Exogenous calcium affects nitrogen metabolism in root-zone hypoxia-stressed muskmelon roots and enhances short-term hypoxia tolerance

We investigated the effects of short-term root-zone hypoxic stress and exogenous calcium application or deficiency in an anoxic nutrient solution on nitrogen metabolism in the roots of the muskmelon cultivar Xiyu No. 1. Seedlings grown in the nutrient solution under hypoxic stress for 6 d displayed significantly reduced plant growth and soluble protein concentrations. However, NO₃⁻ uptake rate and activities of nitrate reductase and glutamate synthase were significantly increased. We also found higher amounts of nitrate, ammonium, amino acids, heat-stable proteins, polyamines, H₂O₂, as well as higher polyamine oxidase activity in the roots. In comparison to the reactions seen under hypoxic stress, exogenous calcium application led to a marked increase in plant weights, photosynthesis parameters, NO₃⁻ uptake rate and contents of nitrate, ammonium, amino acids (e.g., glutamic acid, proline, glycine, cystine, γ-aminobutyric acid), soluble and heat-stable proteins, free spermine, and insoluble bound polyamines. Meanwhile, exogenous calcium application resulted in significantly increased activities for nitrate reductase, glutamine synthetase, and glutamate synthase but decreased activities for diamine and polyamine oxidase, as well as lower H₂O₂ content in roots during exposure to hypoxia. However, calcium deficiency in the nutrient solution decreased plant weight, photosynthesis parameters, NO₃⁻ reduction, amino acids (e.g., alanine, aspartic acid, glutamic acid, γ-aminobutyric acid), protein, all polyamines except for free putrescine, and the activities of glutamate synthase and glutamine synthetase. Additionally, there was an increase in the NO₃⁻ uptake rate, polyamine oxidase activity and H₂O₂ contents under hypoxia-Ca. Simultaneously, exogenous calcium had little effect on nitrate absorption and transformation, photosynthetic parameters, and plant growth under normoxic conditions. These results suggest that calcium confers short-term hypoxia tolerance in muskmelon, most likely by promoting nitrate uptake and accelerating its transformation into amino acids, heat-stable proteins or polyamines, as well as by decreasing polyamine degradation in muskmelon seedlings.     

Changes in the Activity of Antioxidant Enzymes in Wheat Leaves and Roots as a Function of Nitrogen Source and Supply

The activity of enzymes participating in the systems of antioxidant protection was assayed in the second leaf and roots of 21-day-old wheat seedlings (Triticum aestivum L.) grown in a medium with nitrate (NO^– ₃ treatment), ammonium (NH⁺ ₄ treatment), or without nitrogen added (N-deficiency treatment). The activities of superoxide dismutase (SOD), peroxidase, ascorbate peroxidase, glutathione reductase, and catalase in the leaves and roots of the NH⁺ ₄ plants was significantly higher than in the plants grown in the nitrate medium. The activity of SOD decreased and ascorbate peroxidase markedly increased in leaves, whereas the activity of ascorbate peroxidase increased in the roots of N-deficient plants, as compared to the plants grown in nitrate and ammonium. Low-temperature incubation (5°, 12 h) differentially affected the antioxidant activity of the studied plants. Whereas leaf enzyme activities did not change in the NH⁺ ₄ plants, the activities of SOD, peroxidase, ascorbate peroxidase, and catalase markedly increased in the NO^– ₃ plants. In leaves of the N-deficient plant, the activity of SOD decreased; however, the activity of other enzymes increased. In response to temperature decrease, catalase activity increased in the roots of NO^– ₃ and NH⁺ ₄-plants, whereas in the N-deficient plants, the activity of peroxidase increased. Thus, in wheat, both nitrogen form and nitrogen deficiency changed the time-course of antioxidant enzyme activities in response to low temperature.     

Partial nitrate nutrition amends photosynthetic characteristics in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L. var. <Emphasis Type="Italic">japonica</Emphasis>) differing in nitrogen use efficiency

Partial nitrate nutrition (PNN) was found to improve rice (Oryza sativa L. var. japonica) growth. However, how PNN is related to photosynthesis in rice cultivars with different nitrogen use efficiency (NUE) is still not clear. Two rice cultivars, Nanguang (high NUE) and Elio (low NUE), were grown under sole NH₄ ⁺ and PNN at a total nitrogen concentration of 2.86 mM. The dry weight, leaf area, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and gas exchange parameters were measured. Nitrogen and Rubisco contents in the newly expanded leaves of cv. Nanguang were similar to those of cv. Elio when only NH₄ ⁺ was supplemented in the nutrient solution. However, in cv. Nanguang, nitrogen and Rubisco contents increased under PNN than under sole NH₄ ⁺ nutrition. Higher nitrogen and Rubisco contents were recorded in cv. Nanguang than in cv. Elio under PNN. The ratio of carboxylation efficiency (CE) to Rubisco content in cv. Nanguang was 11 and 14% higher than that in cv. Elio under NH₄ ⁺ and PNN, respectively. CE was 14% higher in cv. Nanguang than that in cv. Elio. The results suggest that PNN causes an increase in photosynthesis in cv. Nanguang. It is concluded that differences in Rubisco activity, rather than stomatal limitation, are responsible for the differences in photosynthesis between the two cultivars. The presence of nitrate increases Rubisco content in rice with a high NUE, which leads to faster biomass accumulation at later growth stages.     

The Physiological Mechanisms Underlying N₂-Fixing Common Bean (<i>Phaseolus vulgaris</i> L.) Tolerance to Iron Deficiency

Iron is an essential element for plants as well as all living organisms, functioning in various physiological and biochemical processes such as photosynthesis, respiration, DNA synthesis, and N₂ fixation. In the soil, Fe bioavailability is extremely low, especially under aerobic conditions and at high pH ranges. In contrast, plants with nodules on their roots that fix atmospheric nitrogen need much more iron. To highlight the physiological traits underlying the tolerance of N₂-fixing common bean to iron deficiency, two genotypes were hydroponically cultivated in a greenhouse: Coco nain (CN) and Coco blanc (CB). Plants were inoculated with an efficient strain of Rhizobium tropici, CIAT899, and received a nutrient solution added with 0 µM Fe (severe Fe deficiency, SFeD), 5 µM Fe (moderate Fe deficiency, MFeD) or 45 µM Fe (control, C). Several physiological parameters related to photosynthesis and symbiotic nitrogen fixation were then analyzed. Iron deficiency significantly reduced whole plant and nodule growth, chlorophyll biosynthesis, photosynthesis, leghemoglobin (LgHb), nitrogenase (N₂ase) activity, nitrogen, and Fe nutrition, with some genotypic differences. As compared to CB, CN maintained better Fe allocation to shoots and nodules, allowing it to preserve the integrity of its photosynthetic and symbiotic apparatus, thus maintaining the key functional traits of the plant metabolism (chlorophyll biosynthesis and photosynthesis in shoots, leghemoglobin accumulation, and nitrogenase activity in root nodules). Plant growth depends on photosynthesis, which needs to be supplied with sufficient iron and nitrogen. Fe deficiency stress index (FeD-SI) and Fe use efficiency (FeUE) are two physiological traits of tolerance that discriminated the studied genotypes.     

Effect of nitrogen deficiency on antioxidant status and Cd toxicity in rice seedlings

Effect of nitrogen (N) deficiency on antioxidant status and Cd toxicity in rice seedlings was investigated. N deficiency resulted in a reduction of shoot growth but not root growth. The contents of N-containing compounds such as nitrate, chlorophyll, and protein decreased in leaves of rice seedlings grown under N deficiency. Accumulation of abscisic acid and H₂O₂ in leaves was induced by N deficiency. The content of ascorbate and the activities of ascorbate peroxidase, glutathione reductase, and catalase in N-deficient leaves were lower than their respective control leaves. However, glutathione content was not affected and superoxide dismutase activity was increased by N deficiency. Cd toxicity in N-deficient seedlings was more pronounced than that in N-sufficient ones. Pretreatment with ascorbate or L-galactono-1,4-lactone, a biosynthetic precursor of ascorbate resulted in a reduction of Cd toxicity enhanced by N deficiency. N deficiency also resulted in an enhancement of Cd uptake in rice seedlings. The possible mechanism of Cd toxicity enhanced by N deficiency is discussed.     

GLUTAMINE SYNTHETASE ASE ACTYVITY IN IN CHAETOCEROS AFFINIS (BACILLARIOPHYCEAE): COMPARISON WITH OTHER ESTIMATES OF NITROGEN UTILIZATION DURING NUTRIENT PERTURBATION1

Glutamine synthftase (GS) activity was investigated in a nitratt limited continuous culture of the marine diatom Chaeloccros afTinis (Lauder) Hustedt before and after the perturbation of the culture medium with 10 μM of ¹⁵ N labelled nitrate. Parallel studies were carried out on nitrate reductase(NR). nitrate uptake and assimilation, and Ievels of cellular nitrogen containing compounds with the objective to determine the validity of the GS assay as a measure of nitrate utilization. Activities in N-deficient cells, grown at steady state, correlated well with uptake and assimilation rates. In N-sufftcient celts, however, during the nitrate pertirbation period, they accounted only for about 10% of the two latter rates, when ambient nitrate concentrations were high (0. 7-10 μ). It is proposed that under these growth conditions an alternative pathway via glutamate dehydrogenase (GDH) was operative. At low ambient nitrate concentrations (0.1-0.7 μM), GS activities, uptake and assimilation rates again balanced rather well. Thus, the data support the view that GDH activity is associated with high levels and GS with low levels of external or internal nitrogen.     

Effect of nitrate-nitrogen limitation on photosynthesis of the diatom Phaeodactylum tricornutum Bohlin (Bacillariophyceae)

Abstract Nitrate limited growth of the diatom Phaeodactylum tricornutum in chemostat cultures produced marked changes in biochemical composition and a six-fold reduction in the specific growth rate. This was associated with a reduction in the carbon and chlorophyll a specific light saturated rates, with little effect on light limited photosynthesis. Variations in specific growth rate were quantitatively related to carbon specific net photosynthesis and maximum chlorophyll a specific light saturated rates were positively correlated with cell nitrogen contents. The correlation between nitrogen content and photosynthesis for P. tricornutum and the differential effect of nitrogen supply on the light response curve of photosynthesis is qualitatively and quantitatively similar to published results for terrestrial vascular plants. There was little change in the photon (quantum) yield of photosynthesis which was not significantly different from 0.125mol O₂ mol photon^-1 the theoretical upper limit based on the Z scheme, even under severe nitrate deficiency. The capacity to maintain a high photon yield under nitrate limitation is discussed in relation to the nitrogen requirements of the stromal and membrane components of the photosynthetic apparatus.     

Effect of salt stress on antioxidant system of plants as related to nitrogen nutrition

In plants of wheat (Triticum aestivum L.) grown in the media with nitrate (NO ₃ ^? plants), ammonium (NH ₄ ⁺ plants), and without nitrogen (N-deficient plants), the response to oxidative stress induced by the addition of 300 mM NaCl to the nutrient solution was investigated. Three-day-long salinization induced chlorophyll degradation and accumulation of malondialdehyde (MDA) in the leaves. These signs of oxidative stress were clearly expressed in NO ₃ ^? and N-deficient plants and weakly manifested in NH ₄ ⁺ plants. In none of the treatments, salinization induced the accumulation of MDA in the roots. Depending on the conditions of N nutrition, salt stress was accompanied by diverse changes in the activity of antioxidant enzymes in the leaves and roots. Resistance of leaves of NH ₄ ⁺ plants to oxidative stress correlated with a considerable increase in the activities of ascorbate peroxidase and glutathione reductase. Thus, wheat plants grown on the NH ₄ ⁺ -containing medium were more resistant to the development of oxidative stress in the leaves than those supplied with nitrate.     

Phytoplankton nutrient deficiency in lakes of the McMurdo dry valleys, Antarctica

1. The influence of inorganic nitrogen and phosphorus enrichment on phytoplankton photosynthesis was investigated in Lakes Bonney (east and west lobes), Hoare, Fryxell and Vanda, which lie in the ablation valleys adjacent to McMurdo Sound, Antarctica. Bioassay experiments were conducted during the austral summer on phytoplankton populations just beneath the permanent ice cover in all lakes and on populations forming deep-chlorophyll maxima in the east and west lobes of Lake Bonney. 2. Phytoplankton photosynthesis in surface and mid-depth (13 m) samples from both lobes of Lake Bonney were stimulated significantly (P < 0.01) by phosphorus enrichment (2 μM) with further stimulation by simultaneous phosphorus plus NH₄⁺ (20 μM) enrichment. Similar trends were observed in deeper waters (18 m) from the east lobe of Lake Bonney, although they were not statistically significant at P < 0.05. Photosynthesis in this lake was never enhanced by the addition of 20 μM NH₄⁺ alone. Simultaneous addition of phosphorus plus nitrogen stimulated photosynthesis significantly (P < 0.01) in both Lake Hoare and Lake Fryxell. No nutrient response occurred in Lake Vanda, where activity in nutrient-enriched samples was below unamended controls; results from Lake Vanda are suspect owing to excessively long sample storage in the field resulting from logistic constraints. 3. Ambient dissolved inorganic nitrogen (DIN) (NH4⁺+ NO₂^?+ NO₃^?): soluble reactive phosphorus (SRP) ratios partially support results from bioassay experiments indicating strong phosphorus deficiency in Lake Bonney and nitrogen deficiency in Lakes Hoare and Fryxell. DIN : SRP ratios also imply phosphorus deficiency in Lake Vanda, although not as strong as in Lake Bonney. Particulate carbon (PC): particulate nitrogen (PN) ratios all exceed published ratios for balanced phytoplankton growth, indicative of nitrogen deficiency. 4. Vertical nutrient profiles in concert with low advective flux, indicate that new (sensu Dugdale & Goering, 1967) phytoplankton production in these lakes is supported by upward diffusion of nutrients from deep nutrient pools. This contention was tested by computing upward DIN : SRP flux ratios across horizontal planes located immediately beneath each chlorophyll maximum and about 2 m beneath the ice (to examine flux to the phytoplankton immediately below the ice cover). These flux ratios further corroborated nutrient bioassay results and bulk DIN : SRP ratios indicating phosphorus deficiency in Lakes Bonney and Vanda and potential nitrogen deficiency in Lakes Hoare and Fryxell. 5. Neither biochemical reactions nor physical processes appear to be responsible for differences in nutrient deficiency among the study lakes. The differences may instead be related to conditions which existed before or during the evolution of the lakes.     

Nutrient Biogeochemistry During the Early Stages of Delta Development in the Mississippi River Deltaic Plain

Nutrient biogeochemistry associated with the early stages of soil development in deltaic floodplains has not been well defined. Such a model should follow classic patterns of soil nutrient pools described for alluvial ecosystems that are dominated by mineral matter high in phosphorus and low in carbon and nitrogen. A contrast with classic models of soil development is the anthropogenically enriched high nitrate conditions due to agricultural fertilization in upstream watersheds. Here we determine if short-term patterns of soil chemistry and dissolved inorganic nutrient fluxes along the emerging Wax Lake delta (WLD) chronosequence are consistent with conceptual models of long-term nutrient availability described for other ecosystems. We add a low nitrate treatment more typical of historic delta development to evaluate the role of nitrate enrichment in determining the net dinitrogen (N₂) flux. Throughout the 35-year chronosequence, soil nitrogen and organic matter content significantly increased by an order of magnitude, whereas phosphorus exhibited a less pronounced increase. Under ambient nitrate concentrations (>60 μM), mean net N₂ fluxes (157.5 μmol N m^?2 h^?1) indicated greater rates of gross denitrification than gross nitrogen fixation; however, under low nitrate concentrations (<2 μM), soils switched from net denitrification to net nitrogen fixation (?74.5 μmol N m^?2 h^?1). As soils in the WLD aged, the subsequent increase in organic matter stimulated net N₂, oxygen, nitrate, and nitrite fluxes producing greater fluxes in more mature soils. In conclusion, soil nitrogen and carbon accumulation along an emerging delta chronosequence largely coincide with classic patterns of soil development described for alluvial floodplains, and substrate age together with ambient nitrogen availability can be used to predict net N₂ fluxes during early delta evolution.     

Kinetics of nitrate uptake by different species from nutrient-rich and nutrient-poor habitats as affected by the nutrient supply

The species Urtica dioica L., Plantago major ssp. major L., Plantago lanceolata L., Hypochaeris radicata L. ssp. radicata and Hypochaeris radicata ssp. ericetorum Van Soest were grown under high and low nutrient conditions (1/4 Hoagland and 2% of 1/4 Hoagland further called the 100% and 2% treatment, containing 3.75 mM NO^-₃ and 0.075 mM NO^-₃, respectively). After a certain period half of the plants were transferred from low to high or high to low nutrients, yielding the 100%/2% and the 2%/100% treatments. The kinetics of nitrate uptake in the range of system I of the five species grown under the different nutrient conditions were measured during a three week experimental period. The nitrate uptake of all the species showed the characteristic features of Michaelis-Menten kinetics. Under low nutrient conditions the apparent V_max of U. dioica expressed per g dry root was lower than under high nutrient conditions. For H. radicata ssp. radicata and for H. radicata ssp. ericetorum the reverse was found. The V_max values of P. major ssp. major were almost the same for the two treatments. The apparent V_max in young plants of P. lanceolata was higher in the 100% treatment than in 2%; whereas the reverse was found in mature plants. The results are explained in relation to the relative growth rate, the shoot to root ratio and the natural environment of the species. The apparent K_m values were not influenced by the different treatments. Differences in K_m between the species, if any, were very small. It is suggested that the V_max is a more important parameter for the distribution of plant species in the field than the K_m. The rate of nitrogen accumulation was calculated from growth data and the contents of nitrate and reduced nitrogen. It is concluded that the V_max of system I for nitrate uptake in most cases was sufficient to explain the observed growth rates.     

Influence of temperature and nitrogen concentration on photosynthesis ofDunaliella viridis Teodoresco

The photosynthetic behaviour ofDunaliella viridis has been studied under a combination of three variables: irradiance (0–900 mol m^–2 s^–1), temperature (15, 23, 31, 38, 42 °C) and nitrogen concentration (0.05, 0.5, 1.5, 5, 10 mM NO ₃ ^- ) at a salinity of 2 M NaCl.The highest rates of photosynthesis have been found at 31 °C and a nitrate concentration of 10 mM. There exists a synergistic effect between temperature and nitrogen availability on the photosynthesis ofD. viridis; under nitrogen deficiency oxygen evolution is low, even null at high temperature. The interaction between these two variables of control occurs in a multiplicative way. There is also a general increase in photosynthetic pigments following the increase in nitrogen concentration in the culture medium. The normalization of net photosynthesis data in relation to chlorophylla shows that nitrogen concentration makes an indirect control of the photosynthetic rate ofD. viridis through the variation of pigment concentration.     

Importance of seed Zn content for wheat growth on Zn-deficient soil

Eggplants (Solanum melongena L. cv. Bonica) were grown in a glasshouse during summer under natural light with one unbranched shoot or one shoot with 3 to 4 branches and with or without fruit in quartz sand buffered and not buffered with 0.5% CaCO₃ (w : v), respectively. Nutrient solutions supplied contained nitrate or ammonium as the sole nitrogen source. Compared with nutrient solutions containing nitrate (10 mM), solutions containing ammonium (10 mM) caused a decrease in net photosynthesis of eggplants during early stages of vegetative growth when grown in quartz sand not buffered with CaCO₃. The decrease was not observed before leaves showed interveinal chlorosis. In contrast, net photosynthesis after bloom at first increased more rapidly in eggplants supplied with ammonium than with nitrate nitrogen. However, even in this case, net photosynthesis decreased four weeks later when ammonium nutrition was continued. The decrease was accompanied by epinasty and interveinal chlorosis on the lower leaves and later by severe wilting, leaf drop, stem lesions, and hampered growth of stems, roots, and fruits. These symptoms appeared later on plants not bearing fruits than on plants bearing fruits. If nutrient solutions containing increasing concentrations of ammonium (0.5–30 mM) were supplied after the time of first fruit ripening, shoot growth and set of later flowers and fruits were promoted. In contrast, vegetative growth and reproduction was only slightly affected by increasing the concentration of nitrate in the nutrient solutions. In quartz sand buffered with CaCO₃ ammonium nutrition caused deleterious effects only under low light conditions (shade) and on young plants during rapid fruit growth. If eggplants were supplied with ammonium nitrogen before bloom, vegetative growth was promoted, and set of flowers and fruit occurred earlier than on plants supplied with nitrate. Furthermore, the number of flowers and fruit yield increased. These effects of ammonium nutrition were more pronounced when plants were grown with branched shoots than with unbranched shoots. The results indicate that vegetative and reproductive growth of eggplants may be manipulated without causing injury to the plants by supplying ammonium nitrogen as long as the age of the plants, carbohydrate reserves of the roots, quantity of ammonium nitrogen supplied, and pH of the growth medium are favourable. T W Rufty Section editor     

Interactive effects of elevated CO2 and soil fertility on isoprene emissions from Quercus robur

The effects of global change on the emission rates of isoprene from plants are not clear. A factor that can influence the response of isoprene emission to elevated CO₂ concentrations is the availability of nutrients. Isoprene emission rate under standard conditions (leaf temperature: 30°C, photosynthetically active radiation (PAR): 1000 μmol photons m^?2 s^?1), photosynthesis, photosynthetic capacity, and leaf nitrogen (N) content were measured in Quercus robur grown in well‐ventilated greenhouses at ambient and elevated CO₂ (ambient plus 300 ppm) and two different soil fertilities. The results show that elevated CO₂ enhanced photosynthesis but leaf respiration rates were not affected by either the CO₂ or nutrient treatments. Isoprene emission rates and photosynthetic capacity were found to decrease with elevated CO₂, but an increase in nutrient availability had the converse effect. Leaf N content was significantly greater with increased nutrient availability, but unaffected by CO₂. Isoprene emission rates measured under these conditions were strongly correlated with photosynthetic capacity across the range of different treatments. This suggests that the effects of CO₂ and nutrient levels on allocation of carbon to isoprene production and emission under near‐saturating light largely depend on the effects on photosynthetic electron transport capacity.     

Interaction of nitrogen source and light intensity on the growth and photosynthesis of the brown tide alga Aureococcus anophagefferens

High ratios of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) have been suggested to favor the growth of the brown tide alga Aureococcus anophagefferens. DON could provide a particular advantage in low light levels, as occur when blooms induce self-shading. We examined the effects of varying DON:DIN ratios on the photosynthetic abilities of cultured Aureococcus at two light intensities, 93 and 17 μmol photons m⁻² s⁻¹. Glutamic acid and urea were used as DON sources, and the remainder of the nitrogen was added as nitrate.In experiments examining Aureococcus growth with varying ratios of DON_Glu:DIN_Nitrate at two light intensities in batch culture, higher growth rates and biomass were observed in treatments containing DIN than in those with DON only, which contrasts with the results of previous studies. In semi-continuous growth experiments with varying DON_Urea:DIN_Nitrate ratios, low light cultures with urea had higher growth rates than those without urea. Also, the effective target area for light absorption per cell and photosystem II efficiency were greater for the low light cultures of each nutrient treatment, particularly when DON:DIN mixtures (33 and 67% N_Urea) were used. The same pattern was seen in the maximum photosynthetic rates per cell in the light-saturated (P_m^cell) and in the initial slope (α^cell) of the PE (photosynthesis versus irradiance) curve, and in PON, POC and chlorophyll a cell⁻¹. This indicates that the ability of Aureococcus to acclimate to low light conditions may be enhanced by the presence of both organic and inorganic nitrogen sources. These results suggest that Aureococcus physiology and photosynthesis are different during growth on a mixture of urea-N and nitrate than when either nitrogen source is present alone. Results of this study suggest that Aureococcus may not respond to all DON substrates in the same way, and that mixtures of DON and DIN may provide for higher photosynthetic rates, especially at low light. Our results did not, however, support earlier suggestions that growth on DON alone provides the brown tide alga with a large advantage at low light levels.     

Evidence that Northern Pioneering Pines with Tuberculate Mycorrhizae are Unaffected by Varying Soil Nitrogen Levels

Tuberculate mycorrhizae on Pinus contorta (lodgepole pine) have previously been shown to reduce acetylene, but an outstanding question has been to what degree these structures could meet the nitrogen requirements of the tree. We compared the growth, tissue nitrogen contents, and stable nitrogen isotope ratios of P. contorta growing in gravel pits to the same species growing on adjacent intact soil. Trees growing in severely nitrogen deficient gravel pits had virtually identical growth rates and tissue nitrogen contents to those growing on intact soil that had nitrogen levels typical for the area. δ¹⁵N values for trees in the gravel pits were substantially lower than δ¹⁵N values for trees on intact soil, and isotope ratios in vegetation were lower than the isotope ratios of the soil. The form of soil nitrogen in the gravel pits was almost exclusively nitrate, while ammonium predominated in the intact soil. Discrimination against ¹⁵N during plant uptake of soil nitrate in the highly N-deficient soil should be weak or nonexistent. Therefore, the low δ¹⁵N in the gravel pit trees suggests that trees growing in gravel pits were using another nitrogen source in addition to the soil. Precipitation-borne nitrogen in the study area is extremely low. In conjunction with our other work, these findings strongly suggests that P. contorta and its microbial symbionts or associates fix nitrogen in sufficient amounts to sustain vigorous tree growth on the most nitrogen-deficient soils.     

Temperature Sensitivity of Daily Respiratory Patterns Entrained through Phytochrome Action in Lemna paucicostata Strain 6746

The respiration rate of heterotrophic Lemna paucicostata Hegelm. 6746 cultures at 26–27°C and given a brief pulse of red light, assumes patterns with properties dependent on the nitrogen source supplied. In a search for conditions specifically affecting features of those patterns expressing photperiodic timing, their amplitudes have been measured as relative peak height (RPH) — the increased height at the daily peak as a proportion of the average daily minimum — at both 21°C and 28°C. On nitrate, ammonium or aspartate medium, RPH is reduced roughly 70% at 21°C as compared to 28°C, but on nitrogen-deficient medium the reduction is only 20–30%. Yet growth and the actual intensity of respiration are not differently temperature sensitive on NO₃ and N-deficient media. Nor does RPH itself correlate with growth rate. Previous evidence indicates that patterns on NO₃ and NH₄ media reflect photoperiodic timing and that those on aspartale and N-deficient media do not; hence the temperature sensitivity of RPH does not correlate with whether or not a pattern reflects photo-periodic timing. However, different daily patterns arc elicited by pulses of red or far-red on NO₃. NH₄ and aspartale media but not on the N-deficient. Hence the temperature sensitivity of RPH does correlate with the degree to which the patterns distinguish between red and far-red, which is to say between high anti low levels of Pfr-phytochrome. This suggests that high temperature sensitivity in RPH reflects a reaction limited by N assimilation and saturating only at relatively high levels of Pfr.     

The interaction between elevated carbon dioxide and nitrogen nutrition: the physiological and molecular background

AGPase, ADP glucose pyrophosphorylase
GS, glutamine synthetase
GOGAT, glutamate : oxoglutarate amino transferase
NADP-ICDH, NADP-dependent isocitrate dehydrogenase
NR, nitrate reductase
OPPP, oxidative pentose phosphate pathway
3PGA, glycerate-3-phosphate
PEPCase, phosphoenolpyruvate carboxylase
Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase
SPS, sucrose phosphate-synthase

This review first summarizes the numerous studies that have described the interaction between the nitrogen supply and the response of photosynthesis, metabolism and growth to elevated [CO₂]. The initial stimulation of photosynthesis in elevated [CO₂] is often followed by a decline of photosynthesis, that is typically accompanied by a decrease of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), an accumulation of carbohydrate especially starch, and a decrease of the nitrogen concentration in the plant. These changes are particularly marked when the nitrogen supply is low, whereas when the nitrogen supply is adequate there is no acclimation of photosynthesis, no major decrease in the internal concentration of nitrogen or the levels of nitrogen metabolites, and growth is stimulated markedly. Second, emerging evidence is discussed that signals derived from nitrate and nitrogen metabolites such as glutamine act to regulate the expression of genes involved in nitrate and ammonium uptake and assimilation, organic acid synthesis and starch accumulation, to modulate the sugar-mediated repression of the expression of genes involved in photosynthesis, and to modulate whole plant events including shoot–root allocation, root architecture and flowering. Third, increased rates of growth in elevated [CO₂] will require higher rates of inorganic nitrogen uptake and assimilation. Recent evidence is discussed that an increased supply of sugars can increase the rates of nitrate and ammonium uptake and assimilation, the synthesis of organic acid acceptors, and the synthesis of amino acids. Fourth, interpretation of experiments in elevated [CO₂] requires that the nitrogen status of the plants is monitored. The suitability of different criteria to assess the plant nitrogen status is critically discussed. Finally the review returns to experiments with elevated [CO₂] and discusses the following topics: is, and if so how, are nitrate and ammonium uptake and metabolism stimulated in elevated [CO₂], and does the result depend on the nitrogen supply? Is acclimation of photosynthesis the result of sugar-mediated repression of gene expression, end-product feedback of photosynthesis, nitrogen-induced senescence, or ontogenetic drift? Is the accumulation of starch a passive response to increased carbohydrate formation, or is it triggered by changes in the nutrient status? How do changes in sugar production and inorganic nitrogen assimilation interact in different conditions and at different stages of the life history to determine the response of whole plant growth and allocation to elevated [CO₂]?     

A novel interaction of magnesium translocation with the supply of phosphorus to roots of grapevine (Vitis vinifera L.)*

Abstract. The role of phosphorus (P) in leaf magnesium (Mg) concentrations and photosynthesis was investigated in field and glasshouse experiments with grapevine (Vitis vinifera L., cvs. Chenin blane. Chardonnay, and Carignane). In the field, leaves of vines growing on soil with low available P exhibited symptoms of Mg deficiency and had low P and Mg concentrations. The rate of photosynthesis for leaves of untreated control vines was approximately 0.7 nmol CO₂ cm ² s ¹. When P fertilizer was applied to the soil, Mg deficiency symptoms were eliminated, and leaf P and Mg concentrations increased to above critical levels. When Mg was applied as a foliar spray, leaf Mg increased to above critical levels, but leaf P did not change significantly. In both experiments, the rate of photosynthesis increased to greater than 1.0 nmol CO₂ cm ² s ¹ after nutrient applications. Thus, under low soil P conditions, leaf photosynthesis was limited by leaf Mg concentrations. In glasshouse experiments in which vines were grown with and without P for three seasons, Mg accumulated in large roots of - P vines to approximately twice the concentration found in roots of + P vines. Analysis of the xylem exudate from detopped plants showed that Mg concentration in xylem sap of + P vines was twice as great as that in - P vines. When P was supplied to - P vines, the concentration of Mg increased to the concentration of + P vines within 2 days. The results show that the translocation of Mg from roots to shoots of grapevine is dependent upon P supply to the roots and suggest that Mg translocation is more sensitive than uptake to P supply.     

Effects of nitrate on intracellular nitrite and growth of Microcystis aeruginosa

Although nitrate is a macronutrient and can serve as good nitrogen source for many species of phytoplankton, high nitrate concentrations do not benefit the growth of phytoplankton. We hypothesise that algae cultured under high nitrate concentrations can accumulate intracellular nitrite, which is produced by nitrate reductase (NR) and can inhibit the growth of algae. To assess the validity of this hypothesis, Microcystis aeruginosa was grown under different nitrate concentrations from 3.57 to 21.43 mM in low CO₂ and high CO₂ conditions for 15 days. We observed that, with increasing nitrate concentrations, the intracellular nitrite concentrations of the alga increased and the growth rates and photosynthesis declined. When grown under high CO₂ conditions, M. aeruginosa showed lower intracellular nitrite concentrations and higher growth rates and \textP_\textm^\textchla {\text{P}}_{\text{m}}^{{\text{chl}}a} , \textR_\textd^\textchla {\text{R}}_{\text{d}}^{{\text{chl}}a} , α^chla than under low CO₂ conditions. These results suggest that the accumulation of intracellular nitrite could be the cause of inhibition of algal growth under high nitrate concentrations.