low-N Conditions Research Articles

Combining ability for grain yield and low-N tolerance of intermediate/late maturing maize (Zea mays) inbred lines

ABSTRACT Low soil nitrogen (N) is a significant challenge to maize (Zea mays L.) production in Sub-Saharan Africa, particularly for small-scale farmers. Developing maize hybrids tolerant to low N is crucial for improving yields under such conditions. This study examined the genetic basis of agronomic traits in intermediate/late maturing maize inbred lines under contrasting N conditions, focusing on general combining ability (GCA) and specific combining ability (SCA) effects. Two hundred and thirty-seven F1 hybrids from 79 inbred lines crossed with three testers in a line x tester mating design, were evaluated across eight environments in Nigeria. Line x tester analysis revealed that GCA effects, primarily from lines, predominantly influenced grain yield and other traits across both N conditions, except for ear height and plant aspect under low N, where SCA effects also had significant impact. A significant (p ≤ 0.01) positive correlation was observed between GCA and SCA effects, suggesting a positive relationship between additive and non-additive genetic components. Lines SMLW146 and SMLW147 were identified as valuable resources for breeding low-N tolerant hybrids. Additionally, considering non-additive effects, hybrids SMLW22 × SAM50M and SMLW106 × SAM50M (under low-N) and SMLW3 × IITA1876, SMLW84 × IITA1876, and SMLW165 × IITA1876 (under optimum-N) were promising combinations for high grain yield. Morphological traits like plant height and ear height, phenological traits like days to anthesis and silking, and qualitative traits like ear aspect and plant aspect were identified as important for selecting genotypes with improved grain yield under low-N conditions.

Validation and identification of new QTLs for plant and fruit developmental and composition traits in eggplant under low N conditions

Enhancing plant adaptation to low input conditions is a fundamental goal for implementing sustainable agriculture. In the present study, two eggplant (Solanum melongena) accessions (MEL1 and MEL5), two introgression lines (ILs) derived from eggplant wild relatives S. dasyphyllum (IL-M1-D1) and S. insanum (IL-M5-I9), and a heterozygous version of this last IL (ILHet-M5-I9), along with hybrids among them were evaluated under low N (LN) conditions. IL-M1-D1 carries an introgressed fragment of 4.9 Mb in homozygosis from S. dasyphyllum on chromosome 2, while IL-M5-I9 and ILHet-M5-I9 carry an introgression of 21.5 Mb on chromosome 9 in homozygosis and heterozygosis, respectively, from S. insanum. Multiple quantitative trait loci (QTLs) for several traits of interest were associated with both introgressions under LN conditions in a previous study with segregating advanced backcrosses. Here we evaluated the performance of these materials for 22 agronomic and developmental traits under low N fertilization (LN) conditions. Hybrids with the ILs enabled the study of genetic background effects on QTLs expression. The materials evaluated showed a significant phenotypic variation, particularly within hybrids segregating for the introgression from S. insanum in chromosome 9. Statistical analysis revealed no significant differences among hybrids carrying or not the introgression on chromosome 2 of S. dasyphyllum, and only slight differences were observed between the IL-M1-D1 and its recurrent parent S. melongena MEL1, suggesting a limited impact of this introgression on chromosome 2 on the phenotype variation. However, the differences observed between IL-M5-I9 and its recurrent parent S. melongena MEL5, together with the association between genotypic and phenotypic variation in hybrids segregating for this introgression, allowed the identification of 13 QTLs on chromosome 9. These results successfully validated the previously identified QTLs for flavonol content in leaves, nitrogen balanced index, fruit mean weight, and nitrogen content in leaves and, also revealed nine new QTLs associated with the introgressed genomic region in chromosome 9. This study emphasizes the influence of environmental conditions, genotypes, and genetic backgrounds on the phenotypic expression of eggplant QTLs introgressed from wild relatives and highlights the importance of QTL validation. These findings contribute valuable insights for developing new eggplant cultivars for a more sustainable agriculture, particularly with adaptation to LN conditions.

MicroRNAs Participate in Morphological Acclimation of Sugar Beet Roots to Nitrogen Deficiency.

Nitrogen (N) is essential for sugar beet (Beta vulgaris L.), a highly N-demanding sugar crop. This study investigated the morphological, subcellular, and microRNA-regulated responses of sugar beet roots to low N (LN) stress (0.5 mmol/L N) to better understand the N perception, uptake, and utilization in this species. The results showed that LN led to decreased dry weight of roots, N accumulation, and N dry matter production efficiency, along with damage to cell walls and membranes and a reduction in organelle numbers (particularly mitochondria). Meanwhile, there was an increase in root length (7.2%) and branch numbers (29.2%) and a decrease in root surface area (6.14%) and root volume (6.23%) in sugar beet after 7 d of LN exposure compared to the control (5 mmol/L N). Transcriptomics analysis was confirmed by qRT-PCR for 6 randomly selected microRNAs, and we identified 22 differentially expressed microRNAs (DEMs) in beet root under LN treatment. They were primarily enriched in functions related to binding (1125), ion binding (641), intracellular (437) and intracellular parts (428), and organelles (350) and associated with starch and sucrose metabolism, tyrosine metabolism, pyrimidine metabolism, amino sugar and nucleotide sugar metabolism, and isoquinoline alkaloid biosynthesis, as indicated by the GO and KEGG analyses. Among them, the upregulated miR156a, with conserved sequences, was identified as a key DEM that potentially targets and regulates squamosa promoter-binding-like proteins (SPLs, 104889216 and 104897537) through the microRNA-mRNA network. Overexpression of miR156a (MIR) promoted root growth in transgenic Arabidopsis, increasing the length, surface area, and volume. In contrast, silencing miR156a (STTM) had the opposite effect. Notably, the fresh root weight decreased by 45.6% in STTM lines, while it increased by 27.4% in MIR lines, compared to the wild type (WT). It can be inferred that microRNAs, especially miR156, play crucial roles in sugar beet root's development and acclimation to LN conditions. They likely facilitate active responses to N deficiency through network regulation, enabling beet roots to take up nutrients from the environment and sustain their vital life processes.

Molecular Diversity and Combining Ability in Newly Developed Maize Inbred Lines under Low-Nitrogen Conditions.

Nitrogen is an essential element for maize growth, but excessive application can lead to various environmental and ecological issues, including water pollution, air pollution, greenhouse gas emissions, and biodiversity loss. Hence, developing maize hybrids resilient to low-N conditions is vital for sustainable agriculture, particularly in nitrogen-deficient soils. Combining ability and genetic relationships among parental lines is crucial for breeding superior hybrids under diverse nitrogen levels. This study aimed to assess the genetic diversity of maize inbred lines using simple sequence repeat (SSR) markers and evaluate their combining ability to identify superior hybrids under low-N and recommended conditions. Local and exotic inbred lines were genotyped using SSR markers, revealing substantial genetic variation with high gene diversity (He = 0.60), moderate polymorphism information content (PIC = 0.54), and an average of 3.64 alleles per locus. Twenty-one F1 hybrids were generated through a diallel mating design using these diverse lines. These hybrids and a high yielding commercial check (SC-131) were field-tested under low-N and recommended N conditions. Significant variations (p < 0.01) were observed among nitrogen levels, hybrids, and their interaction for all recorded traits. Additive genetic variances predominated over non-additive genetic variances for grain yield and most traits. Inbred IL3 emerged as an effective combiner for developing early maturing genotypes with lower ear placement. Additionally, inbreds IL1, IL2, and IL3 showed promise as superior combiners for enhancing grain yield and related traits under both low-N and recommended conditions. Notably, hybrids IL1×IL4, IL2×IL5, IL2×IL6, and IL5×IL7 exhibited specific combining abilities for increasing grain yield and associated traits under low-N stress conditions. Furthermore, strong positive associations were identified between grain yield and specific traits like plant height, ear length, number of rows per ear, and number of kernels per row. Due to their straightforward measurability, these relationships underscore the potential of using these traits as proxies for indirect selection in early breeding generations, particularly under low-N stress. This research contributes to breeding nitrogen-efficient maize hybrids and advances our understanding of the genetic foundations for tolerance to nitrogen limitations.

Tryptophan regulates sorghum root growth and enhances low nitrogen tolerance

Over evolutionary time, plants have developed sophisticated regulatory mechanisms to adapt to fluctuating nitrogen (N) environments, ensuring that their growth is balanced with their responses to N stress. This study explored the potential of L-tryptophan (Trp) in regulating sorghum root growth under conditions of N limitation. Here, two distinct sorghum genotypes (low-N tolerance 398B and low-N sensitive CS3541) were utilized for investigating effect of low-N stress on root morphology and conducting a comparative transcriptomics analysis. Our foundings indicated that 398B exhibited longer roots, greater root dry weights, and a higher Trp content compared to CS3541 under low-N conditions. Furthermore, transcriptome analysis revealed substantial differences in gene expression profiles related to Trp pathway and carbon (C) and N metabolism pathways between the two genotypes. Additional experiments were conducted to assess the effects of exogenous Trp treatment on the interplay between sorghum root growth and low-N tolerance. Our observations showed that Trp-treated plants developed longer root and had elevated levels of Trp and IAA under low-N conditons. Concurrently, these plants demonstrated stronger physiological activities in C and N metabolism when subjected to low-N stress. These results underscored the pivotal role of Trp on root growth and low-N stress responses by balancing IAA levels and C and N metabolism. This study not only deepens our understanding of how plants maintain growth plasticity during environmental stress but also provides valuable insights into the availability of amino acid in crops, which could be instrumental in developing strategies for promoting crop resilience to N deficiency.

Low Nitrogen Input Mitigates Quantitative but Not Qualitative Reconfiguration of Leaf Primary Metabolism in Brassica napus L. Subjected to Drought and Rehydration.

In the context of climate change and the reduction of mineral nitrogen (N) inputs applied to the field, winter oilseed rape (WOSR) will have to cope with low-N conditions combined with water limitation periods. Since these stresses can significantly reduce seed yield and seed quality, maintaining WOSR productivity under a wide range of growth conditions represents a major goal for crop improvement. N metabolism plays a pivotal role during the metabolic acclimation to drought in Brassica species by supporting the accumulation of osmoprotective compounds and the source-to-sink remobilization of nutrients. Thus, N deficiency could have detrimental effects on the acclimation of WOSR to drought. Here, we took advantage of a previously established experiment to evaluate the metabolic acclimation of WOSR during 14 days of drought, followed by 8 days of rehydration under high- or low-N fertilization regimes. For this purpose, we selected three leaf ranks exhibiting contrasted sink/source status to perform absolute quantification of plant central metabolites. Besides the well-described accumulation of proline, we observed contrasted accumulations of some "respiratory" amino acids (branched-chain amino acids, lysineand tyrosine) in response to drought under high- and low-N conditions. Drought also induced an increase in sucrose content in sink leaves combined with a decrease in source leaves. N deficiency strongly decreased the levels of major amino acids and subsequently the metabolic response to drought. The drought-rehydration sequence identified proline, phenylalanine, and tryptophan as valuable metabolic indicators of WOSR water status for sink leaves. The results were discussed with respect to the metabolic origin of sucrose and some amino acids in sink leaves and the impact of drought on source-to-sink remobilization processes depending on N nutrition status. Overall, this study identified major metabolic signatures reflecting a similar response of oilseed rape to drought under low- and high-N conditions.

Strong Invasive Mechanism of Wedelia trilobata via Growth and Physiological Traits under Nitrogen Stress Condition

Nitrogen (N) is one of the most crucial elements for plant growth. However, a deficiency of N affects plant growth and development. Wedelia trilobata is a notorious invasive plant species that exhibits superior tolerance to adapt to environmental stresses. Yet, research on the growth and antioxidant defensive system of invasive Wedelia under low N stress, which could contribute to understanding invasion mechanisms, is still limited. Therefore, this study aims to investigate and compare the tolerance capability of invasive and native Wedelia under low and normal N conditions. Native and invasive Wedelia species were grown in normal and low-N conditions using a hydroponic nutrient solution for 8 weeks to assess the photosynthetic parameters, antioxidant activity, and localization of reactive oxygen species (ROS). The growth and biomass of W. trilobata were significantly (p &lt; 0.05) higher than W. chinensis under low N. The leaves of W. trilobata resulted in a significant increase in chlorophyll a, chlorophyll b, and total chlorophyll content by 40.2, 56.2, and 46%, respectively, compared with W. chinensis. W. trilobata significantly enhanced antioxidant defense systems through catalase, peroxidase, and superoxide dismutase by 18.6%, 20%, and 36.3%, respectively, providing a positive response to oxidative stress caused by low N. The PCA analysis showed that W. trilobata was 95.3% correlated with physiological traits by Dim1 (79.1%) and Dim2 (16.3%). This study provides positive feedback on W. trilobata with respect to its comprehensive invasion mechanism to improve agricultural systems via eco-friendly approaches in N deficit conditions, thereby contributing to the reclamation of barren land.

Single-cell transcriptome analyses reveal cellular and molecular responses to low nitrogen in burley tobacco leaves.

Tobacco (Nicotiana tabacum) is cultivated and consumed worldwide. It requires great amounts of nitrogen (N) to achieve the best yield and quality. With a view to sustainable and environmentally friendly agriculture, developing new genotypes with high productivity under low N conditions is an important approach. It is unclear how genes in tobacco are expressed at the cellular level and the precise mechanisms by which cells respond to environmental stress, especially in the case of low N. Here, we characterized the transcriptomes in tobacco leaves grown in normal and low-N conditions by performing scRNA-seq. We identified 10 cell types with 17 transcriptionally distinct cell clusters with the assistance of marker genes and constructed the first single-cell atlas of tobacco leaves. Distinct gene expression patterns of cell clusters were observed under low-N conditions, and the mesophyll cells were the most important responsive cell type and displayed heterogene responses among its three subtypes. Pseudo-time trajectory analysis revealed low-N stress decelerates the differentiation towards mesophyll cells. In combination with scRNA-seq, WGCNA, and bulk RNA-seq results, we found that genes involved in porphyrin metabolism, nitrogen metabolism, carbon fixation, photosynthesis, and photosynthesis-antenna pathway play an essential role in response to low N. Moreover, we identified COL16, GATA24, MYB73, and GLK1 as key TFs in the regulation of N-responsive genes. Collectively, our findings are the first observation of the cellular and molecular responses of tobacco leaves under low N stress and lay the cornerstone for future tobacco scRNA-seq investigations.

Enhancement patterns of potassium on nitrogen transport and functional genes in cotton vary with nitrogen levels

The application of potassium (K) in conjunction with nitrogen (N) has been shown to enhance N use efficiency. However, there is still a need for further understanding of the optimal ratios and molecular regulatory mechanisms, particularly in soil-cotton systems. Here, a field trial was conducted, involving varying rates of N and K, alongside pot and hydroponic experiments. The objective was to assess the impact of N-K interaction on the absorption, transport and distribution of N in cotton. The results showed that K supply at 90 and 240 kg ha−1 had a beneficial impact on N uptake and distribution to both seed and lint, resulting in the highest N use efficiency ranging from 22% to 62% and yield improvements from 20% to 123%. The increase in stem and root diameters, rather than the quantify of xylem vessels and phloem sieve tubes, facilitated the uptake and transport of N due to the provision of K. At the molecular level, K supply upregulated the expression levels of genes encoding GhNRT2.1 transporter and GhSLAH3 channel in cotton roots to promote N uptake and GhNRT1.5/NPF7.3 genes to transport N to shoot under low-N conditions. However, under high-N conditions, K supply induced anion channel genes (GhSLAH4) of roots to promote N uptake and genes encoding GhNRT1.5/NPF7.3 and GhNRT1.8/NPF7.2 transporters to facilitate NO3- unloading from xylem to mesophyll cell in high-N plants. Furthermore, K supply resulted in the upregulation of gene expression for GhGS2 in leaves, while simultaneously downregulating the expression of GhNADH-GOGAT, GhGDH1 and GhGDH3 genes in high-N roots. The enzyme activities of nitrite reductase and glutamine synthetase increased and glutamate dehydrogenase decreased, but the concentration of NO3- and soluble protein exhibited a significant increase and free amino acid decreased in the shoots subsequent to K supply.

Comprehensive Evaluation and Physiological Response of Quinoa Genotypes to Low Nitrogen

The utilization of low-N tolerant and N-efficifent varieties offers significant benefits in terms of reducing the need for excessive N fertilizer input. Quinoa, a resilient crop for agroecological transition, possesses a wide genetic diversity, making it suitable for selecting genotypes that require less N fertilizer. In this study, the growth and physiological characteristics of nine quinoa genotypes were assessed to determine their low-N tolerance using the fuzzy membership function. Based on comprehensive evaluation indices, three genotypes were identified: low-N tolerant (BL22), intermediately tolerant (A29), and sensitive (G68). These genotypes were exposed to varying N concentrations, including normal (4 mM), low (0.8 mM), and deficient N (no N) conditions. The results indicate that low-N conditions altered root phenotype, with reduced biomass, total protein, and chlorophyll content; increased soluble sugar levels; and inhibited N-metabolizing enzymes (nitrate reductase, glutamine synthetase, glutamate synthase) and N uptake. Under low-N conditions, the tolerant genotype exhibited higher maximal efficiency of photosystem II (Fv/Fm), root vitality, and N content compared to the sensitive genotype. Interestingly, the sensitive genotype displayed elongated and thinner shoots and roots in response to low-N, suggesting that plant height and root length are unreliable indicators of low-N tolerance in quinoa. In contrast, shoot and root dry biomass, Fv/Fm, chlorophyll content, N-metabolizing enzymes, and N content proved to be reliable indicators of low-N tolerance during the early growth stage of quinoa. Overall, these findings highlight the potential of utilizing specific growth and physiological parameters as indicators for screening low-N tolerant quinoa genotypes, thereby reducing dependence on N fertilizers.

GmNLP7a inhibits soybean nodulation by interacting with GmNIN1a

Nitrogen (N) is an essential macronutrient for plant growth and productivity. Leguminous plants establish symbiotic relationships with nitrogen-fixing rhizobial bacteria to use atmospheric dinitrogen gas to meet high N demand under low-N conditions. Nodule formation and N fixation are energy-consuming processes and are inhibited by nitrate present in the environment. Previous studies in model leguminous plants characterized NIN-LIKE PROTEIN (NLP) proteins that mediate nitrate control of root nodule symbiosis, but the mechanism by which nitrate regulates soybean root nodules via NLP remains unclear. In the soybean genome we found four homologs of AtNLP7, named GmNLP7a–GmNLP7d. We showed that the expression of GmNLP7s is responsive to nitrate but not to rhizobial infection and localized GmNLP7a to the nucleus. Downregulation of GmNLP7s increased nodule number, and overexpression of GmNLP7a (GmNLP7aOE) reduced nodule number regardless of nitrate availability, suggesting a negative role for GmNLP7s in nodulation. Nitrogenase activity in the GmNLP7aOE line was comparable to that of the wild type, indicating that GmNLP7a does not affect mature nodule activity. Overexpression of GmNLP7a downregulated the expression of GmNIN1a and GmENOD40-1. GmNLP7a interacted with GmNIN1a via the PB1 domain. Our results reveal a new regulator of GmNLP7 in nodulation and a molecular mechanism by which nitrate affects nodule number in soybean.

Nitrogen-Induced Hysteresis in Grassland Biodiversity: A Theoretical Test of Litter-Mediated Mechanisms.

AbstractThe global rise in anthropogenic reactive nitrogen and the negative impacts of N deposition on terrestrial plant diversity are well documented. The R* theory of resource competition predicts reversible decreases in plant diversity in response to N loading. However, empirical evidence for the reversibility of N-induced biodiversity loss is mixed. In a long-term N-enrichment experiment in Minnesota, a low-diversity state that emerged during N addition has persisted for decades after additions ceased. Hypothesized mechanisms preventing recovery of biodiversity include nutrient recycling, insufficient external seed supply, and litter inhibition of plant growth. Here, we present an ordinary differential equation model that unifies these mechanisms, produces bistability at intermediate N inputs, and qualitatively matches the observed hysteresis at Cedar Creek. Key features of the model, including native species' growth advantage in low-N conditions and limitation by litter accumulation, generalize from Cedar Creek to North American grasslands. Our results suggest that effective biodiversity restoration in these systems may require management beyond reducing N inputs, such as burning, grazing, haying, and seed additions. By coupling resource competition with an additional interspecific inhibitory process, the model also illustrates a general mechanism for bistability and hysteresis that may occur in multiple ecosystem types.

Integrated physiological, transcriptomic, and metabolomic analyses reveal that low-nitrogen conditions improve the accumulation of flavonoids in snow chrysanthemum

Nitrogen (N) levels have significant effects on plant growth and chemical composition. Previous studies found that low-N treatment maintained the number of inflorescences and promoted flavonoid accumulation in snow chrysanthemum, but the underlying mechanism was not clear. To investigate the complex flavonoid biosynthesis pathway of snow chrysanthemum under low-N conditions, we compared low-N and normal-N groups across the four harvest stages. Low-N treatment caused early flowering in snow chrysanthemum and maintained the production of flowers. Low-N stress increased the carbon (C)/N ratio in flowers, thus significantly increasing the total flavonoid content in the flower bud stage, early flowering stage and full flowering stage. Moreover, analysis of the flavonoid metabolome showed that low-N treatment resulted in increased levels of kaempferol-3-O-(6''-p-coumaroyl)galactoside, quercetin-3-O-(2''-acetyl)glucuronide, 3,7-dimethylquercetagetin-(3′,4′,5,6-tetrahydroxy-3,7-dime-thoxyflavone), isotamarixin, apigenin-8-C-arabinoside, quercetin-5-O-glucuronide, and cyanidin-3-O-(6''-O-p-coumaroyl)glucoside, which further enhanced antioxidant activity. Combined flavonoid metabolism and transcriptomic data analysis indicated that PAL, 4CL, HCT, C3'H, CHS, DFR and ANR are key genes leading to the accumulation of flavonoids in snow chrysanthemum under low-N conditions. In addition, 6 transcription factors (especially MYB26c, Cluster-16181.96870) and 17 other regulators (mainly including transporter proteins) under low-N conditions are also involved in the regulation of flavonoid accumulation. This research provides a new perspective for studying the regulatory effect of N levels on the growth and flavonoid biosynthesis in snow chrysanthemum. Data availability statementThe data presented in this study are available in the article and supplementary materials.

Combining ability of extra-early maturing pro-vitamin A maize (Zea mays L.) inbred lines and performance of derived hybrids under Striga hermonthica infestation and low soil nitrogen.

Low soil nitrogen (low-N), Striga hermonthica infestation and vitamin A deficiency in normal endosperm maize are major challenges confronting maize production and nutrition of the people of sub-Saharan Africa (SSA). Development of pro-vitamin A (PVA) maize hybrids with combined resistance/tolerance to the two stress factors is crucial in mitigating the food insecurity and nutrition challenges resulting from low-N deficiency and Striga infestation. One hundred and fifty hybrids plus six hybrid checks were evaluated under low-N, Striga-infested and optimal conditions in Nigeria for two years. The study examined the combining ability of the PVA inbreds in contrasting environments, classified them into heterotic groups, examined the inter-relationships of grain yield and other agronomic traits of the inbreds in hybrid combinations and assessed the performance and stability of the PVA hybrids across environments. Additive gene action conditioned the inheritance of grain yield under Striga infestation and optimal conditions while non-additive gene action played the major role in the inheritance of grain yield under low-N. Hybrids TZEEIOR 217 × TZEEIOR 197 and TZEEIOR 245 × TZEEIOR 195 were the top-yielding under Striga and low-N conditions, respectively. Inbred lines were classified into three heterotic groups. Inbreds TZEEIOR 195 and TZEEIOR 221 were identified as testers while TZEEIOR 197 × TZEEIOR 82, TZEEIOR 132 × TZEEIOR 195 and TZEEIOR 205 × TZEEIOR 221 were identified as single-cross testers. Ear aspect had direct contribution to grain yield, justifying its inclusion in the multiple trait base index used for selection of Striga resistant and low-N tolerant maize genotypes in SSA. Hybrids TZEEIOR 195 × TZEEIOR 149, TZEEIOR 195 × TZEEIOR 150, TZEEIOR 245 × TZEEIOR 195, TZEEIOR 30 × TZEEIOR 150 and TZEEIOR 245 × TZEEIOR 30 were high-yielding and stable across test environments. These hybrids should be tested extensively in on-farm trials and commercialized to contribute to food security in SSA.

Estimating the genetic parameters of yield-related traits under different nitrogen conditions in maize.

Understanding the genetic basis responding to nitrogen (N) fertilization in crop production is a long-standing research topic in plant breeding and genetics. Albeit years of continuous efforts, the genetic architecture parameters, such as heritability, polygenicity, and mode of selection, underlying the N responses in maize remain largely unclear. In this study, about n = 230 maize inbred lines were phenotyped under high N (HN) and low N (LN) conditions for 2 consecutive years to obtain 6 yield-related traits. Heritability analyses suggested that traits highly responsive to N treatments were less heritable. Using publicly available SNP genotypes, the genome-wide association study (GWAS) was conducted to identify n = 237 and n = 130 trait-associated loci under HN and LN conditions, n = 164 for N-responsive (NR) traits, and n = 31 for genotype by N interaction (G × N). Furthermore, genome-wide complex trait Bayesian (GCTB) analysis, a method complementary to GWAS, was performed to estimate genetic parameters, including genetic polygenicity and the mode of selection (S). GCTB results suggested that the NR value of a yield component trait was highly polygenic and that 4 NR traits exhibited negative correlations between SNP effects and their minor allele frequencies (or the S value <0)-a pattern consistent with negative selection to purge deleterious alleles. This study reveals the complex genetic architecture underlying N responses for yield-related traits and provides candidate genetic loci for N resilient maize improvement.

Alfalfa and maize intercropping enhances soil nematode structure and food web complexity in low-nitrogen soils

Soil nematodes play key roles in soil ecosystem processes and functions. Although agricultural practices such as intercropping and nitrogen (N) fertilization are known to have a strong influence on soil nematodes, less is known about the responses of the soil nematode community to the combined effects of intercropping and N fertilization. This study was conducted to test whether the intercropping of maize and alfalfa can enhance the nematode community and quantified the effects of intercropping and N fertilization on nematode abundance, diversity, and metabolic footprints compared with soils under monoculture systems. An experiment involving maize and alfalfa was conducted in a 2 × 3 factorial completely randomized block design with four replications. The factors consisted of two N levels, namely, no N fertilization and N fertilization (225 kg N ha−1 yr−1), and three cropping modes: monoculture maize, monoculture alfalfa and maize intercropped with alfalfa. The results showed that, compared with the monoculture systems, intercropping increased total nematode and omnivore–predator abundances but reduced herbivore abundance. The total nematode biomass and metabolic footprint of omnivores and predators significantly increased in the intercropping treatments, regardless of N fertilization. The soil food web in the intercropping system without N fertilization was mature, structured, and enriched, as indicated by high maturity, structure and enrichment index values. However, all these effects decreased in the monoculture treatments, particularly after N fertilization. These results indicated that, compared with the monoculture systems, maize/alfalfa intercropping in low-N soil can improve the structural complexity of the soil food web. The observed complexity was attributed to the high soil moisture content, soil organic carbon and total N under the intercropping and low-N conditions. Therefore, this study provides a promising way to improve soil health and ecosystem stability, revealing that alfalfa intercropping in low-N soils can improve resource flow in the soil food web through the increase in total nematode biomass and the metabolic footprints of nematodes.

Crossbreeding Rootstocks Improve Nitrogen Efficiency of Grafted Watermelon by Inducing Leaf Physiological and Root Morphological Responses

This is the first hydroponic study that evaluated the role of the heterotic plant characters of crossbreeding progenies and accessions which were used as rootstock for watermelon (scion) to improve the nitrogen (N) efficiency of this crop by grafting. The target of the research was to evaluate if grafting could enhance the nitrogen efficiency of watermelon through examining the responses of heterotic plant characters of crossbreeding rootstocks in the shoot development at the agronomical stage, root developments at the morphological stage, and leaf growth at various physiological stages. A hydroponic experiment was conducted by using an aerated deep-water culture (DWC) system in a well-equipped growth chamber of Erciyes University’s Plant Physiology Laboratory located at Kayseri, Türkiye. A watermelon cultivar Crimson Tide (CT) was grafted onto watermelon cultivars of Calhoun Gray (CG), Charleston Gray (Cha. G), and accessions of PI 296341 and PI 271769, the crossbreed progenies of Calhoun Gray × PI 296341, Calhoun Gray × PI 271769, and Charleston Gray × PI 296341. Plants were grown in 8 L plastic containers filled continuously with aired stock nutrient solution under two nitrogen (N) doses (low dose N: 0.3 mM unit N, and high dose N: 3.0 mM unit N) in a completely randomized block design (RBD) which was replicated three times, for six weeks. The grafted plants usually showed a higher crop growth performance than the self-grafted control plants, illustrating that nitrogen efficiency was significantly enhanced with respect to rootstocks of crossbreed progenies under a low N dose and high N dose. The N efficiency of grafted watermelon (CT) was improved by the high manifestation of heterosis in some root morphological characters (vigor root development and active root mechanism) of some of the crossbreeding rootstocks (Calhoun Gray × PI 271769) particularly in low-N conditions. Additionally, some of the crossbreeding rootstocks (Charleston Gray × PI 296341) exhibited high heterosis, which led to improving the N efficiency of grafted watermelon (CT) by inducing leaf physiological responses under high N supply. This clearly indicated that heterosis plays a crucial role in exploiting the genetic diversity in the N efficiency of watermelon. Therefore, these heterotic plant traits may be vital for the selection and breeding of nitrogen-efficient rootstocks for both small-scale and large-scale commercial farming in the nearby future.

GWAS and Transcriptome Analysis Reveal Key Genes Affecting Root Growth under Low Nitrogen Supply in Maize.

Nitrogen (N) is one of the most important factors affecting crop production. Root morphology exhibits a high degree of plasticity to nitrogen deficiency. However, the mechanisms underlying the root foraging response under low-N conditions remain poorly understood. In this study, we analyzed 213 maize inbred lines using hydroponic systems and regarding their natural variations in 22 root traits and 6 shoot traits under normal (2 mM nitrate) and low-N (0 mM nitrate) conditions. Substantial phenotypic variations were detected for all traits. N deficiency increased the root length and decreased the root diameter and shoot related traits. A total of 297 significant marker-trait associations were identified by a genome-wide association study involving different N levels and the N response value. A total of 51 candidate genes with amino acid variations in coding regions or differentially expressed under low nitrogen conditions were identified. Furthermore, a candidate gene ZmNAC36 was resequenced in all tested lines. A total of 38 single nucleotide polymorphisms and 12 insertions and deletions were significantly associated with lateral root length of primary root, primary root length between 0 and 0.5 mm in diameter, primary root surface area, and total length of primary root under a low-N condition. These findings help us to improve our understanding of the genetic mechanism of root plasticity to N deficiency, and the identified loci and candidate genes will be useful for the genetic improvement of maize tolerance cultivars to N deficiency.

Root architecture and leaf photosynthesis traits and associations with nitrogen-use efficiency in landrace-derived lines in wheat

Root system architecture (RSA) is important in optimizing the use of nitrogen. High-throughput phenotyping techniques may be used to study root system architecture traits under controlled environments. A root phenotyping platform, consisting of germination paper-based pouch and wick coupled with image analysis, was used to characterize root seedling traits in 30 landrace-derived bread wheat genotypes and the bread wheat parent Paragon under hydroponic high N (HN) and low N (LN) conditions. In addition, two glasshouse experiments under HN and LN conditions were carried out to measure whole plant performance including flag-leaf photosynthetic rate, N uptake and biomass per plant for 13 wheat genotypes of which eight were common with those in the root phenotyping hydroponic experiment. There were significant differences in RSA traits between genotypes for seminal root number per plant, lateral root number per plant, seminal root length per plant and seminal root angle, with transgressive segregation for landrace-derived lines above the elite parental cultivar Paragon under HN and LN conditions. In the glasshouse experiments, genetic variation in flag-leaf photosynthesis rate was found in landrace-derived genotypes in the range 25.9–33.3 µmol m−2 s−1 under HN and in N uptake in the ranges 0.37–0.48 g N plant−1and 0.21–0.30 g plant−1 under HN and LN conditions, respectively (P < 0.05), with transgressive segregation above Paragon. Plant Nitrogen Nutrition Index also showed transgressive segregation in the landrace-derived lines above Paragon under HN and LN conditions. Greater maximum root depth and more lateral roots per plant in the hydroponic screen were each correlated with increased biomass per plant under LN conditions. Results from this study demonstrated genetic variation for seedling RSA traits in landrace-derived lines above the elite parental cultivar Paragon, which potentially could be utilized to improve N-use efficiency in breeding programmes.

The difference method is not necessarily reliable for comparing the nitrogen use efficiency of hybrid rice cultivars with different tolerance to low nitrogen conditions

The difference method is one of the most commonly used methods to measure crop nitrogen (N) use efficiency. This study aimed to test the reliability of using this method to evaluate the N use efficiency of low-N tolerant hybrid rice. Two years of N fertilization experiments were conducted to compare yield attributes and N use efficiency measured by the difference method between two hybrid rice cultivars with contrasting tolerance to low N conditions, i.e. Deyou 4727 (D4727) with higher tolerance and Y-liangyou 900 (Y900) with relatively lower tolerance. Results showed that D4727 produced 17–24% higher grain yield than Y900 under zero-N fertilization conditions, confirming the higher tolerance of D4727 to low N conditions. Higher grain weight, harvest index, and internal N use efficiency were responsible for the higher grain yield in D4727 than in Y900. However, recovery efficiency of N determined by the difference method was not consistently different between D4727 and Y900, and physiological N use efficiency (PEN) and agronomic N use efficiency (AEN) determined by the difference method were 31–68% lower in D4727 compared to Y900. The lower PEN and AEN in D4727 than in Y900 were mainly due to a higher grain yield in D4727 than in Y900 under unfertilized conditions. This study suggests that the difference method is not necessarily reliable for comparing the N use efficiency of rice cultivars with different tolerance to low-N conditions, and both crop performance under unfertilized conditions and N responses under fertilized conditions should be considered when using this method to evaluate the N use efficiency of rice.