N-inefficient Genotype Research Articles

Integrated BSA-seq and RNA-seq analysis to identify candidate genes associated with nitrogen utilization efficiency (NUtE) in rapeseed (Brassica napus L.)

Rapeseed (Brassica napus L.) is one of the important oil crops, with a high demand for nitrogen (N). It is essential to explore the potential of rapeseed to improve nitrogen utilization efficiency (NUtE). Rapeseed is an allotetraploid crop with a relatively large and complex genome, and there are few studies on the mapping of genes related to NUtE regulation. In this study, we used the combination of bulk segregant analysis sequencing (BSA-Seq) and RNA sequencing (RNA-Seq) to analyze the N-efficient genotype ‘Zheyou 18’ and N-inefficient genotype ‘Sollux’, to identify the genetic regulatory mechanisms. Several candidate genes were screened, such as the high-affinity nitrate transporter gene NRT2.1 (BnaC08g43370D) and the abscisic acid (ABA) signal transduction-related genes (BnaC02g14540D, BnaA03g20760D, and BnaA05g01330D). BnaA05g01330D was annotated as ABA-INDUCIBLE bHLH-TYPE TRANSCRIPTION FACTOR (AIB/bHLH17), which was highly expressed in the root. The results showed that the primary root length of the ataib mutant was significantly longer than that of the wild type under low N conditions. Overexpression of BnaA5.AIB could reduce the NUtE under low N levels in Arabidopsis (Arabidopsis thaliana). Candidate genes identified in this study may be involved in the regulation of NUtE in rapeseed, and new functions of AIB in orchestrating N uptake and utilization have been revealed. It is indicated that BnaA5.AIB may be the key factor that links ABA to N signaling and a negative regulator of NUtE. It will provide a theoretical basis and application prospect for resource conservation, environmental protection, and sustainable agricultural development.

The responses of genotypes with contrasting NUtE to exogenous ABA during the flowering stage in Brassica napus

A large amount of nitrogen (N) fertilizer is applied to maintain a stable yield because of the low N utilization efficiency (NUtE) in rapeseed (Brassica napus L.), which increases production costs and has a negative impact on the environment. Abscisic acid (ABA) has been reported to have crosstalk with nitrate. In this study, exogenous ABA treatment and RNA-seq were used to explore the response patterns of rapeseed with contrasting NUtE to ABA signals. Our results showed that ABA application on leaf promoted the N allocation from vegetative organs to grain and increased the number of seeds per silique, grain yield, N harvest index (NHI), and the NUtE in N-efficient rapeseed. Transcriptome analysis showed the DEGs were mainly enriched in N metabolism, ABC transporters, and MAPK signaling pathways. Among them, the Glutamate dehydrogenase (GS) genes and the Glutamine synthetase gene were more highly expressed in the N-efficient genotype leaf than the N-inefficient genotype after ABA treatment. In contrast, the opposite expression level was observed in seven genes, including nitrate transporter 2.1, Nitrate Reductase genes, and GS genes in the root. In conclusion, the shoot and root of rapeseed may have different response mechanisms to ABA stimulation, and N-efficient rapeseed is more sensitive to ABA treatment, which may be one of the reasons for the difference in NUtE

Genotypic variation in carbon and nitrogen metabolism in the cotton subtending leaves and seed cotton yield under various nitrogen levels.

Nitrogen (N) is the key nutrient required for high cotton production; however, its excessive use can increase the cost of production and environmental problems. Reducing the application of N while sustaining the yield is an important issue to be solved. Therefore, this study was designed to investigate the genotypic variations in subtending leaf physiology and its contribution to seed cotton yield of contrasting N-efficient cotton genotypes under various N levels in pot and field conditions. The results showed that the application of N increased the enzymatic activities related to carbon (C) and N metabolisms. Under the same N level, the C/N metabolisms of the N-efficient genotypes were significantly higher than N-inefficient genotypes, indicating a strong N assimilation and photoassimilation ability in N-efficient genotypes, especially under low N level. Moreover, the antioxidant enzymatic activities were significantly higher, whereas malondialdehyde content was lower in N-efficient cotton genotypes than in N-inefficient ones. Therefore, N-efficient cotton genotypes showed strong resistance, higher C/N metabolisms, and provided sufficient dry matter for boll development. As a result, the yield, N use efficiency, and value cost ratio of the N-efficient cotton genotypes were higher than in the N-inefficient genotypes. It was confirmed that the higher C/N metabolisms in the cotton subtending leaves of N-efficient cotton genotypes could support higher seed cotton yield under relatively low N application. © 2022 Society of Chemical Industry.

Morphological, physiological, and transcriptional responses to low nitrogen stress in Populus deltoides Marsh. clones with contrasting nitrogen use efficiency

BackgroundNitrogen (N) is one of the main factors limiting the wood yield in poplar cultivation. Understanding the molecular mechanism of N utilization could play a guiding role in improving the nitrogen use efficiency (NUE) of poplar.ResultsIn this study, three N-efficient genotypes (A1-A3) and three N-inefficient genotypes (C1-C3) of Populus deltoides were cultured under low N stress (5 μM NH4NO3) and normal N supply (750 μM NH4NO3). The dry matter mass, leaf morphology, and chlorophyll content of both genotypes decreased under N starvation. The low nitrogen adaptation coefficients of the leaves and stems biomass of group A were significantly higher than those of group C (p < 0.05). Interestingly, N starvation induced fine root growth in group A, but not in group C. Next, a detailed time-course analysis of enzyme activities and gene expression in leaves identified 2062 specifically differentially expressed genes (DEGs) in group A and 1118 in group C. Moreover, the sensitivity to N starvation of group A was weak, and DEGs related to hormone signal transduction and stimulus response played an important role in the low N response this group. Weighted gene co-expression network analysis identified genes related to membranes, catalytic activity, enzymatic activity, and response to stresses that might be critical for poplar’s adaption to N starvation and these genes participated in the negative regulation of various biological processes. Finally, ten influential hub genes and twelve transcription factors were identified in the response to N starvation. Among them, four hub genes were related to programmed cell death and the defense response, and PodelWRKY18, with high connectivity, was involved in plant signal transduction. The expression of hub genes increased gradually with the extension of low N stress time, and the expression changes in group A were more obvious than those in group C.ConclusionsUnder N starvation, group A showed stronger adaptability and better NUE than group C in terms of morphology and physiology. The discovery of hub genes and transcription factors might provide new information for the analysis of the molecular mechanism of NUE and its improvement in poplar.

The rapeseed genotypes with contrasting NUE response discrepantly to varied provision of ammonium and nitrate by regulating photosynthesis, root morphology, nutritional status, and oxidative stress response

Ammonium (NH4+) and nitrate (NO3−) are the two predominant inorganic nitrogen (N) forms available to crops in agricultural soils. However, little is known about how the NH4+:NO3− ratio affect the growth of Brassica napus. Here, we investigated the impact of five NH4+:NO3− ratios (100:0, 75:25, 50:50, 25:75, 0:100) on plant growth, photosynthesis, root morphology, ammonium uptake, nutritional status, oxidative stress response, and relative expression of genes involved in these processes in two rapeseed genotypes with contrasting N use efficiency (NUE). Application of NO3− as a N source extremely improved rapeseed growth compare to NH4+. However, the best growth of the N-inefficient genotype was observed under 75:25 NH4+/NO3− ratio, while it happens for the N-efficient genotype only under the sole NO3− environment. The low-NUE genotype exhibited a more developed root system, higher photosynthetic capacity, higher nutrient accumulation, and better NH4+ uptake ability under the 75:25 NH4+/NO3− ratio, resulting in a decrease of malondialdehyde (MDA) in root. However, the high-NUE genotype performed better in the above aspects under the NO3−-only condition. Nitrate decrease MDA by reducing the activities of superoxide dismutase, peroxidase, and catalase in root of the N-efficient genotype. Moreover, significant differences were detected for the expression levels of genes involved in N uptake and oxidative stress response between the two genotypes under two NH4+/NO3− ratios. Taken together, our results indicate that the N-inefficient rapeseed genotype prefers mixed supply of ammonium and nitrate, whereas the genotype with high NUE prefers sole nitrate environment.

Morphological and physiological plasticity response to low nitrogen stress in black cottonwood (Populus deltoides Marsh.)

It is important to evaluate nitrogen use efficiency and nitrogen tolerance of trees in order to improve their productivity. In this study, both were evaluated for 338 Populus deltoides genotypes from six provenances. The plants were cultured under normal nitrogen (750 μM NH4NO3) and low nitrogen (5 μM NH4NO3) conditions for 3 months. Growth, chlorophyll content and glutamine synthetase activity of each genotype were measured. Under low nitrogen, heights, ground diameter, leaf area, leaf and root biomass, and chlorophyll contents were significantly lower than those under normal nitrogen level. Correlation analysis showed that nutrient distribution changed under different nitrogen treatments. There was a negative correlation between leaf traits and root biomass under normal nitrogen level, however, the correlation became positive in low nitrogen treatment. Moreover, with the decrease of nitrogen level, the negative correlation between leaf morphology and chlorophyll levels became weakened. The growth of the genotypes under the two treatments was evaluated by combining principal component analysis with a fuzzy mathematical membership function; the results showed that leaf traits accounted for a large proportion of the variation in the evaluation model. According to the results of comprehensive evaluation of plants under the two treatments, the 338 P. deltoides genotypes could be divided into nine categories, with wide genotypic diversity in nitrogen use efficiency and low nitrogen tolerance. As a result, 26 N-efficient genotypes and 24 N-inefficient genotypes were selected. By comparative analysis of their morphological and physiological traits under the two treatments, leaf traits could be significant indicators for nitrogen use efficiency and nitrogen tolerance, which is of considerable significance for breeding poplar varieties with high nitrogen use efficiencies.

Genotypic variation in nitrogen utilization efficiency in oilseed rape is related to the coordination of leaf senescence and root N uptake during reproductive stage

Due to their low nitrogen utilization efficiency (NUtE) oilseed rape (OSR) crops require large quantities of nitrogen (N) fertilizer to achieve acceptable economic yields. To dissect the underlying mechanisms of oilseed NUtE, we investigated the relationships between NUtE, N uptake, N remobilization and leaf senescence by growing three N-efficient (genotypes with high NUtE at deficient N supplies) and three N-inefficient (ones with low NUtE at deficient N supplies) genotypes in pot experiments. These genotypes showed similar plant N uptake before flowering and similar post-anthesis N remobilization from leaf and stem, but after flowering N-efficient genotypes exhibited higher root N uptake than N-inefficient genotypes. Besides, the pod biomass and leaf N concentration of N-efficient genotypes were higher than those of N-inefficient genotypes at 50 days after flowering, and we observed delayed leaf senescence in N-efficient genotypes. Correspondingly, gene transcripts involved in photosynthesis and carbon metabolism were abundant in green mature leaves of N-efficient genotype, while the expression of genes involved in ubiquitin mediated proteolysis pathway were more abundant in N-inefficient mature leaves. The stay-green phenotype of the N-efficient OSR after flowering provides sufficient photosynthate for root N uptake activity and pod development, and in turn, the prolonged post-anthesis N uptake provides more N to pods, leading to less leaf N remobilization requirement. This work has identified traits important for breeding OSR cultivars with improved NUtE.

Nitrate Uptake and Assimilation Potential Under Limiting N-Status in Wheat Genotypes (Triticum aestivum L.)

The excess use of nitrogen fertilizers causes various problems, including higher costs of crop production, lower nitrogen use efficiency, and environmental damage. Crop breeding for low nitrogen tolerance has become the major focus in solving these issues. Genotypic variation for NUE traits has been reported in cereal crops which include N accumulation, N remobilization and high yield under low N. However, identification of genetic and metabolic control linked to phenotypes NUE is difficult looking to the complexity of N metabolism. The present work with thirteen wheat genotypes was undertaken to understand the basic difference in uptake, assimilation, remobilization of nitrate and other physiological limiting factors with limiting nitrogen (N) supply. The higher leaf nitrate content in limited nitrogen supply in nitrogen-efficient wheat genotypes GW-322, MACS 6222, NIAW-301, NIAW-34, NIAW-3170, NIAW-1994 and HD 281 appears to be low-nitrate tolerant probably indicating that both low and high affinity nitrate transporter are actively operating. Chlorophyll content, leaf nitrate content, in vivo NR, in vitro NR, GS, Fd-GOGAT, NADH-GOGAT and total grain protein were significantly higher in the N-efficient wheat genotypes, even at low dose of applied nitrogen both at pre and post-anthesis stages. The significantly higher in vitro NR activity compared to in vivo NR activity at saturating nitrate concentration (+KNO3) signifies that beside nitrate, NADH appears to be a limiting factor. Glutamine synthetase, Fd-GOGAT and NADH-GOGAT activities were higher at post anthesis stage signifies its importance in remobilization of photorespiratory ammonia. In pre-anthesis, the Nefficient genotypes had higher relative levels of GS2 protein than the N-inefficient genotypes; it was found that GS2 is important in the vegetative growth stage and carbon-nitrogen metabolic balance. In the post-anthesis stage, there were increased relative amounts of the GS1 protein under limiting N condition. The results revealed that the N-efficient genotypes have increased assimilation ability in the pre-anthesis stage and remobilisation ability in the post-anthesis stage. The relative contents of the GS1 and GS2 proteins at pre and post-anthesis could be used as a marker for breeding varieties with high NUE. Study of the activities of nitrate assimilatory enzymes in N-efficient and N-inefficient wheat genotypes showed that nitrate uptake (in vivo) nitrate reductase activity, glutamine synthetase (GS) and glutamate synthase (GOGAT) play important role in N assimilation and remobilization under low-nitrogen conditions.

Variations in Nitrogen Metabolism are Closely Linked with Nitrogen Uptake and Utilization Efficiency in Cotton Genotypes under Various Nitrogen Supplies.

Cotton production is highly sensitive to nitrogen (N) fertilization, whose excessive use is responsible for human and environmental problems. Lowering N supply together with the selection of N-efficient genotypes, more able to uptake, utilize, and remobilize the available N, could be a challenge to maintain high cotton production sustainably. The current study aimed to explore the intraspecific variation among four cotton genotypes in response to various N supplies, in order to identify the most distinct N-efficient genotypes and their nitrogen use efficiency (NUE)-related traits in hydroponic culture. On the basis of shoot dry matter, CCRI-69 and XLZ-30 were identified as N-efficient and N-inefficient genotypes, respectively, and these results were confirmed by their contrasting N metabolism, uptake (NUpE), and utilization efficiency (NUtE). Overall, our results indicated the key role of shoot glutamine synthetase (GS) and root total soluble protein in NUtE. Conversely, tissue N concentration and N-metabolizing enzymes were considered as the key traits in conferring high NUpE. The remobilization of N from the shoot to roots by high shoot GS activity may be a strategy to enhance root total soluble protein, which improves root growth for N uptake and NUE. In future, multi-omics studies will be employed to focus on the key genes and pathways involved in N metabolism and their role in improving NUE.

Growth and nitrogen metabolism are associated with nitrogen-use efficiency in cotton genotypes

Crops, including cotton, are sensitive to nitrogen (N) and excessive use can lead to an increase in production costs and environmental problems. We hypothesized that the use of cotton genotypes with substantial root systems and high genetic potentials for nitrogen-use efficiency (NUE) would best address these problems. Therefore, the interspecific variations and traits contributing to NUE in six cotton genotypes having contrasting NUEs were studied in response to various nitrate concentrations. Large genotypic variations were observed in morphophysiological and biochemical traits, especially shoot dry weight, root traits, and N-assimilating enzyme levels. The roots of all the cotton genotypes were more sensitive to low-than high-nitrate concentrations, and the genotype CCRI-69 had the largest root system irrespective of the nitrate concentration. The root morphological traits were positively correlated with N-utilization efficiency and were more affected by genotype than nitrate concentration. Conversely, growth and N-assimilating enzyme levels were more affected by nitrate concentration and were positively correlated with N-uptake efficiency. Based on shoot dry weight, CCRI-69 and XLZ-30 were identified as N-efficient and N-inefficient genotypes, respectively, and these results were confirmed by their contrasting root systems, N metabolism, and NUEs. In the future, multi-omics techniques will be performed to identify key genes/pathways involved in N metabolism, which may have the potential to improve root architecture and increase NUE.

Genotypic Variation in Nitrogen Utilization Efficiency of Oilseed Rape (Brassica napus) Under Contrasting N Supply in Pot and Field Experiments.

Oilseed rape (Brassica napus) characteristically has high N uptake efficiency and low N utilization efficiency (NUtE, seed yield/shoot N accumulation). Determining the NUtE phenotype of various genotypes in different growth conditions is a way of finding target traits to improve oilseed rape NUtE. The aim of this study was to compare oilseed rape genotypes grown on contrasting N supply rates in pot and field experiments to investigate the genotypic variations of NUtE and to identify indicators of N efficient genotypes. For 50 oilseed rape genotypes, NUtE, dry matter and N partitioning, morphological characteristics, and the yield components were investigated under high and low N supplies in a greenhouse pot experiment and a field trial. Although the genotype rankings of NUtE were different between the pot experiment and the field trial, some genotypes performed consistently in both two environments. N-responder, N-nonresponder, N-efficient and N-inefficient genotypes were identified from these genotypes with consistent NUtE. The correlations between the pot experiment and the field trial in NUtE were only 0.34 at high N supplies and no significant correlations were found at low N supplies. However, Pearson coefficient correlation (r) and principal component analysis showed NUtE had similar genetic correlations with other traits across the pot and field experiment. Among the yield components, only seeds per silique showed strong and positive correlations with NUtE under varying N supply in both experiments (r = 0.47**; 0.49**; 0.47**; 0.54**). At high and low N supply, NUtE was positively correlated with seed yield (r = 0.45**; 0.53**; 0.39**; 0.87**), nitrogen harvest index (NHI, r = 0.68**; 0.82**; 0.99**; 0.89**), and harvest index (HI, r = 0.79**; 0.83**; 0.90**; 0.78**) and negatively correlated with biomass distribution to stem and leaf (r = −0.34**; −0.45**; −0.37**; 0.62**), all aboveground plant section N concentration (r from −0.30* to −0.80**), N distribution to the vegetative parts (silique husk, stem and leaf) (r from −0.40** to −0.83**). N-efficient (N-responder) genotypes produced more seeds per silique and had significantly higher NHI and HI than did N-inefficient (N-nonresponder) genotypes. In conclusion, across the pot and field experiments, the 50 genotypes had similar underlying traits correlated with NUtE and seeds per silique may be a good indicator of NUtE.

The role of glutamine synthetase isozymes in enhancing nitrogen use efficiency of N-efficient winter wheat

Glutamine synthetase (GS) isozymes play critical roles in nitrogen (N) metabolism. However, the exact relationship between GS and nitrogen use efficiency (NUE) remain unclear. We have selected and compared two wheat cultivars, YM49 and XN509, which were identified as the N-efficient and N-inefficient genotypes, respectively. In this study, agronomical, morphological, physiological and biochemical approaches were performed. The results showed that TaGS1 was high expressed post-anthesis, and TaGS2 was highly expressed pre-anthesis in N-efficient genotype compared to N-inefficient genotype. GS1 and GS2 isozymes were also separated by native-PAGE and found that the spatial and temporal distribution of GS isozymes, their expression of gene and protein subunits in source-sink-flow organs during development periods triggered the pool strength and influenced the N flow. According to the physiological role of GS isozymes, we illustrated four metabolic regulation points, by which acting collaboratively in different organs, accelerating the transport of nutrients to the grain. It suggested that the regulation of GS isozymes may promote flow strength and enhance NUE by a complex C-N metabolic mechanism. The relative activity or amount of GS1 and GS2 isozymes could be a potential marker to predict and select wheat genotypes with enhanced NUE.

Expression of nitrogen transporter genes in roots of winter wheat (Triticum aestivum L.) in response to soil drought with contrasting nitrogen supplies

The expression of nitrate and ammonium transporter genes in the roots of winter wheat in response to drought stress is largely unknown. A greenhouse experiment was established to study the expression of five putative nitrate transporter (NRT) genes (TaNRT2.1, TaNRT2.2, TaNRT2.3, TaNRT1.1, TaNRT1.2) and three ammonium transporter (AMT) genes (TaAMT1.1, TaAMT1.2, TaAMT2.1) in the roots of winter wheat in response to soil drought under conditions of limited nitrogen (N) (no N added) and adequate N (addition of 0.3 g N kg–1 soil). Two wheat genotypes with low and high N-uptake efficiencies were used, and water-stress treatments were applied at the vegetative and reproductive growth stages of wheat. Expression of all of the genes was quantified using real-time reverse transcription PCR. The results indicated that wheat plants growing in the N-adequate soil were more sensitive to drought stress than those growing in the N-limited soil. The response of the expression of the NRT and AMT genes to soil drought largely depends on N application, wheat genotype and growth stage. The expression of the two low-affinity NRT genes (i.e. TaNRT1.1 and TaNRT1.2) in the N-inefficient genotype XY6 was mainly induced by drought stress, but the expression of the two genes in the N-efficient genotype XY107 was repressed by drought stress. The expression of the high-affinity NRT gene TaNRT2.1 was repressed by drought stress, but the expression of the other two high-affinity NRT genes, TaNRT2.2 and TaNRT2.3, was induced or repressed by soil drought depending on N application and growth stage. The expression of the genes TaAMT1.1 and TaAMT2.1 was mainly repressed by drought stress, whereas the expression of the gene TaAMT1.2 was induced by drought stress. The expression of TaNRT2.1 in XY107 was significantly higher than in XY6.

In-season expression of nitrate and ammonium transporter genes in roots of winter wheat (Triticum aestivum L.) genotypes with different nitrogen-uptake efficiencies

Although nitrate and ammonium transporter genes of wheat have been cloned, little is known about their expression characteristics. A greenhouse experiment was established to study temporal expression patterns over a growing season for four nitrate-transporter genes (TaNRT2.1, TaNRT2.2, TaNRT2.3 and TaNRT1.2) and two ammonium-transporter genes (TaAMT1.1 and TaAMT1.2) in wheat genotypes with different nitrogen (N)-uptake efficiencies. Genotypes that were N-efficient (XY107) and N-inefficient (XY6) were planted in soils that were N-limited (no N added) and N-adequate (added 0.3 g N kg–1 soil). Roots were sampled at tillering, jointing, heading and grain-filling stages, and the expression of the six genes was quantified using real-time, reverse transcription PCR (polymerase chain reaction). Results indicated that maintaining active N uptake during reproduction was the main strategy used by genotype XY107 to sustain its high N-uptake efficiencies in both N treatments. The expression of all NRT and AMT genes showed significant temporal dynamics, and generally matched the pattern of in-season N uptake of wheat plants. Several NRT or AMT genes (especially TaNRT2.1) showed greater expression at reproduction in the N-efficient genotype, XY107, than in the N-inefficient genotype, XY6, suggesting that nitrate and ammonium transporters play important roles in determining the genotypic variation of N uptake in wheat.

Genotype differences in photosynthetic characteristics and nitrogen efficiency of new-type oilseed rape responding to low nitrogen stress

SUMMARYNew-type oilseed rape (Brassica napus, ArArCcCc) with introgressed exotic subgenomic components from Brassica rapa (ArAr) and Brassica carinata (BcBcCcCc) showed strong heterosis in both vegetative and reproductive growth. The aim of the current study was to analyse the tolerance of the new-type B. napus with different exotic subgenomic contents to low nitrogen (N) stress. Under hydroponic culture and pot experiments, root system parameters, photosynthetic parameters, relative chlorophyll concentration (SPAD values), biomass, seed yield, seed yield components, N concentration and expressions of genes involved in N transport and assimilation were determined with two new-type B. napus genotypes (N-efficient genotype D4-15 and N-inefficient genotype D1-1) under high-N and low-N levels. Furthermore, N accumulation, N transfer efficiency and N use efficiency (NUE) were analysed in the two genotypes. The hydroponic and potted growth tests showed consistent characteristics in N uptake and utilization efficiency at the seedling stage, and N-efficient genotype (D4-15) showed better growth phenotypes across cultured conditions and N levels. Under the low-N condition, D4-15 produced a larger root system and accumulated more N, and had higher N transfer efficiency and NUE than D1-1. Moreover, D4-15 had significantly higher photosynthetic parameters, photosynthetic NUE and expression levels of the N transporter genes, BnNRT1·1, BnNRT2·5, BnNRT2·7 and BnAMT1·1, in roots or leaves, as well as higher seed yield than that of D1-1 under low-N supply. These results indicated that the N-efficient new-type B. napus D4-15 possessed excellent adaptability to low-N stress, which may be attributed to the highly introgressed exotic subgenomic components from B. rapa and B. carinata, suggesting the possibility of identifying high-nutrient-efficiency germplasm from inter-specific hybrids.

Differential response of wheat genotypes to applied nitrogen: biochemical and molecular analysis

Nitrogen-efficient and nitrogen-inefficient wheat genotypes were identified on the basis of the differential response of 16 wheat genotypes with low (1 mM) and high (25 mM) nitrogen (N) supply. Growth performance, measured in terms of fresh weight, dry weight and length of root and shoot, was higher in N-efficient than N-inefficient wheat genotypes at low N levels. Interestingly, although the growth of N-efficient genotypes did not show any change with increasing level of N supply, there was a marked increase in the growth of N-inefficient genotypes with the increase in N level. To work out the basis of this differential response of wheat genotypes to low N levels, we investigated the nitrate uptake rate of root and activities of nitrate assimilatory enzymes in the leaves of N-efficient and N-inefficient wheat genotypes. Nitrate uptake kinetics of these genotypes revealed that the uptake of nitrate in N-efficient genotypes was mediated by high-affinity nitrate transporter systems, whereas those of N-inefficient genotypes was mediated by low-affinity nitrate transporter systems. Study of the activities and expression levels of nitrate assimilatory enzymes in N-efficient and N-inefficient wheat genotypes showed that nitrate reductase (NR) and glutamine synthetase (GS) play important roles in N assimilation under low-nitrogen conditions.

ADAPTABILITY MECHANISM OF NITROGEN-EFFICIENT GERMPLASM OF NATURAL VARIATION TO LOW NITROGEN STRESS IN BRASSICA NAPUS

Previous studies showed that wide genotype differences in nitrogen (N) efficiency exists among cultivars of rapeseed (Brassica napus L.), but the mechanisms behind those differences are still unknown. In the present study, our aim was to analyze the adaptability mechanism of N-efficient rapeseed to low-N stress by employing two genotypes of natural variation in N efficiency. Nitrogen-efficient genotype, ‘BG51’, and N-inefficient genotype, ‘BG88’, were grown in a solution culture experiment under conditions of high-N (6.0 mM N) and low-N (0.6 mM N) supply. After growing 30 d, roots and shoots were sampled for the analysis of dry weight, N concentration and accumulation, N use efficiency (NUE), N transport efficiency (NTE), root system vigor parameters, nitrate redutase (NR) activity, and glutamine synthetase (GS) activity. Nitrogen deficiency decreased shoot and root dry weight significantly, but ‘BG51’ exhibited a significantly lower decrease in shoot dry weight and had significantly higher biomass production than ‘BG88’. Under low N supply ‘BG51’ accumulated more N in shoot, root and whole plant than ‘BG88’, and presented higher NUE in both shoot and root. Low-N stress induced an increase in maximum root length by 28.3% for ‘BG88’ and 55.1% for ‘BG51’ compared with the high-N treatment. And ‘BG51’ presented larger root volume, higher root vigor, larger root total absorbing area and root active absorbing area than ‘BG88’ in low-N treatment. Furthermore, ‘BG51’ had significantly higher NR and GS activity in both leaf and root in low N treatment than ‘BG88’, while there was no evident difference between them in high N treatment. These results suggested that N-efficient rapeseed germplasm of natural variation involves an integrated adaptability mechanism responding to low-N stress. Namely, N-efficient genotype could form more developed root system to accumulate more N, and presented efficient N assimilation by higher NR activity and GS activity than N-inefficient genotype. These ultimately resulted in high tolerance of N-efficient genotype to low-N stress and high biomass production.

Photosynthetic Characteristics of Flag Leaf in Rice Genotypes with Different Nitrogen Use Efficiency

To identify the photosynthetic mechanism of nitrogen (N) absorption and utilization in different rice ( Oryza sativa L.) genotypes, relationships between photosynthetic characteristics of flag leaf and N use efficiency were investigated in 6 N-efficient and 6 N-inefficient rice genotypes with variable N use efficiency. The abilities of seed setting of the genotypes were also analyzed. The N application rate was 225 kg ha −1 for all genotypes. During grain filling, chlorophyll content, N content, net photosynthetic rate, photosynthetic function duration, maximum photochemical efficiency ( F v/ F m), potential photochemical efficiency of photosystem II ( F v/ F o), actual photochemical efficiency of photosystem II ( Φ PSII), photochemical quenching coefficient ( q P), and nonphotochemical quenching coefficient ( q N) of flag leaf were obviously higher in the N-efficient genotypes than those in the N-inefficient genotypes. All these indexes were positively correlated with N use efficiency and seed-setting ability. Compared with the N-inefficient genotypes, the N-efficient genotypes had more favorable characteristics of photosynthesis and longer photosynthetic function duration. Besides, their PSII was steady for the large potential of converting light energy into electrochemical energy, and nonphotochemical quenching could also protect the photosynthetic apparatus. The accumulation of photosynthate and the seed-setting ability were enhanced in the N-efficient genotypes compared to the N-inefficient genotypes. Simultaneously, the N absorption and utilization of N efficient genotypes were improved through feedback regulation of roots and shoots.