The formation of iron plaques on roots and rhizomes of the seagrass Cymodocea serrulata (R. Brown) Ascherson with implications for sulphide intrusion

Despite the fact that iron plaque formation is ubiquitous in aquatic macrophytes and has been known for several decades, there are few reports of plaque occurrence in seagrasses to date. Herein we present the first microscopical observation and chemical quantification of iron (Fe) plaques on the shoots, rhizomes and roots of the seagrass Cymodocea serrulata (R. Brown) Ascherson collected from intertidal seagrass beds in Thailand. Plaques were observed on shoot bases, rhizomes and roots with the highest concentrations of iron in the plaques from the roots, reaching an average of 509 μmol gDW⁻¹. Interestingly, the most negative stable sulphur isotope (δ³⁴S) values, indicating H₂S intrusion into the plants occurred in the sampling site with the most intense root oxidizing capacity, as indicated by a greater Fe plaque formation. These apparently contradictory findings may be attributed to oxidizing capacity of root tips and root hairs sufficient to promote Fe(III) deposition in the rhizosphere, preceding deposition of plaques on the roots. While this rhizosphere oxidation may result in a more efficient sulphide detoxification during the day photosynthetic phase, root tips and hairs may serve as vulnerable sites for sulphide intrusion at night. The presence of Fe plaque on C. serrulata roots and rhizomes reveals the complexity of seagrass–sediment interactions and deserves further attention to understand if this is a local phenomenon or a newly discovered adaptive mechanism in seagrasses.     

Strategies for lead distribution in organs of <Emphasis Type="Italic">Phragmites australis</Emphasis> (Cav.) Trin. ex Steud. (Common reed) subjected to Pb pollution in flood and drought environments

Heavy metal allocation in clonal organs, stems, leaves, and roots has not been systematically studied for rhizomatous perennial plants. Here, pot experiments have been designed to investigate lead (Pb) distribution in different organs of Phragmites australis (Cav.) Trin. ex Steud. Common reed subjected to 0–4500 mg Pb kg^?1 under both flood and drought conditions. In either water treatment, Pb concentrations in offspring shoots were lower than in parent shoots; however, the opposite response was observed for biomass allocation for which parent shoots protected offspring shoots. Lower allocation of Pb to leaves rather than stems in offspring shoots could be a protective strategy of leaves under flood conditions. Lower Pb allocation to rhizomes is better for rhizome growth. This further provides energy for the growth of buds and offspring shoots, because the rhizome biomass and the number of buds and offspring shoots were not significantly inhibited by Pb levels of?≤?3000 mg kg^?1 in the flooded environment. These Pb allocation strategies could enhance the resistance capacity of reeds to Pb contamination by stabilizing population propagation and productivity, especially at Pb levels of?≤?3000 mg kg^?1 under flood conditions.     

Nitrate and nitrite in interstitial waters of eelgrass beds in relation to the rhizosphere

The distribution of nitrate and nitrite in the interstitial water of the sediment of eelgrass (Zostera marina) bed of Izembek Lagoon, Alaska, were investigated. Their concentrations were relatively high (0 to 9.8 μg-at.N·1^?1, average 4.8 for nitrate; 0 to 4.0 μ-at.N·1^?1, average 1.9 for nitrite) although the sediments were anoxic and contained hydrogen sulphide. The rates of bacterial denitrification measured by ¹⁵N tracer technique ranged from 0.49×10^?10 to 1.2 × 10^?9 g-atN·g^?1·h^?1. When a steady state is maintained, the loss of nitrate and nitrite must be balanced by their production by bacterial nitrification. Experimentally determined rate of nitrification in the sediment was of the same order. A model experiment demonstrated that oxygen is transported from leaves to rhizomes and roots of eelgrass and released into the sediment. The oxygen is used for nitrification in the rhizosphere in anoxic sediments.     

Effects of lead contamination on the clonal propagative ability of Phragmites australis (common reed) grown in wet and dry environments

Clonal propagation is important for the survival and maintenance of the common reed Phragmites australis. Pot culture experiments were conducted to investigate the effects of lead (Pb) concentration (0, 500, 1500, 3000, 4500 mg·kg^?1) and water stress on the clonal reproductive ability of this species. The Pb concentration found in plant organs, in decreasing order, was roots >shoots >rhizomes. There was a negative relationship between the growth of clonal propagative modules (excluding axillary shoot buds) and Pb concentrations, which caused a decrease in biomass, rhizome growth and number of axillary and apical rhizome buds. Daughter axillary shoots exhibited a tolerance strategy, with no significant change in their number; the axillary and apical rhizome buds, daughter apical rhizome shoots and rhizomes exhibited compensatory growth during the late stage of Pb (excluding 4500 mg·kg^?1) treatment in a wet environment. Pb applications above 500 mg·kg^?1 reduced these parameters significantly in the drought treatment, except for the number of axillary shoot buds, which did not change. Our results indicate that clonal propagative resistance to Pb contamination can occur via tolerance strategies, compensatory growth and a Pb allocation strategy, enabling these reeds to maintain population stability in wet environments. However, clonal modular growth and reproductive ability were inhibited significantly by the interaction between drought and Pb, which would cause a decline in P. australis populations in a dry environment. Lead concentrations of 4500 and 500 mg·kg^?1 in soils might meet or exceed the Pb tolerance threshold of clonally propagated reeds in wet and dry environments, respectively.     

Induction of non-protein thiols and phytochelatins by cadmium in Eichhornia crassipes

Abstract

Impact of root Cd concentration on production of cysteine, non-protein thiols (NP-SH), glutathione (GSH), reduced glutathione (GSSG), and phytochelatins (PCs) in Eichhornia crassipes exposed to different dilutions of brass and electroplating industry effluent (25%, 50%, and 75%), and synthetic metal solutions of Cd alone (1, 2.5, and 3.5?ppm) and with Cr (1?ppm Cd + 1?ppm Cr, 2.5?ppm Cd + 3?ppm Cr, and 3.5?ppm Cd + 4?ppm Cr) was assessed in a 45?days study. Different treatments were used to understand and compare differential antioxidant defense response of plant under practical drainage (effluent) and experimental synthetic solutions. The production of NP-SH and cysteine was maximum under 2.5?ppm Cd + 3?ppm Cr treatments i.e., 1.78?µmol/g fw and 288?nmol/g fw, respectively. The content of GSH declined whereas that of GSSG increased progressively with exposure duration in all treatments. HPLC chromatograms revealed that the concentrations of PC2, PC3, and PC4 (248, 250, and 288?nmol-SH equiv.g^?1 fw, respectively) were maximum under 1?ppm Cd, 1?ppm Cd + 1?ppm Cr, and 2.5?ppm Cd + 3?ppm Cr treatments, respectively. PC2, PC3, and PC4 concentrations increased with Cd accumulation in the range 812–1354?µg/g dry wt, 1354–2032?µg/g dry wt and 2032–3200?µg/g dry wt, respectively. Thus, the study establishes a direct proportionality relationship between concentration/length of phytochelatins and root Cd concentrations, upto threshold limits, in E. crassipes.     

Development and evaluation of Trichoderma asperellum preparation for control of sheath blight of rice (Oryza sativa L.)

Sheath blight, which is caused by Rhizoctonia solani, is a disease that majorly impacts rice production. A biocontrol agent used for control rice sheath blight must be sprayed on the stem at specific times during rice growth, a process that is labour-intensive and renders the antagonist vulnerable to environmental factors. In this study, Trichoderma asperellum T12 was used to produce preparation by solid-state fermentation using a surface-response method. Rice hull was selected as a carrier based on its ability to sustain the T12 floating in the water and protect T12 from ultraviolet irradiation. The production of a T12-based preparation required 32% wheat bran, 7% inoculum, 2.3 g kg^?1 (NH₄)₂SO₄ and 65% water content, with fermentation at 27.5°C for 30 days and agitation every six days. The preparation demonstrated 90% biocontrol efficacy and significantly (P > 0.05) increased the seed-set rate and 1000-grain weight as compared with the pathogen treatment. The population of Trichoderma on the surface of rice leaf sheath in the treatment applied with T12 preparation increased from 232 cfu (colony forming units) g^?1 fw (fresh weight) to 436 cfu g^?1 fw during rice growth stage, which was significantly (P > 0.05) higher than pathogen treatment. The population of R. solani on the leaf sheath increased from 41 cfu g^?1 fw to 271 cfu g^?1 fw in the pathogen treatment, while remained stable (P > 0.05) at level of 10–23 cfu g^?1 fw in T12 preparation applied treatment. Biocontrol of sheath blight by the addition of the preparation to the soil is effective and decreases the costs of agro-industrial waste disposal.     

Assimilation and translocation of nitrogen and carbon in Curcuma alismatifolia Gagnep

Curcuma or Siam tulip (Curcuma alismatifolia Gagnep.) is an ornamental flowering plant with two underground storage organs, rhizomes and storage roots. Characteristics of N and C assimilation and transport in curcuma were investigated. The plants were treated with ¹⁵NH₄⁺ + ¹⁵NO₃^? and ¹³CO₂ at 10, 13 or 21 weeks after planting. Plants were sampled at several stages up to 32 weeks. The C stored in old storage roots was used rapidly during the first 10 weeks; after which N stored in old rhizomes and old storage roots were used. The daily gain in C depending on photosynthesis was remarkably high between 10 and 21 weeks. However, the daily gain in N was relatively constant throughout the growth period. The ¹⁵N absorbed at 10 weeks was initially accumulated in leaves and roots, but some was transported to flowering organs at 13 weeks. At harvest, 41% of ¹⁵N was recovered in new rhizomes and 17% in new storage roots. After ¹³CO₂ exposure at 10 and 13 weeks, the distribution of ¹³C among organs was relatively constant in subsequent stages. When given ¹³CO₂ at 21 weeks, a large amount of labelled C was recovered in new storage roots and new rhizomes at harvest. Both new rhizomes and new storage roots stored N and C, however, rhizomes played a more important role in supplying N, while storage roots provided C.     

Die-back of Phragmites australis: influence on the distribution and rate of sediment methanogenesis

Methanogenesis was measured during the summer of 1994, in sediment coresand bulk samples from a Phragmites australis wetland in northern Jutland,Denmark. We compared sediment from healthy reed and dying-back reed, andan open lagoon resulting from die-back. Cores revealed variability withdepth and between sites, with the highest rates coinciding with layers oforganic gyttja, and negligible methane production from the underlying sandbase. Methanogenesis rates in the lagoon and die back sites were higher(up to 100–150 nmol h^-1 g^-1dry wt. sediment) than in the healthy reed (50–80 nmolh^-1 g^-1), with the highest rates being recordedfrom May to July. At these times, methanogenesis was markedly temperature-limited; samples incubated at 30 °C anon-limiting temperature, gave rates as high as 200–400nmol h^-1 g^-1 for the lagoon and die-backareas and 150 nmol h^-1 g^-1 for the healthyarea. Addition of 8 mM acetate and H₂/CO₂headspace suggested that both acetate-fermenting andCO₂-reducing bacteria were present. Acetate additions suggested some co-limitation by substrate availability, with acetate limitation occurring in the healthy site during July and in the die-back site during August. Lower rates during August, especially in the healthy area, were associated with low water levels which resulted in more oxidized sediments. The data reveal highly variable methanogenesis in the sediment which, when considered with sediment depths, indicates that sites of Phragmites die-back have significantly greater rates of anaerobic mineralization than surrounding healthy wetland, and may be intense sources of methane.     

TRYPTOPHAN TRANSPORT ACROSS THE BLOOD-BRAIN BARRIER DURING ACUTE HEPATIC FAILURE

Abstract— Tryptophan transport across the blood-brain barrier was studied using a single injection dual isotope label technique, in the following three conditions: normal rats, rats with portacaval shunts, and rats with portacaval shunts followed 65 h later by hepatic artery ligation. In both normal rats and those with acute hepatic failure the tryptophan transport system was found to be comprised of two kinetically distinct components. One component was saturable and obeyed Michaelis-Menten kinetics (normal: V_max= 19.5 nmol.min^?1.g^?1. K_m= 113 μM; hepatic failure: V_max, = 33.8 nmol.min^?1.g^?1, K_m= 108 μM), and the second was a high capacity system which transported tryptophan in direct proportion to concentration over the range tested (normal: K= 0.026 ml.min^?1.g^?1; hepatic failure: K= 0.067 ml.min^?1.g^?1). Since the saturable low capacity component transports several neutral amino acids, and their collective plasma concentration is high in relation to the individual K_ms, tryptophan transport by this component is reduced by competitive inhibition under physiological conditions. Thus it was calculated that in normal rats approx 40% of tryptophan influx occurs via the high capacity system. During acute hepatic failure transport via both components was increased substantially, approximately doubling the rate of tryptophan penetration of the blood-brain barrier at all concentrations tested. The contribution by the high capacity component became even more significant than in normal rats, accounting for about 75% of all tryptophan passage from plasma to brain. Brain tryptophan content was 29.9 nmol/g in normal rats and rose to 45.2 nmol/g in rats with portacaval shunts and 50.5 nmol/g in those with acute hepatic failure, correlating with the increased rate of tryptophan transport. In a previous study we found that plasma competing amino acids were greatly increased during acute hepatic failure. Calculations predict that these increased concentrations would cause a reduction in tryptophan transport by the low capacity system. However, because of the increase in the rate of transport by the high capacity component, net tryptophan entry across the blood-brain barrier was actually increased. This increased rate of transport clearly contributes to the increased content of brain tryptophan found during hepatic failure.     

Fatty acid composition of lipids in vegetative organs of the halophyte Suaeda altissima under different levels of salinity

Qualitative and quantitative composition of fatty acids (FA) in the lipids of vegetative organs of the halophyte Suaeda altissima (L.) Pall. grown at different NaCl concentrations in nutrient solution was studied. Along with this, the biomass of these organs, the content of water and Na⁺, Cl^?, and K⁺ ions in them, and the ultrastructure of root and leaf cells were determined. At both low (1 mM) and high (750 mM) NaCl concentrations in nutrient solution, plants could maintain growth and water content in organs, demonstrating a noticeable increase in the dry weight and a slight increase in the water content at 250 mM NaCl. At all NaCl concentrations in nutrient solution, S. altissima tissues contained a relatively high K⁺ amount. Under salinity, Na⁺ and Cl^? ions contributed substantially into the increase in the cell osmotic pressure, i.e., a decrease in their water potential; in the absence of salinity, K⁺ fulfilled this function. In the cells of both roots and leaves, NaCl stimulated endo- and exocytosis, supposedly involved in the vesicular compound transport. 750 mM NaCl induced plasmolysis and changes in the membrane structure, which can be interpreted as degradation processes. Under optimal NaCl concentration in medium (250 mM), the content of lipids in plant aboveground organs per fresh weight was more than 2.5-fold higher than under 1 or 750 mM NaCl, whereas in the roots opposite patten was observed. When plants were grown under non-optimal conditions, substantial changes occurred in the qualitative and quantitative FA composition in lipids of both aboveground organs and roots. Observed changes are discussed in relation to processes underlying S. altissima salt tolerance and those of disintegration occurring at the high external NaCl concentration (750 mM).     

The effect of zinc on the growth and physiological and biochemical parameters in seedlings of Eruca sativa (L.) (Rocket)

Eruca sativa seedlings were treated with different Zn concentrations (0, 250, 500, 1,000, 2,000 μg g^?1 dried growth medium) under controlled conditions. The seedlings were harvested 20 days after Zn treatment. Physiological parameters, such as root and shoot length, fresh and dry weight, were measured and Zn content of roots and shoots was determined. Furthermore, various biochemical parameters were studied on E. sativa leaves: enzymatic antioxidants, such as superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (POD), ascorbate peroxidase (APX), and non-enzymatic antioxidants, such as ascorbate, non-protein thiols. Malondialdehyde, which is an index of lipid peroxidation, was assayed. Zn treatment did not have any significant effect on fresh and dry weights, whereas 500 μg g^?1 Zn increased root growth significantly (p < 0.05). Zn accumulated in roots 2–8 times more than it did in leaves. Lipid peroxidation increased in proportion with the increase in Zn. Although a decrease in SOD and CAT activities at increased Zn was found, a significant increase in APX and POD was observed at 500 and 1,000 μg g^?1 Zn, respectively. In addition, an increase in the amounts of non-protein thiols and total AsA (Ascorbate) was observed with the increase in Zn.     

Growth and physiological responses to copper stress in a halophyte Spartina alterniflora (Poaceae)

A study quantifying the effects of different copper (Cu) concentrations (50, 200, 800 and 1,000 mg kg^?1 Cu) on Cu bioaccumulation and physiological responses of Spartina alterniflora was conducted. Plant biomass and Cu accumulation were determined. Plant height, tiller number, chlorophyll, leaf electrolyte leakage rate (ELR), malondialdehyde (MDA), proline, soluble sugar, and organic acids were also measured. The results showed that S. alterniflora mainly accumulated Cu in fine roots. No significant changes of biomass of fine roots were detected except for obvious reduction under 1,000 mg kg^?1 Cu. In leaves, rhizomes and fine roots, the highest Cu accumulations were detected under 800 mg kg^?1 Cu. The highest Cu accumulation in stem was revealed under 200 mg kg^?1 Cu. Plant height decreased under 1,000 mg kg^?1 Cu; chlorophyll content reduced under >50 mg kg^?1 Cu; levels of ELR and MDA increased under >200 mg kg^?1 Cu. However, osmotic components such as proline and soluble sugar were accumulated to cope with higher Cu stresses (800 and 1,000 mg kg^?1). Further, oxalic and citric acids were positively related with Cu contents in leaves and stems, suggesting that oxalic and citric acids may be related to Cu detoxification in aboveground parts of S. alterniflora. However, in above and belowground parts, no detoxification function of ascorbic and fumaric acids was observed due to unchanged or decreased trend under Cu stress.     

CADMIUM AND MANGANESE ACCUMULATION IN PHYTOLACCA AMERICANA L. AND THE ROLES OF NON-PROTEIN THIOLS AND ORGANIC ACIDS

Phytolacca americana L. can accumulate large amounts of heavy metals in its aerial tissues, especially cadmium (Cd) and manganese (Mn). It has great potential for use in phytoextraction of metals from multi-metal-contaminated soils. This study was conducted to further investigate the Cd- and Mn-tolerance strategies of this plant. Concentrations of non-protein thiols (NPTs) and phytochelatins (PCs) in leaves and roots increased significantly as the concentration of Cd in solution increased. The molar ratios of PCs:soluble Cd ranged from 1.8 to 3.6 in roots and 8.1 to 31.6 in leaves, suggesting that the cellular response involving PC synthesis was sufficient to complex Cd ions in the cytosol, especially that of leaves. In contrast, excess Mn treatments did not result in a significant increase in NPT or PC concentrations in leaves or roots. Oxalic acid concentrations in leaves of plants exposed to 2 or 20 mM Mn reached 69.4 to 89.3 mg (0.771 to 0.992 mmol) g^–1 dry weight, respectively, which was approximately 3.7- to 8.6-fold higher than the Mn level in the 0.6 M HCl extract. Thus, oxalic acid may play an important role in the detoxification of Mn.     

Mechanisms for tolerance of very high tissue phosphorus concentrations in Ptilotus polystachyus

Study of plants with unusual phosphorus (P) physiology may assist development of more P‐efficient crops. Ptilotus polystachyus grows well at high P supply, when shoot P concentrations ( [P] ) may exceed 40 mg P g^?1 dry matter (DM). We explored the P physiology of P. polystachyus seedlings grown in nutrient solution with 0–5 mM P. In addition, young leaves and roots of soil‐grown plants were used for cryo‐scanning electron microscopy and X‐ray microanalysis. No P‐toxicity symptoms were observed, even at 5 mM P in solution. Shoot DM was similar at 0.1 and 1.0 mM P in solution, but was ～14% lower at 2 and 5 mM P. At 1 mM P, [P] was 36, 18, 14 and 11 mg P g^?1 DM in mature leaves, young leaves, stems and roots, respectively. Leaf potassium, calcium and magnesium concentrations increased with increasing P supply. Leaf epidermal and palisade mesophyll cells had similar [P]. The root epidermis and most cortical cells had senesced, even in young roots. We conclude that preferential accumulation of P in mature leaves, accumulation of balancing cations and uniform distribution of P across leaf cell types allow P. polystachyus to tolerate very high leaf [P].     

Effects of ethanol on growth of rice seedlings

Effects of ethanol, the end product of ethanolic fermentation, on the growth of rice (Oryza sativa L.) seedlings were determined as a means of evaluating growth responses under anoxia. The ethanol concentrations in roots and coleoptiles of the seedlings subjected to 48 h-anoxia, and in their culture medium were 23 and 32 µmol g^–1 fresh weight, and 19 µmol ml^–1, respectively. The growth of the roots and coleoptiles of the seedlings was restricted by exogenous ethanol at concentrations above 50 mM and 100 mM, respectively, suggesting that the roots are more sensitive to ethanol than the coleoptiles.     

Genotypic variation in 'early warning signals' from roots in drying soil: intrinsic differences in ABA synthesising capacity rather than root density determines total ABA 'message' in cowpea (Vigna unguiculata L.)

In a comparison of six cowpea cultivars, we determined the variation in abscisic acid (ABA) production as an ‘early warning signal’ produced in response to drought stress. By imposing drought only to the upper 20 cm rooting zone, we compared the rates of ABA synthesis relative to (i) total root mass and (ii) inherent variation per unit root mass. We were able to relate the intensity of the stress response to these two factors, and determine which is quantitatively more important as the primary signal indicating responsiveness to drought stress. Plants were grown in 1.2 m long columns and a soil drying treatment imposed in such a way that that upper roots were in dry soil and deep roots in soil at field capacity. Relative water contents (RWC) of stressed plants were similar and not significantly different from those of well watered controls. However, roots accumulated ABA in the dehydrated zone, where root water content ranged from 10–12 g g^?1 DW. The soil moisture contents and root ‐water contents in the dry zone were similar for each of the different varieties. However, the ABA contents were significantly different in drought‐stressed (upper) roots and ranged from 7.82 nmol g^?1 DW in cv. APC 689 to 16.02 nmol g^?1 DW in cv. APC 370, such that for varieties with similar overall root weights (e.g. APC 580 and APC 540) the different ABA contents were related to the capacity for ABA synthesis. The relationship between stomatal conductance and total root ABA was assessed, with a negative relation (r= 0.90, n= 24, P= 0.05) suggesting that the intrinsic capacity of cowpea varieties for ABA synthesis could play an important role in regulating stomatal conductance in a drying soil and provide useful selection criteria for tolerance to drought stress.     

Optimization of culture conditions for the production of polysaccharides and kinsenoside from the rhizome cultures of <Emphasis Type="Italic">Anoectochilus roxburghii</Emphasis> (Wall.) Lindl.

The present study designed two sets of experiments by using the uniform design method and investigated the effects of medium components on the accumulation of bioactive compounds (polysaccharide and kinsenoside) in rhizomes, in order to select a suitable culture medium for the rhizome suspension culture of Anoectochilus roxburghii (Wall.) Lindl. Among the combinations of Murashige and Skoog (MS) medium strengths and plant growth regulator (benzylaminopurine, BA; kinetin, KT; and α-naphthaleneacetic acid, NAA) concentrations, and the combinations of nitrogen, phosphorus, and sucrose concentrations, the maximum yield of polysaccharides and kinsenoside was achieved with 0.75 × MS?+?2.0 mg L^?1 BA?+?0.2 mg L^?1 KT?+?0.5 mg L^?1 NAA and 45 mM nitrogen?+?0.93 mM phosphorus?+?35 g L^?1 sucrose, respectively. Therefore, the optimal rhizome suspension culture medium was 0.75 × MS medium supplemented with 2.0 mg L^?1 BA, 0.2 mg L^?1 KT, 0.5 mg L^?1 NAA, and 35 g L^?1 sucrose. Yeast extract (YE) enhanced bioactive compound accumulation in rhizomes. The polysaccharide and kinsenoside production was significantly improved when 75 mg L^?1 YE was added to the culture medium after 30 d of rhizome suspension culture; 8.3 g L^?1 of polysaccharide and 6.1 g L^?1 of kinsenoside were obtained after 4 d of YE treatment. The 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity of YE-treated rhizomes was higher than that of YE-untreated rhizomes, demonstrating enhanced antioxidant activity of the treated bioreactor-cultured rhizomes.     

Characterization of phosphate absorption in maize root cortex segments

Uptake of phosphate ions by 1 mm segments of isolated maize root cortex layers was studied. Cortex segments (from roots of 8 days old maize plants) absorb phosphate ions from 1 mM KH₂PO₄ in 0.2 mM CaSCO₄ at the average rate of 34.3 ±3.2 μg P_i g^?1 (fr. m.) h^?1,i.e. 0.35± 0.02 μmol P_i g^?1 (fr. m.) h^?1. Phosphate uptake considerably increases after a certain period of “augmentation”,i.e. washing in aerated 0.2 mM CaSO₄. This increase is completely blocked by the presence of 10 μg ml^?1 cycloheximide. The relation of uptake rate to phosphate concentration in the medium was shown to have 3 phases in the concentration range of 0.02 - 40 mM. Transition points were found between 0.8–1 mM and 10–20 mM. Following K_m and V_max values were found: K_m[mM] : 0.37 - 3.82 - 27.67 V_max[μg P_i g^?1 (fr. m.) h^?1] : 3.33 - 39.40 - 66.67 We have found no sharp pH optimum for phosphate uptake. It proceeds at almost constant rate till pH 6.0 and then the uptake rate drops with increasing pH. At low phosphate concentrations (1 mM) the lowest uptake rate was found at 5 and 13 °C, while the uptake is higher at 5 °C than at 13 °C at phosphate concentrations higher than 1 mM. At these concentrations uptake rate at 35 °C is lower than at 25 °C. Phosphate uptake considerably decreased in anaerobic conditions. DNP and iodoacetate (0.1 mM) completely blocked phosphate uptake from 1 mM KH₂PO₄, while uptake from 5 and 10 mM KH₂PO₄ was left unaffected by these substances. The inhibitors of active - SH groups NEM and PCMB inhibited phosphate uptake: 10^?3 M NEM by 81.6%, 10⁴ M NEM by 42% and 10^?4 M PCMB by 42%.     

Sulphate reduction in the hypolimnion and sediments of Lake Kinneret, Israel

1. Lake Kinneret is a warm (13–30°C) monomictic lake. Between January and June a heavy annual bloom of the dinoflagellate Peridinium gatunense dominates phytoplankton biomass (250 g m^?2). At the beginning of the summer, degradation and decomposition of the Peridinium biomass occurs, serving as a trigger for intense sulphate reduction in the hypolimnion and sediments. 2. The rates of sulphate reduction in the sediments varied seasonally from 12 to 1700 nmol SO₄.^?2 reduced cm^?3 day^?1 in December and July, respectively. The availability of organic matter and sulphate is high in June after the crash of the Peridinium bloom and the beginning of stratification and is lowest in December before overturn. 3. Sulphate concentrations in the hypolimnion range between 0.52 mM and 0.20 mM during mixing (January-April) and before overturn (December), respectively. The depletion in sulphate in the hypolimnion is stoichiometrically correlated to the increase in sulphide. The lake is not depleted of sulphate at any time, so the sulphate reduction process in Lake Kinneret is not limited by sulphate concentrations except in the sediments just before overturn.     

Comparison of nitrogen uptake in the roots and rhizomes of <Emphasis Type="Italic">Leymus chinensis</Emphasis>

Leymus chinensis (Trin.) Tzvel is a rhizomatous grass species in the Eastern Eurasian steppe zone that is often limited by low soil nitrogen availability. Although a previous study showed that the rhizomes of L. chinensis have the capacity to take up nitrogen, the importance of such uptake for nitrogen nutrition is unclear. Moreover, little is known regarding the inorganic nitrogen uptake kinetics of roots and rhizomes in response to nitrogen status. Here, we first found that ammonium is preferred over nitrate and glycine for L. chinensis growth. Using the ¹⁵N-labelling method, we found that the rate of ion influx into roots was approximately five-fold higher than into rhizomes under the same nitrogen content, and the ion influxes into roots and rhizomes under 0.05 mM N were greater than in the presence of 3 mM N, especially in the form of NH₄⁺. Using a non-invasive micro-test technique, we characterised the patterns of NH₄⁺ and NO₃^– fluxes in the root mature zone, root tip, rhizome mature zone, and rhizome tip following incubation in the solution with different N compounds and different N concentrations. These results suggest a dynamic balance between the uptake, utilisation, and excretion of nitrogen in L. chinensis.