Nitrate Assimilation Capacity Research Articles

Origin, evolution, and future of isoprene and nitric oxide interactions within leaves.

Photolytic generation of nitric oxide (NO), isoprene, and reactive oxygen species (ROS) pre-dated life on Earth (~4 billion years ago). However, isoprene-ROS-NO interactions became relevant to climate chemistry ~50 million years ago, after aquatic and terrestrial ecosystems became dominated by isoprene-emitting diatoms and angiosperms. Today, NO and NO2 (together referred to as NOx) are dangerous biogenic gaseous atmospheric pollutants. In plants, NO, with its multiple sources and sinks, acts as a secondary messenger that regulates development at low doses and induces cell death at high doses. Likewise, biogenic isoprene is a putative antioxidant and hormone 'enabler' that hastens plant (and leaf) growth and reproduction, and improves plant tolerance to transient abiotic stresses. Using examples from controlled-chamber simulation and field studies of isoprene oxidation, we discuss the likely nature and extent of isoprene oxidation within leaves. We argue that isoprene-NO interactions vary greatly among plant species, driven by differences in isoprene emission rate and nitrate assimilation capacity (i.e. NO sink strength), ROS availability, and the within-leaf ratio between free-NO and isoprene. In a warmer and CO2-fertilized future climate, antagonism between isoprene and NO within leaves will probably occur in a NO-rich (relative to present) environment, yielding a greater proportion of isoprene oxidation products, and inducing major changes in NO-mediated growth and stress responses.

Elevated CO2 differentially regulates root nitrate transporter kinetics in a genotype and nitrate dose-dependent manner

The nitrogen (N) and protein concentration of wheat crop and grain often decline as a result of exposure of the crop to elevated CO2 (EC). In our earlier studies, it was found that the exacerbated production of nitric oxide (NO) represses the transcription of nitrate reductase (NR) and high affinity nitrate transporters (HATS) in EC grown wheat seedlings receiving high N. High N supply under EC also resulted in accumulation of reactive oxygen species (ROS), and reactive nitrogen species (RNS; NO and S- nitrosothiols) ensuing faster senescence and reduced N metabolite concentration in wheat. In this study, the effect of short-term exposure to EC on nitrate uptake kinetics was studied. The impact of EC on constitutive and inducible components of high affinity and low affinity nitrate uptake systems (HATS and LATS) were delineated in two wheat genotypes diverse in terms of nitrate uptake and assimilation capacities. Nitrate dose-response of NR was suppressed by EC in both leaf and root tissues. Plants grown under EC displayed a marked reduction in nitrate uptake kinetic components of LATS. Wheat genotype with high leaf nitrate assimilation capacity was able to maintain considerably higher nitrate uptake rate under EC albeit at a lower rate in comparison to ambient CO2. Wheat leaves exposed to EC displayed a comparatively higher abundance of NO and showed incremental abundance depending on increase in nitrate supply. Exogenous NO supply significantly suppressed the nitrate uptake rate of EC grown plants. Hence, EC-induced production of NO downregulates LATS kinetics in a genotype and nitrate dose-dependent manner.

LeNRT1.1 Improves Nitrate Uptake in Grafted Tomato Plants under High Nitrogen Demand.

Grafting has become a common practice among tomato growers to obtain vigorous plants. These plants present a substantial increase in nitrogen (N) uptake from the root zone. However, the mechanisms involved in this higher uptake capacity have not been investigated. To elucidate whether the increase in N uptake in grafted tomato plants under high N demand conditions is related to the functioning of low- (high capacity) or high-affinity (low capacity) root plasma membrane transporters, a series of experiments were conducted. Plants grafted onto a vigorous rootstock, as well as ungrafted and homograft plants, were exposed to two radiation levels (400 and 800 µmol m−2 s−1). We assessed root plasma membrane nitrate transporters (LeNRT1.1, LeNRT1.2, LeNRT2.1, LeNRT2.2 and LeNRT2.3) expression, Michaelis‒Menten kinetics parameters (Vmax and Km), root and leaf nitrate reductase activity, and root respiration rates. The majority of nitrate uptake is mediated by LeNRT1.1 and LeNRT1.2 in grafted and ungrafted plants. Under high N demand conditions, vigorous rootstocks show similar levels of expression for LeNRT1.1 and LeNRT1.2, whereas ungrafted plants present a higher expression of LeNRT1.2. No differences in the uptake capacity (evaluated as Vmax), root respiration rates, or root nitrate assimilation capacity were found among treatments.

The proportion of nitrate in leaf nitrogen, but not changes in root growth, are associated with decreased grain protein in wheat under elevated [CO2

The atmospheric CO2 concentration ([CO2]) is increasing and predicted to reach ∼550ppm by 2050. Increasing [CO2] typically stimulates crop growth and yield, but decreases concentrations of nutrients, such as nitrogen ([N]), and therefore protein, in plant tissues and grains. Such changes in grain composition are expected to have negative implications for the nutritional and economic value of grains. This study addresses two mechanisms potentially accountable for the phenomenon of elevated [CO2]-induced decreases in [N]: N uptake per unit length of roots as well as inhibition of the assimilation of nitrate (NO3−) into protein are investigated and related to grain protein. We analysed two wheat cultivars from a similar genetic background but contrasting in agronomic features (Triticum aestivum L. cv. Scout and Yitpi). Plants were field-grown within the Australian Grains Free Air CO2 Enrichment (AGFACE) facility under two atmospheric [CO2] (ambient, ∼400ppm, and elevated, ∼550ppm) and two water treatments (rain-fed and well-watered). Aboveground dry weight (ADW) and root length (RL, captured by a mini-rhizotron root growth monitoring system), as well as [N] and NO3− concentrations ([NO3−]) were monitored throughout the growing season and related to grain protein at harvest. RL generally increased under e[CO2] and varied between water supply and cultivars. The ratio of total aboveground N (TN) taken up per RL was affected by CO2 treatment only later in the season and there was no significant correlation between TN/RL and grain protein concentration across cultivars and [CO2] treatments. In contrast, a greater percentage of N remained as unassimilated [NO3−] in the tissue of e[CO2] grown crops (expressed as the ratio of NO3− to total N) and this was significantly correlated with decreased grain protein. These findings suggest that e[CO2] directly affects the nitrate assimilation capacity of wheat with direct negative implications for grain quality.

Quantifying and reducing the water footprint of rain-fed potato production, part I: measuring the net use of blue and green water

Abstract Agriculture imposes demands on freshwater resources. Water footprinting has been proposed to quantify those impacts. To date, water footprints have been assessed only through modelling. We quantified the water footprint (WF) using measured soil water content and drainage under potatoes ( Solanum tuberosum ) grown in the Manawatu region of New Zealand. The net uses of groundwater and soil-water storage were considered to be the blue and green WFs, respectively. The drainage below the root zone was measured using tension fluxmeters over a cropping season and nitrate-nitrogen (NO 3 -N) concentrations were analysed. The mean of the cumulative drainage was 263.8 mm (SE ± 84). Irrigation was not used, so the blue-WF was equal to the drainage. It was negative at −58.6 L/kg indicating that potato cultivation contributes to groundwater recharge. The green-WF was 15.8 L/kg. However, this green-water deficit would certainly be replenished, by winter rainfall. The grey-WF, the water needed to ‘dilute’ the leachate to the drinking water standard, was 133.1 L/kg of potato. The water quality impact based on the used-fraction of the nitrate assimilation capacity of groundwater was just 4.82 × 10 −11 for a kg of potato. The NO 3 -N leaching pattern reveals that some of the losses could be minimized through changed fertilizer practices. We estimated that 20 fluxmeters, rather than the six we used, would be needed to reduce the SE to 12.5% of the mean. Therefore, modelling will be the cost-effective means of quantifying the WFs. Nonetheless, model validation will always be necessary.

Optimisation of potassium chloride nutrition for proper growth, physiological development and bioactive component production in Prunella vulgaris L.

Prunella vulgaris L. is an important medicinal plant with a variety of pharmacological activities, but limited information is available about its response to potassium chloride (KCl) supplementation. P. vulgaris seedlings were cultured in media with four different KCl levels (0, 1.00, 6.00 and 40.00 mM). Characteristics relating to the growth, foliar potassium, water and chlorophyll content, photosynthesis, transpiration, nitrogen metabolism, bioactive constituent concentrations and yield were determined after three months. The appropriate KCl concentration was 6.00 mM to result in the highest values for dry weight, shoot height, spica and root weight, spica length and number in P. vulgaris. The optimum KCl concentration resulted in a maximum net photosynthetic rate (Pn) that could be associated with the highest chlorophyll content and fully open stomata conductance. A supply of surplus KCl resulted in a higher concentration of foliar potassium and negatively correlated with the biomass. Plants that were treated with the appropriate KCl level showed a greater capacity for nitrate assimilation. The Pn was significantly and positively correlated with nitrate reductase (NR) and glutamine synthetase (GS) activities and was positively correlated with leaf-soluble protein and free amino acid (FAA) contents. Both KCl starvation (0 mM) and high KCl (40.00 mM) led to water loss through a high transpiration rate and low water absorption, respectively, and resulted in increased concentrations of ursolic acid (UA), oleanolic acid (OA) and flavonoids, with the exception of rosmarinic acid (RA). Moreover, the optimum concentration of KCl significantly increased the yields of RA, UA, OA and flavonoids. Our findings suggested that significantly higher plant biomass; chlorophyll content; Pn; stronger nitrogen anabolism; lower RA, UA, OA and flavonoid accumulation; and greater RA, UA, OA and flavonoid yields in P. vulgaris could be expected in the presence of the appropriate KCl concentration (6.00 mM).

Genotypic differences in nitrate uptake, translocation and assimilation of two Chinese cabbage cultivars [Brassica campestris L. ssp. Chinensis (L.)

A hydroponic trial was conducted to investigate genotypic differences in nitrate uptake, translocation and assimilation between low nitrate accumulator L18 and high accumulator H96 of Chinese cabbage [Brassica campestris L. ssp. Chinensis (L.)]. The results suggested that H96 could uptake more nitrate than L18 in the root but lower transport inside leaf cells and assimilation in the leaf. It was showed that root morphology parameters - length, surface area and volume of H96 were 18.0%, 31.6% and 46.5% higher than L18. Nitrate transporters NRT1.1 and NRT2.1 transcription levels were 41.6% and 269.6% higher than those of L18 respectively. NRT1.1 and NRT2.1 expression amount in leaf blade of two cultivars were opposite to in the root, L18 NRT1.1 and NRT2.1 expressions were 279.2% and 80.0% higher than H96. In addition, nitrate assimilation capacity of L18 was significantly higher than H96 in leaf. It was showed that Nitrate Reductase (NR; EC 1.7.1.1) activity and NIA expression level of L18 leaf were 234 0.4% and 105.4% higher than those of H96, Glutamine Synthetase (GS; EC 6.3.1.2) activity, Gln1 and Gln2 expression levels in the leaf blade of L18 were 43.9%, 331.5% and 124.8% higher than those of H96 respectively. Nitrate assimilation products-Glu, total amino acid, soluble protein content in the leaf of L18 were all significantly higher than those of H96. The results above suggested that nitrate accumulation differences were due to differential capacities to uptake, mechanisms for nitrate transport in leaves and assimilate nitrate. Comparing contribution of three aspects in nitrate accumulation, translocation and assimilation were contributed more in low nitrate concentration in the leaf blade.

Genotypic variations in photosynthetic and physiological adjustment to potassium deficiency in cotton (Gossypium hirsutum)

A hydroponic culture experiment was conducted to determine genotypic variation in photosynthetic rate and the associated physiological changes in response to potassium (K) deficiency in cotton (Gossypium hirsutum L.) seedlings with contrasting two cotton cultivars in K efficiency. The K-efficient Liaomian18 produced 66.7% more biomass than the K-inefficient NuCOTN99B under K deficiency, despite their similar biomass under K sufficiency. Compared with NuCOTN99B, Liaomian18 showed 19.4% higher net photosynthetic rate (Pn, per unit leaf area) under K deficient solutions and this was associated with higher photochemical efficiency and faster export of soluble sugars from the phloem. The lower net Pn of NuCOTN99B was attributed to higher capacity for nitrate assimilation and lower export of soluble sugars. Furthermore, NuCOTN99B showed 38.4% greater ETR/Pn than Liaomian18 under K deficiency, indicating that more electrons were driven to other sinks. Higher superoxide dismutase (SOD) and lower catalase (CAT) and ascorbate peroxidase (APX) activities resulted in higher levels of reactive oxygen species (ROS; e.g. O2−and H2O2) in NuCOTN99B relative to Liaomian18. Thus, the K inefficiency of NuCOTN99B, indicated by lower biomass and net Pn under K deficiency, was associated with excessively high nitrogen assimilation, lower export of carbon assimilates, and greater ROS accumulation in the leaf.

Nitrate reductase activity (NRA) in Asphodelus aestivus Brot. (Liliaceae): distribution among organs, seasonal variation and differences among populations

Nitrate reductase activity (NRA) in different compartments (leaves, inflorescence stalks, flowers and tuberous roots) of Asphodelus aestivus Brot. (Liliaceae) and actual mineral nitrogen (NO 3 −-N and NH 4 +-N) in soil surrounding the roots were investigated over one year. Although the highest NRA was found in the leaves, the other plant compartments, such as flowers and tuberous roots, also have nitrate assimilation capacity. High nitrate assimilation capacity under suitable conditions is considered to be a good strategy for development and dominance of this species in Mediterranean environments. There was a seasonal variation in nitrate assimilation in leaves and actual NO 3 −-N content of soils. Depending on actual nitrate content of soils, nitrate assimilation increased in winter.

Physiological responses of intertidal marine brown algae to nitrogen deprivation and resupply of nitrate and ammonium.

Intertidal macroalgae Fucus and Laminaria experience seasonally fluctuating inorganic N supply. This study examined the effects of long-term N deprivation, recovery following N resupply, and effects of elevated ammonium and nitrate exposure on N acquisition in intertidal algae using manipulations of N supply in tank culture. Over 15 weeks of N deprivation, internal N and nitrate reductase activity (NRA) declined, but maximum quantum yield of PSII was unaffected in Fucus serratus and Fucus vesiculosus. Low NRA was maintained despite no external nitrate availability and depletion of internal pools, suggesting a constitutive NRA, insensitive to N supply. Nitrate resupplied to N-starved thalli was rapidly taken up and internal nitrate pools and NRA increased. Exposure to elevated (50 microM) nitrate over 4 days stimulated nitrate uptake and NRA in Laminaria digitata and F. serratus. Exposure to elevated ammonium suppressed NRA in L. digitata but not in F. serratus. This novel insensitivity of NRA to ammonium in Fucus contrasts with regulation of NRA in other algae and higher plants. Ammonium suppression of NRA in L. digitata was not via inhibition of nitrate uptake and was independent of nitrate availability. L. digitata showed a higher capacity for internal nitrate storage when exposed to elevated ambient nitrate, but NRA was lower than in Fucus. All species maintained nitrate assimilation capacity in excess of nitrate uptake capacity. N uptake and storage strategies of these intertidal macroalgae are adaptive to life in fluctuating N supply, and distinct regulation of N metabolism in Fucus vs Laminaria may relate to position in the intertidal zone.

Construction and characterization of a cyanobacterial bioreporter capable of assessing nitrate assimilatory capacity in freshwaters

The use of cyanobacterial whole-cell luminescent bioreporters has enhanced our ability to monitor nutrient availability in aquatic ecosystems. We have constructed a Synechocystis sp. strain PCC6803 bioluminescent reporter for the assessment of nitrate bioavailability. Specifically, a 380—base pair DNA fragment containing the NtcA/B-dependent nitrate/nitrite-activated nirA promoter (regulating expression of genes encoding nitrite reductase) was fused to the bacterial luciferase genes, luxAB, and introduced into Synechocystis by genetic transformation. Characterization of this strain, designated AND100, yielded dose-dependent increased bioluminescence coincident with increased nitrate added to the growth medium from 1 to 100 µM. Bioluminescence in response to nitrate addition was light dependent up to 50 µmol quanta m−2 s−1. Assessing environmental samples collected from oligotrophic Lake Superior, we demonstrated that the onset of luminescence coincided with the drawdown of nitrate by simultaneously monitoring nitrate depletion from reaction vessels. Nitrate in the Lake Superior samples was consistently underestimated by the bioreporter. Only by following amendment of these samples with phosphate and iron was total nitrate accurately reflected by the cyanobacterial bioreporter. Thus, strain AND100 can be used to elucidate factors that constrain use of nitrate in freshwaters. This is pertinent to a system such as Lake Superior where the concentration of nitrate has increased 6-fold in the last century. Indeed, pilot experiments with the bioreporter suggest that nutrient co-limitation (P and Fe), as well as low light, may reduce the capacity for nitrate assimilation in field samples from Lake Superior.

Impacts of increased nitrogen supply on high Arctic heath: the importance of bryophytes and phosphorus availability.

• This study investigates effects of nitrogen and phosphorus on high Arctic heath vegetation, particularly bryophytes. • Heath communities received factorial combinations of nitrogen (0, 10 and 50kgha-1 yr-1 ) and phosphorus (0 and 5kgha-1 yr-1 ) in five applications per growing season, for 8yr. • Nitrogen decreased lichen cover but did not affect cover of any other functional type. However, just 10kgha-1 yr-1 increased the proportion of physiologically active bryophte shoots, and decreased their nitrate assimilation capacity. Phosphorus had greater effects, and the combination of both nutrients altered species composition. Individual bryophyte species displayed contrasting responses to fertilization, suggesting that they should not be grouped as a single functional type. • The 'critical load' of nitrogen for Arctic heath lies below 10kgha-1 yr-1 . Nitrogen and phosphorus are colimiting in this sytem, so the critical load of nitrogen will be lower where phosphorus availability is greater. Responses of vegetation to any increase in net mineralisation due to soil warming will depend on the ratio in which nitrogen and phosphorus availabilities increase. The effects of nutrient enhancement are very persistent.

Nitrogen Utilization in Creeping Bentgrass

Abstract N utilization, including plant dry weight (DW) production, total N and nitrate N (NO3-N) content accumulation, N utilization efficiency (NUE), root absorption efficiency (RAE), reduced N accumulation, and nitrate assimilation capacity (NAC) were determined for creeping bentgrass cultivars grown in hydroponic culture. Possible mechanisms affecting N utilization including nitrate reductase activity (NRA) and root morphology were also examined. Four cultivars, selected from an initial screening of creeping bentgrasses were grown under both low and high levels of N in a flowing solution culture system. The relationship between NUE and plant growth for two different creeping bentgrass cultivars was investigated through tissue culture, The results indicate a genotypic variation in N utilization and absorption. The NAC was not the primary factor involved in genotypic differences in NUE. However, NRA was probably one of the mechanisms for the regulation of NUE. N utilization was affected by the level of ...

Tobacco mutants with a decreased number of functional nia genes compensate by modifying the diurnal regulation of transcription, post-translational modification and turnover of nitrate reductase.

Although nitrate reductase (NR. EC 1.6.6.1) is thought to control the rate of nitrate assimilation, mutants with 40-45% of wildtype (WT) NR activity (NRA) grow as fast as the WT. We have investigated how tobacco (Nicotiana tabacum L. cv. Gatersleben) mutants with one or two instead of four functional nia genes compensate. (i) The nia transcript was higher in the leaves of the mutants. However, the diurnal rhythm was retained in the mutants, with a maximum at the end of the night and a strong decline during the photoperiod. (ii) Nitrate reductase protein and NRA rose to a maximum after 3-4 h light in WT leaves, and then decreased by 50-60% during the second part of the photoperiod and the first part of the night. Leaves of mutants contained 40-60% less NR protein and NRA after 3-4 h illumination, but NR did not decrease during the photoperiod. At the end of the photoperiod the WT and the mutants contained similar levels of NR protein and NRA. (iii) Darkening led to a rapid inactivation of NR in the WT and the mutants. However, in the mutants, this inactivation was reversed after 1-3 h darkness. Calyculin A prevented this reversal. When magnesium was included in the assay to distinguish between the active and inactive forms of NR, mutants contained 50% more activity than the WT during the night. Conversion of [15N]-nitrate to organic compounds in leaves in the first 6 h of the night was 60% faster in the mutants than in the WT. (iv) Growth of WT plants in enhanced carbon dioxide prevented the decline of NRA during the second part of the photoperiod, and led to reactivation of NR in the dark. (v) Increased stability of NR in the light and reversal of dark-inactivation correlated with decreased levels of glutamine in the leaves. When glutamine was supplied to detached leaves it accelerated the breakdown of NR, and led to inactivation of NR, even in the light. (vi) Diurnal changes were also investigated in roots. In the WT, the amount of nia transcript rose to a maximum after 4 h illumination and then gradually decreased. The amplitude of the changes in transcript amount was smaller in roots than in leaves, and there were no diurnal changes in NRA. In mutants, nia transcript levels were high through the photoperiod and the first part of the night. The NRA was 50% lower during the day but rose during the night to an activity almost as high as in the WT. The rate of [15N]-nitrate assimilation in the roots of the mutants resembled that in the WT during the first 6 h of the night. (vii) Diurnal changes were also compared in Nia30(145) transformants with very low NRA, and in nitrate-deficient WT plants. Both sets of plants had similar low growth rates. Nitrate reductase did not show a diurnal rhythm in leaves or roots of Nia30(145), the leaves contained very low glutamine, and NR did not inactivate in the dark. Nitrate-deficient WT plants were watered each day with 0.2 mM nitrate. After watering, there was a small peak of nia transcript NR protein and NRA and, slightly later, a transient increase of glutamine and other amino acids in the leaves. During the night glutamine was low, and NR did not inactivate. In the roots, there was a very marked increase of nitrate, nia transcript and NRA 2-3 h after the daily watering with 0.2 mM nitrate. (viii) It is concluded that WT plants have excess capacity for nitrate assimilation. They only utilise this potential capacity for a short time each day, and then down-regulate nitrate assimilation in response, depending on the conditions, to accumulation of the products of nitrate assimilation or exhaustion of external nitrate. Genotypes with a lower capacity for nitrate assimilation compensate by increasing expression of NR and weakening the feedback regulation, to allow assimilation to continue for a longer period each day.

Seasonal variation in nitrate reductase activity in needles of high-elevation red spruce trees

To assess seasonal and site variation in foliar nitrate reductase activity and its utility as a biochemical marker for the uptake of nitrogen oxide pollutants in high-elevation forests, we measured nitrate reductase activity in current-year needles of red spruce (Picearubens Sarg.) saplings at two high-elevation stands (1935 and 1720 m) in the Great Smoky Mountains, North Carolina. Measurements spanned two growing seasons between September 1987 and September 1988. Nitrate reductase activity peaked near 60 nmol•g−1•h−1 at both sites in September and October 1987 and August 1988 and declined 80% in November 1987 and 65% in September 1988. Although nitrate reductase activity was 30% greater in saplings at the higher site relative to the lower site in September and October 1987, activity dropped to approximately 10 nmol•g−1•h−1 at both sites in November 1987. No differences among sites were evident the following year. Comparing deposition of nitric acid vapor at a nearby site to nitrate reductase activity suggests that needle nitrate reductase activity is not an unequivocal marker for foliar uptake of nitrogen oxides during air pollutant episodes. The changes in soil nitrate levels in this system provide preliminary evidence that foliar nitrate assimilation may, in part, include nitrate taken up from the soil, as the highest activity occurred during periods of higher A-horizon nitrate concentrations in 1988. These measurements of nitrate reductase activity suggest that red spruce are capable of assimilating nitrate in foliage in the field and that the nitrate assimilation capacity varies throughout the year.

Effects of nitrogen oxides on nitrate assimilation in bryophytes

summaryNitrate assimilation and photosynthetic responses of Ctentdium mulluscum (Hedw.) Mitt. Homalothecium sericeum (Hedw.) Br.Eur. Pleurusium schreberi (Brid.) Mitt. Hylocomium splendens (Hedw.) Br.Eur. were studied during and after controlled exposures to 35 nl 1−1NO2 and 35 nl 1−1NO. NO2 induced nitrate reductase activity in all species 24 hours after the start of fumigation. The magnitude of the inductions varied between 23 and 89%. In contrast, exposure to NO caused a rapid decline in activity within 24 hours. After 21 days continuous exposure to each of the gases, the plants were supplied with a single application of 10 mM, NaNO3. Nitrate reductase activity was induced only in plants exposed to clean air. NO8exposure caused a loss of nitrate inducibility of NR. There were no marked effects on rates of oxygen evolution. After a three‐week pollution‐free period, the bryophytes regained the capacity for nitrate assimilation.

Short-term nitrate (nitrite) inhibition of nitrogen fixation in Azotobacter chroococcum.

Nitrate-grown Azotobacter chroococcum ATCC 4412 cells lack the ability to fix N2. Nitrogenase activity developed after the cells were suspended in a combined nitrogen-free medium and was paralleled by a concomitant decrease in nitrate assimilation capacity. In such treated cells exhibiting transitory nitrate assimilation and N2-fixation capacity, nitrate or nitrite caused a short-term inhibitory effect on nitrogenase activity which ceased once the anion was exhausted from the medium. The analog L-methionine-DL-sulfoximine, an inhibitor of glutamine synthetase, prevented inhibition of nitrogenase activity by nitrate or nitrite without affecting the uptake of these antions, which were reduced and stoichiometrically released into the external medium as ammonium. Inhibition of nitrogenase by nitrate (nitrite) did not take place in A. chroococcum MCD1, which is unable to assimilate either. We conclude that the short-term inhibitory effect of nitrate (nitrite) on nitrogenase activity is due to some organic product(s) formed during the assimilation of the ammonium resulting from nitrate (nitrite) reduction.

Effect of Nitrate Level in Nutrient Solutions on Growth and Nitrate Accumulation in Two Plum Clones

Marianna 2624 (Prunus cerasifera EHRH. × P. munsoniana Wight and Hedr.) and Myrobolan 3-J (P. cerasifera) plum were grown in aerated nutrient solutions to study the relationship between nitrate level and vegetative growth. Shoot-to-root ratios increased in Marianna 2624 and Myrobolan 3-J by 2 and 3.5 times, respectively, as the nitrate concentration of the nutrient solution was raised from 0.25 to 2 mM. The number of lateral shoots on Myrobolan 3-J plants increased significantly at increasing nitrate concentrations, but release of lateral buds from correlative inhibition was not apparent among Marianna 2624 plants. Leaf production and total leaf area also increased significantly as Myrobolan 3-J plants were exposed to 0.75 mM or 2 mM nitrate. The failure of Marianna 2624 plants to respond similarly at higher nitrate levels may be due to a more limited capacity for nitrate assimilation. This interpretation is based on significantly higher concentrations of nitrate in Marianna 2624 roots and foliage than in those organs of Myrobolan 3-J plants.