Nitrate Reductase Activity In Roots Research Articles

Effects of Nitrate Assimilation in Leaves and Roots on Biomass Allocation and Drought Stress Responses in Poplar Seedlings

Knowledge of tree biomass allocation is fundamental for estimating forest acclimation and carbon stock for global changes in the future. Optimal partitioning theory (OPT) and allometric partitioning theory (APT) are two major patterns of biomass allocation, and occurrences have been tested on taxonomical, ontogenetic, geographic and environmental scales, showing conflicting results and unclear ecophysiological mechanisms. Here, we examine the biomass allocation patterns of two young poplar (Populus) clones varying greatly in drought resistance under different soil water and nitrogen availabilities and the major physiological processes involved in biomass partitioning. We found that Biyu, a drought-sensitive hybrid poplar clone, had significant relations among biomass of leaf, stem and root, showing allometric partitioning. Xiaoye, a drought-tolerant poplar clone native to semi-arid areas, on the contrary, showed tightly regulated biomass allocation following optimal partitioning theory. Biyu had higher nitrate reductase activity in the fine roots, while Xiaoye had higher nitrate reductase activity in the leaves. Biochemical analyses and measurements of fluorescence and gas exchange showed that Xiaoye maintained more stable chloroplast membranes and photosystem electron flow, showing higher water use efficiency and a higher resistance to drought. A nitrogen addition could improve leaf photosynthesis and growth both in Biyu and Xiaoye seedlings under drought conditions. We concluded that the two poplar clones showed different biomass allocation patterns and suggest that the site of nitrate assimilation may play a role in biomass partitioning under varying water and nitrogen availabilities.

Trichoderma asperellum boosts nitrogen accumulation and photosynthetic capacity of wolfberry (Lycium chinense) under saline soil stress.

Trichoderma can promote plant growth under saline stress, but the mechanisms remain to be revealed. In this study, we investigate photosynthetic gas exchange, photosystem II (PSII) performance, nitrogen absorption and accumulation in a medicinal plant wolfberry (Lycium chinense) in saline soil supplemented with Trichoderma biofertilizer (TF). Larger nitrogen and biomass accumulation were found in plants supplemented with TF than with organic fertilizer (OF), suggesting that Trichoderma asperellum promoted plant growth and nitrogen accumulation under saline stress. T. asperellum strengthened root nitrogen (N) absorption according to greater increased root NH4+ and NO3- influxes under supplement with TF than OF, while nitrogen assimilative enzymes such as nitrate reductase, nitrite reductase and glutamine synthetase activities in roots and leaves were also stimulated. Thus, the elevated N accumulation derived from the induction of T. asperellum on nitrogen absorption and assimilation. Greater increased photosynthetic rate (Pn) and photosynthetic N-use efficiency under supplement with TF than OF illustrated that T. asperellum enhanced photosynthetic capacity and N utilization under saline stress. Although increased leaf stomatal conductance contributed to carbon (C) isotope fractionation under TF supplement, leaf 13C abundance was significantly increased by supplement with TF rather than OF, indicating that T. asperellum raised CO2 assimilation to a greater extent, reducing C isotope preference. Trichoderma asperellum optimized electron transport at PSII donor and acceptor sides under saline stress because of lower K and J steps in chlorophyll fluorescence transients under supplement with TF than OF. The amount of PSII active reaction centers was also increased by T. asperellum. Thus, PSII performance was upgraded, consistent with greater heightened delayed chlorophyll fluorescence transients and I1 peak under supplement with TF than OF. In summary, TF acted to increase N nutrient acquisition and photosynthetic C fixation resulting in enhanced wolfberry growth under saline soil stress.

Exogenous Putrescine Modulates Nitrate Reductase-Dependent NO Production in Cucumber Seedlings Subjected to Salt Stress.

Polyamines (PAs) are small aliphatic compounds that participate in the plant response to abiotic stresses. They also participate in nitric oxide (NO) production in plants; however, their role in this process remains unknown. Therefore, the study aimed to investigate the role of putrescine (Put) in NO production in the roots of cucumber seedlings subjected to salt stress (120 mM NaCl) for 1 and 24 h. In salinity, exogenous Put can regulate NO levels by managing NO biosynthesis pathways in a time-dependent manner. In cucumber roots exposed to 1 h of salinity, exogenous Put reduced NO level by decreasing nitrate reductase (NR)-dependent NO production and reduced nitric oxide synthase-like (NOS-like) activity. In contrast, during a 24 h salinity exposure, Put treatment boosted NO levels, counteracting the inhibitory effect of salinity on the NR and plasma membrane nitrate reductase (PM-NR) activity in cucumber roots. The role of endogenous Put in salt-induced NO generation was confirmed using Put biosynthesis inhibitors. Furthermore, the application of Put can modulate the NR activity at the genetic and post-translational levels. After 1 h of salt stress, exogenous Put upregulated CsNR1 and CsNR2 expression and downregulated CsNR3 expression. Put also decreased the NR activation state, indicating a reduction in the level of active dephosphorylated NR (dpNR) in the total enzyme pool. Conversely, in the roots of plants subjected to 24 h of salinity, exogenous Put enhanced the NR activation state, indicating an enhancement of the dpNR form in the total NR pool. These changes were accompanied by a modification of endogenous PA content. Application of exogenous Put led to an increase in the amount of Put in the roots and reduced endogenous spermine (Spm) content in cucumber roots under 24 h salinity. The regulatory role of exogenous Put on NO biosynthesis pathways may link with plant mechanisms of response to salt stress.

Nitrate reductase regulation in wheat seedlings by exogenous nitrate: A possible role in tolerance to salt stress

AbstractBackgroundSoil salinity is a major abiotic stress causing severe damage to plants. Thus, proper management approaches need to be developed to lessen the detrimental effect of salinity on crop growth and productivity.AimsThis study was conducted to investigate the putative role of nitrate (NO3−) and nitrate reductase (NR) in mitigating the adverse effects of salt stress on the growth of durum wheat seedlings.MethodsNitrogen nutrition has been modified, in the presence of 100 mM NaCl, either by increasing the NO3− availability, the deprivation of NO3− or by the addition of sodium tungstate—an inhibitor of NR—in the culture medium.ResultsObtained results showed that, in the presence of 2.5 mM NO3−, salt stress significantly decreased all studied growth traits (biomass production, relative growth rate, and water content). This was associated with a noteworthy reduction in total chlorophyll pigment, total carbohydrates, and protein contents. Concomitantly, NR activity was remarkably decreased. However, proline and malondialdehyde (MDA) were significantly accumulated. In the absence of NO3− as well as in the presence of tungstate, NR activities were noticeably reduced, and wheat seedling growth was further disturbed in comparison to salt‐stressed seedlings grown under 2.5 mM NO3−. Increasing the NO3− availability to 7.5 mM significantly restricted Cl− uptake, markedly increased root and leaf NR activities, and alleviated salt stress–induced seedling growth inhibition as compared to salt‐stressed seedlings grown under 2.5 mM NO3−. Such effects were associated with an increase in leaf chlorophyll and protein concentrations and in root and leaf carbohydrate concentrations. Nevertheless, MDA concentrations were sharply decreased.ConclusionsThis study provides strong arguments highlighting the potential role of NO3− reduction in mitigating the adverse effects of salt stress on the growth of wheat plants at the early seedling stage. The enhancement of the NR activity through increasing the NO3− availability may, therefore, represent a potential strategy to overcome the salinity‐mediated impairment of wheat seedlings to some extent.

Nitrogen uptake preference and allocation in Populus cathayana in response to drought stress

The preference for different nitrogen (N) forms may enhance plant survival and fitness. However, little information is known about how soil water availability, N addition, and their interaction dynamically impact plant N uptake preference and allocation. Here, we conducted a pot experiment using Populus cathayana cuttings, consisting of soil water regime (well watering control/mild drought/intensive drought: 80/60/40 % of field capacity) combined with N addition (N0 as control/N4/N8: 0/4/8 g N m−2 yr−1) treatments based on 15N isotope tracer technology. Two tracer solution (15NH4NO3/NH415NO3, both at 10.2 atom%) were used in N4/N8 treatments. Root, stem and leaf were collected at 6 h, 1, 3, 7, 14, and 31 days after labeling. Results showed that N addition significantly increased plant biomass and N accumulation, and decreased N allocation to leaf. Drought significantly decreased plant biomass, N accumulation, and N uptake (indicated by δ15N), and intensive drought reduced more than mild drought did. Nitrogen allocation to leaf was increased by drought. NO3- uptake by P. cathayana (indicated by δ15N in NH415NO3 treatment) was correlated significantly positively with total root length (TRL), total root surface area (TRA), specific root length (SRL), specific root area (SRA), root vitality, and root nitrate reductase (NR) activity. Intensive drought significantly reduced TRL, TRA, SRL, SRA, root vitality, and root NR activity under N4 and N8 treatments, which resulted δ15N in NH415NO3 treatment reduction and lower than in 15NH4NO3 treatment, so P. cathayana exhibited ammonium preference under intensive drought treatment. Our study provides an essential insight into how plants adapt and prefer different N forms under drought stress in nature.

Wheat Straw Burial Improves Physiological Traits, Yield and Grain Quality of Rice by Regulating Antioxidant System and Nitrogen Assimilation Enzymes under Alternate Wetting and Drying Irrigation

Wheat straw burial has great potential to sustain rice production under alternate wetting and drying (AWD) irrigation. A field experiment was conducted with three wheat straw burial treatments, including without straw burial (NSB), with light straw burial of 300 kg/hm2 (LSB) and dense straw burial of 800 kg/hm2 (DSB), as well as three AWD regimes: alternate wetting/moderate drying (AWMD), alternate wetting/severe drying (AWSD) and alternate wetting/critical drying (AWCD). The rice growth and grain quality were higher in LSB and NSB than those in NSB under the same AWD regime. The AWMD × DSB treatment resulted in the highest yield, brown rice rate, milled rice rate, amylose content and protein content. Conversely, the AWCD × NSB treatment led to the lowest yield, brown rice rate, milled rice rate, amylose content and protein content. The active absorption area and nitrate reductase activity of roots were higher in the AWMD × DSB treatment than those in the AWCD × NSB treatment, as the former increased organic carbon and nitrogen contents in the rhizosphere, whereas the latter reduced their availability. Total soluble protein content and glutamine synthetase activity were greater in the AWMD × DSB treatment than those in the AWCD × NSB treatment. The activities of superoxide dismutase and catalase were higher in the AWMD × DSB treatment compared with the AWCD × NSB treatment, leading to the amelioration of oxidative cell injury, as shown by a lower malonaldehyde level. This study suggested that farmers should implement AWMD irrigation after leaving the straw residues in the field, followed by deep tillage to improve soil quality and mitigate the drought stress cycles of AWD. This approach can improve rice growth and grain quality and alleviate the problems of disposal of straw residues and water scarcity for sustainable rice production.

Humic Acid Modified by Being Incorporated Into Phosphate Fertilizer Increases Its Potency in Stimulating Maize Growth and Nutrient Absorption.

Humic acid-enhanced phosphate fertilizer (HAP) is widely applied in Chinese agriculture due to its high efficiency. Although the structural composition and physicochemical properties of humic acid (HA) are significantly altered during HAP production, a clear understanding of the mechanisms underlying the biological effects of HA extracted from HAP fertilizer (PHA) on plant growth is still lacking. In the current study, we extracted PHA from HAP and assessed its effects on the dry biomass, phosphorus (P) and nitrogen (N) uptake, and P absorption rate of maize seedlings when supplied at different concentrations (2.5, 5, 10, and 25 mg C L−1) in the hydroponic culture. The root vigor, root plasma membrane H+-ATPase activity, and root nitrate reductase activity were also determined as the representative indicators of the root capacity for nutrient absorption, and used to clarify the mechanism by which PHA affects the maize growth and nutrient absorption. The results showed that the dry biomass, phosphorus uptake, nitrogen uptake, and average phosphorus absorption rates were significantly higher by 14.7–27.9%, 9.6–35.1%, 17.9–22.4%, and 22.1–31.0%, respectively, in plants treated with 2.5–5 mg C L−1 PHA compared to untreated controls. Application of 10–25 mg C L−1 raw HA resulted in similar stimulatory effects on plant growth and nutrient absorption. However, higher levels of PHA (10–25 mg C L−1) negatively impacted these indicators of plant growth. Furthermore, low PHA or high raw HA concentrations similarly improved root vigor and root plasma membrane H+-ATPase and nitrate reductase (NR) activities. These results indicate that lower concentrations of PHA can stimulate maize seedling growth and nutrient absorption to an extent that is comparable to the effect of higher concentrations of raw HA. Thus, the proportion of HA incorporated into HAP could be lower than the theoretical amount estimated through assays evaluating the biological effects of raw HA.

Arbuscular Mycorrhizal Fungi Promote Gleditsia sinensis Lam. Root Growth under Salt Stress by Regulating Nutrient Uptake and Physiology

Towards the improvement of plant productivity in saline–alkali soils, the application of arbuscular mycorrhizal fungi (AMF) is an intensive topic of research. For this study, three inoculation treatments, namely, autoclaved AMF inocula (CK), Funneliformis mosseae (FM), and Corymbiglomus tortuosum (CT), and four NaCl levels, namely, 0, 50, 100, and 150 mM were established to investigate the growth and physiological responses of mycorrhizal Gleditsia sinensis Lam. root systems to increase salinity through root dry weight, morphology, nutrient content, and physiology, and soil nutrient content. As NaCl levels increased, root dry weight, morphology, and nutrient content under the CK treatment exhibited a downward trend, while FM and CT treatments weakened this trend and significantly improved root dry weight and morphology, which increased by more than 200%. Under high NaCl levels, root activity under the FM treatment was significantly higher than that under the CK, with an average increase of 120.86%. In contrast to the activity of nitrate reductase, niacinamide adenine dinucleotide oxidase activity under CK was significantly less than that in FM and CT treatments. Moreover, inoculation with AMF significantly affected soil alkali-hydrolyzable nitrogen (AN), total nitrogen (TN), and phosphorus (TP), while NaCl had no significant impact on soil nutrients. Further, both soil salinity and mycorrhizal colonization rate had significant direct effects on root growth. However, soil salinity primarily influenced root growth through indirect effects on root nitrogen content, while mycorrhizal colonization rate indirectly impacted root nitrate reductase activity, and root nitrogen and phosphorus content. Our results suggested that the use of suitable AMF (e.g., Funneliformis mosseae) might effectively improve the currently unfavorable situation of economic tree species production on land with saline soils, which may greatly optimize the utility of these areas.

Comparison of nitrogen assimilation components among Agrostis species and cultivars

AbstractEnhanced nitrogen use efficiency (NUE) in turfgrass systems comprises several factors, including more efficient uptake and assimilation of applied nitrogen (N), as well as reduced N losses to the environment. The objectives of the current research were to evaluate the inter‐ and intraspecific variation in N use parameters in two Agrostis species, creeping bentgrass (Agrostis stolonifera L.) and velvet bentgrass (Agrostis canina L.), under field and controlled environmental conditions. Measurements included leaf NUE (g dry matter mg–1 N), leaf N content (mg g–1 dry matter), leaf and root nitrate reductase activity (NRA)(μmol NO2– g–1 fresh weight h–1), and root nitrate uptake rate (NUR). Velvet bentgrass cultivars exhibited higher leaf N content and therefore lower leaf NUE than creeping bentgrass cultivars in the field and in a nutrient solution. Differences in leaf NRA were only observed in Agrostis species and cultivars in the field, with higher NRA observed in creeping bentgrass. Velvet bentgrass cultivars grown in a nutrient solution exhibited a 50% higher root NUR than creeping bentgrass, with leaf NRA and N content increasing and leaf NUE decreasing in Agrostis under higher NO3– supply.

Soil amendments for improving grain quality of grass pea (Lathyrus sativus L.) under drought

AbstractBackgroundUse of soil amendments in agriculture is an old practice and with increase in demand of organic agriculture, it is gaining more attention. Work on the quality of the produce more particularly if crop faces any adverse situation like drought are scarce. This paper provides a comparative assessment of organic (farm yard manure) and inorganic (biochar) soil amendments on grain quality of grass pea (Lathyrus sativus L.) when crop faces drought. Different grain quality parameters were studied along with crop health and grain yield under the application of soil amendments during drought.ResultsBiochar and farm yard manure (FYM) lowered drought‐induced leaf water potential (upto 22%) and root nitrate reductase activity (by 62% and 67% respectively). Both inorganic and organic soil amendments improved in vitro protein digestibility (up to 16% and 37% respectively). Whereas higher grain phytic acid content (increased up to 40%) was noted from application of organic amendment (FYM) compared to inorganic (16% increase in biochar treated plots). Protein yield is enhanced (8%) under the application of Biochar‐enhanced protein yield by 8% when crops experienced drought at vegetative stage, whereas FYM reduced it by 9% under the same situation.ConclusionTested soil amendments can improve grain quality of grass pea. However, they were not equally efficient when the crop experienced drought at either stage of growth. Thus, screening of soil amendment is important when added under stressful situation like drought to ascertain better grain quality for consumers.

Interactions between species change the uptake of ammonium and nitrate in Abies faxoniana and Picea asperata.

Plant nitrogen (N) uptake is affected by plant-plant interactions, but the mechanisms remain unknown. A 15N-labeled technique was used in a pot experiment to analyze the uptake rate of ammonium (NH4+) and nitrate (NO3-) by Abies faxoniana Rehd. et Wils and Picea asperata Mast. in single-plant mode, intraspecific and interspecific interactions. The results indicated that the effects of plant-plant interactions on N uptake rate depended on plant species and N forms. Picea asperata had a higher N uptake rate of both N forms than A. faxoniana, and both species preferred NO3-. Compared with single-plant mode, intraspecific interaction increased NH4+ uptake for A. faxoniana but reduced that for P. asperata, while it did not change NO3- uptake for the two species. The interspecific interaction enhanced N uptake of both N forms for A. faxoniana but did not affect the P. asperata compared with single-plant mode. NH4+ and NO3- uptake rates for the two species were regulated by root N concentration, root nitrate reductase activity, root vigor, soil pH and soil N availability under plant-plant interactions. Decreased NH4+ uptake rate for P. asperata under intraspecific interaction was induced by lower root N concentration and nitrate reductase activity. The positive effects of interspecific interaction on N uptake for A. faxoniana could be determined mainly by positive rhizosphere effects, such as high soil pH. From the perspective of root-soil interactions, our study provides insight into how plant-plant interactions affect N uptake, which can help to understand species coexistence and biodiversity maintenance in forest ecosystems.

Influence of Seaweed Liquid Fertilizer on the Growth of Trigonellafoenum-graecum L.

The effect of seaweed liquid fertilizer (SLF) of Sargassum wightii on germination, growth and biochemical constituents of Trigonellafoenum-graecum L. (Fenugreek) was studied. Sargassum wightii was collected from Kanyakumari (longitude 8.0883° N, latitude 77.5385° E) for the present study. Different concentrations (2%, 4%, 6%, 8% and 10%) of SLF were prepared using distilled water. Maximum germination percentage (100 ± 0.00%), fresh weight (3.63±0.18 g), dry weight (0.22±0.04 g), shoot length (6.83 ± 1.04 cm), root length (3.26 ± 0.20 cm), recorded at 6% concentration of Sargassum wightii liquid fertilizer treated fenugreek. Pigments such as total chlorophyll (11.95±0.22 mg.g/fr.wt) and carotenoids (4.9±0.1 mg.g/fr.wt) contents in leaves recorded at 6% SLF were the highest. The 6% SLF treated seedling showed the highest content of protein (6.8±0.1mg.g/fr.wt), aminoacid (3.63 ± 0.05mg.g/fr.wt), carbohydrate (2.6 ± 0.1 mg.g/fr.wt), ascorbic acid (0.36±0.015 mg.g/fr.wt), phenol (0.8±0.01 mg.g/fr.wt), nitrate (9.96± 1.35 mg.g/fr.wt) and nitrate reductase (14.96±0.85 mg.g/fr.wt) activity of shoots and roots respectively. The SLF showed better responses at lower concentration when compared to control and higher concentrations showed declining trend. The present study demonstrated that Sargassum wightii liquid extract could serve as an alternative biofertilizer as it is eco-friendly, cheaper, deliver substantial economic and environmental benefits to farmers.

Post-flowering nitrogen uptake leads to the genotypic variation in seed nitrogen accumulation of oilseed rape

Increasing post-flowering nitrogen (N) uptake is likely to improve seed N accumulation, ultimately leading to greater seed yield and N use efficiency (NUE). A comprehensive study on contrasting N utilization efficiency (NUtE) winter oilseed rape genotypes was conducted for three years (2017–2020) to unravel the effect of post-flowering N uptake on seed N accumulation. Compared to the low NUtE genotype, the high NUtE genotype displayed respectively 31%, 28% and 70% greater root biomass, length and volume, along with 40%, 44%, 46% and 82% higher root nitrate reductase (NR), glutamine synthetase (GS), glutamate synthetase (GOGAT), and glutamate dehydrogenase (GDH) activities after flowering. These were accompanied by a significant increase (P < 0.05) in catalases (134%), peroxidase (45%), glutathione (45%) and ascorbate peroxidase (41%), leading to 27% higher seed N accumulation, 23% higher seed yield and 60% higher NUE. Stronger post-flowering N uptake potential for high NUtE genotype was a main contributor to the enhanced seed N accumulation, and ultimately increased seed yield and NUE.

Cd胁迫下SA和NO互作对小麦幼苗根生长和叶绿素含量的影响

以"陇春27"号水培小麦幼苗为研究材料，外源添加水杨酸（SA）、一氧化氮（NO）清除剂（carboxy-PTIO，c-PTIO）、NO供体硝普钠（SNP）、硝酸还原酶（NR）抑制剂钨酸盐（Tungstate）及NO合成酶（NOS）抑制剂（L-NAME）进行不同预处理，分析其在镉（Cd）胁迫下根的生长和叶片叶绿素含量的变化，探讨SA和NO互作对小麦幼苗Cd胁迫的缓解机制。结果表明：随着Cd处理时间的延长，小麦幼苗根中SA含量显著降低，NO含量则呈现先增加（6 h和12 h）后减少（24 h和48 h）的趋势；Cd胁迫抑制了小麦幼苗根的生长，减少了叶片叶绿素的含量，而一定浓度的SA或SNP预处理可以缓解Cd胁迫对小麦幼苗根长的抑制作用，增加叶绿素的含量。c-PTIO、L-NAME和Tungstate单独预处理显著抑制了小麦幼苗根的生长，减少了NO的含量，但不影响叶绿素含量。SA400+L-NAME预处理可以缓解Cd胁迫对小麦幼苗根长的抑制作用以及叶绿素和NO含量的减少作用；SA400+c-PTIO或SA400+Tungstate预处理可增加Cd胁迫下叶绿素的含量，但对根的伸长无影响。进一步研究发现，Cd胁迫抑制了NR的活性，而SA400预处理可以使Cd胁迫下NR的活性增强，不同处理对NOS的活性均无影响。综上所述，Cd胁迫导致小麦幼苗根内源SA含量降低和NO含量先升高再降低；外源添加SA或SNP预处理缓解了Cd胁迫对根生长的抑制和叶绿素含量降低的作用；外源SA通过影响NO的产生从而提高小麦幼苗对Cd胁迫的耐受性，最终缓解了Cd对小麦幼苗的毒害作用。;Soil pollution of heavy metals is very severe in China, especially the cadmium pollution. Excessive cadmium not only causes severe damage to the growth and development of plants, leading to disorders of their physiological functions, reduced photosynthesis, and reduced organic matter accumulation and so on, but also greatly endangers human health. In this research, the growth of roots and the content of chlorophyll in the seedlings of wheat (Triticum aestivum L. cv. Longchun 27) were analyzed after being pretreated with exogenous salicylic acid (SA), nitric oxide (NO) scavenger (Carboxy-PTIO, c-PTIO), NO donor sodium nitratol (SNP), nitrate reductase (NR) inhibitor (Tungstate), NO synthase inhibitor (L-NAME) and then being treated with 100 μmol/L CdCl₂, to explore the mitigation mechanism of SA and NO interaction on Cd-induced phytotoxicity. The results showed that with the prolongation of cadmium treatment time, the content of SA decreased gradually, when the treatment time was extended to 24h, the SA content no longer changed, but the content of NO increased firstly (6 h and 12 h), then decreased (24 h and 48 h) in the roots of wheat seedlings. Cd stress significantly inhibited the root growth and reduced the chlorophyll content of the leaves. However, pretreatment with suitable SA or SNP could moderate the inhibition of Cd stress on the growth of roots and increase the content of chlorophyll in wheat seedlings. The single pretreatment of c-PTIO, L-NAME and Tungstate restrained the growth of roots significantly and reduced the content of NO, but had little effect on the content of chlorophyll. The pretreatment of SA400+L-NAME could alleviate the inhibition of root elongation and the reduction of chlorophyll as well as NO content in wheat seedlings were induced by Cd stress; the pretreatment SA400+c-PTIO or SA400+Tungstate could augment the content of chlorophyll, but there was no effect on root elongation under Cd stress. Further studies showed that Cd stress significantly weaken NR activity, while the pretreatment of SA400 could increase the NR activity of wheat seedlings roots under Cd stress; NOS activity was not affected by different treatments. In summary, Cd stress caused the decrease of endogenous SA content and the first increase of NO content and then decrease of wheat seedlings; the pretreatment with exogenous SA or SNP reduced the inhibition of root growth and the decrease of chlorophyll content by Cd stress; exogenous SA improved the tolerance of wheat seedlings to Cd stress by affecting the production of NO, ultimately, the phytotoxic effects of Cd on wheat seedlings were alleviated.

Effects of Olive Mill Wastewater and Two Natural Extracts as Nitrification Inhibitors on Activity of Nitrifying Bacteria, Soil Nitrate Leaching Loss, and Nitrogen Metabolism of Celery (Apium graveolens L.)

Minimizing nitrification of fertilizer ammonium (NH4+) can reduce nitrate (NO3−) contamination of groundwater and increase nitrogen (N) use efficiency (NUE). Olive mill wastewater (OMW), hydroalcoholic extracts of Mentha piperita L. (Mp) and Artemisia annua L. (Aa), and synthetic nitrification inhibitor (NI) dicyandiamide (DCD) were investigated. All NIs reduced activity of nitrifying bacteria and NO3− leached and increased efficiency of N metabolism of celery (Apium graveolens L.) during 56 days in a soil mixture fertilized with ammonium sulfate [(NH4)2SO4]. Soil NO3− leaching losses of DCD, OMW, Mp, and Aa treatments were 24%, 26%, 67%, and 78% of the untreated control loss, respectively, at 35 and 56 days after planting (DAP). Decreased nitrification by NIs resulted in greater concentrations of soil NH4+ correlated with less nitrate reductase (NR) activity in roots and leaves and less soil acidification compared to the control. At 35 and 56 DAP, DCD and OMW treatments decreased NR and increased glutamine synthetase activities in leaves and roots, compared to the control. NIs increased leaf and root protein and amino acids. OMW significantly decreased leaching loss of NO3− to 10% of fertilizer N applied to the soil, compared to 38% of applied NH4+-N leached from the control. OMW proved an effective alternative to DCD to improve NUE of NH4+-fertilizers.

Elevated CO2 alters tissue balance of nitrogen metabolism and downregulates nitrogen assimilation and signalling gene expression in wheat seedlings receiving high nitrate supply.

Tissue and canopy-level evidence suggests that elevated carbon dioxide (EC) inhibits shoot nitrate assimilation in plants and thereby affects nitrogen (N) and protein content of the economic produce. It is speculated that species or genotypes relying more on root nitrate assimilation can adapt better under EC due to the improved/steady supply of reductants required for nitrate assimilation. A study was conducted to examine the effect of EC on N assimilation and associated gene expression in wheat seedlings. Wheat genotypes, BT-Schomburgk (BTS) with comparatively high leaf nitrate reductase (NR) activity and Gluyas Early (GE) with high root NR activity were grown in hydroponic culture for 30days with two different nitrate levels (0.05mM and 5mM) in the climate controlled growth chambers maintained at either ambient (400 ± 10μmolmol-1) or EC (700 ± 10μmolmol-1) conditions. Exposure to EC downregulated the activity of enzyme NR and glutamate synthase (GOGAT) in leaf tissues, whereas in roots, activities of both the enzymes were upregulated by exposure to EC. In addition, EC downregulated N assimilation and signalling gene expression under high N availability. Root N assimilation was less affected in comparison with shoot N assimilation; thereby, the proportion of root contribution towards total assimilation was higher. The results suggest that EC could alter and re-programme N assimilation and signalling in wheat seedlings. The genotype and tissue-specific effects of EC on N assimilation also warrants the need for identification of suitable genotypes and revision of fertiliser regime for tapping the beneficial effects of EC conditions.

Enhanced Nitric Oxide Synthesis Through Nitrate Supply Improves Drought Tolerance of Sugarcane Plants.

Nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. Nitrate reductase (NR) not only mediates the reduction of NO3− to NO2− but also reduces NO2− to NO, a relevant pathway for NO production in higher plants. Herein, we hypothesized that sugarcane plants supplied with more NO3− as a source of N would produce more NO under water deficit. Such NO would reduce oxidative damage and favor photosynthetic metabolism and growth under water limiting conditions. Sugarcane plants were grown in nutrient solution and received the same amount of nitrogen, with varying nitrate:ammonium ratios (100:0 and 70:30). Plants were then grown under well-watered or water deficit conditions. Under water deficit, plants exhibited higher root [NO3−] and [NO2−] when supplied with 100% NO3−. Accordingly, the same plants also showed higher root NR activity and root NO production. We also found higher photosynthetic rates and stomatal conductance in plants supplied with more NO3−, which was associated with increased root growth. ROS accumulation was reduced due to increases in the activity of catalase in leaves and superoxide dismutase and ascorbate peroxidase in roots of plants supplied with 100% NO3− and facing water deficit. Such positive responses to water deficit were offset when a NO scavenger was supplied to the plants, thus confirming that increases in leaf gas exchange and plant growth were induced by NO. Concluding, NO3− supply is an interesting strategy for alleviating the negative effects of water deficit on sugarcane plants, increasing drought tolerance through enhanced NO production. Our data also provide insights on how plant nutrition could improve crop tolerance against abiotic stresses, such as drought.

Nitrate-mediated maintenance of photosynthetic process by modulating hypoxic metabolism of common bean plants

Nitrate has been reported to improve tolerance of plants under flooding by modulating carbon and nitrogen metabolism of root cells; however, the extent to which nitrate modulates the photosynthetic process is not well understood. This work aimed to evaluate the photosynthetic process by nitrate-mediated modulation of hypoxic metabolism in the root of common bean plants (Phaseolus vulgaris L.) under flooding and recovery conditions. Three groups of common beans [N2-fixing (N2)], N2-fixing pre-hypoxic nitrate-treated (N2 + NO3−), and nitrate (NO3−)-supplied plants] were grown in vermiculite and the root system was subjected to flooding for 48 h at the early reproductive stage. After flooding, plants returned to normoxic conditions per 24 and 72 h for recovery. Plants from N2 + NO3− and mainly NO3− groups maintained transpiration rate (E) and CO2 assimilation under root-flooding, which was not shown by N2-fixing plants. No changes in the dynamic dissipation of photosynthetic energy were evidenced in leaves of NO3− plants upon flooding, besides an increase in nitrate reductase (NR) activity and a decrease in fermentative enzymes in roots. Reactive oxygen species (ROS) and antioxidative enzymes increased in leaves and decreased in roots. Nitrate-mediated maintenance of the photosynthetic process may be related to induction in NR activity in roots to alleviate the toxic effects of fermentation through a decrease of fermentative enzyme activity.

Nitrogen source utilization in co-existing canopy tree and dwarf bamboo in a northern hardwood forest in Japan

Understory dwarf bamboo mitigated soil N competition with co-existing canopy oak trees by foraging in deeper soils and increasing dependence on N forms that differ from those used by canopy trees. Nitrogen (N) competition among co-existing plant species utilizing different mycorrhiza types was explored through the investigation of N sources of oak trees and dwarf bamboo. Vertical distribution of fine roots, soil N pools, δ15N of leaves, and possible soil N sources and nitrate reductase activity (NRA) were all quantified. The fine roots of canopy trees were more concentrated in the surface soils than roots of the understory dwarf bamboo. Soil NH4+ and extractable organic N (EON) content (based on unit weight) decreased from the organic horizon (O horizon) to the deep soils, the size of the NH4+ pool per unit volume increased with soil depth, and the EON was approximately constant. Soil NO3− was not detected at any soil depth or was not significant in value, while NO3− captured by ion-exchange resin (IER) buried at a 10 cm soil depth and net nitrification were observed via laboratory incubation at all soil depths. The δ15N of the NH4+ and EON pools increased with soil depth and the δ15N of NO3− of IER was lower than that of other N forms, except for the δ15N of NH4+ in the O horizon. Furthermore, root NRA tended to be lower in canopy trees than in the understory, implying lower dependency on NO3− by canopy trees. The pattern of root distribution and mycorrhizal fungi association of the understory vegetation (as well as the high root NRA) suggested that dependence on N in deeper soils was higher in understory plants than in canopy trees. These findings indicate that understory vegetation mitigates soil N competition against co-existing canopy trees via the use of alternative N sources.

Nitrate reductase activities in plants from different ecological and taxonomic groups grown in Japan

AbstractPlants generally use soil inorganic nitrogen, ammonium (NH4+–N), and nitrate (NO3−–N) as sources of nitrogen, an essential nutrient. The assimilation processes after uptake differ considerably from ammonium to nitrate. Nitrate must be reduced to ammonium in plant tissue before it is synthesized to amino acids, while ammonium is directly and immediately synthesized to amino acids after its uptake. Nitrate reductase is an enzyme that catalyzes the first and rate‐limiting step of nitrate assimilation, reducing nitrate to nitrite. It is a substrate‐inducible enzyme, and the capacity to induce nitrate reductase varies greatly among plant species. In vivo nitrate reductase activity (NRA) is generally measured as a nitrite production rate during incubation using fine cut pieces of plant tissue, and it is applicable as an indicator on plant nitrate use. Here we present in vivo NRA of leaves from a total of 108 species including arboreal trees, small trees, shrubs, herbs, a vine and a moss. For 75 of the species, NRA in fine roots was also determined. At least 20 species in sampled plants were imported and planted for scientific or industrial purposes, but most sampled species were native to Japan. Several inventory studies of plant NRA have been conducted, mainly in Europe, and they provided information on the species‐specific capacity of nitrate use by plants in Europe. However, to our best knowledge, there has been hitherto no published inventory of the NRA of plants in Japan where many endemic species are distributed. Our dataset contains plant NRA with species, family name, life form, leaf lifespan (evergreen or deciduous), growth stage, the season of sample collection, growth conditions (natural or cultivated) and other treatments/conditions when applicable. The data provided by this study may contribute to future works that require information regarding the plant species characteristics for nitrate use capacity or nitrate preference.The complete data set for this abstract published in the Data Paper section of the journal is available in electronic format in MetaCat in JaLTER at http://db.cger.nies.go.jp/JaLTER/metacat/metacat/ERDP-2019-06.1/jalter-en. [Correction added on 7 September 2020, after first online publication: JaLTER URL has been updated.]