Short-term studies of NO 3 − uptake in Pisum using 13NO 3 −

Influx, efflux and net uptake of NO ₃ ^– was studied in Pisum sativum L. cv. Marma in short-term experiments where ¹³NO ₃ ^– was used to trace influx. The influx rate in N-limited plants was similar both during net uptake at external concentrations of around 50 M, and at low external NO ₃ ^– concentrations (4–6 M) when net uptake was practically zero. Efflux could be inferred from discrepancies between influx and net uptake but was never very high in the N-limited plants during net uptake. Close to the threshold concentration for not NO ₃ ^– uptake, efflux was high and equalled influx. Thus, the threshold concentration can be regarded as a NO ₃ ^– compensation point. The inclusion of NH ₄ ⁺ in the outer medium decreased influx by about 40% but did not significantly affect efflux. The roles of NO ₃ ^– fluxes and nitrate-reductase activity in regulating/limiting NO ₃ ^– utilization are discussed.Abbreviations DW dry weight - FW fresh weight - R_N relative nitrogen addition rate     

Electrogenic calcium transport in plasma membrane of rat pancreatic acinar cells

Summary ATP-dependent⁴⁵Ca²⁺ uptake was investigated in purified plasma membranes from rat pancreatic acinar cells. Plasma membranes were purified by four subsequent precipitations with MgCl₂ and characterized by marker enzyme distribution. When compared to the total homogenate, typical marker enzymes for the plasma membrane, (Na⁺,K⁺)-ATPase, basal adenylate cyclase and CCK-OP-stimulated adenylate cyclase were enriched by 43-fold, 44-fold, and 45-fold, respectively. The marker for the rough endoplasmic reticulum was decreased by fourfold compared to the total homogenate. Comparing plasma membranes with rough endoplasmic reticulum, Ca²⁺ uptake was maximal with 10 and 2 mol/liter free Ca²⁺, and half-maximal with 0.9 and 0.5 mol/liter free Ca²⁺. It was maximal at 3 and 0.2 mmol/liter free Mg²⁺ concentration, at an ATP concentration of 5 and 1 mmol/liter, respectively, and at pH 7 for both preparations. When Mg²⁺ was replaced by Mn²⁺ or Zn²⁺ ATP-dependent Ca²⁺ uptake was 63 and 11%, respectively, in plasma membranes; in rough endoplasmic reticulum only Mn²⁺ could replace Mg²⁺ for Ca²⁺ uptake by 20%. Other divalent cations such as Ba²⁺ and Sr²⁺ could not replace Mg²⁺ in Ca²⁺ uptake. Ca²⁺ uptake into plasma membranes was not enhanced by oxalate in contrast to Ca²⁺ uptake in rough endoplasmic reticulum which was stimulated by 7.3-fold. Both plasma membranes and rough endoplasmic reticulum showed cation and anion dependencies of Ca²⁺ uptake. The sequence was K⁺>Rb⁺>Na⁺>Li⁺>choline⁺ in plasma membranes and Rb⁺K⁺Na⁺>Li⁺>choline⁺ for rough endoplasmic reticulum. The anion sequence was Cl^–Br^–I^–>SCN^–>NO ₃ ^– >isethionate^– >cyclamate^–>gluconate^–>SO ₄ ^2– glutarate^– and Cl^–>Br>gluconate>SO ₄ ^2– >NO ₃ ^– >I^–>cyclamate^–SCN^–, respectively. Ca²⁺ uptake into plasma membranes appeared to be electrogenic since it was stimulated by an inside-negative K⁺ and SCN diffusion potential and inhibited by an inside-positive diffusion potential. Ca²⁺ uptake into rough endoplasmic reticulum was not affected by diffusion potentials. We assume that the Ca²⁺ transport mechanism in plasma membranes as characterized in this study represents the extrusion system for Ca²⁺ from the cell that might be involved in the regulation of the cytosolic Ca²⁺ level.     

Effects of supplied nitrogen form on growth and water uptake of French bean (Phaseolus vulgaris L.) plants

In order to investigate the effect of N form on dry matter (DM) formation and water uptake rate, French bean (Phaseolus vulgaris L. `Sotaxa') plants were grown with a split-root system. Three treatments were compared: sole nitrate (NO^– ₃) supply (NN), sole ammonium (NH⁺ ₄) supply (AA) and spatially separated supply of NO^– ₃ and NH⁺ ₄ (NA). The pH of the nutrient solutions was kept constant at 6.3 using a pH-stat system. 9 days after onset of the treatments, NN plants had higher root (36%) and shoot dry matter (11%) than AA plants. N form drastically influenced partitioning of assimilates: in the NA treatment, the root half exposed to NO^– ₃ revealed a 170% higher DM than the root half exposed to NH⁺ ₄. N form affected stable carbon-isotope discrimination () of leaf tissue. In leaves of plants which were supplied with NH⁺ ₄ (AA; NA) was significantly more negative (–29.4, –29.6) than in NN treatment (–28.2). We explain this effect by differences in stomatal conductance. We suppose that the significantly less negative of root tissue under NH⁺ ₄ supply is most probably related to higher PEP-case activity. The water uptake rate was higher in NN than in AA grown plants. This effect was found in both, short- and long-term experiments. In case of NA plants, the water uptake in the root part being exposed to NO^– ₃ was 104% higher than in those receiving NH⁺ ₄. At least in the case of the NA treatment we can exclude shoot growth effects as being responsible for differences in water uptake. We therefore assume that differences in root hydraulic conductivity are responsible for the observed effects.     

Ammonium and nitrate uptake in gap,generalist and understory species of the genus Piper

Summary We studied root net uptake of ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) in species of the genus Piper (Piperaceae) under high, intermediate and low photosynthetically active photon flux densities (PFD). Plants were grown hydroponically, and then transferred to temperature controlled (25° C) root cuvettes for nutrient uptake determinations. Uptake solutions provided NH ₄ ⁺ and NO ₃ ^– simultaneously (both) or separately (single). In the first experiment, seven species of Piper, from a broad range of rainforest light habitats ranging from gap to understory, were screened for mineral nitrogen preference (100 M NH ₄ ⁺ and/or 100 M NO ₃ ^– ) at intermediate PFD (100 mol m^–2 s^–1). Preference for NH ₄ ⁺ relative to NO ₃ ^– , defined as the ratio of NH ₄ ⁺ (both):NO ₃ ^– (both) net uptake, was higher in understory species than in gap species. Ammonium repression of NO ₃ ^– uptake, defined as the ratio of NO ₃ ^– (single): NO ₃ ^– (both) net uptake, was also higher in understory species as compared to gap species. In a second set of experiments, we examined the effect of nitrogen concentration (equimolar, 10 to 1000 M) on NH ₄ ⁺ preference and NH ₄ ⁺ repression of NO ₃ ^– net uptake at high (500 mol m^–2 s^–1) and low (50 mol m^–2 s^–1) PFD in a gap (P. auritum), generalist (P. hispidum) and understory species (P. aequale). All species exhibited negligible NH ₄ ⁺ repression of NO ₃ ^– net uptake at high PFD. At low PFD, NH ₄ ⁺ preference and repression of NO ₃ ^– net uptake occurred in all species (understory > generalist > gap), but only at intermediate nitrogen concentrations, i.e. between 10 and 200 M. Ammonium repression of net NO ₃ ^– uptake decreased or increased rapidly (in < 48 h) after transitions from low to high or from high to low PFD respectively. No significant diurnal patterns in NO ₃ ^– or NH ₄ ⁺ net uptake were observed.CIWDPB publication # 1130     

Cadmium uptake kinetics and plants factors of shoot Cd concentration

Background and aims

Accumulation of Cd in the shoots of plants grown on Cd contaminated soils shows considerable variation. A previous preliminary experiment established that one major reason for this variation was the rate of Cd influx into the roots (mol Cd cm^?2 root s^?1). However, this experiment did not distinguish between solubilization of soil Cd on the one hand and difference in Cd uptake kinetics on the other. The main objectives of the present study were thus to characterize Cd uptake kinetics of plants continuously exposed to Cd concentrations similar to those encountered in soils. Furthermore we determined the factors responsible for differences in shoot Cd concentration such as net Cd influx, root area-shoot dry weight ratio, shoot growth rate and proportion of Cd translocated to the shoot.

Materials and methods

Maize, sunflower, flax and spinach were grown in nutrient solution with five constant Cd concentrations varying from 0 to 1.0 μmol?L^?1. Root and shoot parameters as well as Cd uptake were determined at two harvest dates and from these data Cd net influx and shoot growth rates were calculated.

Results and conclusions

Cadmium uptake kinetics, i.e. the net Cd influx vs. Cd solution concentration followed a straight line. Its slope is the root absorbing power, α, $ \left( {\alpha ={{{\mathrm{Cd}\;\mathrm{net}\;\mathrm{influx}}} \left/ {{\mathrm{Cd}\;\mathrm{solution}\;\mathrm{concentration}}} \right.}} \right) $ . The α values of spinach and flax were about double that of maize and sunflower (5?×?10^?6?cm?s^?1 vs. 2.5?×?10^?6?cm?s^?1). Spinach and flax had a 3–5 times higher shoot Cd concentration than maize and sunflower. The difference in shoot Cd concentration was partly due to the higher Cd influx but also to a higher translocation of Cd from root to shoot and also to a slower shoot growth rate.     

Kinetic analyses of waterborne Ca and Cd transport and their interactions in the gills of rainbow trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>) and yellow perch (<Emphasis Type="Italic">Perca flavescens</Emphasis>), two species differing greatly in acute waterborne Cd sensitivity

We evaluated the differential nature of interactions between waterborne Ca and Cd transport in the gills of yellow perch (Perca flavescens) and rainbow trout (Oncorhynchus mykiss), two species with a more than 400-fold difference in acute waterborne Cd tolerance. The J_max (maximum rate of uptake) and K_m (inverse of affinity) for Ca uptake, in the absence of Cd, were significantly lower in yellow perch (120.48 nM g^–1 wet wt h^–1 and 92.17 M, respectively) relative to rainbow trout (188.68 nM g^–1 wet wt h^–1 and 243.90 M, respectively). Similarly, the J_max for Cd uptake, at the lowest waterborne Ca level (100 M) tested, was significantly lower in yellow perch (0.27 nM g^–1 wet wt h^–1) relative to rainbow trout (0.40 nM g^–1 wet wt h^–1), but no significant difference was observed in the K_m values between the two species (yellow perch: 32.47 nM; rainbow trout: 31.27 nM). Waterborne Cd (0–890 nM) as well as waterborne Ca (100–1,000 M) competitively inhibited branchial uptake of each other in both species. However, analyses of inhibitor constants for branchial Ca uptake by waterborne Cd ( ) revealed that the inhibition was about 1.8 times more potent in rainbow trout compared to yellow perch. In contrast, analyses of inhibitor constants for branchial Cd uptake by waterborne Ca ( ) indicated that the inhibition was more than three fold more potent in yellow perch than in rainbow trout. Higher branchial Ca uptake and more potent inhibition by Cd as well as higher branchial Cd uptake and less potent inhibition by Ca were also reflected in whole-body measurements of Ca and Cd influx in trout relative to perch. Overall, whole-body effects were in accord with the branchial kinetic analyses. These results further strengthen the conclusion that branchial influxes of Ca and Cd occur through common pathways. Moreover, interspecific differences in acute waterborne Cd sensitivity can be explained, at least in part, by the differential nature of interactions between waterborne Ca and Cd transport in fish gills.Abbreviations FAAS flame atomic absorption spectrophotometer - GFAAS graphite furnace atomic absorption spectrophotometer - J _max maximum rate of uptake - K _i inhibitor constant - K _m substrate concentration at which the rate of uptake is half of the J_max - 96 h LC50 concentration at which 50% mortality occurs after 96 h Communicated by L.C.-H. WangThis revised version was published online in February 2004 with corrections to the abbreviation .     

A new approach to quantify the utilization of non-exchangeable soil potassium by plants

To predict the contribution of soil K fractions of different mobility to K supply of plants, the kinetics of K release from soil was related to the kinetics of K uptake of young sugar beet and wheat plants. For this purpose K release rates from soil were measured by continuously percolating samples of a luvisol with 0.01 M CaCl₂ solution and effective diffusion coefficients, D_e, were determined. Two soil K fractions of different mobility were obtained. D_e values of the more mobile exchangeable K and the less mobile non-exchangeable K fraction were found to be 58.9 × 10^–9 and 8.2 × 10^–9 cm² s^–1, respectively. In a pot experiment, sugar beet and wheat plants were grown, for 15 days and both root growth and K uptake were measured. K uptake kinetics of both crops was determined in a separate experiment using flowing solution culture. To integrate these data quantitatively, the simulation model of Claassen et al. (1986) was applied. Results show that calculated total K uptake agreed closely with real K uptake of the plants. On this basis, 64 and 79% of the K taken up by wheat and sugar beet plants was derived from the rapidly released exchangeable and 21–36% from the less mobile non-exchangeable soil K fraction.     

Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [Picea abies (L.) Karst.]

Summary Relationships between root zone temperature, concentrations and uptake rates of NH ₄ ⁺ and NO ₃ ^– were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH ₄ ⁺ and NO ₃ ^– uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (C_min) range of 100–150 M uptake rates of NH ₄ ⁺ were 3–4 times higher than those of NO ₃ ^– The preference for NH ₄ ⁺ uptake was also reflected in the minimum concentration (C_min) values. Supplying NH₄NO₃, the rate of NO ₃ ^– uptake was very low until the NH ₄ ⁺ concentrations had fallen below about 100 M. The shift from NH ₄ ⁺ to NO ₃ ^– uptake was correlated with a corresponding shift from net H⁺ production to net H⁺ consumption in the external solution. The uptake rates of NH ₄ ⁺ were correlated with equimolar net production of H⁺. With NO ₃ ^– nutrition net consumption of H⁺ was approximately twice as high as uptake rates of NO ₃ ^– In the forest stand the NO ₃ ^– concentration in the soil solution was more than 10 times higher than the NH ₄ ⁺ concentration (<100 M), and the rhizosphere pH of non-mycorrhizal roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root zones where the rates of water and NO ₃ ^– uptake and nitrate reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and uptake by, non-mycorrhizal roots of Norway spruce in a forest stand. Model calculations indicate that delivery to the roots by mass flow may meet most of the plant demand of nitrogen and calcium, and that non-mycorrhizal root tips have the potential to take up most of the delivered nitrate and calcium.     

Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh

It is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO ₃ ^– deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO ₃ ^– uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO ₃ ^– additions equivalent to 30 kg N ha^–1 year^–1. We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used ¹⁵N to determine the kinetic parameters of NO ₃ ^– uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0–3.5 g NO ₃ ^– –N m^–2 and determined the kinetic parameters of NO ₃ ^– and NH ₄ ⁺ uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO ₃ ^– fertilization (range = non-detectable to 33 nmol NO ₂ ^– g^–1 h^–1). Maximal rates of NO ₃ ^– uptake in overstory trees also were low, ranging from 0.2 to 1.0 mol g^–1 h^–1. In seedlings, the mean V _max for NO ₃ ^– uptake in fine roots (1 mol g^–1 h^–1) was approximately 30 times lower than the V _max for NH ₄ ⁺ uptake (33 mol g^–1 h^–1). Our results suggest that A. saccharum satisfies its N demand through rapid NH ₄ ⁺ uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO ₃ ^– .     

Photosynthetic Response to Irradiance in Valeriana jatamansi Jones,a Threatened Understorey Medicinal Herb of Western Himalaya

Net photosynthetic rate (P _N) of Valeriana jatamansi plants, grown under nylon net shade or under different tree canopies, was saturated with photons at 1 000 mol m^–2 s^–1 photosynthetic photon-flux-density (PPFD), whereas open-grown plants were able to photosynthesise even at higher PPFD, e.g. of 2 000 mol m^–2 s^–1. Plants grown under net shade had higher total chlorophyll (Chl) content per unit area of leaf surface. However, Chl a/b ratio was maximal in open-grown plants, but remained unchanged in plants grown in nylon net shade and under different tree canopies. Sun-grown plants had thicker leaves (higher leaf mass per leaf area unit), higher wax content, and higher P _N than shade grown plants. Thus V. jatamansi is able to acclimate to high PPFD and therefore this Himalayan species may be cultivated in open habitat to meet the ever-increasing industrial demand.     

Simultaneous oxygen production and nitrogenase activity of an ammonia-excreting mutant of the cyanobacterium Anabaena variabilis in a co-culture with wheat

Summary A mutant strain of Anabaena variabilis, strain SA-1 that supported growth of wheat plants in a hydroponic co-culture in nitrogen (N) free medium also produced enough oxygen (O₂) to support root respiration. The steady-state concentration of net O₂ in the co-culture was dependent on incident light intensity. At an incident photosynthetic photoflux (PPF) of 1000 mol photons·m^–2·s^–1, net O₂ evolution by the co-culture in the root zone reached a maximum value of about 220 mol O₂ evolved·h^–1·mg chlorophyll (Chl)^–1. The O₂ concentration in the rhizosphere of the co-culture stayed above the ambient air level. O₂ uptake in the dark by strain SA-1-supplemented wheat roots washed free of cyanobacterium was higher than the root respiration of nitrate-grown plants. Nitrate-grown plants required aeration for maximum growth while the wheat-cyanobacterial co-culture can be cultured without aeration. These results show that O₂ produced by strain SA-1 can be used to supply the O₂ needs for root respiration of wheat. Respiration reduced net O₂ evolution by the mutant SA-1, decreasing the partial pressure of O₂ at the sites of cyanobacterial attachment to the roots. This led to an increase in the specific activity of nitrogenase of the co-culture at the high light intensities used to support wheat growth. This activity of about 30 mol ethylene produced·mg Chl^–1·h^–1 was three-fold higher than the activities obtained with the free-living strain SA-1 assayed at the same light intensity. In the co-culture, ammonia produced by the mutant strain SA-1 was not detectable. The NH _inf4 ^sup+ produced by strain SA-1 was used by the wheat plants and, under these conditions, the total N content of the plants reached as high as 85% of the total N content of nitrate-grown plants. In the co-culture system the metabolism of wheat and the cyanobacterium complemented each other, leading to higher plant growth in N-free medium. Offprint requests to: M. Gunasekaran     

Carbon assimilation in contrasting streamside and inland Spartina alterniflora salt marsh

Carbon assimilation and standing crop biomass of Spartina alterniflora were studied in a contrasting streamside and inland salt marsh in Louisiana Gulf coast, USA. A substantially lower leaf dry weight, leaf area index, and standing crop biomass were recorded for inland plants as compared to streamside plants. Net assimilation rates ranged between 8 to 25 mol m^–2 s^–1 for streamside and between 4 to 19 mol m^–2 s^–1 for inland plants. The average photosynthetic rates were significantly lower for inland plants which were growing in an apparently more stressed environment. In addition, the differences were more profound with progression of the growing season. The reduced photosynthetic activity in the inland marsh was attributed to greater soil waterlogging, increased anaerobic root respiration, plant toxins (sulfide), restricted nutrient uptake or a combination of these factors.Abbreviations E_h = redox potential - g_w = stomatal conductance - LAI = leaf area index - P_n = net photosynthesis - PPFD = photosynthetic photon flux density - T₁ = leaf temperature     

Effects of constitutive expression of nitrate reductase in transgenic Nicotiana plumbaginifolia L. in response to varying nitrogen supply

Transformed Nicotiana plumbaginifolia plants with constitutive expression of nitrate reductase (NR) activity were grown at different levels of nitrogen nutrition. The gradients in foliar NO ₃ ^– content and maximum extractable NR activity observed with leaf order on the shoot, from base to apex, were much decreased as a result of N-deficiency in both the transformed plants and wild type controls grown under identical conditions. Constitutive expression of NR did not influence the foliar protein and chlorophyll contents under any circumstances. A reciprocal relationship between the observed maximal extractable NR activity of the leaves and their NO ₃ ^– content was observed in plants grown in nitrogen replete conditions at low irradiance (170 mol photons·m^–2 ·s^–1). This relationship disappeared at higher irradiance (450 mol photons·m^–2·S^–1) because the maximal extractable NR activity in the leaves of the wild type plants in these conditions increased to a level that was similar to, or greater than that found in constitutive NR-expressors. Much more NO ₃ ^– accumulated in the leaves of plants grown at 450 mol photons·m^–2·s^–1 than in those grown at 170 mol photons·m^–2·s^–1 in N-replete conditions. The foliar NO ₃ ^– level and maximal NR activity decreased with the imposition of N-deficiency in all plant types such that after prolonged exposure to nitrogen depletion very little NO ₃ ^– was found in the leaves and NR activity had decreased to almost zero. The activity of NR decreased under conditions of nitrogen deficiency. This regulation is multifactoral since there is no regulation of NR gene expression by NO ₃ ^– in the constitutive NR-expressors. We conclude that the NR protein is specifically targetted for destruction under nitrogen deficiency. Consequently, constitutive expression of NR activity does not benefit the plant in terms of increased biomass production in conditions of limiting nitrogen.Abbreviations Chl chlorophyll - N nitrogen - NR NADH-nitrate reductase - WT wild type     

Inhibition of N2 fixation by white clover (Trifolium repens L.) at low concentrations of NO inf3 sup− in flowing solution culture

The impact of sustained low external concentrations of NO ₃ ^– (0, 10, 100 and 1000 mmol m^–3) on plant growth and the relative acquisition of N through N₂ fixation and NO ₃ ^– uptake by established, nodulated white clover (Trifolium repens L. cv. Blanca) was studied over 28 days in flowing solution culture. Nitrogen fixation was measured by N difference and ¹⁵N dilution methods. Plants supplied with NO ₃ ^– achieved higher relative growth rates (% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmiEayaara% aaaa!3702!\[\bar x\]=0.091 d^–1) compared with control plants dependent on N₂ fixation (0.073 d^–1). Nitrate plants showed progressive increases in shoot: root d.w. ratios from 4 to 6.5–7.6 between days 0–28, compared with 5.1 on day 28 for control plants. Increases in both nodule d.w. and numbers per plant were inhibited after day seven at all concentrations of NO ₃ ^– . The severity of inhibition of N₂ fixation increased with increasing NO ₃ ^– concentration and with time. The total amounts of N₂ fixed per plant between days 0–7 after supplying 10, 100 and 1000 mmol m^–3 NO ₃ ^– , respectively, were 37–39, 28–30 and 0–13%, of the total N acquired. Between days 7–28 the proportional contributions of N₂ fixation to total N acquisition declined to 3, 0.5 and 0%, respectively, in these treatments. The corresponding mean specific rates of N₂ fixation between days 0–7 were, respectively, 5.4, 3.2, and 2.0 mmol N d^–1 g^–1 nodule d.w., compared with 7.9 mmol N d^–1 g^–1 nodule d.w. for zero NO ₃ ^– plants. There was no evidence of a transitory increase in N₂ fixation following the addition of NO ₃ ^– , even at the lowest supply concentration.     

Effects of a clearcut on the net rates of nitrification and N mineralization in a northern hardwood forest,Catskill Mountains,New York,USA

The Catskill Mountains of southeastern New York receive among the highest rates of atmospheric nitrogen (N) deposition in eastern North America, and ecosystems in the region may be sensitive to human disturbances that affect the N cycle. We studied the effects of a clearcut in a northern hardwood forest within a 24-ha Catskill watershed on the net rates of N mineralization and nitrification in soil plots during 6 years (1994–1999) that encompassed 3-year pre- and post-harvesting periods. Despite stream NO₃^– concentrations that increased by more than 1400 mol l^–1 within 5 months after the clearcut, and three measures of NO₃^– availability in soil that increased 6- to 8-fold during the 1^st year after harvest, the net rates of N mineralization and nitrification as measured by in situ incubation in the soil remained unchanged. The net N-mineralization rate in O-horizon soil was 1– 2 mg N kg^–1 day^–1 and the net nitrification rate was about 1 mg N kg^–1 day^–1, and rates in B-horizon soil were only one-fifth to one-tenth those of the O-horizon. These rates were obtained in single 625 m² plots in the clearcut watershed and reference area, and were confirmed by rate measurements at 6 plots in 1999 that showed little difference in N-mineralization and nitrification rates between the treatment and reference areas. Soil temperature increased 1 ± 0.8 °C in a clearcut study plot relative to a reference plot during the post-harvest period, and soil moisture in the clearcut plot was indistinguishable from that in the reference plot. These results are contrary to the initial hypothesis that the clearcut would cause net rates of these N-cycling processes to increase sharply. The in situ incubation method used in this study isolated the samples from ambient roots and thereby prevented plant N uptake; therefore, the increases in stream NO₃^– concentrations and export following harvest largely reflect diminished uptake. Changes in temperature and moisture after the clearcut were insufficient to measurably affect the net rates of N mineralization and nitrification in the absence of plant uptake. Soil acidification resulting from the harvest may have acted in part to inhibit the rates of these processes. The US Governments right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Effect of Cl on Cd uptake by Swiss chard in nutrient solutions

Swiss chard (Beta vulgaris L., cv. Fordhook Giant) was grown in nutrient solution with Cl concentrations varying between 0.01 mM and 120 mM. Solution Na concentration and ionic strength were maintained in all treatments by compensating with NaNO₃. All solutions contained Cd (50 nM, spiked with ¹⁰⁹Cd). Three different Cd²⁺ buffering systems were used. In one experiment, Cd²⁺ activity was unbuffered; its activity decreased with increased Cl concentration as a result of the formation of CdCl_n ^2–n species. In the other experiments, Cd²⁺ activity was buffered by the chelator nitrilotriacetate (NTA, 50 M) and ethylene-bis-(oxyethylenenitrilo)-tetraacetate (EGTA, 50 M) at about 10^–9 M and 10^–11 M, respectively. Plant growth was generally unaffected by increasing Cl concentrations in the three experiments. In unbuffered solutions, Cd concentrations in plant tissue decreased significantly (p<0.01) (approximately 2.4-fold) as solution Cl concentration increased from 0.01 mM to 120 mM. However, this decrease was smaller in magnitude than the 4.7-fold decrease in Cd²⁺ activity as calculated by the GEOCHEM-PC program for the same range of Cl concentrations. In solutions where Cd²⁺ activity was buffered by NTA, Cd concentrations in plant tissue increased approximately 1.4-fold with increasing Cl concentration in solution, while the Cd²⁺ activity was calculated to decrease 1.3-fold. In solutions where Cd²⁺ activity was buffered by EGTA, Cd concentrations in the roots increased 1.3-fold with increasing Cl concentration in solution but there was no effect of Cl on shoot Cd concentrations. The data suggest that either CdCl_n ^2–nspecies can be taken up by plant roots or that Cl enhances uptake of Cd²⁺ through enhanced diffusion of the uncomplexed metal to uptake sites.Abbreviations DAS days after sowing - EGTA ethylene-bis-(oxyethylenenitrilo)-tetraacetate - HBED N,N-bis(2-hydroxybenzyl)-ethylenediamine-N,N-diacetate - NTA nitrilotriacetate     

Permeability of ammonia,methylamine and ethylamine in the cyanobacterium,Synechococcus R-2 (Anacystis nidulans) PCC 7942

Summary Permeabilities of ammonia (NH₃), methylamine (CH₃NH₂) and ethylamine (CH₃CH₂NH₂) in the cyanobacterium (cyanophyte)Synechococcus R-2 (Anacystis nidulans) have been measured. Based on net uptake rates of DCMU (dichlorophenyldimethylurea) treated cells, the permeability of ammonia was 6.44±1.22 m sec^–1 (n=13). The permeabilities of methylamine and ethylamine, based on steady-state¹⁴C labeling were more than ten times that of ammonia (P _methylamine=84.6±9.47 m sec^–1 (76),P _ethylamine=109±11 m sec^–1 (55)). The apparent permeabilities based on net uptake rates of methylamine and ethylamine uptake were significantly lower, but this effect was partially reversible by ammonia, suggesting that net amine fluxes are rate limited by proton fluxes to an upper limit of about 700 nmol m^–2 sec^–1. Increasing concentrations of amines in alkaline conditions partially dissipated the pH gradient across the cell membrane, and this property could be used to calculate the relative permeabilities of different amines. The ratio of ethylamine to methylamine permeabilities was not significantly different from that calculated from the direct measurements of permeabilities; ammonia was much less effective in dissipating the pH gradient across the cell membrane than methylamine or ethylamine. An apparent permeability of ammonia of 5.7±0.9 m sec^–1 could be calculated from the permeability ratio of ammonia to methylamine and the experimentally measured permeability of methylamine. The permeability properties of ammonia and methylamine are very different; this poses problems in the interpretation of experiments where¹⁴C-methylamine is used as an ammonia analogue.     

CO2 gas exchange of the understory plantAsarum europaeum L. in November

The CO₂ gas exchange rates of the Central European perennial understory plantAsarum europaeum L. were measured in late autumn (October 30 to November 30) in its natural habitat day and night.During these measurements the temperature ranged from 0 to 15°C and the absolute air humidity from 3 to 10 mg H₂O·1^–1. Temperature and absolute air humidity over these ranges did not affect CO₂ net assimilation which was determined almost entirely by quantum flux density.CO₂ net assimilation was light saturated at about 100 M·m^–2·s^–1 quantum flux density. The uptake rate at this point was 4.3 mg·dm^–2·h^–1. The compensation point occurred at approximately 1 M·m^–2·s^–1.     

Comparison of the growth rates of dinitrogen fixing subterranean clover swards with those assimilating nitrate ions

Summary Subterranean clover plants were grown as swards (about 2000 plants/m²) under controlled conditions with N provided either by N₂-fixation (NO ₃ ^– withheld) or by assimilation of NO ₃ ^– (NO ₃ ^– supplied). Crop growth rates were measured by dry matter sampling over periods of up to 70 days at PPFD values of 400–1000 mole quanta/m²/s. When NO ₃ ^– was supplied from sowing the swards grew more rapidly than when the swards were not supplied with NO ₃ ^– and plants had to establish an N₂-fixing apparatus. When inter-plant competition was reduced within the sward, a difference in growth rate in favour of NO ₃ ^– -fed plants continued for at least 50 days. When however, a closed canopy was allowed to form, the NO ₃ ^– -fed swards had more dry weight than the N₂-fed swards at the time of canopy closure but thereafter the two swards grew at similar rates at light flux densities of above about 800 mole quanta/m²/s. At light flux densities of about 400 mole quanta/m²/s N₂-fed swards had a growth rate 70–80% of that of NO ₃ ^– -fed plants. NO ₃ ^– -fed plants had a higher organic N content than did N₂-fed plants under all conditions.