Whole Plant Regulation of Sulfur Nutrition of Deciduous Trees-Influences of the Environment

Abstract: The current view of sulfur nutrition is based on the source‐to‐sink relationship of carbohydrates. SO₄^2‐ reduction is thought to occur mainly in leaves. Surplus reduced sulfur must be transported out of the leaves, loaded into the phloem and transported to other tissues, in particular tissues assumed to be sink organs. However, it has not been proved that tissues which are sinks for carbohydrates are also sink organs for reduced sulfur. It is evident that sinks must communicate with sources, and vice versa, to signal demand and to transport the surplus of reduced sulfur that is produced. The demand‐driven control model of sulfur nutrition proposes that the tripeptide glutathione is the signal which regulates S nutrition of the whole plant at the level of SO₄^2‐ uptake. Acclimatization to environmental changes has been shown to result in several changes in S nutrition of deciduous trees: (i) Drought stress diminished SO₄^2‐ transport into the xylem, although the GSH content in lateral roots remained unaffected, possibly due to an overall reduction in water status. (ii) Flooding decreased APS reductase activity in the anoxic roots. This may be due to enhanced GSH transport to the roots, but it is more likely to be the result of a change in metabolism leading to diminished energy gain in the roots. (iii) Mycorrhization enhanced the GSH content in the phloem, while SO₄^2‐ uptake was not affected. This clearly goes against the demand‐driven control model. (iv) Under both short‐ and long‐term exposure to elevated pCO₂, the APS reductase activity in leaves and lateral roots did not correlate with the GSH contents therein. Therefore, it must be assumed that, under these conditions, regulation of S nutrition goes beyond the demand‐driven control model, and occurs within the network of other nutrient metabolism. (v) Atmospheric S in the form of H₂S enhanced the reduced sulfur content of the phloem and lateral roots. Under these conditions, the SO₄^2‐ loaded into the xylem decreased. It would appear that the demand‐driven control model of sulfur nutrition is not always valid in the case of deciduous trees.     

Phloem flow and sugar transport in Ricinus communis L. is inhibited under anoxic conditions of shoot or roots

Anoxic conditions should hamper the transport of sugar in the phloem, as this is an active process. The canopy is a carbohydrate source and the roots are carbohydrate sinks. By fumigating the shoot with N₂ or flooding the rhizosphere, anoxic conditions in the source or sink, respectively, were induced. Volume flow, velocity, conducting area and stationary water of the phloem were assessed by non‐invasive magnetic resonance imaging (MRI) flowmetry. Carbohydrates and δ¹³C in leaves, roots and phloem saps were determined. Following flooding, volume flow and conducting area of the phloem declined and sugar concentrations in leaves and in phloem saps slightly increased. Oligosaccharides appeared in phloem saps and after 3 d, carbon transport was reduced to 77%. Additionally, the xylem flow declined and showed finally no daily rhythm. Anoxia of the shoot resulted within minutes in a reduction of volume flow, conductive area and sucrose in the phloem sap decreased. Sugar transport dropped to below 40% by the end of the N₂ treatment. However, volume flow and phloem sap sugar tended to recover during the N₂ treatment. Both anoxia treatments hampered sugar transport. The flow velocity remained about constant, although phloem sap sugar concentration changed during treatments. Apparently, stored starch was remobilized under anoxia.     

Influence of glutathione (GSH) on net uptake of sulphate and sulphate transport in tobacco plants

Net uptake of sulphate by the roots, and its transport to theshoot was inhibited in intact tobacco plants by 1 h exposureof its roots to 0.1 or 1.0 mM GSH. The relative amount of sulphatetaken up that was transported to the shoot was not affectedby this treatment. Apparently, sulphate uptake, but not xylemloading of sulphate, was inhibited by GSH. Similar results wereobtained when L-cysteine was used instead of GSH. As L-cysteine,mediated inhibition of sulphate, net uptake was counteractedby exposure of the roots to BSO, GSH synthesis seems to be requiredfor inhibition of sulphate uptake by L-cysteine. However, exposureto 0.1 or 1.0mM GSH did not result In an enhanced GSH levelin roots or shoots. Also the cysteine contents remained unchanged.Small changes in metabolically active pools of GSH or cysteineseem to be responsible for the inhibition of sulphate uptakeand its transport to the shoot. Flap-feeding of GSH to a matureleaf resulted in an inhibition of net uptake of sulphate bythe roots and its transport to the shoot. These findings supportthe hypothesis that GSH produced in the leaves acts as a signalto control sulphur nutrition of plants. Key words: Sulphate transport, sulphate uptake, xylem loading, Nicotiana, Solanaceae, glutathione, cysteine, buthionine sulphoximine     

Over‐expression of bacterial γ‐glutamylcysteine synthetase (GSH1) in plastids affects photosynthesis,growth and sulphur metabolism in poplar (Populus tremula × Populus alba) dependent on the resulting γ‐glutamylcysteine and glutathione levels

We compared three transgenic poplar lines over‐expressing the bacterial γ‐glutamylcysteine synthetase (GSH1) targeted to plastids. Lines Lggs6 and Lggs12 have two copies, while line Lggs20 has three copies of the transgene. The three lines differ in their expression levels of the transgene and in the accumulation of γ‐glutamylcysteine (γ‐EC) and glutathione (GSH) in leaves, roots and phloem exudates. The lowest transgene expression level was observed in line Lggs6 which showed an increased growth, an enhanced rate of photosynthesis and a decreased excitation pressure (1‐qP). The latter typically represents a lower reduction state of the plastoquinone pool, and thereby facilitates electron flow along the electron transport chain. Line Lggs12 showed the highest transgene expression level, highest γ‐EC accumulation in leaves and highest GSH enrichment in phloem exudates and roots. This line also exhibited a reduced growth, and after a prolonged growth of 4.5 months, symptoms of leaf injury. Decreased maximum quantum yield (F_v/F_m) indicated down‐regulation of photosystem II reaction centre (PSII RC), which correlates with decreased PSII RC protein D1 (PsbA) and diminished light‐harvesting complex (Lhcb1). Potential effects of changes in chloroplastic and cytosolic GSH contents on photosynthesis, growth and the whole‐plant sulphur nutrition are discussed for each line.     

Interactive effects of elevated atmospheric CO2, mycorrhization and drought on long-distance transport of reduced sulphur in young pedunculate oak trees (Quercus robur L.)

Pedunculate oak (Quercus robur L.) was germinated and grown under nutrient non-limiting conditions for a total of 10–15 weeks at ambient CO₂ concentration and 1100 μmol mol^–1 CO₂ either in the presence or the absence of the mycorrhizal fungus Laccaria laccata. Half of the oak trees of these treatments were exposed to drought during final growth by suspending the water supply for 21 d. Mycorrhization and elevated atmospheric CO₂ each enhanced total plant biomass per tree. Whereas additional biomass accumulation of trees grown under elevated CO₂ was mainly attributed to increased growth of lateral roots, mycorrhization promoted shoot growth. Water deficiency reduced biomass accumulation without affecting relative water content, but this effect was more pronounced in mycorrhizal as compared to non-mycorrhizal trees. Elevated CO₂ partially prevented the development of drought stress, as indicated by leaf water potential, but did not counteract the negative effects of water deficiency on growth during the time studied. Enhanced biomass accumulation requires adaption in protein synthesis and, as a consequence, enhanced allocation of reduced sulphur produced in the leaves to growing tissues. Therefore, allocation of reduced sulphur from oak leaves was studied by flap-feeding radiolabelled GSH, the main long-distance transport form of reduced sulphur, to mature oak leaves. Export of radiolabel proceeded almost exclusively in basipetal direction to the roots. The rate of export of radioactivity out of the fed leaves was significantly enhanced under elevated CO₂, irrespective of mycorrhization. A higher proportion of the exported GSH was transported to the roots than to basipetal stem sections under elevated CO₂ as compared to ambient CO₂. Mycorrhization did not affect ³⁵S export out of the fed leaves, but the distribution of radiolabel between stem and roots was altered in preference of the stem. Trees exposed to drought did not show appreciable export of the ³⁵S radioactivity fed to the leaves when grown under ambient CO₂. Apparently, drought inhibited basipetal transport of reduced sulphur at the level of phloem loading and/or phloem transport. Elevated CO₂ seemed to counteract this effect of drought stress to some extent, since higher leaf water potentials and improved ³⁵S export out of the fed leaves was observed in oak trees exposed to drought and elevated CO₂ as compared to trees exposed to drought and ambient CO₂.     

Supply and partitioning of assimilates to roots of Medicago sativa L. and Lotus corniculatus L. under anoxia

Abstract A current explanation of the mechanism of flooding injury to roots suggests that oxygen deficiency depresses the supply of respirable carbohydrates sufficiently to inhibit fermentation. However, even though it has been shown that phloem transport of assimilate is sharply reduced to anaerobic roots, inhibition of assimilate metabolism has also been suggested to be an important factor. This study examines these hypotheses by relating assimilate supply and metabolic activity in anoxic roots of alfalfa (Medicago sativa L.), a flood-intolerant species, and birdsfoot trefoil (Lotus corniculatus L.), a flood-tolerant plant. Roots were made anoxic (severe O₂ deficiency) for 2, 4 or 6 d and shoots were labelled with ¹⁴CO₂. Assimilate transport to the roots and metabolism to structural components were significantly decreased in both species in response to anoxia. Trefoil exhibited significantly greater ¹⁴C incorporation into the residue fraction at 4 d anoxia than did alfalfa, and this was consistent with the greater flooding tolerance of trefoil. When assimilate supply to O₂-deficient roots was decreased by shoot shading, shoot fresh weight was reduced by both anoxia and light treatments. Root-soluble sugars were significantly decreased by shading but were greatly increased in response to anoxia. Root starch concentration also increased under anoxia. Root K⁺ concentration was reduced by anoxia only. The energy status (ATP/ADP) of roots was significantly decreased by shading; however, anoxia reduced the energy status only in unshaded plants. The data indicate that carbohydrate supply to anaerobic roots does not appear to be a limiting factor in the metabolic response of alfalfa roots. Alternatively, metabolism of assimilate in anoxic roots may be an important determinant of survival.     

Cysteine, gamma-Glutamylcysteine, and Glutathione Levels in Maize Seedlings : Distribution and Translocation in Normal and Cadmium-Exposed Plants

The levels of cysteine (Cys), γ-glutamylcysteine (γEC), and glutathione (GSH) were measured in the endosperms, scutella, roots, and shoots of maize (Zea mays L.) seedlings. GSH was the major thiol in roots, shoots, and scutella, Cys predominated in endosperms. The endosperm, scutellum, and functional phloem translocation were required for maintenance of GSH pools in roots and shoots of 6-day-old seedlings. Exposure of roots to 3 micromolar Cd, besides causing a decline in GSH, caused an accumulation of γEC, as if the activity of GSH synthetase was reduced in vivo. [³⁵S]Cys injected into endosperms of seedlings was partly metabolized to [³⁵S]sulfate. The scutella absorbed both [³⁵S]sulfate and [³⁵S]Cys and transformed 68 to 87% of the radioactivity into [³⁵S]GSH. [³⁵S]GSH was translocated to roots and shoots in proportion to the tissue fresh weight. Taken together, the data supported the hypothesis that Cys from the endosperm is absorbed by the scutellum and used to synthesize GSH for transfer through the phloem to the root and shoot. The estimated flux of GSH to the roots was 35 to 60 nanomoles per gram per hour, which totally accounted for the small gain in GSH in roots between days 6 and 7. For Cd-treated roots the GSH influx was similar, yet the GSH pool did not recover to control levels within 24 hours. The estimated flux of GSH to the entire shoot was like that to the roots; however, it was low (11-13 nanomoles per gram per hour) to the first leaf and high (76-135 nanomoles per gram per hour) to the second and younger leaves.     

Interactive effects of mycorrhization and elevated atmospheric CO2 on sulphur nutrition of young pedunculate oak (Quercus robur L.) trees

Pedunculate oak (Quercus robur L.) was germinated and grown at ambient CO₂ concentration and 650 μmol mol^?1 CO₂ in the presence and absence of the ectomycorrhizal fungus Laccaria laccata for a total of 22 weeks under nonlimiting nutrient conditions. Sulphate uptake, xylem loading and exudation were analysed in excised roots. Despite a relatively high affinity for sulphate (K_M= 1.6 mmol m^?3), the rates of sulphate uptake by excised lateral roots of mycorrhizal oak trees were low as compared to herbaceous plants. Rates of sulphate uptake were similar in mycorrhizal and non-mycorrhizal roots and were not affected by growth of the trees at elevated CO₂. However, the total uptake of sulphate per plant was enhanced by elevated CO₂ and further enhanced by elevated CO₂ and mycorrhization. Sulphate uptake seemed to be closely correlated with biomass accumulation under the conditions applied. The percentage of the sulphate taken up by mycorrhizal oak roots that was loaded into the xylem was an order of magnitude lower than previously observed for herbaceous plants. The rate of xylem loading was enhanced by mycorrhization and, in roots of mycorrhizal trees only, by growth at elevated CO₂. On a whole-plant basis this increase in xylem loading could only partially be explained by the increased growth of the trees. Elevated CO₂ and mycorrhization appeared to increase greatly the sulphate supply of the shoot at the level of xylem loading. For all treatments, calculated rates of sulphate exudation were significantly lower than the corresponding rates of xylem loading of sulphate. Radiolabelled sulphate loaded into the xylem therefore seems to be readily diluted by unlabelled sulphate during xylem transport. Allocation of reduced sulphur from oak leaves was studied by flap-feeding radiolabelled GSH to mature oak leaves. The rate of export of radioactivity from the fed leaves was 4–5 times higher in mycorrhizal oak trees grown at elevated CO₂ than in those grown at ambient CO₂. Export of radiolabel proceeded almost exclusively in a basipetal direction to the roots. From these experiments it can be concluded that, in mycorrhizal oak trees grown at elevated CO₂, the transport of sulphate to the shoot is increased at the level of xylem loading to enable increased sulphate reduction in the leaves. Increased sulphate reduction seems to be required for the enhanced allocation of reduced sulphur to the roots which is observed in trees grown at elevated CO₂. These changes in sulphate and reduced sulphur allocation may be a prerequisite for the positive effect of elevated CO₂ on growth of oak trees previously observed.     

Impact of pedospheric and atmospheric sulphur nutrition on sulphur metabolism of Allium cepa L., a species with a potential sink capacity for secondary sulphur compounds

Onion (Allium cepa L.) was able to use atmospheric H(2)S as sole sulphur source for growth. The foliarly absorbed H(2)S was rapidly metabolized into water-soluble, non-protein thiol compounds, including cysteine, and subsequently into other sulphur compounds in the shoots. In H(2)S-exposed plants, the accumulation of sulphur compounds in the shoots was nearly linear with the concentration (0.15-0.6 microl l(-1)) and duration of the exposure. Exposure of onion to H(2)S for up to 1 week did not affect the sulphur content of the roots. Secondary sulphur compounds formed a sink for the foliarly absorbed sulphide, and the sulphur accumulation upon H(2)S exposure could, for a great part, be ascribed to enhancement of the content of gamma-glutamyl peptides and/or alliins. Furthermore, there was a substantial increase in the sulphate content in the shoots upon H(2)S exposure. The accumulation of sulphate originated both from the pedosphere and from the oxidation of absorbed atmospheric sulphide, and/or from the degradation of accumulated secondary sulphur compounds. From studies on the interaction between atmospheric and pedospheric sulphur nutrition it was evident that H(2)S exposure did not result in a down-regulation of the sulphate uptake by the roots.     

Sulla carnosa modulates root invertase activity in response to the inhibition of long‐distance sucrose transport under magnesium deficiency

• Being the principal product of photosynthesis, sucrose is involved in many metabolic processes in plants. As magnesium (Mg) is phloem mobile, an inverse relationship between Mg shortage and sugar accumulation in leaves is often observed.
• Mg deficiency effects on carbohydrate contents and invertase activities were determined in Sulla carnosa Desf. Plants were grown hydroponically at different Mg concentrations (0.00, 0.01, 0.05 and 1.50 mM Mg) for one month.
• Mineral analysis showed that Mg contents were drastically diminished in shoots and roots mainly at 0.01 and 0.00 mM Mg. This decline was adversely associated with a significant increase of sucrose, fructose and mainly glucose in shoots of plants exposed to severe deficiency. By contrast, sugar contents were severely reduced in roots of these plants indicating an alteration of carbohydrate partitioning between shoots and roots of Mg‐deficient plants. Cell wall invertase activity was highly enhanced in roots of Mg‐deficient plants, while the vacuolar invertase activity was reduced at 0.00 mM Mg. This decrease of vacuolar invertase activity may indicate the sensibility of roots to Mg starvation resulting from sucrose transport inhibition. ¹⁴CO₂ labeling experiments were in accordance with these findings showing an inhibition of sucrose transport from source leaves to sink tissues (roots) under Mg depletion.
• The obtained results confirm previous findings about Mg involvement in photosynthate loading into phloem and add new insights into mechanisms evolved by S. carnosa to cope with Mg shortage in particular the increase of the activity of cell wall invertase.

     

Tetrathionate Disproportionation by Thiomonas intermedia K12

Thiomonas intermedia K12, a moderately acidophilic bacterium, which oxidises sulphur compounds, – exhibited the capability to use tetrathionate under oxic and anoxic conditions. Whereas under oxic conditions, the reduced sulphur tetrathionate compound was oxidised, under anoxic conditions, the organism disproportionated the compound. In both cases, trithionate and sulphate were produced but in different amounts. The results of the tetrathionate degradation experiments under oxic conditions pointed towards a cyclic degradation process with a transient formation of trithionate and sulphate as the final products, similar to the mechanism described for acidophilic sulphur compound oxidising bacteria. The results of the tetrathionate degradation experiments under anoxic conditions hinted to a partial reduction of tetrathionate to thiosulphate and a fractional oxidation to trithionate and sulphate. 4 M tetrathionate were converted to 6 M thiosulphate, 1 M trithionate, 1 M sulphate, and 8 M protons. The ΔG₀' of this reaction was found to be –16.1 kJ per mol tetrathionate degraded. Additionally, Thiomonas intermedia K12 grew under anoxic conditions with tetrathionate as the sole energy source. The cell numbers increased from 10⁵ as the start value to 10⁷/mL at the end. Organic compounds, excluding traces of yeast extract, did not enhance growth. Therefore, it is proposed that tetrathionate disproportionation is a novel lithotrophic metabolism, which allowed Thiomonas intermedia K12 to survive changing conditions of oxygen supply in sulphur‐compound‐rich environments and even to grow during this reaction. The extensive sulphur compound analysis was carried out by ion‐pair chromatography.     

Implications of nitrogen phloem loading for carbon metabolism and transport during Arabidopsis development

Metabolite transport processes and primary metabolism are highly interconnected. This study examined the importance of source-to-sink nitrogen partitioning, and associated nitrogen metabolism for carbon capture, transport and usage. Specifically, Arabidopsis aap8(AMINO ACID PERMEASE 8) mutant lines were analyzed to resolve the consequences of reduced amino acid phloem loading for source leaf carbon metabolism,sucrose phloem transport and sink development during vegetative and reproductive growth phase. Results showed that decreased amino acid transport had a negative effect on sink development of aap8 lines throughout the life cycle, leading to an overall decrease in plant biomass. During vegetative stage, photosynthesis and carbohydrate levels were decreased in aap8 leaves, while expression of carbon metabolism and transport genes, as well as sucrose phloem transport were not affected despite reduced sink strength.However, when aap8 plants transitioned to reproductive phase, carbon fixation and assimilation as well as sucrose partitioning to siliques were strongly decreased. Overall,this work demonstrates that phloem loading of nitrogen has varying implications for carbon fixation, assimilation and source-to-sink allocation depending on plant growth stage. It further suggests alterations in source-sink relationships, and regulation of carbon metabolism and transport by sink strength in a development-dependent manner.     

Bacterial oxidation of sulphide under denitrifying conditions

Anoxic H2S oxidation under denitrifying conditions produced sulphur and sulphate in almost equal proportions by an isolated Thiobacillus denitrificans. Under nitrate reducing conditions the rate of sulphide oxidation was approximately 0.9 g sulphide/g biomass h. Nitrate was reduced to nitrite and accumulated during sulphide oxidation. Above 100 mg nitrite/l, the sulphide oxidation rate declined and at 500 mg/l it was totally arrested. The optimum pH for the anoxic sulphide oxidation was around 7.5. Concentrations of sulphate 1500 mg/l and acetate 400 mg/l had no effect on anoxic sulphide oxidation.     

Effects of total submergence in saltwater on growth and leaf ion content of preflooded and unflooded Taxodium distichum saplings

Preconditioning by hypoxic condition would contribute higher tolerance to anoxic condition in physiological and metabolic aspects of various plant species. In expectation of improvement of salinity tolerance, effects of preconditioning with soil flooding on growth responses and photosynthetic activity in relation to Na⁺ content in leaves and roots were studied using 2-year-old Taxodium distichum saplings. A submergence experiment was performed at three salt concentrations (0, 4000, and 8000 ppm NaCl), and preconditioning was started 2 months prior to the beginning of submergence treatments. Continuous soil flooding with freshwater as a control did not inhibit growth in height and stimulated diameter increment of the saplings. Saplings submerged in saltwater and freshwater also showed no morphological changes during submergence treatments. However, leaf injury and shoot dieback were observed in drained saplings that had been submerged in saltwater. Na⁺ and K⁺ ion contents increased with increases in salt concentration in both preconditioned and unconditioned saplings, whereas ion contents in preconditioned saplings tended to be higher than those of unconditioned saplings.     

Manipulation of sucrose phloem and embryo loading affects pea leaf metabolism,carbon and nitrogen partitioning to sinks as well as seed storage pools

Seed development largely depends on the long‐distance transport of sucrose from photosynthetically active source leaves to seed sinks. This source‐to‐sink carbon allocation occurs in the phloem and requires the loading of sucrose into the leaf phloem and, at the sink end, its import into the growing embryo. Both tasks are achieved through the function of SUT sucrose transporters. In this study, we used vegetable peas (Pisum sativum L.), harvested for human consumption as immature seeds, as our model crop and simultaneously overexpressed the endogenous SUT1 transporter in the leaf phloem and in cotyledon epidermal cells where import into the embryo occurs. Using this ‘Push‐and‐Pull’ approach, the transgenic SUT1 plants displayed increased sucrose phloem loading and carbon movement from source to sink causing higher sucrose levels in developing pea seeds. The enhanced sucrose partitioning further led to improved photosynthesis rates, increased leaf nitrogen assimilation, and enhanced source‐to‐sink transport of amino acids. Embryo loading with amino acids was also increased in SUT1‐overexpressors resulting in higher protein levels in immature seeds. Further, transgenic plants grown until desiccation produced more seed protein and starch, as well as higher seed yields than the wild‐type plants. Together, the results demonstrate that the SUT1‐overexpressing plants with enhanced sucrose allocation to sinks adjust leaf carbon and nitrogen metabolism, and amino acid partitioning in order to accommodate the increased assimilate demand of growing seeds. We further provide evidence that the combined Push‐and‐Pull approach for enhancing carbon transport is a successful strategy for improving seed yields and nutritional quality in legumes.     

The effect of irradiance, p-chloromercuribenzenesulphonic acid and fusicoccin on the long distance transport in Zea mays L. seedlings

The system consisting of a few proportional detectors with appropriate electronic components was earlier developed for in vivo studies of long distance transport in whole maize seedlings. ¹⁴CO₂ assimilation rate (P_a), time of radioactivity appearing in the loading region (AT), transport speed in the leaf (TS_l), transport speed between the leaf and the roots (TS_r), the maximum radioactivity values detected in the leaf below the feeding area (R_l) and in the mesocotyl (R_r) from leaves to roots in maize seedlings were calculated from the obtained temporal profiles of radioactivity. The study was undertaken to follow the changes in separate steps of long distance transport in maize seedlings as affected by two light irradiances and application of p-chloromercuribenzenesulphonic acid and fusicoccin, with the aim to investigate different steps of long distance transport, particularly phloem loading. The method used allows to study in vivo the different aspects of long distance transport in maize seedlings, both qualitatively and quantitatively. It was shown that the characteristics obtained from the radioactivity profiles corresponded to different steps of long distance transport, as assimilate synthesis, phloem loading, and phloem translocation. It was also demonstrated that although active phloem loading participate in assimilate export from the leaves, assimilate transport along the maize seedling might undergo accordingly to assimilate gradient, particularly under light irradiance higher than during the growth.     

Utilization of host‐derived cysteine‐containing peptides overcomes the restricted sulphur metabolism of Campylobacter jejuni

The non‐glycolytic food‐borne pathogen Campylobacter jejuni successfully colonizes the intestine of various hosts in spite of its restricted metabolic properties. While several amino acids are known to be used by C. jejuni as energy sources, none of these have been found to be essential for growth. Here we demonstrated through phenotype microarray analysis that cysteine utilization increases the metabolic activity of C. jejuni. Furthermore, cysteine was crucial for its growth as C. jejuni was unable to synthesize it from sulphate or methionine. Our study showed that C. jejuni compensates this limited anabolic capacity by utilizing sulphide, thiosulphate, glutathione and the dipeptides γGlu–Cys, Cys–Gly and Gly–Cys as sulphur sources and cysteine precursors. A panel of C. jejuni mutants in putative peptidases and peptide transporters were generated and tested for their participation in the catabolism of the cysteine‐containing peptides, and the predicted transporter protein CJJ81176_0236 was discovered to facilitate the growth with the dipeptide Cys–Gly, Ile–Arg and Ile–Trp. It was named Campylobacter peptide transporter A (CptA) and is the first representative of the oligopeptide transporter OPT family demonstrated to participate in the glutathione‐derivative Cys–Gly catabolism in prokaryotes. Our study provides new insights into how host‐ and microbiota‐derived substrates like sulphide, thiosulphate and short peptides are used by C. jejuni to compensate its restricted metabolic capacities.     

Ecophysiological adaptations of black spruce (<Emphasis Type="Italic"> Picea mariana</Emphasis>) and tamarack (<Emphasis Type="Italic"> Larix laricina</Emphasis>) seedlings to flooding

Black spruce [ Picea mariana (Mill.) B.S.P.] and tamarack [ Larix laricina (Du Roi) K. Koch] are the predominant tree species in boreal peatlands. The effects of 34 days of flooding on morphological and physiological responses were investigated in the greenhouse for black spruce and tamarack seedlings in their second growing season (18 months old). Flooding resulted in reduced root hydraulic conductance, net assimilation rate and stomatal conductance and increased needle electrolyte leakage in both species. Flooded tamarack seedlings maintained a higher net assimilation rate and stomatal conductance compared to flooded black spruce. Flooded tamarack seedlings were also able to maintain higher root hydraulic conductance compared to flooded black spruce seedlings at a comparable time period of flooding. Root respiration declined in both species under flooding. Sugar concentration increased in shoots while decreasing in roots in both species under flooding. Needles of flooded black spruce appeared necrotic and electrolyte leakage increased over time with flooding and remained significantly higher than in flooded tamarack seedlings. No visible damage symptoms were observed in flooded tamarack seedlings. Flooded tamarack seedlings developed adventitious roots beginning 16 days after the start of flooding treatment. Adventitious roots exhibited significantly higher root hydraulic conductivity than similarly sized flooded tamarack roots. Flooded black spruce lacked any such morphological adaptation. These results suggest that tamarack is better able to adjust both morphologically and physiologically to prolonged soil flooding than black spruce seedlings.     

Sulphur metabolism in Thiorhodaceae. II. stoichiometric relationship of CO2 fixation to oxidation of hydrogen sulphide and intracellular sulphur inChromatium okenii

Stoichiometry of sulphide and intracellular sulphur oxidation in connection with CO₂ fixation was studied inChromatium okenii. The equipment used was a special stirred cuvette with a rapid-sampling arrangement, which allowed short-time experiments with illuminated bacterial suspensions under anaerobic conditions. Turnover of the sulphur compounds is controlled by a linear CO₂ fixation rate which amounts to 0.069µmoles of CO₂/min mg of cell protein at light saturation. Van Niel's equations for bacterial photosynthesis could be confirmed for short periods under the condition that sulphate is produced during increase of intracellular sulphur; i.e., oxidation of sulphide and of intracellular sulphur do not occur consecutively but simultaneously. The full oxidation rate of intracellular sulphur starts after complete consumption of sulphide. The time during which sulphide is oxidized to intracellular sulphur amounts to 1/3–1/4 of the time necessary for the complete quantitative oxidation of the sulphide to sulphate.     

Influence of gibberellic acid on <Superscript>14</Superscript>CO<Subscript>2</Subscript> metabolism,growth, and production of alkaloids in <Emphasis Type="Italic">Catharanthus roseus</Emphasis>

Changes in growth parameters, carbon assimilation efficiency, and utilization of ¹⁴CO₂ assimilate into alkaloids in plant parts were investigated at whole plant level by treatment of Catharanthus roseus with gibberellic acid (GA). Application of GA (1 000 g m⁻³) resulted in changes in leaf morphology, increase in stem elongation, leaf and internode length, plant height, and decrease in biomass content. Phenotypic changes were accompanied by decrease in contents of chlorophylls and in photosynthetic capacity. GA application resulted in higher % of total alkaloids accumulated in leaf, stem, and root. GA treatment produced negative phenotypic response in total biomass production but positive response in content of total alkaloids in leaf, stem, and roots. ¹⁴C assimilate partitioning revealed that ¹⁴C distribution in leaf, stem, and root of treated plants was higher than in untreated and variations were observed in contents of metabolites as sugars, amino acids, and organic acids. Capacity to utilize current fixed ¹⁴C derived assimilates for alkaloid production was high in leaves but low in roots of treated plants despite higher content of ¹⁴C metabolites such as sugars, amino acids, and organic acids. In spite of higher availability of metabolites, their utilization into alkaloid production is low in GA-treated roots.