular matrix.     

Anti-mitotic and anti-genotoxic effects of Plantago lanceolata aqueous extract on Allium cepa root tip meristem cells

Plantago is the most important genus of Plantaginaceae family and is used in traditional medicine around the world for different purposes. Plantago coronopus L., Plantago major L., Plantago media L. and Plantago lanceolata L. are most commonly used species of Plantago in traditional medicine in Turkey. The main goal of this study was to investigate the eventual anti-mitotic and anti-genotoxic effects of P. lanceolata L. leaf aqueous extracts (15 g/L and 30 g/L) on Allium cepa L. root tip meristem cells which were treated with 0.7% hydrogen peroxide. For this purpose, two different experiments were performed under the same conditions. In the first experiment, Allium cepa onion bulbs were treated with 0.7% H₂O₂ for 1 h. After the H₂O₂ treatment, the onion bulbs were treated with two different concentrations (15 g/L and 30 g/L) of P. lanceolata extracts for 24 h. In the second experiment, A. cepa onion bulbs were treated with two different extract concentrations (15 g/L and 30 g/L) for 24 h and then with 0.7% H₂O₂ for 1 h. The test concentrations were determined according to doses which are recommended in alternative medicinal usage by people. As positive and negative control 0.7% H₂O₂ and tap water was used, respectively. As a result, it was determined that aqueous extracts reduced mitotic index and chromosome aberrations in treatment groups in comparison with controls. These results showed that P. lanceolata aqueous extracts have anti-mitotic and anti-genotoxic effects.     

Enhanced seed development in the interspecific cross Allium cepa x A. sphaerocephalon through ovary culture

Allium sphaerocephalon pollen tubes grew into styles and penetrated micropyles of Allium cepa, but ovules started to degenerate about 16 days after pollination and no seeds developed. Seeds developed in vitro in ovaries excised from flowers 4 and 7 days after pollination. Seven weeks after culture initiation, seeds had grown in 4 of 96 excised ovaries, cultured on BDS medium supplemented with GA₃. Although the culture medium supported seed maturation within excised ovaries of self-pollinated A. cepa flowers, no viable hybrid seeds were recovered from crosses with A. sphaerocephalon. Extended post-fertilization barriers may have restrained development of hybrid embryos in vitro. Ovary culture followed by in ovulo embryo rescue may be feasible for distant-species hybridization in Allium.     

Oxydation of substrates in organic acids utilization negative mutants and the wild typeRhizobium meliloti strain S'14

Two mutants defective in succinate utilization were isolated by NTG mutagenesis of the effective wild typeRhizobium meliloti strain S₁₄. The mutants used carbon sources in a fashion similar to strain S₁₄, but they were not able to grow on succinate, fumarate or malate. The mutants nodulated alfalfa plants but did not exhibit any nitrogenase activity. The mutants oxidized glucose and fructose, but were not able to oxidize organic acids. Cultured free-living bacteria of strain S₁₄ appeared to have an inducible C₄-dicarboxylic acid uptake system and a constitutive glucose uptake system. When S₁₄ cells were grown on glucose in the presence of 5mM or more succinate or malate, the rate of glucose-dependent O₂ consumption significantly decreased suggesting the presence of a catabolite repression like phenomenom. Contribution no. 301, Station de Recherches, Agriculture Canada.     

The effect of lead on the cell cycle in the root meristem ofAllium cepa L.

Summary Allium cepa (L.) adventitious roots were treated with lead (2.5 mg of Pb²⁺ [from Pb(NO₃)₂] per dm³) for 30–72 h. The cell cycle was studied by pulse labeling with [³H]thymidine. Mitotic activity kinetics, occurrence of disturbed mitoses (c-mitoses), and level of DNA synthesis were examined. It was found that lead prolonged the cell cycle and that cells in two phases of the cycle, G₂ and S, differed in their sensitivity to lead. Cells in G₂ were more sensitive; lead lengthened their cycle by 216% and disturbed the course of cell division by causing c-mitoses. Cells in S phase were less sensitive. Their cell cycle was longer by 55%. They went through their G₂ phase without major disturbances, mitosis in these cells was normal. During treatment ofA. cepa with lead, its destructive effects on cells were exerted only during the first few hours (around 6 h) of incubation. That is when the inhibition of mitotic activity, numerous disturbances of cell division, a decline in the number of cells synthesizing DNA, and a lower level of DNA synthesis were observed. As the incubation continued, the above processes were found to return to normal. In the discussion, data are presented supporting the hypothesis that during the initial period of exposure ofA. cepa to lead, this metal enters both the root apoplast and symplast, exerting a destructive effect on cells, while later, lead penetrates only into the root apoplast, and in this way remains harmless to cells.     

Variations in short term products of inorganic carbon fixation in exponential and stationary phase cultures of Aphanocapsa 6308

Aphanocapsa 6308 metabolizes both NaHCO₃ and Na₂CO₃. The short term incorporation (5-s) metabolic pattern and the patterns of incorporation of bicarbonate for exponential versus stationary phase cultures differ, however. Cells were equilibrated for 10 min in air and distilled water prior to injection of either NaH¹⁴CO₃ at pH 8.0, or Na₂ ¹⁴CO₃ at pH 11.0. Hot ethanol extracts were analyzed via paper chromatography and autoradiography for products of CO₂ fixation. At 5 s, malate (51.5%) predominates slightly as a primary bicarbonate fixation product over 3-phosphoglycerate (40.3%); 3-phosphoglycerate is the primary product of carbonate fixation. At 60 s, the carbonate and bicarbonate labelling patterns are similar. Cells in stationary phase fix in 5 s a greater proportion of bicarbonate into malate (36% vs. 14% for 3-phosphoglycerate) than do cells in exponential growth. Likewise, 60 s incorporations show a large amount of bicarbonate fixed into aspartate (30.9%) in stationary phase cells over that of exponential phase (11.6%). These data suggest an operative C₄ pathway for purposes not related to carbohydrate synthesis but rather as compensation for the incomplete tricarboxylic acid cycle in cyanobacteria. The enhancement of both aspartate fixation and CO₂ fixation into citrulline in stationary phase correlates with an increase in cyanophycin granule production which requires both aspartate and arginine.Nonstandard Abbreviations 3-PGA 3-phosphoglyceric acid - TCA tricarboxylic acid