Combining the critical nitrogen concentration and machine learning algorithms to estimate nitrogen deficiency in rice from UAV hyperspectral data

Rapid and large area acquisition of nitrogen(N)deficiency status is important for achieving the optimal fertilization of rice.Most existing studies,however,focus on the use of unmanned aerial vehicle(UAV)remote sensing to diagnose N nutrition in rice,while there are fewer studies on the quantitative description of the degree of N deficiency in rice,and the effects of the critical N concentration on the spectral changes in rice have rarely been explored.Therefore,based on the canopy spectral data obtained by remotely-sensed UAV hyperspectral images,the N content in rice was obtained through field sampling.The construction method of the rice curve for the northeastern critical N concentration was studied,and on this basis,N deficiency was determined.Taking the spectrum of the critical N concentration state as the standard spectrum,the spectral reflectivity data were transformed by the ratios and differences,and the feature extraction of the spectral data was carried out by the successive projections algorithm(SPA).Finally,by taking the characteristic band as the input variable and N deficiency as the output variable,a set of multivariate linear regression(MLR),long short-term memory(LSTM)inversion models based on extreme learning machine(ELM),and the nondominated sorting genetic algorithmⅢextreme learning machine(NSGA-Ⅲ-ELM)were constructed.The results showed two key aspects of this system:1)The correlation between the N deficiency data and original spectrum was poor,but the correlation between the N deficiency data and N deficiency could be improved by a difference change and ratio transformation;2)The inversion results based on the ratio spectrum and NSGA-Ⅲ-ELM algorithm were the best,as the R2values of the training set and validation set were 0.852 and 0.810,and the root mean square error(RMSE)values were 0.291 and 0.308,respectively.From the perspective of the spectral data,the inversion accuracy of the ratio spectrum was better than the accuracy of the original spectrum or difference spectrum.At the algorithm level,the model inversion results based on LSTM algorithms showed a serious overfitting phenomenon and poor inversion effect.The inversion accuracy based on the NSGA-Ⅲ-ELM algorithm was better than the accuracy of the MLR algorithm or the ELM algorithm.Therefore,the inversion model based on the ratio spectrum and NSGA-Ⅲ-ELM algorithm could effectively invert the N deficiency in rice and provide critical technical support for accurate topdressing based on the N status in the rice.

Comparative Proteomic Analysis of Cucumber Fruits Under Nitrogen Deficiency At the Fruiting Stage

Nitrogen deficiency is a major factor that affects the yield and quality of horticultural crops.Differentially expressed proteins were identified in cucumber fruit under nitrogen deficiency treatment,and are expected to be highly useful for dissecting carbon,nitrogen and ascorbate metabolism in cucumber and for improving nitrogen fertilizer utilization.Cucumber plants grown hydroponically in Hoagland solution were transferred to nitrogen-free conditions for 3 or 5 d after the blossom of female flowers.Fruit proteome profiles were assessed using twodimensional electrophoresis.The relative expression of the genes encoding 50 selected differentially expressed proteins and 4 key enzymes in carbon metabolism was analyzed by qRT-PCR.Resulted showed that most of the identified differentially expressed proteins were mainly associated with carbon metabolism,amino acid biosynthesis,ascorbate metabolism,and proteasomes.Glucose phosphorylation was enhanced,while the pentose phosphate pathway was inhibited.Carbon metabolism and the synthesis of most amino acids was affected significantly,and ascorbate might be accumulated under nitrogen deficiency in cucumber fruit.

Combining nitrogen effects and metabolomics to reveal the response mechanisms to nitrogen stress and the potential for nitrogen reduction in maize

The physiological and metabolic differences in maize under different nitrogen(N)levels are the basis of reasonable N management,which is vital in improving fertilizer utilization and reducing environmental pollution.In this paper,on the premise of defining the N fertilizer efficiency and yield under different long-term N fertilization treatments,the corresponding differential metabolites and their metabolic pathways were analyzed by untargeted metabolomics in maize.N stress,including deficiency and excess,affects the balance of carbon(C)metabolism and N metabolism by regulating C metabolites(sugar alcohols and tricarboxylic acid(TCA)cycle intermediates)and N metabolites(various amino acids and their derivatives).L-alanine,L-phenylalanine,L-histidine,and L-glutamine decreased under N deficiency,and L-valine,proline,and L-histidine increased under N excess.In addition to sugar alcohols and the above amino acids in C and N metabolism,differential secondary metabolites,flavonoids(e.g.,kaempferol,luteolin,rutin,and diosmetin),and hormones(e.g.,indoleacetic acid,trans-zeatin,and jasmonic acid)were initially considered as indicators for N stress diagnosis under this experimental conditions.This study also indicated that the leaf metabolic levels of N2(120 kg ha–1 N)and N3(180 kg ha–1 N)were similar,consistent with the differences in their physiological indexes and yields over 12 years.This study verified the feasibility of reducing N fertilization from 180 kg ha–1(locally recommended)to 120 kg ha–1 at the metabolic level,which provided a mechanistic basis for reducing N fertilization without reducing yield,further improving the N utilization rate and protecting the ecological environment.

Postponed and reduced basal nitrogen application improves nitrogen use efficiency and plant growth of winter wheat

Excessive nitrogen(N) fertilization with a high basal N ratio in wheat can result in lower N use efficiency(NUE) and has led to environmental problems in the Yangtze River Basin, China. However, wheat requires less N fertilizer at seedling growth stage, and its basal N fertilizer utilization efficiency is relatively low; therefore, reducing the N application rate at the seedling stage and postponing the N fertilization period may be effective for reducing N application and increasing wheat yield and NUE. A 4-year field experiment was conducted with two cultivars under four N rates(240 kg N ha–1(N240), 180 kg N ha–1(N180), 150 kg N ha–1(N150), and 0 kg N ha–1(N0)) and three basal N application stages(seeding(L0), fourleaf stage(L4), and six-leaf stage(L6)) to investigate the effects of reducing the basal N application rate and postponing the basal N fertilization period on grain yield, NUE, and N balance in a soil-wheat system. There was no significant difference in grain yield between the N180 L4 and N240 L0(control) treatments, and the maximum N recovery efficiency and N agronomy efficiency were observed in the N180 L4 treatment. Grain yield and NUE were the highest in the L4 treatment. The leaf area index, flag leaf photosynthesis rate, flag leaf nitrate reductase and glutamine synthase activities, dry matter accumulation, and N uptake post-jointing under N180 L4 did not differ significantly from those under N240 L0. Reduced N application decreased the inorganic N content in the 0–60-cm soil layer, and the inorganic N content of the L6 treatment was higher than those of the L0 and L4 treatments at the same N level. Surplus N was low under the reduced N rates and delayed basal N application treatments. Therefore, postponing and reducing basal N fertilization could maintain a high yield and improve NUE by improving the photosynthetic production capacity, promoting N uptake and assimilation, and reducing surplus N in soil-wheat systems.

Responses in growth, lipid accumulation,and fatty acid composition of four oleaginous microalgae to different nitrogen sources and concentrations

Nitrogen deficiency is an effective strategy for enhancing lipid production in microalgae. Close relationships exist among lipid production, microalgal species, and nitrogen sources. We report growth, lipid accumulation, and fatty acid composition in four microalgae （Chloroeoccum ellipsoideum UTEX972, Chlorococcum nivale LB2225, Chlorococcum tatrense UTEX2227, and Scenedesmus deserticola JNU19） under nitrate- and urea-nitrogen deficiencies. We found three patterns of response to nitrogen deficiency： Type-A （decrease in biomass and increase in lipid content）, Type-B （reduction in both biomass and lipid content）, and Type-C （enhancement of both biomass and lipid content）. Type-C microalgae are potential candidates for large-scale oil production. Chlorococcum ellipsoideum, for example, exhibited a neutral lipid production of up to 239.6 mg/（L＇d） under urea-nitrogen deficiency. In addition, nitrogen deficiency showed only a slight influence on lipid fractions and fatty acid composition. Our study provides useful information for further screening hyper-lipid microalgal strains for biofuel production.

Fertilizer-Induced Advances in Corn Growth Stage and Quantitative Definitions of Nitrogen Deficiencies

Evidence that nitrogen (N) fertilization tends to accelerate maturation as well as increase rates of growth has received little attention when diagnosing N deficiencies in corn (Zea mays L.).Such a tendency could be a potential source of errors when the diagnosis is solely based on comparing plants with different rates of growth.Whether N fertilization could accelerate rates of growth and maturation was tested in a field study with 12 paired plots representing relatively large variability in soil properties and landscape positions.The plots were located under conditions where preplant N fertilization reduced or avoided temporary N shortages for some plants but did not reduce for other plants early in the season.We measured corn heights to the youngest leaf collar,stages of growth and chlorophyll meter readings (CMRs). The added N advanced growth stages as well as increased corn heights and CMRs at any given time.Fertilization effects on corn heights,growth stages and ear weights were statistically significant (P<0.05) despite substantial variability associated with landscape.Reductions in growth due to a temporary shortage of N within a growth stage might be partially offset by longer periods of growth within that stage to physiological maturity.Temporary shortages of N,therefore,may produce symptoms of N deficiency in situations where subsequent additions of N should not be expected to increase yields.Recognition of these two somewhat different effects (i.e.,increase growth rates and advance growth stages) on corn growth could help to define N deficiency more precisely and to improve the accuracy of diagnosing N status in production agriculture.

The mitigation effects of exogenous dopamine on low nitrogen stress in Malus hupehensis

Dopamine plays numerous physiological roles in plants.We explored its role in the regulation of growth,nutrient absorption,and response to nitrogen(N)deficiency in Malus hupehensis Rehd.Under low N condition,plant growth slowed,and the net photosynthetic rates,chlorophyll contents,and maximal quantum yield of PSII(Fv/Fm)decreased significantly.However,the application of 100μmol L−1 exogenous dopamine significantly reduced the inhibition of low N stress on plant growth.In addition to modifying root system architecture under low N supply,exogenous dopamine also changed the uptake,transport,and distribution of N,P,and K.Furthermore,exogenous dopamine enhances the tolerance to low nitrogen stress by increasing the activity of enzymes(nitrate reductase,nitrite reductase,glutamic acid synthase and glutamine synthetase)involved in N metabolism.We also found that exogenous dopamine promoted the expression of ethylene signaling genes(ERF1,ERF2,EIL1,ERS2,ETR1,and EIN4)under low N stress.Therefore,we hypothesized that ethylene might be involved in dopamine response to low N stress in M.hupehensis.Our results suggest that exogenous dopamine can mitigate low N stress by regulating the absorption of mineral nutrients,possibly through the regulation of the ethylene signaling pathway.

Determination of Internal Controls for Quantitative Gene Expression of Isochrysis zhangjiangensis at Nitrogen Stress Condition

lsochrysis zhangfiangensis is a potential marine microalga for biodiesel production, which accumulates lipid under ni- trogen limitation conditions, but the mechanism on molecular level is veiled. Quantitative real-time polymerase chain reaction （qPCR） provides the possibility to investigate the gene expression levels, and a valid reference for data normalization is an essential prerequisite for firing up the analysis. In this study, five housekeeping genes, actin （ACT）, α-tubulin （TUA）, β-tubulin （TUB）, ubiquitin （UBI）, 18S rRNA （18S） and one target gene, diacylglycerol acyltransferase （DGAT）, were used for determining the reference. By analyzing the stabilities based on calculation of the stability index and on operating the two types of software, geNorm and bestkeeper, it showed that the reference genes widely used in higher plant and microalgae, such as UBI, TUA and 18S, were not the most stable ones in nitrogen-stressed 1. zhangjiangensis, and thus are not suitable for exploring the mRNA expression levels under these experi- mental conditions. Our results show that ACT together with TUB is the most feasible internal control for investigating gene expres- sion under nitrogen-stressed conditions. Our findings will contribute not only to future qPCR studies of/. zhangfiangensis, but also to verification of comparative transcriptomics studies of the microalgae under similar conditions.

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(750μM NH_4 NO_3)and low nitrogen(5μM NH_4 NO_3)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.

Differences in Physiological Age Affect Diagnosis of Nitrogen Deficiencies in Cornfields

Many studies have shown that chlorophyll meter readings （CMRs） can be used to diagnose deficiencies of nitrogen （N） during the growth of corn （Zea mays L.） in small-plot trials, but there is need to address additional problems encountered when diagnoses are made in fields of the size managed in production agriculture. A noteworthy difference between smallplot trials and production agriculture is the extent to which the effects of N are confounded with the effects of other factors such as tillage, landscape, soil organic matter and moisture content. We illustrate how some of these factors can cause differences in the physiological age of plants and introduce errors in the diagnoses of N deficiencies. We suggest methods （measuring the height to the youngest leaf collar and assigning leaf numbers by using the first leaf with pubescence and the ear leaf as references to identify growth stages） for minimizing these errors. The simplified method of growth stage identification can be used to select appropriate plants and leaves for making diagnoses in fields and to distinguish the effects of N from the effects of other factors that influence plant growth.

Benzyl Amino Purine and Gibberellic Acid Coupled to Nitrogen-Limited Stress Induce Fatty Acids,Biomass Accumulation,and Gene Expression in Scenedesmus Obliquus

The need for renewable energy sources makes microalgae an essential feedstock for biofuels production.The molecular aspects and the response to nitrogen(N)-limited conditions with a phytohormone stimulus in microalgae have been slightly explored.In this work,Scenedesmus obliquus was used as a study model to analyze the effect of benzyl amino purine(BAP)and gibberellic acid(GA)coupled to nitrogen limitation on cell growth,biomass and fatty acids.The selected 10^(-5)M BAP increased the biomass by 1.44-fold,and 10^(-6)M GA by 1.35-fold.The total lipids also increased by 2.8 and 1.11-fold,respectively.The 10^(-5)M BAP and 10^(-6)M GA addition to S.obliquus cultures at different initial nitrogen percentages(N-0,N-25,and N-50)showed a significant increase in cell growth and biomass productivity compared to the unstimulated cultures.BAP N-0 and GA N-0 produced the highest lipid yields with 55%and 50%,respectively.The lipid profile analysis revealed an increase,particularly in C18:1 and C16:0 fatty acids.Gene expression analysis showed an over-expression of acyl carrying protein(ACP),stearoyl-ACP desaturase(SAD),fatty acid acyl-ACP thioesterase(FATA),and diacylglycerol acyltransferase(DGAT)genes,which were mainly induced by nitrogen limitation.Furthermore,BAP and GA produced a significant over-expression on these genes in the N-replete cultures.This study shows that BAP and GA,coupled to N limitation stress,can be used to increase the biomass and lipid production in S.obliquus for sustainable biofuels.

Growth and Carotenoid Production in Dunafiella spp. Isolated from Hypersaline Habitats in the Region of Essaouira （Morocco）： Effect of NaNO3 Concentration

In order to determine the impact of nitrogen deficiency in medium, growth rate and carotenoids contents were followed during 15 days in two strain Dunaliella spp. （DUN2 and DUN3）, isolated respectively from Azla and Idao Iaaza saltworks in the Essaouira region （Morocco）. These microalgae were incubated at 25 ± 1 ℃ with a salinity of 35960 and continuous light in four growth media with different concentrations of sodium nitrate （NaNO3）： 18.75 g/L, 2.5 g/L, 37.5 g/L and 75 g/L. Maximum of cell density was observed under high sodium nitrate concentration during logarithmic phase of growth. The highest specific growth rate was 0.450 × 10^6 ± 0.006 cells/mL and 2.680 × 10^6 ± 0.216 cells/mL, respectively for DUN2 and DUN3. Carotenoids production mean were not stimulated under nitrogen deficiency, and the highest content was showed in DUN2 at high nitrogen concentration （3.210 ± 0.261 μg·mL^-1） compared with DUN3 strain.

Abscisic acid signaling negatively regulates nitrate uptake via phosphorylation of NRT1.1 by SnRK2s in Arabidopsis

Nitrogen(N)is a limiting nutrient for plant growth and productivity.The phytohormone abscisic acid(ABA)has been suggested to play a vital role in nitrate uptake in fluctuating N environments.However,the molecular mechanisms underlying the involvement of ABA in N deficiency responses are largely unknown.In this study,we demonstrated that ABA signaling components,particularly the three subclass Ⅲ SUCROSE NON-FERMENTING1(SNF1)-RELATED PROTEIN KINASE 2 S(SnRK2)proteins,function in root foraging and uptake of nitrate under N deficiency in Arabidopsis thaliana.The snrk2.2 snrk2.3 snrk2.6 triple mutant grew a longer primary root and had a higher rate of nitrate influx and accumulation compared with wild-type plants under nitrate deficiency.Strikingly,SnRK2.2/2.3/2.6 proteins interacted with and phosphorylated the nitrate transceptor NITRATE TRANSPORTER1.1(NRT1.1)in vitro and in vivo.The phosphorylation of NRT1.1 by SnRK2 s resulted in a significant decrease of nitrate uptake and impairment of root growth.Moreover,we identified NRT1.1Ser585 as a previously unknown functional site:the phosphomimetic NRT1.1S585 D was impaired in both low-and high-affinity transport activities.Taken together,our findings provide new insight into how plants fine-tune growth via ABA signaling under N deficiency.