Genotypic Variation in Cotton Genotypes for Phosphorus-Use Efficiency

Moderate nitrogen application facilitates Bt cotton growth and suppresses population expansion of aphids (Aphis gossypii) by altering plant physiological characteristics

Introduction

Excessive application of nitrogen fertilizer in cotton field causes soil and water pollution as well as significant increase of aphid population. Reasonable fertilization is an important approach to improve agricultural production efficiency and reduce agriculture-derived pollutions. This study was aimed to explore the effects of nitrogen fertilizer on the Bt cotton physiological characteristics and the growth and development of A. gossypii, a sap-sucking cotton pest.

Methods

Five different levels of Ca(NO3)2 (0.0 g/kg, 0.3 g/kg, 0.9 g/kg, 2.7 g/kg and 8.1 g/kg) were applied into vermiculite as nitrogen fertilizer in order to explore the effects of nitrogen fertilizer on the growth and development of Bt cotton and aphids.

Results

The results showed that the medium level of nitrogen fertilizer (0.9 g/kg) effectively facilitated the growth of Bt cotton plant and suppressed the population expansion of aphids, whereas high and extremely high nitrogen application (2.7 and 8.1 g/kg) significantly increased the population size of aphids. Both high and low nitrogen application benefited aphid growth in multiple aspects such as prolonging nymph period and adult lifespan, enhancing fecundity, and improving adult survival rate by elevating soluble sugar content in host Bt cotton plants. Cotton leaf Bt toxin content in medium nitrogen group (0.9 g/kg) was significantly higher than that in high (2.7 and 8.1 g/kg) and low (0.3 g/kg) nitrogen groups, but Bt toxin content in aphids was very low in all the nitrogen treatment groups, suggesting that medium level (0.9 g/kg) might be the optimal nitrogen fertilizer treatment level for promoting cotton seedling growth and inhibiting aphids.

Discussion

Overall, this study provides insight into trophic interaction among nitrogen fertilizer levels, Bt cotton, and cotton aphid, and reveals the multiple effects of nitrogen fertilizer levels on growth and development of cotton and aphids. Our findings will contribute to the optimization of the integrated management of Bt cotton and cotton aphids under nitrogen fertilization.

Optimal phosphorus management strategies to enhance crop productivity and soil phosphorus fertility in rapeseed-rice rotation

Optimal phosphorus (P) managements can improve the crop yield without reducing soil P supply capacity over the long term. In this study, the rapeseed-rice rotation experiments were conducted to evaluate the effect of five optimal P fertilizer managements, including the addition of RA (rooting agents), PSB (phosphate solubilizing bacteria), CMP (calcium and magnesium phosphate fertilizer), DP1 (starter P) and DP2 (foliar fertilizer) with the reduction of 40% (in the 1st rapeseed season) and 75% (in the 2nd rapeseed season) P fertilizers of farmers' fertilizer practice (FFP) on crop productivity and soil P fertility in low and high P fertility soils. Seed yield, P partial factor productivity, and P recovery efficiency of both cultivars, Shengguang168 (SG168) and Zhongshuang 11 (ZS11), were significantly improved under optimal P managements, and the increase of them in low P fertility soil was more than that in high P fertility soil. Total P surplus was lower under optimal P managements than under FFP in both P fertility soils. The increasing amount of crop yields under optimal P managements for both cultivars was equivalent to that of 16.0-38.3 kg P₂O₅ hm^-2 of P fertilizer application, and the order of the optimal P managements was as follows: RA > PSB > CMP > DP1 > DP2. In addition, the grain yield of rotated rice cultivar Longliangyou1212 (LLY1212) without P supply was not reduced in both fertility soils. Compared with low P fertility soil, yields of SG168, ZS11 and LLY1212 in high P fertility soil increased by 28.1%-71.7%, 28.3%-78.9% and 26.2%-47.2% at the same treatment, respectively. In summary, optimal P managements in the rapeseed season could stabilize the crop yield, promote P use efficiency and the capacity of soil P supply in the rapeseed-rice rotation, especially in low P fertility soil.

Differential responses of contrasting low phosphorus tolerant cotton genotypes under low phosphorus and drought stress

BACKGROUND

Drought is one of the main reasons for low phosphorus (P) solubility and availability.

AIMS

The use of low P tolerant cotton genotypes might be a possible option to grow in drought conditions.

METHODS

This study investigates the tolerance to drought stress in contrasting low P-tolerant cotton genotypes (Jimian169; strong tolerant to low P and DES926; weak tolerant to low P). In hydroponic culture, the drought was artificially induced with 10% PEG in both cotton genotypes followed by low (0.01 mM KH₂PO₄) and normal (1 mM KH₂PO₄) P application.

RESULTS

The results showed that under low P, PEG-induced drought greatly inhibited growth, dry matter production, photosynthesis, P use efficiency, and led to oxidative stress from excessive malondialdehyde (MDA) and higher accumulation of reactive oxygen species (ROS), and these effects were more in DES926 than Jimian169. Moreover, Jimian169 alleviated oxidative damage by improving the antioxidant system, photosynthetic activities, and an increase in the levels of osmoprotectants like free amino acids, total soluble proteins, total soluble sugars, and proline.

CONCLUSIONS

The present study suggests that the low P-tolerant cotton genotype can tolerate drought conditions through high photosynthesis, antioxidant capacity, and osmotic adjustment.

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

BACKGROUND

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.

RESULTS

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.

CONCLUSION

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.

Biochemical and Molecular Responses Underlying the Contrasting Phosphorus Use Efficiency in Ryegrass Cultivars

Improving plant ability to acquire and efficiently utilize phosphorus (P) is a promising approach for developing sustainable pasture production. This study aimed to identify ryegrass cultivars with contrasting P use efficiency, and to assess their associated biochemical and molecular responses. Nine ryegrass cultivars were hydroponically grown under optimal (0.1 mM) or P-deficient (0.01 mM) conditions, and P uptake, dry biomass, phosphorus acquisition efficiency (PAE) and phosphorus utilization efficiency (PUE) were evaluated. Accordingly, two cultivars with high PAE but low PUE (Ansa and Stellar), and two cultivars with low PAE and high PUE (24Seven and Extreme) were selected to analyze the activity and gene expression of acid phosphatases (APases), as well as the transcript levels of P transporters. Our results showed that ryegrass cultivars with high PAE were mainly influenced by root-related responses, including the expression of genes codifying for the P transporter LpPHT1;4, purple acid phosphatase LpPAP1 and APase activity. Moreover, the traits that contributed greatly to enhanced PUE were the expression of LpPHT1;1/4 and LpPHO1;2, and the APase activity in shoots. These outcomes could be useful to evaluate and develop cultivars with high P-use efficiency, thus contributing to improve the management of P in grassland systems.

Integrative physiological, transcriptome and metabolome analysis reveals the involvement of carbon and flavonoid biosynthesis in low phosphorus tolerance in cotton

Phosphorus (P) is an essential nutrient controlling plant growth and development through the regulation of basic metabolic processes; however, the molecular details of these pathways remain largely unknown. In this study, physiological, transcriptome, and metabolome analysis were compared for two cotton genotypes with different low P tolerance under P starvation and resupply. The results showed that the glucose, fructose, sucrose, and starch contents increased by 18.2%, 20.4%, 20.2%, and 14.3% in the roots and 18.3%, 23.3%, 11.0%, and 13.6% in the shoot of Jimian169 than DES926, respectively. Moreover, the activities of enzymes related to carbon and phosphorus metabolism were higher in the roots and shoots of Jimian169 than DES926. In addition, transcriptome analysis revealed that the number of differentially expressed genes (DEGs) was higher in both roots (830) and shoots (730) under P starvation and the DEGs drastically reduced upon P resupply. The KEGG analysis indicated that DEGs were mainly enriched in phenylpropanoid biosynthesis, carbon metabolism, and photosynthesis. The metabolome analysis showed the enrichment of phenylpropanoid, organic acids and derivatives, and lipids in all the pairs at a given time point. The combined transcriptome and metabolome analysis revealed that carbon metabolism and flavonoid biosynthesis are involved in the P starvation response in cotton. Moreover, co-expression network analysis identified 3 hub genes in the roots and shoots that regulate the pathways involved in the P starvation response. This study provides the foundation for understanding the mechanisms of low P tolerance and the hub genes as a potential target for the development of low P tolerant genotypes.

Phosphorus and carbohydrate metabolism contributes to low phosphorus tolerance in cotton

Low phosphorus (P) is one of the limiting factors in sustainable cotton production. However, little is known about the performance of contrasting low P tolerant cotton genotypes that might be a possible option to grow in low P condition. In the current study, we characterized the response of two cotton genotypes, Jimian169 a strong low P tolerant, and DES926 a weak low P tolerant genotypes under low and normal P conditions. The results showed that low P greatly inhibited growth, dry matter production, photosynthesis, and enzymatic activities related to antioxidant system and carbohydrate metabolism and the inhibition was more in DES926 as compared to Jimian169. In contrast, low P improved root morphology, carbohydrate accumulation, and P metabolism, especially in Jimian169, whereas the opposite responses were observed for DES926. The strong low P tolerance in Jimian169 is linked with a better root system and enhanced P and carbohydrate metabolism, suggesting that Jimian169 is a model genotype for cotton breeding. Results thus indicate that the Jimian169, compared with DES926, tolerates low P by enhancing carbohydrate metabolism and by inducing the activity of several enzymes related to P metabolism. This apparently causes rapid P turnover and enables the Jimian169 to use P more efficiently. Moreover, the transcript level of the key genes could provide useful information to study the molecular mechanism of low P tolerance in cotton.

Phosphorus Availability Affects the Photosynthesis and Antioxidant System of Contrasting Low-P-Tolerant Cotton Genotypes

Phosphorus (P) is an essential macronutrient, and an important component of plant metabolism. However, little is known about the effects of low P availability on P absorption, the photosynthetic electron transport chain, and the antioxidant system in cotton. This study used cotton genotypes (sensitive FJA and DLNTDH and tolerant BX014 and LuYuan343) with contrasting low-P tolerance in a hydroponic experiment under 15 µM, 50 µM, and 500 μM P concentrations. The results showed that low P availability reduced plant development and leaf area, shoot length, and dry weight in FJA and DLNADH, compared to BX014 and LuYuan343. The low P availability decreased the gas-exchange parameters such as the net photosynthetic rate, transpiration rate, and stomatal conductance, and increased the intercellular CO₂ concentration. Chlorophyll a fluorescence demonstrated that the leaves' absorption and trapped-energy flux were largely steady. In contrast, considerable gains in absorption and trapped-energy flux per reaction center resulted from decreases in the electron transport per reaction center under low-P conditions. In addition, low P availability reduced the activities of antioxidant enzymes and increased the content of malondialdehyde in the cotton genotypes, especially in FJA and DLNTDH. Moreover, low P availability reduced the activity of PEPC and generated a decline in the content of ATP and NADPH. Our research can provide a theoretical physiological basis for the growth and tolerance of cotton under low-P conditions.

Genetic variation in morphological traits in cotton and their roles in increasing phosphorus-use-efficiency in response to low phosphorus availability

Phosphorus (P) is an essential macronutrient required for fundamental processes in plants. Trait plasticity is crucial for plant adaptation to environmental change. Variations in traits underlie diverse phosphorus (P) acquisition strategies among plants. Nevertheless, how the intraspecific plasticity and integration of morphological traits contribute to Phosphorus-Use-Efficiency (PUE) in cotton is unknown. In this study, 25 morphological traits were evaluated in 384 cotton genotypes grown with low P (LP, 10μmol. L^-1) and normal nutrition (CK, 500μmol. L^-1) to assess the genetic variability of morphological traits and their relationship to phosphorus use efficiency. Results revealed a large genetic variation in mostly morphological traits under low P. Significant enhancement in root traits and phosphorus efficiency-related traits like PUE was observed at LP as compared to CK conditions. In response to low P availability, cotton genotypes showed large plasticity in shoot and total dry biomass, phosphorus, and nitrogen efficiency-related traits (i.e., phosphorus/nitrogen use efficiency, phosphorus/nitrogen uptake efficiency), and most root traits, but a limited response in root dry biomass, taproot length, root surface area, root volume, and SPAD value. In addition, significant correlations were observed between PUtE (phosphorus uptake efficiency), NUE (nitrogen use efficiency), TDB (total dry biomass), and RTD (root tissue density) with PUE under both P supply level and phosphorus stress index, which may be a key indicator for improving PUE under LP conditions. Most root traits are most affected by genotypes than nutrition level. Conserved PUE is more affected by the nutrition level than the genotype effect. Principal component analysis depicted the comprehensive indicators under two P supply conditions were mainly reflected in root-related traits and morphological indicators such as dry matter biomass. These results indicate that interspecific variations exist within these cotton genotypes and traits. Our study provides suggestions for future research to enhance the ability of the earth system model to predict how crops respond to environmental interference and provide target quality for cotton breeding in phosphorus-deficient areas.

Comparative Transcriptome Profiling Reveals Potential Candidate Genes, Transcription Factors, and Biosynthetic Pathways for Phosphite Response in Potato (Solanum tuberosum L.)

The study was conducted with C31 and C80 genotypes of the potato (Solanum tuberosum L.), which are tolerant and susceptible to phosphite (Phi, H₂PO₃), respectively. To decipher the molecular mechanisms underlying tolerance and susceptibility to Phi in the potato, RNA sequencing was used to study the global transcriptional patterns of the two genotypes. Media were prepared with 0.25 and 0.50 mM Phi, No-phosphorus (P), and 1.25 mM (phosphate, Pi as control). The values of fragments per kilobase of exon per million mapped fragments of the samples were also subjected to a principal component analysis, grouping the biological replicates of each sample. Using stringent criteria, a minimum of 819 differential (DEGs) were detected in both C80-Phi-0.25_vs_C80-Phi-0.50 (comprising 517 upregulated and 302 downregulated) and C80-Phi-0.50_vs_C80-Phi-0.25 (comprising 302 upregulated and 517 downregulated) and a maximum of 5214 DEGs in both C31-Con_vs_C31-Phi-0.25 (comprising 1947 upregulated and 3267 downregulated) and C31-Phi-0.25_vs_C31-Con (comprising 3267 upregulated and 1947 downregulated). DEGs related to the ribosome, plant hormone signal transduction, photosynthesis, and plant–pathogen interaction performed important functions under Phi stress, as shown by the Kyoto Encyclopedia of Genes and Genomes annotation. The expressions of transcription factors increased significantly in C31 compared with C80. For example, the expressions of Soltu.DM.01G047240, Soltu.DM.08G015900, Soltu.DM.06G012130, and Soltu.DM.08G012710 increased under P deficiency conditions (Phi-0.25, Phi-0.50, and No-P) relative to the control (P sufficiency) in C31. This study adds to the growing body of transcriptome data on Phi stress and provides important clues to the Phi tolerance response of the C31 genotype.

Genotypic variation in root architectural traits under contrasting phosphorus levels in Mediterranean and Indian origin lentil genotypes

The development of phosphorus-efficient crop cultivars boosts productivity while lowering eutrophication in the environment. It is feasible to improve the efficiency of phosphorus (P) absorption in lentils by enhancing phosphorus absorption through root architectural traits. The root architectural traits of 110 diverse lentil genotypes of Indian and Mediterranean origin were assessed, and the relationships between traits were investigated. In a hydroponics experiment, the lentil lines were examined at the seedling stage under two conditions: adequate P supply and deficient P supply. The Pearson correlation coefficients between root architectural traits and genetic diversity among lentil lines were assessed. To estimate variance components, a model (fixed factor) was used. In this experiment, both phosphorus (P) and genotype were fixed variables. Our lentil lines showed significant genetic variability and considerable genetic diversity for all traits under both treatments. The TRL (total root length) and PRL (primary root length) showed strong positive associations with all other characteristics excluding root average diameter (RAD) in both P treatments. In both P treatments, the RAD revealed a negative significant association with Total Root Tips (TRT), as well as total root volume (TRV) and total root forks (TRF) in the deficit conditions of P. Total root volume (TRV), total surface area (TSA), and total root tips had higher coefficient variance values. The first two principal components represented 67.88% and 66.19% of the overall variance in the adequate and deficit P treatments respectively. The Shannon-Weaver diversity index (H') revealed that RAD, PRL, and TSA had more variability than TRT and TRF under both treatments. According to the Comprehensive Phosphorus Efficiency Measure (CPEM), the best five highly efficient genotypes are PLL 18-09, PLS 18-01, PLL 18-25, PLS 18-23, and PLL 18-07, while IG112131, P560206, IG334, L11-231, and PLS18-67 are highly inefficient genotypes. The above contrasting diverse lentil genotypes can be utilized to produce P-efficient lentil cultivars. The lentil germplasm with potentially favorable root traits can be suggested to evaluated for other abiotic stress to use them in crop improvement programme. The scientific breakthroughs in root trait phenotyping have improved the chances of establishing trait-allele relationships. As a result, genotype-to-phenotype connections can be predicted and verified with exceptional accuracy, making it easier to find and incorporate favourable nutrition-related genes/QTLs in to breeding programme.

Comparison of different selection traits for identification of phosphorus use efficient lines in mungbean

Phosphorus (P) is one of the major constraints for crop growth and development, owing to low availability and least mobility in many tropical soil conditions. Categorization of existing germplasm under P deficient conditions is a prerequisite for the selection and development of P efficient genotypes in the mungbean. In the present investigation, 36 diverse genotypes were categorized for phosphorus use efficiency traits using four different techniques for identification of phosphorus use efficient mungbean genotypes. The studied genotypes were categorized for P efficiency based on efficiency, responsiveness, and stress tolerance score of genotypes under normal and low P conditions. The mean values of traits, root dry mass, root to shoot ratio, and P utilization efficiency are significantly higher under low P conditions indicating the high responsiveness of traits to P deficiency. The presence of significant interaction between genotypes and P treatment indicates the evaluated genotypes were significantly affected by P treatment for studied traits. The total P uptake showed significant and positive correlations with root dry mass, shoot dry mass, total dry mass,and P concentration under both P regimes. Out of the four techniques used for the categorization of genotypes for P efficiency, three techniques revealed that the genotype PUSA 1333, followed by Pusa Vishal, PUSA 1031, and Pusa Ratna is efficient. The categorization based on stress tolerance score is the finest way to study variation and for the selection of contrasting genotypes for P efficiency. The identified P efficient genotypes would be valuable resources for genetic enhancement of P use efficiency in mungbean breeding.

Phosphorus Nutrition and Growth of Cotton Plants Inoculated With Growth-Promoting Bacteria Under Low Phosphate Availability

The low availability of phosphorus (P) in the soil drastically limits the world productivity of crops such as cotton. In order to contribute sustainably to the solution of this problem, the current study aimed to evaluate the capacity of phosphate-solubilising bacteria to improve plant growth and its relationship with physiological parameters, as well as the shoot P content in cotton plants in a soil with low P availability amended with rock phosphate. The results showed that, of the six plant growth-promoting bacteria strains evaluated under greenhouse conditions, the Rhizobium strain B02 significantly promoted growth, shoot P content and photosynthetic rate. This strain also improved the transpiration rate and the relative content of chlorophyll but without significant differences. Remarkably, Rhizobium sp. B02 had a more significant effect on plant growth compared to the P nutrition. Furthermore, the effect of its inoculation was more pronounced on the roots' growth compared to the shoot. Finally, application of Rhizobium strain B02 showed the capacity to optimize the use of low-solubility fertilizer as the rock phosphate. These findings could be associated with the metabolic activities of plant growth promotion exhibited by phosphate-solubilising strains, such as phosphate solubilisation, production of indole compounds and siderophores synthesis. In conclusion, this research provides evidence of the biotechnological potential of the Rhizobium genus as phosphate-solubilising bacteria with multiple plant growth-promoting activities capable of improving the plant growth and phosphate nutrition of non-leguminous crops such as cotton in soil with low P availability amended with rock phosphate.

Pre evaluation of cassava (Manihot esculenta Crantz) germplasm for genotypic variation in the identification of K efficient genotypes through different statistical tools

Among the tropical tuber crops, cassava (Manihot esculenta Crantz) deserves special attention as regards to its higher biological efficiency in terms of dry matter production which incidentally implies to the higher amount of nutrient extraction from the soil resulting in better response to the application of manures and fertilizers. Among the major nutrients, Potassium (K) is considered as the key nutrient for cassava owing to its influence both in tuber yield and tuber quality. The above facts as well as the availability of sufficient cassava genotypes in the germplasm collection of ICAR-CTCRI made us to initiate research work to screen cassava germplasm including the pre breeding lines. The objective being to identify K efficient genotypes which can yield well under limited availability of K so that the external application of K can be reduced. This paper describes the wide variation noticed during the pre evaluation of 83 elite genotypes which was done as a prelude in the screening and identification of K efficient genotypes. The characters studied were tuber yield, tuber characters, plant dry matter percentage, plant K content, tuber quality (starch, cyanogenic glucosides) attributes, physiological efficiency and plant biometric characters. The variation among the genotypes for the above traits was assessed by making some yardstick for classification which in turn helped in determining the percent distribution of genotypes in each category. The variation among the genotypes were further affirmed through principal component analysis, wherein the first five components explained more than 77% of variability and the cluster analysis performed grouped these genotypes into five clusters. The biplot showed the traits which are closely linked to the genotypes. The dendrogram constructed indicated similar genotypes to that of the clusters to the extent of more than 50% revealing the association of members with similar traits in clusters and dendrograms. The study helped in establishing the drastic variation among the genotypes along with identification of six genotypes viz., Aniyoor, 7 Sahya (2), 7 III E3-5, W-19, CR 43-8, 6-6 for further detailed experimentation to identify K efficient genotypes.

Nitrogen preference and genetic variation of cotton genotypes for nitrogen use efficiency

BACKGROUND

Although nitrogen (N) availability is a major determinant of cotton production, little is known about the importance of plants' preference for ammonium versus nitrate for better growth and nitrogen use efficiency (NUE). We aimed to assess the growth, physiology, and NUE of contrasting N-efficient cotton genotypes (Z-1017, N-efficient and GD-89, N-inefficient) supplied with low and high concentrations of ammonium- and nitrate-N.

RESULTS

The results revealed that ammonium fed plants showed poor root growth, lower dry biomass, N content, leaf chlorophyll and gas exchange than those under nitrate irrespective of the concentration. However, the highest N uptake and utilization efficiency were obtained with nitrate fed plants, which also resulted in the highest dry biomass, N content, leaf chlorophyll and gas exchange as well as root growth. The results further confirmed that N-efficient (Z-1017) genotype performed better under both N sources, showing more flexibility to contrasting N condition by increasing growth and NUE in either source of N. Moreover, multivariate analysis showed a strong relationship of root morphological traits with N utilization efficiency, suggesting the physiological importance of roots over shoots in response to low nitrate concentration.

CONCLUSION

Thus, it was confirmed that nitrate-N is superior to ammonium-N and the use of nitrate and N-efficient genotype is the best option for optimum cotton growth and NUE. Further, field evaluation is required to confirm the hypothesis that nitrate is a preferred N source for better cotton production and NUE. © 2020 Society of Chemical Industry.

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.

Transcriptome Analysis Reveals Differences in Key Genes and Pathways Regulating Carbon and Nitrogen Metabolism in Cotton Genotypes under N Starvation and Resupply

Nitrogen (N) is the most important limiting factor for cotton production worldwide. Genotype-dependent ability to cope with N shortage has been only partially explored in cotton, and in this context, the comparison of molecular responses of cotton genotypes with different nitrogen use efficiency (NUE) is of particular interest to dissect the key molecular mechanisms underlying NUE. In this study, we employed Illumina RNA-Sequencing to determine the genotypic difference in transcriptome profile using two cotton genotypes differing in NUE (CCRI-69, N-efficient, and XLZ-30, N-inefficient) under N starvation and resupply treatments. The results showed that a large genetic variation existed in differentially expressed genes (DEGs) related to amino acid, carbon, and nitrogen metabolism between CCRI-69 and XLZ-30. Further analysis of metabolic changes in cotton genotypes under N resupply showed that nitrogen metabolism and aromatic amino acid metabolism pathways were mainly enriched in CCRI-69 by regulating carbon metabolism pathways such as starch and sucrose metabolism, glycolysis/gluconeogenesis, and pentose phosphate pathway. Additionally, we performed an expression network analysis of genes related to amino acid, carbon, and nitrogen metabolism. In total, 75 and 33 genes were identified as hub genes in shoots and roots of cotton genotypes, respectively. In summary, the identified hub genes may provide new insights into coordinating carbon and nitrogen metabolism and improving NUE in cotton.

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.

High Nitrogen Enhance Drought Tolerance in Cotton through Antioxidant Enzymatic Activities, Nitrogen Metabolism and Osmotic Adjustment

Drought is one of the most important abiotic stresses and hampers many plant physiological processes under suboptimal nitrogen (N) concentration. Seedling tolerance to drought stress is very important for optimum growth and development, however, the enhancement of plant stress tolerance through N application in cotton is not fully understood. Therefore, this study investigates the role of high N concentration in enhancing drought stress tolerance in cotton. A hydroponic experiment supplying low (0.25 mM) and high (5 mM) N concentrations, followed by 150 g L-1 polyethylene glycol (PEG)-induced stress was conducted in a growth chamber. PEG-induced drought stress inhibited seedling growth, led to oxidative stress from excessive malondialdehyde (MDA) generation, and reduced N metabolism. High N concentrations alleviated oxidative damage and stomatal limitation by increasing antioxidant enzymatic activities, leaf relative water content, and photosynthesis in cotton seedlings under drought stress. The results revealed that the ameliorative effects of high N concentration may be ascribed to the enhancement of N metabolizing enzymes and an increase in the amounts of osmoprotectants like free amino acids and total soluble protein. The present data suggest that relatively high N concentrations may contribute to drought stress tolerance in cotton through N metabolism, antioxidant capacity, and osmotic adjustment.