Nitrate and ammonium uptake and solution pH changes for Al-tolerant and Al-sensitive sorghum (Sorghum bicolor) genotypes grown with and without aluminium

Plant tolerance to Al toxicity has been associated with differential nitrate and ammonium uptake and solution pH changes. Sorghum [Sorghum bicolor (L.) Moench] genotypes with tolerance (SC283) and sensitivity (ICA-Nataima) to Al toxicity were grown with different nitrate/ammonium ratios (39:1, 9:1, and 3:1) at 0 and 300 μM Al to determine genotypic differences in nitrate and ammonium uptake, changes in nutrient solution pH, and relationships of these traits to Al toxicity tolerance in the genotypes. ICA-Nataima had greater reductions in nitrate and ammonium uptake than SC283 when plants were grown with Al, but SC283 had higher nitrate and ICA-Nataima had higher ammonium uptake when plants were grown without Al. Differences in nitrate and ammonium uptake were associated with changes in solution pH; pH decreased as long as ammonium was in solution and increased when ammonium was depleted from solution. Greater changes in solution pH occurred when plants were grown with 39:1 compared to 9:1 and 3:1 nitrate/ammonium ratios. Solution pH values were lower when plants were grown with than without Al. The genotypes maintained their relative differences in Al toxicity tolerance when plants were grown separately or together in the same container with Al and with different nitrate/ammonium ratios.     

Nitrate Reductase in Barley Roots under Sterile, Low Oxygen Conditions

Levels of nitrate reductase activity (EC 1.9.6.1.) as high as 11 μmoles nitrite produced/hour gram fresh weight were found in barley (Hordeum vulgare cv. Compana) roots grown under low oxygen conditions. Roots of plants given identical treatment under sterile conditions did not develop the high levels of nitrate reductase activity. The results suggest that the buildup of particulate, reduced viologen-utilizing nitrate reductase reported in barley roots may be caused by bacterial contamination. The nitrate reductase activity in roots grown under low oxygen conditions was not specific for reduced nicotinamide adenine dinucleotide like the assimilatory nitrate reductase (EC 1.6.6.1.) normally found in aerated plant roots.     

The Relationship of Abscisic Acid to Nitrate Reductase Activity in the Potato Plant

Abscisic acid (ABA) at 3.8 µM suppressed both in vivoand in vitro nitrate reductase activity in roots, stems andleaves of potato plants grown in solution culture. Suppressionwas maximal between 24 and 48 h, followed by recovery of activityat 72 h in roots and leaves and at 96 h in stems. Removal from ABA after 24 h resulted in complete recovery ofnitrate reductase activity in roots by 24 h and partial recoveryin leaves. ABA treatment enhanced nitrate accumulation in roots,decreased that of leaves, but had no effect on stem nitratecontent. ABA enhanced decay of the enzyme following nitrate removal;by 7 h activity in roots was 22.5% of the initial value comparedto 55% in the control. ABA showed a less drastic effect on lossof activity in leaves and stems. These results indicate thatABA suppression of nitrate reductase activity is not dependenton nitrate uptake, and although it reduced leaf nitrate contentthere was no clear relationship between tissue nitrate levelsand the ABA response. (Received September 13, 1984; Accepted July 1, 1985)     

Responses of nitrate assimilation and N translocation in tomato (Lycopersicon esculentum Mill) to reduced ambient air humidity

The impact of low humidity in ambient air on water relations,nitrate uptake, and translocation of recently absorbed nitrogen,was investigated in 5-week-old tomato (Lycopersicon esculentumMill cv. Ailsa Craig) plants grown hydroponically in a completenutrient solution. Plants were subjected to dry air (relativehumidity 2–4% for 6 h. The transpiration rate increasedseveral-fold and the shoot water content decreased by almost20%, whereas root water content was unaffected. No effect onin vitro nitrate reductase (NR) activity was detected when usingan EDTA-contraining assay buffer. Replacement of EDTA with Mg²⁺revealed a significant decline in shoot NR activity, which suggestsphosphorylation of the enzyme during the stress treatment. Plantswere grown in a split-root system, in which one root half wasfed ¹⁵N-nitrate during the treatment, in order to determinenitrate uptake and translocation of recently absorbed nitrogenin the plants. Uptake of nitrate was substantially inhibited,but the proportion of absorbed ¹⁵N that was translocated tothe shoots was only slightly affected. In untreated plants,71% of the ¹⁵N recovered in roots had been retranslocated fromthe shoots, whereas in plants subjected to stress the deliveryof ¹⁵N from shoots to roots appeared to be completely inhibited.The data show that lowered humidity in air has significant effectson both uptake of nitrate as well as translocation of nitrogenwithin the plants. Some of these effects appear to be commonwith those observed in plants subjected to reduced water potentialsin the root environment and point to the possibility of theshoot water relations being highly influential on nitrogen uptakeand translocation. Key words: Air humidity, nitrate assimilation, nitrate reductase activity, nitrogen translocation, tomato, water stress     

Effect of withdrawal of phosphorus on nitrate assimilation and PEP carboxylase activity in tomato

Tomato plants (Lycopersicon esculentum) grown in a complete nutrient solution for 8 days were transferred to a P-free solution of pH 6.0. Within 2 days of transfer the rate of alkalinization of the nutrient solution declined and by 4 days the solution had become acid. Nitrate transferred from roots to leaves was depressed over this period, and the rate of nitrate reductase activity in the leaves (the main site of assimilation of nitrate in tomato) had declined by 60% within 5 days of transfer. The activity of PEP carboxylase in the leaves of the P-deficient plants increased after 3 days, eventually becoming 3 times greater than in the leaves of plants adequately supplied with P. The PEP carboxylase activity in the roots of the P-deficient plants increased within 2 days, becoming 4 times greater after 8 days' growth. These results are discussed in relation to mechanisms for enhancement of P acquisition and maintenance of cation and anion uptake during P-deficiency.     

Effect of one night with "low light'on uptake, reduction and storage of nitrate in spinach

During the night, shoot nitrate concentration in spinach (Spinacia oleracea L. cv. Vroeg Reuzenblad) increased due to increased uptake of nitrate by the roots. When the plants were subjected to a one night “low light’period at 35 μmol m^?2 s^?1, the shoot nitrate concentration did not increase and was reduced by 25% compared to control plants in the dark. The major contribution to this decrease was located in the leaf blades, where the nitrate concentration was decreased by 60%, while the petiole nitrate concentration decreased by only 9%. Nitrate accumulated in the leaf blade vacuoles during a dark night, but this was not the case during the “low light’period. This decrease in vacuolar nitrate concentration, compared to control plants in the dark, was not caused by increased amounts of leaf blade nitrate reductase (NR; EC 1.6.6.1). During a “low light’night period, the cytoplasmic soluble carbohydrate concentration was increased compared to the control plants in the dark. Calculations showed in situ NR activity to be higher than in the control plants in the dark. This increase in NR activity, however, was not large enough to account for the total difference found in the shoot nitrate concentration. Net uptake of nitrate by the roots was increased during the initial hours of the dark night, while vacuolar nitrate concentration in the leaf blades increased at the same time. During the “low light’night period, however, net uptake of nitrate by the roots did not increase, and vacuolar nitrate concentration did not change. We conclude that nitrate uptake by the roots and vacuolar nitrate concentration in the leaf blades are tightly coupled. The decreased shoot nitrate concentration is mainly caused by a reduction in net uptake of nitrate by the roots. During the “low light’night period, carbohydrates and malic acid partly replaced vacuolar nitrate. A “low light’period one night prior to harvest provides a valuable tool to reduce shoot nitrate concentrations in spinach grown in greenhouses in the winter months.     

Uptake of Nitrate by Mutants of Arabidopsis thaliana, Disturbed in Uptake or Reduction of Nitrate

A study of nitrate and chlorate uptake by Arabidopsis thaliana was made with a wildtype and two mutant types, both mutants having been selected by resistance to high chlorate concentrations. All plants were grown on a nutrient solution with nitrate and/or ammonium as the nitrogen source. Uptake was determined from depletion in the ambient solution. Nitrate and chlorate were able to induce their own uptake mechanisms. Plants grown on ammonium nitrate showed a higher subsequent uptake rate of nitrate and chlorate than plants grown on ammonium alone. Mutant B25, which has no nitrate reductase activity, showed higher rates of nitrate and chlorate uptake than the wildtype, when both types were grown on ammonium nitrate. Therefore, the uptake of nitrate is not dependent on the presence of nitrate reductase. Nitrate has a stimulating effect on nitrate and chlorate uptake, whereas some product of nitrate and ammonium assimilation inhibits uptake of both ions by negative feedback. Mutant B 1, which was supposed to have a low chlorate uptake rate, also has disturbed uptake characteristics for nitrate.     

Nitrate Reductase Activity in Calcifugous and Calcicolous Tomatoes as Affected by Iron Stress

Calcicolous plants are generally more Fe-efficient than calcifugous plants, because they respond to Fe stress by releasing H-ions and “reductants” from their roots that causes Fe to become available. The objective of our study was to determine if differential response to Fe stress in calcicolous and calcifugous varieties affects nitrate reductase activity. T3238FER (Fe-efficient) and T3238fer (Fe-inefficient) tomato (Lycopersicon esculentum Mill.) cultivars were grown in nutrient solutions supplied with N as NH₄⁺-N plus NO₃^?-N, and as NO₃^?-N only. The chemical reactions induced by Fe stress concomitantly increased nitrate reductase activity in roots and tops of calcicolous, but not in calcifugous tomato. This nitrate reductase activity decreased, however, when Fe was made available to the plants. When Fe stress was eliminated by adding Fe, nitrate reductase activity was comparable in the two cultivars.     

Regulation of aldehyde oxidase and nitrate reductase in roots of barley (Hordeum vulgare L.) by nitrogen source and salinity

The molybdenum cofactor (MoCo) is a component of aldehyde oxidase (AO EC 1.2.3.1), xanthine dehydrogenase (XDH EC 1.2.1.37) and nitrate reductase (NR, EC 1.6.6.1). The activity of AO, which catalyses the last step of the synthesis of abscisic acid (ABA), was studied in leaves and roots of barley (Hordeum vulgare L.) plants grown on nitrate or ammonia with or without salinity. The activity of AO in roots was enhanced in plants grown with ammonium while nitrate-grown plants exhibited only traces. Root AO in barley was enhanced by salinity in the presence of nitrate or ammonia in the nutrient medium while leaf AO was not significantly affected by the nitrogen source or salinity of the medium.Salinity and ammonium decreased NR activity in roots while increasing the overall MoCo content of the tissue. The highest level of AO in barley roots was observed in plants grown with ammonium and NaCl, treatments that had only a marginal effect on leaf AO. ABA concentration in leaves of plants increased with salinity and ammonium.Keywords: ABA, aldehyde oxidase, ammonium, nitrate, salinity.      

Differential effects of low concentrations of aluminium on the growth of four genotypes of white clover

Summary The effects of aluminium (Al³⁺) on the growth of four cultivars of white clover dependent upon NO₃ ⁻−N were examined. Plants were grown in flowing solution culture with carefully maintained low concentrations (0, 12.5, 25 and 50 mmolm⁻³) of Al, and with P and pH (4.5) also held constant and appropriately low. A three-week treatment period resulted in major effects on the growth and elemental composition of shoots and roots at all concentrations of added Al. There were inherent differences between the cultivars in growth but the relative effects of Al were similar in each case. Examination by S.E.M. and x-ray microanalysis of one cultivar grown at 50 mmolm⁻³ Al, indicated that Al in the roots was associated with P, especially in old, outer epidermal cells. Aluminium reduced NO₃ ⁻ uptake and there were significant effects of Al on nitrate reductase activity (NRA). In contrast to the other characteristics, there were differential effects between the cultivars in NRA, both in the presence and absence of Al.     

Role of sugars in nitrate utilization by roots of dwarf bean

Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen-depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium. The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m^?3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m^?3). When plants were decapitated after an 18 h NO₃^? pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG^?₃ uptake. Presumably due to a difference in NO₃^? due to a difference in NO₃^? uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar-stimulation of, oxygen consumption as well as the release of ¹⁴CO₂ from freshly absorbed (U-¹⁴C) sugar was the same for glucose and fructose. Therefore, we propose a glucose-specific effect on NO₃^? uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose-fructose effect on nitrate reductase activity independent of the effect on NO₃^? uptake was not indicated. A constant level of NRA occurred in roots of NO₃^? induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half-life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots.     

Effects of NaCl on the nitrogen metabolism of the halophyteArthrocnemum fruticosum (L.) Moq., grown in a greenhouse

Summary Arthrocnemum fruticosum (L.)Moq., a halophyte from the shore of the Dead Sea in Jordan was grown in a greenhouse with nutrient solution supplemented with various concentrations of NaCl. It was shown that with increasing salinity the plants became more succulent, mainly due to an accumulation of sodium and water. Sodium was taken up into the roots in equal amounts to chloride, but in the shoots far more sodium than chloride was found, suggesting a control of these ions either in the excretion into the xylem, or in the uptake by the shoot out of the xylem. Ammonium and nitrate in the plants decreased with time on nutrient solution more or less independently of the salt concentration. However, more nitrate appeared again in the plants when they started flowering. After an initial period of adaptation the nitrate reductase activityin vivo was not inhibited by a salinity of up to 2%, but at higher NaCl concentrations a shift of nitrate reductase activity occurred from the roots to the shoots. This was consistent with earlier observations in the field. In the vegetative phase of the plants the nitrate reductase in the roots was influenced by the soil water potential, but in the shoot it was mainly dependent on the supply of nitrate from the roots. High NaCl concentrations delayed flower initiation. During flowering the nitrate reductase was involved in the re-allocation of nitrogenous compounds from the roots to the developing flowers, and it became effectively independent from salinity.     

The Influence of Ambient Nitrate, Temperature, and Light on Nitrate Assimilation in Sudangrass Seedlings

Seedlings of Sundangrass (Sorghum Sudanese [Piper] Stapf.) were grown 10 to 13 days of age in a nutrient solution containing nitrate and then placed under treatment conditions for 24 h before assays of nitrate assimilation were begun. Nitrate uptake was determined by its disappearance from the ambient solution. In vivo reduction of nitrate was determined by the overall balance between the amount taken up and the change in tissue concentration of nitrate during the experiments. Nitrate reductase activity was determined from tissue slices. In vivo reduction was strongly regulated by uptake in response to time and ambient nitrate concentration, temperature and light. Nitrate reduction responded to the concentration of nitrate supplied by uptake and by a storage pool, since reduction often exceeded uptake. Nitrate reductase activity in tissue slices was exponential in initial response to increasing temperature. After a 24-h equilibration period at each temperature, the activity was lower at higher temperatures. In contrast, actual reduction of nitrate increased linearly with increasing temperature between 15 and 24°C in the plants equilibrated 24 h at each temperature. Nitrate uptake and reduction were greatly inhibited under low light conditions, with reduction inhibited more than uptake., The effect of ambient nitrate, temperature, and light on the nitrate assimilatory processes help to explain observations reported on nitrate accumulation by Sudangrass forage.     

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.     

Effects of sodium on mineral nutrition in rose plants

The effects of sodium (Na⁺) ion concentration on shoot elongation, uptake of ammonium (NH₄⁺) and nitrate (NO₃^?) and the activities of nitrate reductase (NR) and glutamine synthetase (GS) were studied in rose plants (Rosa hybrida cv. “Lambada”). The results showed that shoot elongation was negatively correlated with sodium concentration, although no external symptoms of toxicity were observed. Nitrate uptake decreased at high sodium levels, specifically at 30 meq litre4 of sodium. As flower development was normal under high saline conditions, this could suggest that nitrogen was being mobilised from shoot and leaf reserves. Ammonium uptake was not affected by any of the salt treatments applied probably because it diffuses through the cell membrane at low concentrations. Nitrate reductase activity was reduced by 50% at 30 meq litre 1 compared with control treatment, probably due to a decrease in the free nitrate related to nitrate uptake pattern. None of the salt treatments used affected total leaf GS activity (both chloroplastic and cytosolic isoforms) or leaf NPK mineral contents. Nitrate reductase activity in leaves increased at 10 meq litre^?1 of sodium and GS activity in roots (cytosolic isoform only) followed the same pattern as NR. It is suggested that the activation of both enzymes at low salt level could be attributed to the beneficial effect of increased sulphur in the nutrient solutions.     

Ontogenetic Variation of Nitrogenase, Nitrate Reductase, and Glutamine Synthetase Activities in Oryza sativa

The relationship between the rates of nitrogenase, nitrate reductase, and glutamine synthetase activities, and plant ontogeny in rice (Oryza sativa L.), cultivar `M9', grown in salt marsh sediment with and without nitrate treatment was studied. In both treatments, nitrogenase activity measured as the immediate linear rate of acetylene reduction by bacteria associated with the roots varied with plant age. In control plants, the nitrogenase activity developed during the vegetative stage, peaked during early reproductive growth and then declined. The application of 10 kilograms N per hectare as KNO₃ once every 2 weeks delayed the development of and decreased the nitrogenase activity. The nitrogenase activity in both treatments developed as leaf nitrate reductase activity declined. The per cent nitrogen of roots was negatively correlated with the rates of acetylene reduction during the life cycles of control and nitrate-treated plants. This suggests that the concentration of combined nitrogen in the plants controlled the development and rate of root-associated nitrogenase activity. During reproductive growth, no nitrate reductase activity was detected in the roots from either treatment. In control plants, the patterns of nitrogenase activity and glutamine synthetase activity in the roots were similar. Thus, rice roots have the potential to assimilate ammonia while fixing N₂. During the vegetative and early reproductive stages of growth, the development of maximal rates of nitrogenase activity coincided with an increase of total nitrogen of the plants in both treatments.     

Influence of NO3 - and NH4 + nutrition on fermentation,nitrate reductase activity and adenylate energy charge of roots of Carex pseudocyperus L. and Carex sylvatica Huds. exposed to anaerobic nutrient solutions

The adenylate energy charge, production of ethanol and lactate, and nitrate reductase activity were determined in order to study the influence of different nitrogen sources on the metabolic responses of roots of Carex pseudocyperus L. and Carex sylvatica HUDS. exposed to anaerobic nutrient solutions. Determination of adenylates was carried out by means of a modified HPLC technique. Total quantity of adenylates was higher in Carex pseudocyperus than in Carex sylvatica under all conditions. In contrast, the adenylate energy charge was only slightly different between the species and decreased more or less in relation to the applied nitrogen source under oxygen deficiency. The adenylate energy charge in roots of plants under nitrate nutrition showed a smaller decrease under anaerobic environmental conditions than plants grown with ammonium or nitrate/ammonium. Roots of nitrate-fed plants showed a lower ethanol and lactate production than ammonium/nitrate- and ammonium-fed plants. Ethanol production was higher in C. pseudocyperus, formation of lactate was lower compared to that in Carex sylvatica. The activity of enzymes involved in fermentation processes (ADH, LDH and PDC) was enhanced significantly after 24 hours of exposure to anaerobic nutrient solutions in roots of both species. The induction of these enzymes was only slightly influenced by different nitrogen supply. In vivo nitrate reductase activity increased almost 3-fold compared to the aerobic treatment in both species and overcompensated loss of NADH reoxidation capacity caused by decrease of ethanol and lactate development. Induction of in vitro nitrate reductase activity was enhanced 313% in C. pseudocyperus and 349% in C. sylvatica under anaerobic environmental conditions and nitrate supply. These results indicate that nitrate may serve as an alternative electron acceptor in anaerobic plant root metabolism and that the nitrate-supported energy charge may be due to an accelerated glycolytic flux resulting from a more effective NADH reoxidation capacity by nitrate reduction plus fermentation than by fermentation alone.Abbreviations ADH alcohol dehydrogenase - AEC adenylate energy charge - DMSO dimethyl sulfoxide - EDTA ethylen diamine tetraacetic acid - HPLC high performance liquid chromatography - LDH lactate dehydrogenase - NRA nitrate reductase activity - PCA perchloric acid - PDC pyruvate decarboxylase - PVP polyvinylpyrrolidone - PVPP polyvinylpolypyrrolidone - TCA trichloroacetic acid, Tris-tris(hydroxymethyl)aminomethane     

Differences in nitrate and ammonium uptake between Scots pine and European larch

In forest soils, ammonium is usually the predominant form of inorganic nitrogen. However, the capacity of trees to utilize both NO₃ ^- and NH₃ ⁺ may provide greater flexibility in responding to changes of nitrogen supply from the environment. Such capacity has been studied in seedlings of Scots pine (Pinus sylvestris L.) and European larch (Larix decidua Mill.) grown in the presence or absence of either nitrate or ammonium. Nitrate-induced plants showed a higher nitrate uptake rate than non-induced plants; this difference was almost negligible after 24 h of exposure to NO₃ ^-. Ammonium uptake in both species was consistently higher than that of nitrate, regardless of prior nitrogen provision. In both nutrient conditions, larch showed a more efficient transport system in comparison with Scots pine, with higher ammonium and nitrate uptake rates in both induced and non-induced plants. This was consistent also with the activity of nitrate reductase, measured in vivo in roots and leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of nitrate,ammonium and pH on the growth of conifer seedlings and their production of nitrate reductase

Summary Lodgepole pine (Pinus contorta Dougl.), Engelmann spruce (Picea engelmanni Parry), and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were grown in open-ended tube cultures of sand and perlite, irrigated with nitrate, ammonium, and a 1∶1 mixture of ammonium and nitrate, combined factorially with pH values of 4.6, 5.3 and 6.0 giving a total of nine treatments. Douglas-fir showed intolerance to ammonium which was especially marked in root weight. Lodgepole pine and Engelmann spruce made poor growth with nitrate, but showed little difference between ammonium and mixed sources. Only Douglas-fir showed a significant response to pH treatments with pH 5.3 plants being largest. Contamination of the sand with carbonate-bicarbonate, apparently caused seedlings grown in ammonium solutions to be larger in sand than in perlite. Douglas-fir grown in perlite cultures showed a growth response like the first experiment and nitrate reductase activity in the order nitrate > nitrateammonium mixture > ammonium. Plastic bead cultures had poor growth response due to low retention of water by the substrate, but the nitrate reductase assays produced results like the perlite cultures. Lodgepole pine grown in water culture demonstrated the well known pH shift associated with different nitrogen forms, and when assayed for nitrate reductase these seedlings had larger relative activities than Douglas-fir, but the order of activity remained nitrate > mixed source > ammonium.     

Effects of CO 2 enrichment on photosynthesis,growth, and nitrogen metabolism of the seagrass Zostera noltii

Seagrass ecosystems are expected to benefit from the global increase in CO ₂ in the ocean because the photosynthetic rate of these plants may be C_i‐limited at the current CO ₂ level. As well, it is expected that lower external pH will facilitate the nitrate uptake of seagrasses if nitrate is cotransported with H⁺ across the membrane as in terrestrial plants. Here, we investigate the effects of CO ₂ enrichment on both carbon and nitrogen metabolism of the seagrass Zostera noltii in a mesocosm experiment where plants were exposed for 5 months to two experimental CO ₂ concentrations (360 and 700 ppm). Both the maximum photosynthetic rate (P_m) and photosynthetic efficiency (α) were higher (1.3‐ and 4.1‐fold, respectively) in plants exposed to CO ₂‐enriched conditions. On the other hand, no significant effects of CO ₂ enrichment on leaf growth rates were observed, probably due to nitrogen limitation as revealed by the low nitrogen content of leaves. The leaf ammonium uptake rate and glutamine synthetase activity were not significantly affected by increased CO ₂ concentrations. On the other hand, the leaf nitrate uptake rate of plants exposed to CO ₂‐enriched conditions was fourfold lower than the uptake of plants exposed to current CO ₂ level, suggesting that in the seagrass Z. noltii nitrate is not cotransported with H⁺ as in terrestrial plants. In contrast, the activity of nitrate reductase was threefold higher in plant leaves grown at high‐CO ₂ concentrations. Our results suggest that the global effects of CO ₂ on seagrass production may be spatially heterogeneous and depend on the specific nitrogen availability of each system. Under a CO ₂ increase scenario, the natural levels of nutrients will probably become limiting for Z. noltii. This potential limitation becomes more relevant because the expected positive effect of CO ₂ increase on nitrate uptake rate was not confirmed.