Effect of four heavy metals on the biology of Nostoc muscorum

Summary This study presents the effects of Cr, Pb, Ni and Ag on growth, pigments, protein, DNA, RNA, heterocyst frequency, uptake of NH₄ ⁺ and N0₃ ^–, loss of electrolytes (Na⁺ and K⁺), nitrate reductase and glutamine synthetase activities ofNostoc muscorum. The statistical tests revealed a direct positive correlation between the metal concentration and inhibition of different processes. Ni was found to be more toxic against growth, pigments and heterocyst differentiation compared to the other metals. Inhibition of pigment showed the following trend: chlorophyll > phycocyanin > carotenoid. No generalized trend for inhibition of macromolecules was observed. The loss of K⁺ and Na⁺ as affected by Cr, Ni and Pb was similar but more pronounced for K⁺ than Na⁺. The inhibition of physiological variables depicted the following trend: Na⁺ loss > K⁺ loss > glutamine synthetase > NH₄ uptake > growth > N0₃ ^– uptake > nitrate reductase > heterocyst frequency. This study therefore suggests that loss of electrolytes can be used as a first signal of metal toxicity in cyanobacteria. However, further study is needed to confirm whether the abnormality induced by nickel (branch formation) is a physiological or genetic phenomenon.     

AMMONIUM,NITRATE, PHOSPHATE,AND INORGANIC CARBON UPTAKE IN AN OLIGOTROPHIC LAKE: SEASONAL VARIATIONS AMONG LIGHT RESPONSE VARIABLES1

The dependence of substrate saturated uptake of ¹⁵NH₄⁺, ¹⁵NO₃^?, ³²PO₄^3?, and ¹⁴CO₂ on photosynthetic photon flux density (PPFD or photsynthetically active radiation, 400–700 nm) was characterized seasonally in oligotrophic Flathead Lake, Montana. PO₄^3? uptake was not dependent upon PPFD at any time of the year, whereas NH₄⁺, NO₃^?, and CO₂ uptake were consistently dependent on PPFD over all seasons. Maximal rates of NH₄⁺, NO₃^? and CO₂ uptake usually occurred near 40% of surface PPFD, which corresponded to about 5 m in the lake; inhibition was evident at PPFD levels greater than 40%. NH₄⁺, NO₃^? and PO₄^3? were incorporated in the dark at measurable rates most of the year, whereas dark CO₂ uptake was always near 0 relative to light uptake. CO₂ and NO₃^? uptake were more strongly influenced by PPFD than was NH₄^3? uptake. The PPFD dependence of PO₄^3?, NH₄⁺, NO₃^? and CO₂ uptake may affect algal growth and nutrient status by influencing the balance in diel and seasonal C:N:P uptake ratios.     

Response ofAnabaena doliolum to bimetallic combinations of Cu,Ni and Fe with special reference to sequential addition

This study reports physiological features of a N₂-fixing cyanobacteriumAnabaena doliolum in response to metal mixtures. Exposure of the cyanobacterium to Cu, Ni and Fe individually, as well as in combinations (Cu + Ni, Cu + Fe, Ni + Fe), showed marked differences in growth inhibition, nutrient uptake (NH₄ ⁺ and NO₃ ⁻), photosynthesis, ATP content, nitrate reductase, glutamine synthetase and urease activities. The response to metal combinations was also dependent upon the order in which the metals were added. The Cu-Ni combination resulted in synergistic interaction, in contrast to the antagonism of Cu-Fe and Ni-Fe. Pre-addition of Fe protected the cyanobacterium against Cu and Ni toxicity. Statistically significant (P < 0.005) inhibition of all the processes following metal supplementation was observed. This study suggests that carbon fixation is the most suitable variable for assessing heavy metal toxicity.     

Nitrate nutrition enhances nickel accumulation and toxicity in Arabidopsis plants

Background and aims

Nickel (Ni) has become a major heavy metal contaminant. The form of nitrogen nutrition remarkably affects IRT1 expression in roots. IRT1 has an activity of transporting Ni²⁺ into root cells. Therefore, nitrogen-form may affect Ni accumulation and toxicity in plants. The assumption was investigated in this study.

Methods

The Arabidopsis plants were treated in Ni-contained growth solutions with either nitrate (NO₃ ^?) or ammonium (NH₄ ⁺) as the sole N source. After 7-day treatments, Ni concentration, IRT1 expression, Ni-induced toxic symptoms and oxidative stress in plants were analyzed.

Results

The NO₃ ^?-fed plants contained a higher Ni concentration, had a greater IRT1 expression in roots, and developed more severe toxic symptoms in the youngest fully expanded leaves, compared with the NH₄ ⁺-fed plants. The Ni-induced growth inhibition was also more significant in NO₃ ^?-fed plants. Interestingly, Ni exposure resulted in greater hydrogen peroxide (H₂O₂) and superoxide radical (O₂ ^{. ?}) accumulations, more severe lipid peroxidation and more cell death in NO₃ ^?-fed plants, whereas the opposite was true for NH₄ ⁺-fed plants. Furthermore, the Ni-enhanced peroxidase (POD) and superoxide dismutase (SOD) activities were greater in NO₃ ^?-fed plants

Conclusion

NO₃ ^? nutrition promotes Ni uptake, and enhances Ni-induced growth inhibition and oxidative stress in plants compared with NH₄ ⁺ nutrition.     

Elevated CO2 plus chronic warming reduce nitrogen uptake and levels or activities of nitrogen‐uptake and ‐assimilatory proteins in tomato roots

Atmospheric CO₂ enrichment is expected to often benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Most plants primarily procure N as inorganic nitrate (NO₃^?) or ammonium (NH₄⁺), using membrane‐localized transport proteins in roots, which are key targets for improving N use. Although interactive effects of elevated CO₂, chronic warming and N form on N relations are expected, these have not been studied. In this study, tomato (Solanum lycopersicum) plants were grown at two levels of CO₂ (400 or 700 ppm) and two temperature regimes (30 or 37°C), with NO₃^? or NH₄⁺ as the N source. Elevated CO₂ plus chronic warming severely inhibited plant growth, regardless of N form, while individually they had smaller effects on growth. Although %N in roots was similar among all treatments, elevated CO₂ plus warming decreased (1) N‐uptake rate by roots, (2) total protein concentration in roots, indicating an inhibition of N assimilation and (3) shoot %N, indicating a potential inhibition of N translocation from roots to shoots. Under elevated CO₂ plus warming, reduced NO₃^?‐uptake rate per g root was correlated with a decrease in the concentration of NO₃^?‐uptake proteins per g root, reduced NH₄⁺ uptake was correlated with decreased activity of NH₄⁺‐uptake proteins and reduced N assimilation was correlated with decreased concentration of N‐assimilatory proteins. These results indicate that elevated CO₂ and chronic warming can act synergistically to decrease plant N uptake and assimilation; hence, future global warming may decrease both plant growth and food quality (%N).     

Toxicity of chromium and tin toAnabaena doliolum Interaction with bivalent cations

Summary The toxicity of chromium and tin on growth, photosynthetic carbon-fixation, oxygen evolution, heterocyst differentiation and nitrogenase activity ofAnabaena doliolum and its interaction with bivalent cations has been studied. Some interacting cations, viz. Ca²⁺, Mg²⁺ and Mn²⁺, substantially antagonised the toxic effects of chromium and tin with reference to growth, heterocyst differentiation and nitrogenase activity in the following hierarchal sequence: Ca²⁺ > Mg²⁺ > Mn²⁺. However, the sequence of hierarchy was Mg²⁺ > Ca²⁺ > Mn²⁺ for carbon fixation and Mn²⁺ > Mg²⁺ > Ca²⁺ for photosynthetic oxygen evolution. Synergistically inhibitory patterns were noticed for all the parameters, viz. growth,¹⁴CO₂ uptake, oxygen evolution, heterocyst differentiation and nitrogenase activity ofA. doliolum when Ni²⁺, Co²⁺ and Zn²⁺ were combined with the test metals in the growth medium. These cations followed the following sequence of synergistic inhibition: Ni²⁺ > Co²⁺ > Zn²⁺. Among all the interacting cations, Ca²⁺, Mg²⁺ and Mn²⁺ exhibited antagonistic effects which relieved the test cyanobacterium from metal toxicity. In contrast to this, Ni²⁺, CO²⁺ and Zn²⁺ showed synergistic inhibition which potentiating the toxicity of test metals in the N₂-fixing cyanobacteriumA. doliolum. It is evident from the present study that bivalent cations, viz. Ca²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Co²⁺ and Zn²⁺, may appreciably regulate the toxicity of heavy metals in N₂-fixing cyanobacteria if present in aquatic media.     

The influence of enriched rhizosphere CO2 on N uptake and metabolism in wild-type and NR-deficient barley plants

Positive influences of high concentrations of dissolved inorganic carbon (DIC) in the growth medium of salinity-stressed plants are associated with carbon assimilation through phosphoenolpyruvate carboxylase (PEPc) activity in roots; and also in salinity-stressed tomato plants, enriched CO₂ in the rhizosphere increases NO^?₃uptake. In the present study, wild-type and nitrate reductase-deficient plants of barley (Hordeum vulgare L. cv. Steptoe) were used to determine whether the influence of enriched CO₂ on NO^?₃ uptake and metabolism is dependent on the activity of nitrate reductase (NR) in the plant. Plants grown in NH₄⁺and aerated with ambient air, were transferred to either NO₃^? or NH₄⁺ solutions and aerated with air containing between 0 and 6 500 μmol mol^?1 CO₂. Nitrogen uptake and tissue concentrations of NO₃^? and NH₄⁺ were measured as well as activities of NR and PEPc. The uptake of NO^?₃ by the wild-type was increased by increasing CO₂. This was associated with increased in vitro NR activity, but increased uptake of NO₃^? was found also in the NR-deficient genotype when exposed to high CO₂ concentrations; so that the influence of CO₂ on NO₃^? uptake was independent of the reduction of NO₃^? and assimilation into amino acids. The increase in uptake of NO₃^? in wild-type plants with enriched CO₂ was the same at pH 7 as at pH 5, indicating that the relative abundance of HCO₃^? or CO₂ in the medium did not influence NO₃^? uptake. Uptake of NH₄⁺ was decreased by enriched CO₂ in a pH (5 or 7) independent fashion. Thus NO₃^? and NH⁺₄ uptakes are influenced by the CO₂ component of DIC independently of anaplerotic carbon provision for amino acid synthesis, and CO₂ may directly affect the uptake of NO₃^? and NH₄⁺ in ways unrelated to the NR activity in the tissue.     

NUTRITIONAL STUDIES OF TWO RED ALGAE. II. KINETICS OF AMMONIUM AND NITRATE UPTAKE1, 2

Similar NH₄₊ and NO₃^?.uptake kinetic patterns were observed in Neoagardhiella baileyi (Harvey ex Kiitzing) Wyinne & Taylor and Gracilaria foliifera (Forssk?l) Borgesen. NO₃^? was taken up in a rate-sturating fashion described by the Michaelis-Menten equation. NH₄₊ uptake was multicomponent: a saturable component was accompanied by a diffusive or a high K component showing no evidence of saturation (at ≤50 μM [NH₄₊]). Nitrogen starved plantsi(C/N atom ratios > ca. 10) showed higher transient rates of NH₄₊ uptake at a given concentration than plants not N-Iimited. Only plants with high N content exhibited diel changes inNH₄₊ uptake rates, and showed transient rates of NH₄₊ accumulation which did not greatly exceed the capacity to incorporate N in steady-state growth. NH₄₊ was preferred over NO_3?even in plants preconditioned on NO_3?as the sole N. source, NO_3? uptake was suppressed at 5μM [NH₄₊], but simultaneous uptake occurred at unsurpressed rates at lower concentrations. Potential for N accumulation was greater via NH₄₊uptake than via NO_3?uptake. Changing capacity for NH₄₊ uptake with N content appears to be a mechanism whereby excessive accumulation of N was avoided by N-.satiated plants but a large accumulation was possible for N-depleted plants.     

Interspecies variation in nitrogen uptake kinetic responses of temperate forest species to elevated CO2: potential causes and consequences

Despite the recognition that the capacity to acquire N is critical in plant response to CO₂ enrichment, there is little information on how elevated CO₂ affects root N uptake kinetics. The few available data indicate a highly variable pattern of response to elevated CO₂, but it is presently unclear if the observed inconsistencies are caused by differences in experimental protocols or by true species differences. Furthermore, if there are interspecific variations in N uptake responses to elevated CO₂, it is not clear whether these are associated with different functional groups. Accordingly, we examined intact root‐system NH₄⁺ and NO₃^– uptake kinetic responses to elevated CO₂ in seedlings of six temperate forest tree species, representing (i) fast‐ vs. slow‐growers and (ii) broad‐leaves vs. conifers, that were cultured and assayed in otherwise similar conditions. In general, the species tested had a higher uptake capacity (V_max) for NH₄⁺ than for NO₃^–. Species substantially differed in their NO₃^– and NH₄⁺ uptake capacities, but the interspecific differences were markedly greater for NO₃^– than NH₄⁺ uptake. Elevated CO₂ had a species‐dependent effect on root uptake capacity for NH₄⁺ ranging from an increase of 215% in Acer negundo L. to a decrease of about 40% in Quercus macrocarpa Michx. In contrast, NO₃^– uptake capacity responded little to CO₂ in all the species except A. negundo in which it was significantly down‐regulated at elevated CO₂. Across species, the capacity for NH₄⁺ uptake was positively correlated with the relative growth rate (RGR) of species; however, the CO₂ effect on NH₄⁺ uptake capacity could not be explained by changes in RGR. The observed variation in NH₄⁺ uptake response to elevated CO₂ was also inconsistent with life‐form differences. Other possible mechanisms that may explain why elevated CO₂ elicits a species‐specific response in root N uptake kinetics are discussed. Despite the fact that the exact mechanism(s) for such interspecific variation remains unresolved, these differences may have a significant implication for competitive interactions and community responses to elevated CO₂ environment. We suggest that differential species responses in nutrient uptake capacity could be one potential mechanism for the CO₂‐induced shifts in net primary productivity and species composition that have been observed in experimental communities exposed to elevated levels of CO₂.     

Correlation between nitrogenase and glutamine synthetase expression in the cyanobacterium Anabaena cylindrical

Distribution pattern and levels of nitrogenase (EC 1.7.99.2) and glutamine synthetase (GS, EC 6.3.1.2) were studied in N₂-, NO₃^? and NH₄⁺ grown Anabaena cylindrica (CCAP 1403/2a) using immunogold electron microscopy. In N₂- and NO₃^? grown cultures, heterocysts were formed and nitrogenase activity was present. The nitrogenase antigen appeared within the heterocysts only and showed an even distribution. The level of nitrogenase protein in the heterocysts was identical with both nitrogen sources. In NO₃^? grown cells the 30% reduction in the nitrogenase activity was due to a corresponding decrease in the heterocyst frequency and not to a repressed nitrogenase synthesis. In NH₄^? grown cells, the nitrogenase activity was almost zero and new heterocysts were formed to a very low extent. The heterocysts found showed practically no nitrogenase protein throughout the cytoplasm, although some label occurred at the periphery of the heterocyst. This demonstrates that heterocyst differentiation and nitrogenase expression are not necessarily correlated and that while NH₄⁺ caused repression of both heterocyst and nitrogenase synthesis, NO₃^? caused inhibition of heterocyst differentiation only. The glutamine synthetase protein label was found throughout the vegetative cells and the heterocysts of all three cultures. The relative level of the GS antigen varied in the heterocysts depending on the nitrogen source, whereas the GS level was similar in all vegetative cells. In N₂- and NO₃⁺ grown cells, where nitrogenase was expressed, the GS level was ca 100% higher in the heterocysts compared to vegetative cells. In NH₄⁺ grown cells, where nitrogenase was repressed, the GS level was similar in the two cell types. The enhanced level of GS expressed in heterocysts of N₂ and NO₃^? grown cultures apparently is related to nitrogenase expression and has a role in assimilation of N₂derived ammonia.     

Effect of elevated CO2 concentration and nitrate: ammonium ratios on gas exchange and growth of cassava (Manihot esculenta Crantz)

Aims

This study evaluated how different nitrogen forms affect growth and photosynthetic responses of cassava to CO₂ concentration.

Methods

Cassava was grown in 14-L pots in a greenhouse at 390 or 750 ppm of CO₂. Three nitrogen treatments were applied: (a) 12?mM NO₃ ^?, (b) 6?mM NO₃ ^??+?6?mM NH₄ ⁺, and (c) 12?mM NH₄ ⁺.

Results

Thirty-six days after treatments began, plants grown under elevated CO₂ and fertilized only with NO₃ ^? (750_NO₃ ^?) had photosynthetic rates similar to plants grown under 390_NO₃ ^?, indicating significant photosynthetic acclimation to CO₂. In contrast, photosynthetic rates at elevated CO₂ increased as NH₄ ⁺ increased in the nutrient solution, such that photosynthetic acclimation was reduced for plants fertilized with only NH₄ ⁺. However, this positive effect of NH₄ ⁺ on photosynthesis was not observed in more advanced growth stages, and the toxic effects of NH₄ ⁺ severely reduced total dry mass for these plants measured at the end of the experiment.

Conclusions

Our results indicate that cassava will respond with increased biomass accumulation in response to raising atmospheric CO₂ levels, and that N form can have an important impact on the photosynthetic response. However, the positive effect of NH₄ ⁺ fertilization on cassava photosynthetic CO₂ response eventually led to a toxicity problem that reduced biomass production. The challenge is to determine how to manage NH₄ ⁺ fertilization so that the photosynthetic benefit observed in the initial phase may persist throughout the crop cycle.     

Dissolved Nitrogen Uptake by a Cyanobacterial Bloom (Anabaena flos-aquae) in a Subarctic Lake

Uptake of dissolved nitrogen (NH₄⁺ + NO₃^- + urea + N₂) by a cyanobacterial [Anabaena flos-aquae (Lyngb.)] De Brèb population in Smith Lake, Alaska, was measured every 2 to 4 days during the spring of 1990. Total dissolved nitrogen uptake ranged from 0.34 to 24.75 μmol liter^-1 h^-1, with a mean of 5.75 μmol liter^-1 h^-1; the euphotic zone accounted for 91% of the uptake. The mean turnover time for dissolved combined nitrogen (NH₄⁺ + NO₃^- + urea) in the euphotic zone was less than 14 h, and that for NH₄⁺ was only 3.6 h. The mean relative preference indices for NH₄⁺ (2.4), NO₃^- (0.4), and urea (0.5) established NH₄⁺ as the preferred nitrogenous nutrient. The uptake rates were apparently dependent on biomass, temperature, and light. Regeneration, probably due to zooplankton excretion and bacterial remineralization of dissolved organic nitrogen, was the main source of NH₄⁺ for the cyanobacterial growth. The high half-saturation constant for NH₄⁺ with low ambient NH₄⁺ concentration nevertheless resulted in the simultaneous utilization of several forms of nitrogen.     

Differential responses of root uptake kinetics of NH4+ and NO3− to enriched atmospheric CO2 concentration in field-grown loblolly pine

The nitrogen requirement of plants is predominantly supplied by NH₄⁺ and/or NO₃^? from the soil solution, but the energetic cost of uptake and assimilation is generally higher for NO₃^? than for NH₄⁺. We found that CO₂ enrichment of the atmosphere enhanced the root uptake capacity for NO₃^?, but not for NH₄⁺, in field-grown loblolly pine saplings. Increased preference for NO₃^? at the elevated CO₂ concentration was accompanied by increased carbohydrate levels in roots. The results have important implications for the potential consequences of global climate change on plant-and ecosystem-level processes in many temperate forest ecosystems.     

Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth,transpiration, and water and potassium uptake and flow in tobacco

NH₄ ⁺-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO₃ ^–, NH₄NO₃, NH₄ ⁺ or NH₄ ⁺+6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO₃ ^–-N by NH₄ ⁺-N (NH₄NO₃) did not change growth, transpiration, uptake and flow of water and K⁺ compared with the NO₃ ^–-N treatment. However, NH₄ ⁺-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO₃ ^–-N and NH₄NO₃ treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (¹³C) compared with the NO₃ ^– treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K⁺ uptake to 10%. Under NH₄ ⁺ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K⁺ currently taken up, indicating massive mobilisation of K⁺ from lower leaves and even the roots. Phloem retranslocation of K⁺ from the shoot and cycling through the root contributed 67% to the xylem transport of K⁺, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K⁺) caused by NH₄ ⁺-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10^–8 M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH₄ ⁺-fed plants are discussed.     

Nitrogen uptake and allocation in <Emphasis Type="Italic">Populus simonii</Emphasis> in different seasons supplied with isotopically labeled ammonium or nitrate

Key message

The total uptake of ¹⁵ NO ₃ -N was twofold higher than that of ¹⁵ NH ₄ -N when supplied with ammonium and/or nitrate in different seasons; the seedlings fertilized with NO ₃ -N had good growth with high photosynthetic rate and total biomass.

Abstract

Appropriate fertilization is crucial for maximum plant growth and improving nitrogen use efficiency. Poplar is an important fast-growing tree species for biomass production, however, little is known about fertilizer management of poplar plantations growing on barren soil in different seasons. To understand nitrogen uptake and allocation of Populus simonii supplied with different forms of nitrogen in different seasons, we determined nitrogen uptake and allocation of P. simonii potted seedlings after a 4-day supply of ¹⁵NH₄-N, ¹⁵NO₃-N, ¹⁵NH₄NO₃, and NH ₄ ¹⁵ NO₃ in May, July, and September. The total ¹⁵N uptake was twofold higher when supplied with sole ¹⁵NO₃-N compared to sole ¹⁵NH₄-N in all the investigated seasons. In the presence of ammonium nitrate (¹⁵NH₄NO₃ and NH ₄ ¹⁵ NO₃), the total ¹⁵N uptake was two times higher when supplied with NH ₄ ¹⁵ NO₃ compared to ¹⁵NH₄NO₃. Per unit biomass, the ¹⁵N-uptake ability of fine roots was higher in May and July compared to that in September. ¹⁵N was present mainly in leaves in May and July, and was mainly stored in roots and stems in autumn. The effect of nitrogen on the growth of P. simonii seedlings was studied by fertilizing with NH₄-N, NO₃-N, and NH₄NO₃ for 8 weeks. The seedlings fertilized with NO₃-N had good growth with high photosynthetic rate and total biomass indicating that NO₃-N is crucial for P. simonii growth. These data contribute to understand the nitrogen uptake in different seasons in trees supplied with different forms of nitrogen. This provides important theoretical bases for fertilizer management of poplar plantations.

     

Impact of bimetallic combinations of Cu,Ni and Fe on growth rate,uptake of nitrate and ammonium,14C02 fixation,nitrate reductase and urease activity ofChlorella vulgaris

Summary The toxicity of Cu, Ni and Fe individually, as well as in combination (Cu + Ni, Cu + Fe, Ni + Fe), on growth-rate depression, uptake of NO₃ ^– and NH₄ ⁺, photosynthesis, nitrate reductase and urease activity ofChlorella vulgaris has been studied. All the test metals when used individually showed pronounced toxicity on all the parameters studied. However, their interactive effect was mostly antagonistic except for Cu + Ni (synergism). Pre-addition of Fe offered more protection to the cells against copper and nickel toxicity. The data of statistical analysis reconfirmed that¹⁴C0₂ uptake is the most sensitive parameter (significant atP<0.005, both for time and treatment) than others in metal toxicity assessment. However, these results suggest further that exposure time and sequence of metal addition are very important in biomonitoring of heavy metal toxicity.     

Soil acidification as caused by the nitrogen uptake pattern of Scots pine (Pinus sylvestris)

Three-year-old Scots pine (Pinus sylvestris) trees were grown on a sandy forest soil in pots, with the objective to determine their NH₄/NO₃ uptake ratio and proton efflux. N was supplied in three NH₄-N/NO₃-N ratios, 3:1, 1:1 and 1:3, either as ¹⁵NH₄+¹⁴NO₃ or as ¹⁴NH₄+¹⁵NO₃. Total N and ¹⁵N acquisition of different plant parts were measured. Averaged over the whole tree, the NH₄/NO₃ uptake ratios throughout the growing season were found to be 4.2, 2.5, and 1.5 for the three application ratios, respectively. The excess cation-over-anion uptake value (C_a-A_a) appeared to be linearly related to the natural logarithm of the NH₄/NO₃ uptake ratio. Further, this uptake ratio was related to the NH₄/NO₃ ratio of the soil solution. From these relationship it was estimated that Scots pine exhibits an acidifying uptake pattern as long as the contribution of nitrate to the N nutrition is lower than 70%. Under field circumstances root uptake may cause soil acidification in the topsoil, containing the largest part of the root system, and soil alkalization in deeper soil layers.     

Short-term studies of 15NO3 - and 15NH4 + uptake by micropropagated strawberry shoots cultured with or without CO2 enrichment

The uptake of ¹⁵NO₃ ^- and ¹⁵NH₄ ⁺ has been examined in 5-,10- and 28-day-old micropropagated strawberry (Fragaria x ananassa Duch. cv. Kent) shoots rooted in one-half strength Murashige and Skoog (MS) liquid medium on cellulose plugs (Sorbarods). The results indicated that the plantlets absorbed both NO₃ ^- and NH₄ ⁺ during the culture with a greater uptake of NH₄ ⁺ at 5 days of culture. Furthermore, a pronounced reduction in NO₃ ^- and NH₄ ⁺ uptake at 10 and 28 days of culture was observed within 6 h of the short-term uptake study. This reduction could be explained by the low CO₂ concentration in test tubes during the photoperiod, since no reduction in nitrogen uptake occurred in the CO₂ enriched condition. The results are interpreted as an indication of the important role for photosynthetic CO₂ fixation in the process of nitrogen uptake by the plantlets during the rooting stage.Contribution No. CRH 82, Centre de Recherche en Horticulture, F.S.A.A., Université Laval, Québec.     

Photosynthetic energy supply for NO3− assimilation in Scenedesmus

NO₃^?-dependent O₂ in synchronous Scenedesmus obtusiusculus Chod. in the absence of CO₂ is stoichiometric with NH₄⁺ excretion, indicating a close coupling of NO₃^? reduction to non-cyclic electron flow. Also in the presence of CO₂, NO₃^? stimulates O₂ evolution as manifested by an increase in the O₂/CO₂ ratio from 0.96 to 1.11. This quotient was increased to 1.36 by addition of NO₂^?, without competitive interaction with CO₂ fixation, indicating that the capacity for non-cyclic electron transport at saturating light is non-limiting for simultaneous reduction of NO₃^? and CO₂ at high rates. During incubation with NO₃^?+ CO₂, no NH₄⁺ is released to the outer medium, whereas during incubation with NO₂^?+ CO₂, excess NH₄⁺ is formed and excreted. NO₃^? uptake is stimulated by CO₂, and this stimulation is also significant when the cellular energy metabolism is restricted by moderate concentrations of carbonyl cyanide-p-trifluoromethoxyphenylhydrazone, whereas NO₃^? uptake in the absence of CO₂ is severely inhibited by the uncoupler. Also under energy-restricted conditions NO₃^? uptake is not competitive with CO₂ fixation. Antimycin A is inhibitory for NO₃^? uptake in the absence of CO₂, and there is no enhancement of NO₃^? uptake by CO₂ in the presence of antimycin A. It is assumed that the energy demand for NO₃^? uptake is met by energy fixed as triosephosphates in the Calvin cycle. Antimycin A possibly affects the transfer of reduced triose phosphates from the chloroplast to the cytoplasm. Active carbon metabolism also seems to exert a control effect on NO₃^? assimilation, inducing complete incorporation of all NO₃^? taken up into amino acids. This control effect is not functional when NO₂^? is the nitrogen source. Active carbon metabolism thus seems to be essential both for provision of energy for NO₃^? uptake and for regulation of the process.     

Uptake and inhibition kinetics of nitrogen in Microcystis aeruginosa: Results from cultures and field assemblages collected in the San Francisco Bay Delta,CA

The nitrogen (N) uptake kinetic parameters for Microcystis field assemblages collected from the San Francisco Bay Delta (Delta) in 2012 and non-toxic and toxic laboratory culture strains of M. aeruginosa were assessed. The ¹⁵N tracer technique was used to investigate uptake of ammonium (NH₄⁺), nitrate (NO₃⁻), urea and glutamic acid over short-term incubations (0.5–1 h), and to study inhibition of NO₃⁻, NH₄⁺ and urea uptake by NH₄⁺, NO₃⁻ and NH₄⁺, respectively. This study demonstrates that Delta Microcystis can utilize different forms of inorganic and organic N, with the greatest capacity for NH₄⁺ uptake and the least for glutamic acid uptake, although N uptake did not always follow the classic Michaelis–Menten hyperbolic relationship at substrate concentrations up to 67 μmol N L⁻¹. Current ambient N concentrations in the Delta may be at sub-saturating levels for N uptake, indicating that if N loading (especially NH₄⁺) were to increase, Delta Microcystis assemblages have the potential for increased N uptake rates. Delta Microcystis had the highest specific affinity, α, for NH₄⁺ and the lowest for NO₃⁻. In culture, N uptake by non-toxic and toxic M. aeruginosa strains was much higher than from the field, but followed similar N utilization trends to those in the field. Neither strain showed severe inhibition of NO₃⁻ uptake by NH₄⁺ or inhibition of NH₄⁺ uptake on NO₃⁻, but both strains showed some inhibition of urea uptake by NH₄⁺.