Growth and nutrient uptake by two species of Elodea in experimental conditions and their role in nutrient accumulation in a macrophyte-dominated lake

The capacity of Elodea nuttallii (Planch.) St. John and Elodea canadensis Michx. to remove nitrogen from water was evaluated in laboratory experiment. The growth rate of plants and their effect on the nitrogen level of hypertrophic Lake Zwemlust (the Netherlands) as well as on lake water enriched with nitrogen were investigated. The plants grew best in water enriched with up to 2 mg NH₄-Nl^–1 and 2 mg NH₄-Nl^–1 plus 2 mg NO₃ Nl^–1. During a 14 day experiment, plants absorbed from 75% to 90% of nitrogen. Higher nitrogen concentration than 4 mg l^–1 had a negative effect on growth of both species. Elodea nuttallii and E. canadensis prefer NOinf4/^p+ over NOinf4/^p– when both ions were present in water in equal concentrations.     

A New Sensitive Bioassay for Determination of Microbially Available Phosphorus in Water

The content of assimilable organic carbon has been proposed to control the growth of microbes in drinking water. However, recent results have shown that there are regions where it is predominantly phosphorus which determines the extent of microbial growth in drinking waters. Even a very low concentration of phosphorus (below 1 μg of P liter⁻¹) can promote extensive microbial growth. We present here a new sensitive method to determine microbially available phosphorus concentrations in water down to 0.08 μg of P liter⁻¹. The method is a bioassay in which the analysis of phosphorus in a water sample is based on maximum growth of Pseudomonas fluorescens P17 when the energy supply and inorganic nutrients, with the exception of phosphorus, do not limit bacterial growth. Maximum growth (CFU) in the water sample is related to the concentration of phosphorus with the factor 373,200 ± 9,400 CFU/μg of PO₄-P. A linear relationship was found between cell growth and phosphorus concentration between 0.05 to 10 μg of PO₄-P liter⁻¹. The content of microbially available phosphorus in Finnish drinking waters varied from 0.1 to 10.2 μg of P liter⁻¹ (median, 0.60 μg of P liter⁻¹).     

Responses of three invasive aquatic macrophytes to nutrient enrichment do not explain their observed field displacements

In some eutrophic inland waters the invasive aquatic macrophyte Elodea canadensis has been displaced by the morphologically similar species Elodea nuttallii and subsequently E. nuttallii by Lagarosiphon major. We investigated whether differences in the responses of these species and their associated epiphytic floras to five nutrient loadings in the range 30–480 μg L⁻¹ P and 0.21–3.36 mg L⁻¹ N could explain their observed field displacements. The mean relative growth rate (RGR) of E. nuttallii (RGR 0.086 d⁻¹) was significantly higher than that of either E. canadensis (RGR 0.066 d⁻¹) or L. major (RGR 0.063 d⁻¹). All three species exhibited a plastic morphological response to increasing nutrient loadings with mean root weights reduced at the highest nutrient loading compared with the lowest loading by 33, 75 and 56% for E. canandensis, E. nuttallii and L. major, respectively. Mean tissue nitrogen concentrations increased significantly with increasing nutrient loading, with concentrations in E. canadensis (1.83–2.10% dry wt.) significantly higher than either E. nuttallii (1.56–2.10% dry wt.) or L. major (1.50–1.90% dry wt.). Tissue phosphorus concentrations likewise increased with increasing nutrient loadings although this trend was not as pronounced. Epiphyte biomass per unit photosynthetic surface area (PSA) was significantly higher on E. canadensis than on either E. nuttallii or L. major, but did not increase significantly with increasing nutrient loadings. We suggest that differences in species responses to nutrient enrichment do not explain the species displacements observed in the field. E. nuttallii's higher RGR may, regardless of nutrient supply, enable this species to shade out neighbouring species and outpace the establishment of algae on its leaves.     

Effects of phosphorus deficiency on the photosynthesis and respiration of leaves of sugar beet

Phosphorus deficiency was induced in sugar beet plants (Beta vulgaris L. var. F5855441), cultured hydroponically under standardized environmental conditions, by removal of phosphorus from the nutrient supply at the ten leaf stage 28 days after germination. CO₂ and water vapor exchange rates of individual attached leaves were determined at intervals after P cutoff. Leaves grown with an adequate nutrient supply attained net rates of photosynthetic CO₂ fixation of 125 ng CO₂ cm⁻² sec⁻¹ at saturating irradiance, 25 C, and an ambient CO₂ concentration of about 250 μl l⁻¹. After P cutoff, leaf phosphorus concentrations decreased as did net rates of photosynthetic CO₂ uptake, photorespiratory evolution of CO₂ into CO₂-free air, and dark respiration, so that 30 days after cutoff these rates were about one-third of the control rates. The decrease in photosynthetic rates during the first 15 days after cutoff was associated with increased mesophyll resistance (r_m) which increased from 2.4 to 4.9 sec cm⁻¹, while from 15 to 30 days there was an increase in leaf (mainly stomatal) diffusion resistance (r_l′) from 0.3 to 0.9 sec cm⁻¹, as well as further increases in r_m to 8.5 sec cm⁻¹. Leaf diffusion resistance (r_l′) was increased greatly by low P at low but not at high irradiance, r_l′ for plants at low P reaching values as high as 9 sec cm⁻¹.     

Photosynthetic HCO(3) Utilization and OH Excretion in Aquatic Angiosperms: LIGHT-INDUCED pH CHANGES AT THE LEAF SURFACE

The utilization of HCO₃⁻ as carbon source for photosynthesis by aquatic angiosperms results in the production of 1 mole OH⁻ for each mole CO₂ assimilated. The OH⁻ ions are subsequently released to the medium. In several Potamogeton and Elodea species, the site of the HCO₃⁻ influx and OH⁻ efflux are spatially separated. Described here are light- and dark-induced pH changes at the lower and upper epidermis of the leaves of Potamogeton lucens, Elodea densa, and Elodea canadensis.     

Phosphorus Accumulation and Sorption in Calcareous Soil under Long-Term Fertilization

Application of phosphorus (P) fertilizers to P-deficient soils can also result in P accumulation. In this study, soil P status and P uptake by apple trees were investigated in 5-, 10-, and 15-year-old orchards in the semi-arid Loess Plateau, China, and subset soils with different soil P statuses (14–90 Olsen-P mg kg⁻¹) were selected to evaluate the characteristic P adsorption. Due to the low P-use efficiency (4–6%), total soil P increased from 540 mg kg⁻¹ to 904 mg kg⁻¹, Olsen-P ranged from 3.4 mg kg⁻¹ to 30.7 mg kg⁻¹, and CaCl₂-P increased from less than 0.1 mg kg⁻¹ to 0.66 mg kg⁻¹ under continuous P fertilization. The P sorption isotherms for each apple orchard were found to fit the Langmuir isotherm model (R ² = 0.91–0.98). K (binding energy) and Q _m (P sorption maximum) decreased, whereas DPS (degree of phosphorus sorption) increased with increasing P concentration. CaCl₂-P increased significantly with the increase of Olsen-P, especially above the change point of 46.1 mg kg⁻¹. Application of surplus P could result in P enrichment in P-deficient soil which has high P fixation capacity, thus posing a significant environmental risk.     

Dynamics of Nutrient Contents (Phosphorus,Nitrogen) in Water,Sediment and Plants After Restoration of Connectivity in Side‐Channels of the River Rhine

During the last century, canalization of the Rhine river led to disconnection of side‐arms, over‐sedimentation of these channels, loss of the fluvial dynamics, and aquatic vegetation change or disappearance. Recent restoration projects aim to reconnect disconnected arms to the main channel. The objective of this study was to assess the nutrient dynamics in restored channels during the vegetation colonization process. In spring, summer, and autumn 2009, the phosphorus and nitrogen contents were measured in water, sediment, and plants, sampled in six channels, two reference sites and four restored ones at different dates. Aquatic vegetation was monitored during the same period. Sites were mesotrophic related to the water nutrient concentrations. However, vegetation communities indicated a eutrophic level, as they were dominated by species like Elodea nuttallii, Myriophyllum spicatum, and Potamogeton perfoliatus. Sites were discriminated by P content and mineral nitrogen in the sediment. We showed an effect of species and season on the plant nutrient content, but there was no relationship between plant nutrient content and nutrients in water and sediment. A negative correlation between mean N plant content and the cover of each species was found. Vegetation characteristics (species richness and cover) and bioavailable phosphorus in the sediment were also correlated. In the restored side‐arms of the river Rhine, phosphorus‐rich sediment seems to be important in the recolonization dynamics, as it was linked to higher species richness, whereas nitrogen played a role in the colonization patterns as a growth limiting factor.     

Present distribution of the genus Elodea in the Alsatian Upper Rhine floodplain (France) with a special focus on the expansion of Elodea nuttallii St. John during recent decades

Three species of Elodea (Elodea canadensis Michaux, E. nuttallii St John and E. ernstiae St John) have colonized Europe from the American continent. All three arrived in the Alsatian Rhine floodplain (north-eastern France) soon after their arrival in Europe, i.e. in the mid-19th century for E. canadensis, and in the mid-20th century for E. nuttallii and E. ernstiae. The paper investigates the present distribution of Elodea spp. in the floodplain by quantifying the species’ respective occurrences and by describing their habitats. The study further focuses on E. nuttallii which is presently colonizing other parts of Europe. It analyses whether it has continued to expand in the Alsatian Rhine floodplain during recent decades, and it checks whether changes in the abundance of E. nuttallii have had an impact on species richness of water plant communities. E.␣nuttallii has been found to be at present one of the most dominant and most frequent aquatic plant species in the study sector, while E. canadensis and E. ernstiae are less abundant. The species’ distributions differ with regard to water chemistry and water temperature: E. canadensis occurs in oligo-mesotrophic, rather stenothermic habitats, whereas E. nuttallii and E. ernstiae can be encountered in meso- to eutrophic sites with little or no arrival of stenothermic ground water. By comparing successive vegetation relevés from the same sites the study revealed further that the distribution of E. nuttallii has been stable in recent decades, despite local fluctuations in abundance. No relationship could be established between those fluctuations and changes in species richness or type of local plant communities. The sum of the results suggests that the expansion of E. nuttallii in the Alsatian Rhine floodplain had been completed prior to the study period. The species’ present distribution in the study sector as well as its position in local plant communities might therefore be considered a model for what can be expected to happen in areas where E.␣nuttallii has only recently arrived.     

Growth and Nitrogen Uptake Kinetics in Cultured Prorocentrum donghaiense

We compared growth kinetics of Prorocentrum donghaiense cultures on different nitrogen (N) compounds including nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), urea, glutamic acid (glu), dialanine (diala) and cyanate. P. donghaiense exhibited standard Monod-type growth kinetics over a range of N concentraions (0.5–500 μmol N L⁻¹ for NO₃ ⁻ and NH₄ ⁺, 0.5–50 μmol N L⁻¹ for urea, 0.5–100 μmol N L⁻¹ for glu and cyanate, and 0.5–200 μmol N L⁻¹ for diala) for all of the N compounds tested. Cultures grown on glu and urea had the highest maximum growth rates (μ_m, 1.51±0.06 d⁻¹ and 1.50±0.05 d⁻¹, respectively). However, cultures grown on cyanate, NO₃ ⁻, and NH₄ ⁺ had lower half saturation constants (K_μ, 0.28–0.51 μmol N L⁻¹). N uptake kinetics were measured in NO₃ ⁻-deplete and -replete batch cultures of P. donghaiense. In NO₃ ⁻-deplete batch cultures, P. donghaiense exhibited Michaelis-Menten type uptake kinetics for NO₃ ⁻, NH₄ ⁺, urea and algal amino acids; uptake was saturated at or below 50 μmol N L⁻¹. In NO₃ ⁻-replete batch cultures, NH₄ ⁺, urea, and algal amino acid uptake kinetics were similar to those measured in NO₃ ⁻-deplete batch cultures. Together, our results demonstrate that P. donghaiense can grow well on a variety of N sources, and exhibits similar uptake kinetics under both nutrient replete and deplete conditions. This may be an important factor facilitating their growth during bloom initiation and development in N-enriched estuaries where many algae compete for bioavailable N and the nutrient environment changes as a result of algal growth.     

Removing Constraints on the Biomass Production of Freshwater Macroalgae by Manipulating Water Exchange to Manage Nutrient Flux

Freshwater macroalgae represent a largely overlooked group of phototrophic organisms that could play an important role within an industrial ecology context in both utilising waste nutrients and water and supplying biomass for animal feeds and renewable chemicals and fuels. This study used water from the intensive aquaculture of freshwater fish (Barramundi) to examine how the biomass production rate and protein content of the freshwater macroalga Oedogonium responds to increasing the flux of nutrients and carbon, by either increasing water exchange rates or through the addition of supplementary nitrogen and CO₂. Biomass production rates were highest at low flow rates (0.1–1 vol.day⁻¹) using raw pond water. The addition of CO₂ to cultures increased biomass production rates by between 2 and 25% with this effect strongest at low water exchange rates. Paradoxically, the addition of nitrogen to cultures decreased productivity, especially at low water exchange rates. The optimal culture of Oedogonium occurred at flow rates of between 0.5–1 vol.day⁻¹, where uptake rates peaked at 1.09 g.m⁻².day⁻¹ for nitrogen and 0.13 g.m⁻².day⁻¹ for phosphorous. At these flow rates Oedogonium biomass had uptake efficiencies of 75.2% for nitrogen and 22.1% for phosphorous. In this study a nitrogen flux of 1.45 g.m⁻².day⁻¹ and a phosphorous flux of 0.6 g.m⁻².day⁻¹ was the minimum required to maintain the growth of Oedogonium at 16–17 g DW.m⁻².day⁻¹ and a crude protein content of 25%. A simple model of minimum inputs shows that for every gram of dry weight biomass production (g DW.m⁻².day⁻¹), Oedogonium requires 0.09 g.m⁻².day⁻¹ of nitrogen and 0.04 g.m⁻².day⁻¹ of phosphorous to maintain growth without nutrient limitation whilst simultaneously maintaining a high-nutrient uptake rate and efficiency. As such the integrated culture of freshwater macroalgae with aquaculture for the purposes of nutrient recovery is a feasible solution for the bioremediation of wastewater and the supply of a protein resource.     

Effects of decenylsuccinic Acid on p uptake and translocation by barley and winter wheat

After 3 days of exposure to 10⁻³ and 10⁻⁴ M decenylsuccinic acid, winter wheat plants wilted and died. Decenylsuccinate at 10⁻³ M inhibited ³²P uptake by barley roots and wheat roots and resulted in significant (P ≤ 0.05) leakage of previously absorbed ³²P and total phosphorus (barley roots). Decenylsuccinate effects on ³²P uptake and retention were attributed to increased permeability resulting from injury. Decenylsuccinate at 10⁻⁴ M did not inhibit root uptake of ³²P but decreased movement into the shoot. This could be interpreted as an indication of reduced transpiration or inhibition of ³²P loading into the transpiration stream. Decenylsuccinate did not increase cold hardiness in winter wheat in a nonhardening environment.     

Effects of macrophyte-associated nitrogen cycling bacteria on denitrification in the sediments of the eutrophic Gonghu Bay, Taihu Lake

Integrated Elodea nuttallii-immobilized nitrogen cycling bacteria (INCB) technology was used for ecological restoration in the eutrophic Gonghu Bay, Taihu Lake. Sediment denitrification was investigated through microcosm incubations with four different treatments: bare sediment core as control without restoration, sediment + E. nuttallii, sediment + E. nuttallii + INCB, and sediment + INCB. The sediments with E. nuttallii-INCB assemblage (E-INCB) had the highest denitrification rates among all the treatments, and the E-INCB increased the denitrification rate by 162% in the sediments. The presence of macrophytes yielded a penetration depth of O₂ to more than 20 mm below the sediment–water interface (SWI), while the depth was only 4 mm in the sediments without macrophytes. The quantity of denitrifier in E-INCB sediments (within ~2 cm below the SWI) showed a significant increasing trend during one-month incubation, which was one order of magnitudes higher than that in the sediments without INCB. Macrophytes caused deeper O₂ penetration and increased oxic-anoxic interface, which could stimulate the coupled nitrification–denitrification. The high denitrification rate of the E-INCB treatment may result from the increased inorganic nitrogen content in the vicinity of the SWI, causing more nitrate to reach the anoxic denitrification zone. The results showed that E-INCB assemblage could increase benthic N removal by stimulating denitrification via combined O₂ penetration and enhanced microbial N cycling processes. E-INCB might be used as a potential restoration method for controlling fresh water system eutrophication.     

Production of Superoxide Anions by Keratinocytes Initiates P. acnes-Induced Inflammation of the Skin

Acne vulgaris is a chronic inflammatory disorder of the sebaceous follicles. Propionibacterium acnes (P. acnes), a gram-positive anareobic bacterium, plays a critical role in the development of these inflammatory lesions. This study aimed at determining whether reactive oxygen species (ROS) are produced by keratinocytes upon P. acnes infection, dissecting the mechanism of this production, and investigating how this phenomenon integrates in the general inflammatory response induced by P. acnes. In our hands, ROS, and especially superoxide anions (O₂ ^•−), were rapidly produced by keratinocytes upon stimulation by P. acnes surface proteins. In P. acnes-stimulated keratinocytes, O₂ ^•− was produced by NAD(P)H oxidase through activation of the scavenger receptor CD36. O₂ ^•− was dismuted by superoxide dismutase to form hydrogen peroxide which was further detoxified into water by the GSH/GPx system. In addition, P. acnes-induced O₂ ^•− abrogated P. acnes growth and was involved in keratinocyte lysis through the combination of O₂ ^•− with nitric oxide to form peroxynitrites. Finally, retinoic acid derivates, the most efficient anti-acneic drugs, prevent O₂ ^•− production, IL-8 release and keratinocyte apoptosis, suggesting the relevance of this pathway in humans.     

Phosphorus stress effects on assimilation of nitrate

An experiment was conducted to investigate alterations in uptake and assimilation of NO₃⁻ by phosphorus-stressed plants. Young tobacco plants (Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were deprived of an external phosphorus (P) supply for 12 days. On selected days, plants were exposed to ¹⁵NO₃⁻ during the 12 hour light period to determine changes in NO₃⁻ assimilation as the P deficiency progressed. Decreased whole-plant growth was evident after 3 days of P deprivation and became more pronounced with time, but root growth was unaffected until after day 6. Uptake of ¹⁵NO₃⁻ per gram root dry weight and translocation of absorbed ¹⁵NO₃⁻ out of the root were noticeably restricted in −P plants by day 3, and effects on both increased in severity with time. Whole-plant reduction of ¹⁵NO₃⁻ and ¹⁵N incorporation into insoluble reduced-N in the shoot decreased after day 3. Although the P limitation was associated with a substantial accumulation of amino acids in the shoot, there was no indication of excessive accumulation of soluble reduced-¹⁵N in the shoot during the 12 hour ¹⁵NO₃⁻ exposure periods. The results indicate that alterations in NO₃⁻ transport processes in the root system are the primary initial responses limiting synthesis of shoot protein in P-stressed plants. Elevated amino acid levels evidently are associated with enhanced degradation of protein rather than inhibition of concurrent protein synthesis.     

Induction of Acid Metabolism in Portulacaria afra

Portulacaria afra, a succulent plant, shifts from a predominantly C₃ mode of gas exchange to a typical Crassulacean acid metabolism type CO₂ uptake in response to water or NaCl stress. Control plants in the absence of water stress assimilated CO₂ during the light (about 7-8 mg CO₂ dm⁻² hr⁻¹), transpiration (about 1.5 g dm⁻² hr⁻¹) was predominantly during the day, stomates were open during the day, and there was little diurnal organic acid fluctuation. Stressed plants showed only dark CO₂ uptake and dark water loss, nocturnal stomatal opening, and an increased diurnal fluctuation of titratable acidity. Within 2 weeks after rewatering, stressed plants returned to the control acid fluctuation levels indicating that the response to stress was reversible.     

Macrophyte-related shifts in the nitrogen and phosphorus contents of the different trophic levels in a biomanipulated shallow lake

Lake Zwemlust, a small highly eutrophic lake, was biomanipulated without reducing the external nutrient loading, and the effects were studied for four years. In this paper we pay special attention to the shifts in relative distribution of nitrogen and phosphorus in the different trophic levels and to the changes in growth limitation of the autotrophs.Despite of the high external nutrient loads to the lake (ca 2.4 g P m^–2 y^–1 and 9.6 g N m^–2 y^–1), the effects of biomanipulation on the lake ecosystem were pronounced. Before biomanipulation no submerged vegetation was present in the lake and P and N were stored in the phytoplankton (44% N, 47% P), fish (33% N, 9% P) and in dissolved forms (23% N, 44% P). P and N contents in sediments were not determined. In the spring and summer following the biomanipulation (1987), zooplankton grazing controlled the phytoplankton biomass and about 90% of N and P were present in dissolved form in the water. From 1988 onwards submerged macrophyte stands continue to thrive, reducing the ammonium and nitrate concentrations in the water below detection levels. In July 1989 storage of N and P in the macrophytes reached 86% and 80%, respectively. Elodea nuttallii (Planchon) St.John, the dominant species in 1988 and 1989, acted as sink both for N and P during spring and early summer, withdrawing up to ca 60% of its N and P content from the sediment. At the end of the year only part of the N and P from the decayed macrophytes (ca 30% of N and 60% of P) was recovered in the water phase of the ecosystem (chiefly in dissolved forms). The rest remained in the sediment, although some N may have been released from the lake by denitrification.In summer 1990 only 30% of the N and P was found in the macrophytes (dominant species Ceratophyllum demersum L.), while ca 30% of N and P was again stored in phytoplankton and fish.     

The Influence of Nitrate and Chloride Uptake on Expressed Sap pH, Organic Acid Synthesis, and Potassium Accumulation in Higher Plants

The influence of NO₃⁻ uptake and reduction on ionic balance in barley seedlings (Hordeum vulgare, cv. Compana) was studied. KNO₃ and KCl treatment solutions were used for comparison of cation and anion uptake. The rate of Cl⁻ uptake was more rapid than the rate of NO₃⁻ uptake during the first 2 to 4 hours of treatment. There was an acceleration in rate of NO₃⁻ uptake after 4 hours resulting in a sustained rate of NO₃⁻ uptake which exceeded the rate of Cl⁻ uptake. The initial (2 to 4 hours) rate of K⁺ uptake appeared to be independent of the rate of anion uptake. After 4 hours the rate of K⁺ uptake was greater with the KNO₃ treatment than with the KCl treatment, and the solution pH, cell sap pH, and organic acid levels with KNO₃ increased, relative to those with the KCl treatment. When absorption experiments were conducted in darkness, K⁺ uptake from KNO₃ did not exceed K⁺ uptake from KCl. We suggest that the greater uptake and accumulation of K⁺ in NO₃⁻-treated plants resulted from (a) a more rapid, sustained uptake and transport of NO₃⁻ providing a mobile counteranion for K⁺ transport, and (b) the synthesis of organic acids in response to NO₃⁻ reduction increasing the capacity for K⁺ accumulation by providing a source of nondiffusible organic anions.     

Linking Stoichiometric Homeostasis of Microorganisms with Soil Phosphorus Dynamics in Wetlands Subjected to Microcosm Warming

Soil biogeochemical processes and the ecological stability of wetland ecosystems under global warming scenarios have gained increasing attention worldwide. Changes in the capacity of microorganisms to maintain stoichiometric homeostasis, or relatively stable internal concentrations of elements, may serve as an indicator of alterations to soil biogeochemical processes and their associated ecological feedbacks. In this study, an outdoor computerized microcosm was set up to simulate a warmed (+5°C) climate scenario, using novel, minute-scale temperature manipulation technology. The principle of stoichiometric homeostasis was adopted to illustrate phosphorus (P) biogeochemical cycling coupled with carbon (C) dynamics within the soil-microorganism complex. We hypothesized that enhancing the flux of P from soil to water under warming scenarios is tightly coupled with a decrease in homeostatic regulation ability in wetland ecosystems. Results indicate that experimental warming impaired the ability of stoichiometric homeostasis (H) to regulate biogeochemical processes, enhancing the ecological role of wetland soil as an ecological source for both P and C. The potential P flux from soil to water ranged from 0.11 to 34.51 mg m⁻² d⁻¹ in the control and 0.07 to 61.26 mg m⁻² d⁻¹ in the warmed treatment. The synergistic function of C-P acquisition is an important mechanism underlying C∶P stoichiometric balance for soil microorganisms under warming. For both treatment groups, strongly significant (p<0.001) relationships fitting a negative allometric power model with a fractional exponent were found between n-H_C∶P (the specialized homeostatic regulation ability as a ratio of soil highly labile organic carbon to dissolved reactive phosphorus in porewater) and potential P flux. Although many factors may affect soil P dynamics, the n-H_C∶P term fundamentally reflects the stoichiometric balance or interactions between the energy landscape (i.e., C) and flow of resources (e.g., N and P), and can be a useful ecological tool for assessing potential P flux in ecosystems.     

Early effects of salinity on nitrate assimilation in barley seedlings

The effect of NaCl and Na₂SO₄ salinity on NO₃⁻ assimilation in young barley (Hordeum vulgare L. var Numar) seedlings was studied. The induction of the NO₃⁻ transporter was affected very little; the major effect of the salts was on its activity. Both Cl⁻ and SO₄²⁻ salts severely inhibited uptake of NO₃⁻. When compared on the basis of osmolality of the uptake solutions, Cl⁻ salts were more inhibitory (15-30%) than SO₄²⁻ salts. At equal concentrations, SO₄²⁻ salts inhibited NO₃⁻ uptake 30 to 40% more than did Cl⁻ salts. The absolute concentrations of each ion seemed more important as inhibitors of NO₃⁻ uptake than did the osmolality of the uptake solutions. Both K⁺ and Na⁺ salts inhibited NO₃⁻ uptake similarly; hence, the process seemed more sensitive to anionic salinity than to cationic salinity.

Unlike NO₃⁻ uptake, NO₃⁻ reduction was not affected by salinity in short-term studies (12 hours). The rate of reduction of endogenous NO₃⁻ in leaves of seedlings grown on NaCl for 8 days decreased only 25%. Nitrate reductase activity in the salt-treated leaves also decreased 20% but its activity, determined either in vitro or by the `anaerobic' in vivo assay, was always greater than the actual in situ rate of NO₃⁻ reduction. When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in situ nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO₃⁻ transporter, rather than that of the NO₃⁻ reduction system, may be a critical factor to plant survival during salt stress.

     

Comparative Kinetics and Reciprocal Inhibition of Nitrate and Nitrite Uptake in Roots of Uninduced and Induced Barley (Hordeum vulgare L.) Seedlings

Nitrate and NO₂⁻ transport by roots of 8-day-old uninduced and induced intact barley (Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of NO₃⁻ and NO₂⁻ was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low K_m, possibly a constitutive uptake system, and a linear system for both NO₃⁻ and NO₂⁻. The low K_m system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both NO₃⁻ and NO₂⁻ uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low K_ms of NO₃⁻ and NO₂⁻ uptake were 7 and 9 micromolar, respectively, and were similar to those of the low K_m system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high K_m values of NO₃⁻ uptake ranged from 34 to 36 micromolar and of NO₂⁻ uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that NO₃⁻ and NO₂⁻ inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, K_i values showed that NO₃⁻ was a more effective inhibitor than NO₂⁻. Nitrate and NO₂⁻ transport by both the low and high K_m systems were greatly inhibited by p-hydroxymercuribenzoate, whereas the linear system was only slightly inhibited.