Effect of ammonium and nitrate nutrition on some physiological processes in higher plants - growth, photosynthesis, photorespiration, and water relations

Impact of nitrogen source and supply level on growth, yield and nutritional value of two contrasting ecotypes of Cichorium spinosum L. grown hydroponically

BACKGROUND

In the present study, two contrasting stamnagathi (Cichorium spinosum L.) ecotypes originating either from a mountainous or from a seaside habitat were grown hydroponically and supplied with a nutrient solution differing in the total-N level (4 or 16 mmol L^-1 ) and the N source (NH₄⁺ -N/total-N: 0.05, 0.25 or 0.50). The aim was to search for genotypic differences in nitrogen nutrition.

RESULTS

At commercial maturity, the dry weight of mountainous plants was higher than that of seaside plants. The shoot mineral concentrations were higher in seaside plants than in mountainous plants in both harvests. The leaf nitrate concentration was influenced by the levels of both total-N and NH₄⁺ -N/total-N at both harvests, whereas plants with a seaside origin exhibited higher nitrate concentrations than those originating from a mountainous site in all total-N and NH₄⁺ -N/total-N treatments.

CONCLUSION

The two stamnagathi ecotypes differed considerably in their responses to nitrogen nutrition and tissue nitrate content. The mountainous ecotype was superior in terms of growth, tissue nitrate concentration and antioxidant capacity, whereas the seaside ecotype accumulated more nutrient microcations in leaves. A low total-N concentration (up to 4 mmol L^-1 ) combined with a high NH₄⁺ -N/total-N ratio (up to 0.05) could minimize tissue NO₃^- concentrations without compromising yield. © 2017 Society of Chemical Industry.

Aquaporin Expression and Water Transport Pathways inside Leaves Are Affected by Nitrogen Supply through Transpiration in Rice Plants

The photosynthetic rate increases under high-N supply, resulting in a large CO₂ transport conductance in mesophyll cells. It is less known that water movement is affected by nitrogen supply in leaves. This study investigated whether the expression of aquaporin and water transport were affected by low-N (0.7 mM) and high-N (7 mM) concentrations in the hydroponic culture of four rice varieties: (1) Shanyou 63 (SY63), a hybrid variant of the indica species; (2) Yangdao 6 (YD6), a variant of indica species; (3) Zhendao 11 (ZD11), a hybrid variant of japonica species; and (4) Jiuyou 418 (JY418), another hybrid of the japonica species. Both the photosynthetic and transpiration rate were increased by the high-N supply in the four varieties. The expressions of aquaporins, plasma membrane intrinsic proteins (PIPs), and tonoplast membrane intrinsic protein (TIP) were higher in high-N than low-N leaves, except in SY63. Leaf hydraulic conductance (Kleaf) was lower in high-N than low-N leaves in SY63, while Kleaf increased under high-N supply in the YD6 variant. Negative correlations were observed between the expression of aquaporin and the transpiration rate in different varieties. Moreover, there was a significant negative correlation between transpiration rate and intercellular air space. In conclusion, the change in expression of aquaporins could affect Kleaf and transpiration. A feedback effect of transpiration would regulate aquaporin expression. The present results imply a coordination of gas exchange with leaf hydraulic conductance.

Evaluation of Fertilizer Potential of Different K Compounds Prepared Utilizing Sea Bittern as Feed Stock

Aim: Many countries import potassic fertilizers due to dearth of K-mineral deposits. Therefore processes to obtain K-nutrient sources from sea bittern were developed by our Institute. The present investigation evaluated the fertilizer potential of three different sea bittern-derived (SBD) potassium forms developed viz., potassium schoenite, potassium nitrate and potassium ammonium sulfate on maize productivity in two cropping seasons. Methods: The pot and field experiments consisted of four treatments, wherein the three K forms were applied at the recommended rate of 40 kg K2O ha-1 and were compared with commercially used sulfate of potash. The effect of these fertilizers on different parameters of plant and soil were evaluated. Results: The application of SBD-potassic fertilizers led to enhancement in growth, productivity and quality of maize which related well with higher photosynthesis, nutrient uptake and soil quality parameters. On an average all the three forms of sea bittern-derived potash enhanced yield of maize over control by 22.3 and 23.8%, respectively, in pot and field trials. The best performance was under SBD-KNO3, which also recorded the highest benefit: cost ratio of 1.76. Conclusion: The K-fertilizers derived from sea-bittern-a waste product of salt industry-can thus be economically used to improve crop production sustainably.

Drought stress obliterates the preference for ammonium as an N source in the C4 plant Spartina alterniflora

The C₄ grass Spartina alterniflora is known for its unique salt tolerance and strong preference for ammonium (NH₄⁺) as a nitrogen (N) source. We here examined whether Spartina's unique preference for NH₄⁺ results in improved performance under drought stress. Manipulative greenhouse experiments were carried out to measure the effects of variable water availability and inorganic N sources on plant performance (growth, photosynthesis, antioxidant, and N metabolism). Drought strongly reduced leaf number and area, plant fresh and dry weight, and photosynthetic activity on all N sources, but the reduction was most pronounced on NH₄⁺. Indeed, the growth advantage seen on NH₄⁺ in the absence of drought, producing nearly double the biomass compared to growth on NO₃^-, was entirely obliterated under both intermediate and severe drought conditions (50 and 25% field capacity, respectively). Both fresh and dry weight became indistinguishable among N sources under drought. Major markers of the antioxidant capacity of the plant, the activities of the enzymes superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase, showed higher constitutive levels on NH₄⁺. Catalase and glutathione reductase were specifically upregulated in NH₄⁺-fed plants with increasing drought stress. This upregulation, however, failed to protect the plants from drought stress. Nitrogen metabolism was characterized by lower constitutive levels of glutamine synthetase in NH₄⁺-fed plants, and a rise in glutamate dehydrogenase (GDH) activity under drought, accompanied by elevated proline levels in leaves. Our results support postulates on the important role of GDH induction, and its involvement in the synthesis of compatible solutes, under abiotic stress. We show that, despite this metabolic shift, S. alterniflora's sensitivity to drought does not benefit from growth on NH₄⁺ and that the imposition of drought stress equalizes all N-source-related growth differences observed under non-drought conditions.

Nitrogen use strategies of seedlings from neotropical tree species of distinct successional groups

Few studies have analyzed the strategies of neotropical tree seedlings for absorbing, translocating and assimilating the nitrogen. Here, we compared the nitrogen use strategies of seedlings from six tree species that are native to the Brazilian Atlantic Forest and that belong to different successional groups: Trema micrantha, Heliocarpus popayanensis and Cecropia pachystachya (pioneers), Cariniana estrellensis, Eugenia brasiliensis and Guarea kunthiana (non-pioneers). The effects of cultivating seedlings with nitrate or ammonium on the growth, physiology and nitrogen metabolism were analyzed. Nitrate-grown pioneer species had much higher leaf nitrate reductase activity than non-pioneer ones, but non-pioneer seedlings were also able to use nitrate as a nitrogen source. In addition to this remarkable difference between the groups in the capacity for leaf nitrate assimilation, substantial variations in the nitrogen use strategies were observed within the successional classes. Differently from the other non-pioneers, the canopy species C. estrellensis seemed to assimilate nitrate mainly in the leaves. Morphophysiological analyses showed a gradient of ammonium toxicity response, with E. brasiliensis as the most tolerant species, and T. micrantha and H. popayanensis as the most sensitive ones. Guarea kunthiana showed a relatively low tolerance to ammonium and an unusual high translocation of this cation in the xylem sap. In contrast to the other pioneers, C. pachystachya had a high plasticity in the use of nitrogen sources. Overall, these results suggest that nitrogen use strategies of neotropical tree seedlings were not determined solely by their successional position.

Inorganic nitrogen form: a major player in wheat and Arabidopsis responses to elevated CO2

Critical for predicting the future of primary productivity is a better understanding of plant responses to rising atmospheric carbon dioxide (CO2) concentration. This review considers recent results on the role of the inorganic nitrogen (N) forms nitrate (NO3-) and ammonium (NH4+) in determining the responses of wheat and Arabidopsis to elevated atmospheric CO2 concentration. Here, we identify four key issues: (i) the possibility that different plant species respond similarly to elevated CO2 if one accounts for the N form that they are using; (ii) the major influence that plant-soil N interactions have on plant responses to elevated CO2; (iii) the observation that elevated CO2 may favor the uptake of one N form over others; and (iv) the finding that plants receiving NH4+ nutrition respond more positively to elevated CO2 than those receiving NO3- nutrition because elevated CO2 inhibits the assimilation of NO3- in shoots of C3 plants. We conclude that the form and amount of N available to plants from the rhizosphere and plant preferences for the different N forms are essential for predicting plant responses to elevated CO2.

Plant nitrate transporters: from gene function to application

We summarize nitrate transporters and discuss their potential in breeding for improved nitrogen use efficiency and yield.

Nitrate increases ethylene production and aerenchyma formation in roots of lowland rice plants under water stress

Ethylene increases root cortical aerenchyma formation in maize (Zea mays L.), rice (Oryza sativa L.) and other species. To further investigate the effects of nitrate, ammonium and water stress on ethylene production and aerenchyma formation in roots, two lowland rice cultivars (Shanyou 63, hybrid indica, and Yangdao 6, inbred indica) were cultured hydroponically with 10% (w/v) polyethylene glycol to simulate water stress. Water stress decreased shoot biomass, stomatal conductivity and leaf water potential in cultivars fed with nitrate but not with ammonium. Water stress induced more aerenchyma formation in cultivars fed with nitrate rather than ammonium, and increased cortical aerenchyma was found in Yangdao 6. Endogenous ethylene production by roots increased significantly under water stress in plants fed with nitrate rather than ammonium. Exogenous ethylene stimulated root cortical aerenchyma formation. Expression of the ethylene biosynthesis gene 1-aminocyclo-propane-1-carboxylic acid (ACC) synthase (ACS5) was greater in roots fed with nitrate rather than ammonium in the presence and absence of water stress. The expression of ethylene signalling pathway genes involved in programmed cell death (lesion-simulating disease (L.S.D.)1.1 and L.S.D.2; enhanced disease susceptibility (EDS) and phytoalexin-deficient (PAD4)) were regulated by the N form and water stress. In plants of cultivars fed with ammonium, L.S.D.1.1 expression increased under water stress, whereas L.S.D.2, EDS and PAD4 expression decreased. In conclusion, nitrate increases ethylene production and cortical aerenchyma formation in roots of water-stressed lowland rice. However, ammonium increased L.S.D.1.1 expression in water-stressed roots, and decreased ACS5, EDS and PAD4 expression, which would inhibit ethylene production and aerenchyma formation.

The redox control of photorespiration: from biochemical and physiological aspects to biotechnological considerations

Photorespiration is a complex and tightly regulated process occurring in photosynthetic organisms. This process can alter the cellular redox balance, notably via the production and consumption of both reducing and oxidizing equivalents. Under certain circumstances, these equivalents, as well as reactive oxygen or nitrogen species, can become prominent in subcellular compartments involved in the photorespiratory process, eventually promoting oxidative post-translational modifications of proteins. Keeping these changes under tight control should therefore be of primary importance. In order to review the current state of knowledge about the redox control of photorespiration, we primarily performed a careful description of the known and potential redox-regulated or oxidation sensitive photorespiratory proteins, and examined in more details two interesting cases: the glycerate kinase and the glycine cleavage system. When possible, the potential impact and subsequent physiological regulations associated with these changes have been discussed. In the second part, we reviewed the extent to which photorespiration contributes to cellular redox homeostasis considering, in particular, the set of peripheral enzymes associated with the canonical photorespiratory pathway. Finally, some recent biotechnological strategies to circumvent photorespiration for future growth improvements are discussed in the light of these redox regulations.

Nitrate Increased Cucumber Tolerance to Fusarium Wilt by Regulating Fungal Toxin Production and Distribution

Cucumber Fusarium wilt, induced by Fusarium oxysporum f. sp. cucumerinum (FOC), causes severe losses in cucumber yield and quality. Nitrogen (N), as the most important mineral nutrient for plants, plays a critical role in plant-pathogen interactions. Hydroponic assays were conducted to investigate the effects of different N forms (NH₄⁺ vs. NO₃‒) and supply levels (low, 1 mM; high, 5 mM) on cucumber Fusarium wilt. The NO₃‒-fed cucumber plants were more tolerant to Fusarium wilt compared with NH₄⁺-fed plants, and accompanied by lower leaf temperature after FOC infection. The disease index decreased as the NO₃‒ supply increased but increased with the NH₄⁺ level supplied. Although the FOC grew better under high NO₃- in vitro, FOC colonization and fusaric acid (FA) production decreased in cucumber plants under high NO₃- supply, associated with lower leaf membrane injury. There was a positive correlation between the FA content and the FOC number or relative membrane injury. After the exogenous application of FA, less FA accumulated in the leaves under NO₃- feeding, accompanied with a lower leaf membrane injury. In conclusion, higher NO₃- supply protected cucumber plants against Fusarium wilt by suppressing FOC colonization and FA production in plants, and increasing the plant tolerance to FA.

Glycine increases cold tolerance in rice via the regulation of N uptake, physiological characteristics, and photosynthesis

To investigate the response of rice growth and photosynthesis to different nitrogen (N) sources under cold stress, hydroponic cultivation of rice was done in greenhouse, with glycine, ammonium, and nitrate as the sole N sources. The results demonstrate that exposure to low temperature reduced the rice biomass and leaf chlorophyll content, but their values in the glycine-treated plants were significantly higher than in the ammonium- and nitrate-treated plants. This might be attributed to the higher N uptake rate and root area and activity in the glycine-treated plants. The glycine-treated plants also maintained high contents of soluble proteins, soluble sugars, and proline as well as enhanced antioxidant enzyme activities to protect themselves against chilling injury. Under cold stress, reduced stomatal conductance (g_s) and effective quantum efficiency of PSII (Φ_PSII) significantly inhibited the leaf photosynthesis; however, glycine treatment alleviated these effects compared to the ammonium and nitrate treatments. The high non-photochemical quenching (qN) and excess energy dissipative energy (E_x) in the glycine-treated plants were beneficial for the release of extra energy, thereby, strengthening their photochemical efficiency. We, therefore, conclude that the strengthened cold tolerance of glycine-treated rice plants was closely associated with the higher accumulation of dry matter and photosynthesis through the up-regulation of N-uptake, and increase in the content of osmoprotectants, activities of the antioxidant defense enzymes, and photochemical efficiency. The results of the present study provide new ideas for improving the plant tolerance to extreme temperatures by nutrient resource management in the cold regions.

Appropriate NH4+: NO3- ratio improves low light tolerance of mini Chinese cabbage seedlings

BACKGROUND

In northwest of China, mini Chinese cabbage (Brassica pekinensis) is highly valued by consumers, and is widely cultivated during winter in solar-greenhouses where low light (LL) fluence (between 85 and 150 μmol m^-2 s^-1 in day) is a major abiotic stress factor limiting plant growth and crop productivity. The mechanisms with which various NH₄⁺: NO₃^- ratios affected growth and photosynthesis of mini Chinese cabbage under normal (200 μmol m^-2 s^-1) and low (100 μmol m^-2 s^-1) light conditions was investigated. The four solutions with different ratios of NH₄⁺: NO₃^- applied were 0:100, 10:90, 15:85 and 25:75 with the set up in a glasshouse in hydroponic culture. The most appropriate NH₄⁺: NO₃^- ratio that improved the tolerance of mini Chinese cabbage seedlings to LL was found in our current study.

RESULTS

Under low light, the application of NH₄⁺: NO₃^- (10:90) significantly stimulated growth compared to only NO₃^- by increasing leaf area, canopy spread, biomass accumulation, and net photosynthetic rate. The increase in net photosynthetic rate was associated with an increase in: 1) maximum and effective quantum yield of PSII; 2) activities of Calvin cycle enzymes; and 3) levels of mRNA relative expression of several genes involved in Calvin cycle. In addition, glucose, fructose, sucrose, starch and total carbohydrate, which are the products of CO₂ assimilation, accumulated most in the cabbage leaves that were supplied with NH₄⁺: NO₃^- (10:90) under LL condition. Low light reduced the carbohydrate: nitrogen (C: N) ratio while the application of NH₄⁺: NO₃^- (10:90) alleviated the negative effect of LL on C: N ratio mainly by increasing total carbohydrate contents.

CONCLUSIONS

The application of NH₄⁺:NO₃^- (10:90) increased rbcL, rbcS, FBA, FBPase and TK expression and/or activities, enhanced photosynthesis, carbohydrate accumulation and improved the tolerance of mini Chinese cabbage seedlings to LL. The results of this study would provide theoretical basis and technical guidance for mini Chinese cabbage production. In practical production, the ratio of NH₄⁺:NO₃^- should be adjusted with respect to light fluence for successful growing of mini Chinese cabbage.

Nutrient constraints on terrestrial carbon fixation: The role of nitrogen

Carbon dioxide (CO₂) concentrations in the earth's atmosphere are projected to rise from current levels near 400ppm to over 700ppm by the end of the 21st century. Projections over this time frame must take into account the increases in total net primary production (NPP) expected from terrestrial plants, which result from elevated CO₂ (eCO₂) and have the potential to mitigate the impact of anthropogenic CO₂ emissions. However, a growing body of evidence indicates that limitations in soil nutrients, particularly nitrogen (N), the soil nutrient most limiting to plant growth, may greatly constrain future carbon fixation. Here, we review recent studies about the relationships between soil N supply, plant N nutrition, and carbon fixation in higher plants under eCO₂, highlighting key discoveries made in the field, particularly from free-air CO₂ enrichment (FACE) technology, and relate these findings to physiological and ecological mechanisms.

Nitrate Protects Cucumber Plants Against Fusarium oxysporum by Regulating Citrate Exudation

Fusarium wilt causes severe yield losses in cash crops. Nitrogen plays a critical role in the management of plant disease; however, the regulating mechanism is poorly understood. Using biochemical, physiological, bioinformatic and transcriptome approaches, we analyzed how nitrogen forms regulate the interactions between cucumber plants and Fusarium oxysporum f. sp. cucumerinum (FOC). Nitrate significantly suppressed Fusarium wilt compared with ammonium in both pot and hydroponic experiments. Fewer FOC colonized the roots and stems under nitrate compared with ammonium supply. Cucumber grown with nitrate accumulated less fusaric acid (FA) after FOC infection and exhibited increased tolerance to chemical FA by decreasing FA absorption and transportation in shoots. A lower citrate concentration was observed in nitrate-grown cucumbers, which was associated with lower MATE (multidrug and toxin compound extrusion) family gene and citrate synthase (CS) gene expression, as well as lower CS activity. Citrate enhanced FOC spore germination and infection, and increased disease incidence and the FOC population in ammonium-treated plants. Our study provides evidence that nitrate protects cucumber plants against F. oxysporum by decreasing root citrate exudation and FOC infection. Citrate exudation is essential for regulating disease development of Fusarium wilt in cucumber plants.

Use of transcriptomics and co-expression networks to analyze the interconnections between nitrogen assimilation and photorespiratory metabolism

Nitrogen is one of the most important nutrients for plants and, in natural soils, its availability is often a major limiting factor for plant growth. Here we examine the effect of different forms of nitrogen nutrition and of photorespiration on gene expression in the model legume Lotus japonicus with the aim of identifying regulatory candidate genes co-ordinating primary nitrogen assimilation and photorespiration. The transcriptomic changes produced by the use of different nitrogen sources in leaves of L. japonicus plants combined with the transcriptomic changes produced in the same tissue by different photorespiratory conditions were examined. The results obtained provide novel information on the possible role of plastidic glutamine synthetase in the response to different nitrogen sources and in the C/N balance of L. japonicus plants. The use of gene co-expression networks establishes a clear relationship between photorespiration and primary nitrogen assimilation and identifies possible transcription factors connected to the genes of both routes.

A Combination of Biochar-Mineral Complexes and Compost Improves Soil Bacterial Processes, Soil Quality, and Plant Properties

Organic farming avoids the use of synthetic fertilizers and promises food production with minimal environmental impact, however this farming practice does not often result in the same productivity as conventional farming. In recent years, biochar has received increasing attention as an agricultural amendment and by coating it with minerals to form biochar-mineral complex (BMC) carbon retention and nutrient availability can be improved. However, little is known about the potential of BMC in improving organic farming. We therefore investigated here how soil, bacterial and plant properties respond to a combined treatment of BMC and an organic fertilizer, i.e., a compost based on poultry manure. In a pakchoi pot trial, BMC and compost showed synergistic effects on soil properties, and specifically by increasing nitrate content. Soil nitrate has been previously observed to increase leaf size and we correspondingly saw an increase in the surface area of pakchoi leaves under the combined treatment of BMC and composted chicken manure. The increase in soil nitrate was also correlated with an enrichment of bacterial nitrifiers due to BMC. Additionally, we observed that the bacteria present in the compost treatment had a high turnover, which likely facilitated organic matter degradation and a reduction of potential pathogens derived from the manure. Overall our results demonstrate that a combination of BMC and compost can stimulate microbial process in organic farming that result in better vegetable production and improved soil properties for sustainable farming.

Temperature determines size and direction of effects of elevated CO2 and nitrogen form on yield quantity and quality of Chinese cabbage

Rising atmospheric CO2 concentrations (e[CO2 ]) are presumed to have a significant impact on plant growth and yield and also on mineral nutrient composition, and therefore, on nutritional quality of crops and vegetables. To assess the relevance of these effects in future agroecosystems it is important to understand how e[CO2 ] interacts with other environmental factors. In the present study, we examined the interactive effects of e[CO2 ] with temperature and the form in which nitrogen is supplied (nitrate or ammonium nitrate) on growth, amino acid content and mineral nutrient composition of Chinese cabbage (Brassica pekinensis Rupr.), a crop characterised by its high nutritional value and increasing relevance for human nutrition in many developing countries. Higher temperature, ammonium nitrate and e[CO2 ] had a positive impact on net photosynthesis and growth. A stimulating effect of e[CO2 ] on growth was only observed if the temperature was high (21/18 °C, day/night), and an interaction of e[CO2 ] with N form was only observed if the temperature was ambient (15/12 °C, day/night). Mineral nutrient composition was affected in a complex manner by all three factors and their interaction. These results demonstrate how much the effect of e[CO2 ] on mineral quality of crops depends on other environmental factors. Changes in temperature, adapting N fertilisation and the oxidation state of N have the potential to counteract the mineral depletion caused by e[CO2 ].

Transgenic Rice Expressing Ictb and FBP/Sbpase Derived from Cyanobacteria Exhibits Enhanced Photosynthesis and Mesophyll Conductance to CO2

To find a way to promote the rate of carbon flux and further improve the photosynthetic rate in rice, two CO2-transporting and fixing relevant genes, Ictb and FBP/Sbpase, which were derived from cyanobacteria with the 35SCaMV promotor in the respective constructs, were transformed into rice. Three homologous transgenic groups with Ictb, FBP/Sbpase and the two genes combined were constructed in parallel, and the functional effects of these two genes were investigated by physiological, biochemical and leaf anatomy analyses. The results indicated that the mesophyll conductance and net photosynthetic rate were higher at approximately 10.5-36.8% and 13.5-34.6%, respectively, in the three groups but without any changes in leaf anatomy structure compared with wild type. Other physiological and biochemical parameters increased with the same trend in the three groups, which showed that the effect of FBP/SBPase on improving photosynthetic capacity was better than that of ICTB and that there was an additive effect in ICTB+FBP/SBPase. ICTB localized in the cytoplasm, whereas FBP/SBPase was successfully transported to the chloroplast. The two genes might show a synergistic interaction to promote carbon flow and the assimilation rate as a whole. The multigene transformation engineering and its potential utility for improving the photosynthetic capacity and yield in rice were discussed.

Fertilization with urea, ammonium and nitrate produce different effects on growth, hydraulic traits and drought tolerance in Pinus taeda seedlings

Urea fertilization decreases Pinus taeda L. growth in clay soils of subtropical areas. The negative effect of urea is related to changes in some hydraulic traits, similar to those observed in plants growing under drought. The aims of this work were (i) to determine whether different sources of nitrogen applied as fertilizers produce similar changes in growth and hydraulic traits to those observed by urea fertilization and (ii) to analyze the impact of those changes in plant drought tolerance. Plants fertilized with urea, nitrate [Formula: see text] or ammonium [Formula: see text] were grown well watered or with reduced water supply. Urea and [Formula: see text] fertilization reduced plant growth and increased root hydraulic conductance scaled by root dry weight (DW). [Formula: see text] fertilization did not reduce plant growth and increased shoot hydraulic conductance and stem hydraulic conductivity. We conclude that [Formula: see text] is the ion involved in the changes linked to the negative effect of urea fertilization on P. taeda growth. [Formula: see text] fertilization does not change drought susceptibility and it produces changes in shoot hydraulic traits, therefore plants avoid the depressive effect of fertilization. Urea and [Formula: see text] fertilizers induce changes in DW and root hydraulic conductance and consequently plants are less affected by drought.

The enhanced drought tolerance of rice plants under ammonium is related to aquaporin (AQP)

Previously, we demonstrated that drought resistance in rice seedlings was increased by ammonium (NH4(+)) treatment, but not by nitrate (NO3(-)) treatment, and that the change was associated with root development. To study the effects of different forms of nitrogen on water uptake and root growth under drought conditions, we subjected two rice cultivars (cv. 'Shanyou 63' hybrid indica and cv. 'Yangdao 6' indica, China) to polyethylene glycol-induced drought stress in a glasshouse using hydroponic culture. Under drought conditions, NH4(+) significantly stimulated root growth compared to NO3(-), as indicated by the root length, surface area, volume, and numbers of lateral roots and root tips. Drought stress decreased the root elongation rate in both cultivars when they were supplied with NO3(-), while the rate was unaffected in the presence of NH4(+). Drought stress significantly increased root protoplast water permeability, root hydraulic conductivity, and the expression of root aquaporin (AQP) plasma intrinsic protein (PIP) genes in rice plants supplied with NH4(+); these changes were not observed in plants supplied with NO3(-). Additionally, ethylene, which is involved in the regulation of root growth, accumulated in rice roots supplied with NO3(-) under conditions of drought stress. We conclude that the increase in AQP expression and/or activity enhanced the root water uptake ability and the drought tolerance of rice plants supplied with NH4(+).

Heme-heme oxygenase 1 system is involved in ammonium tolerance by regulating antioxidant defence in Oryza sativa

Despite substantial evidence showing the ammonium-altered redox homeostasis in plants, the involvement and molecular mechanism of heme-heme oxygenase 1 (heme-HO1), a novel antioxidant system, in the regulation of ammonium tolerance remain elusive. To fill in these gaps, the biological function of rice HO1 (OsSE5) was investigated. Results showed that NH4 Cl up-regulated rice OsSE5 expression. Oxidative stress and subsequent growth inhibition induced by excess NH4 Cl was partly mitigated by pretreatment with carbon monoxide (CO, a by-product of HO1 activity) or intensified by zinc protoporphyrin (ZnPP, a potent inhibitor of HO1 activity). Pretreatment with HO1 inducer hemin, not only up-regulated OsSE5 expression and HO activity, but also rescued the down-regulation of antioxidant transcripts, total and related isozymatic activities, thus significantly counteracting the excess NH4 Cl-triggered reactive oxygen species overproduction, lipid peroxidation and growth inhibition. OsSE5 RNAi transgenic rice plants revealed NH4 Cl-hypersensitive phenotype with impaired antioxidant defence, both of which could be rescued by CO but not hemin. Transgenic Arabidopsis plants over-expressing OsSE5 also exhibited enhanced tolerance to NH4 Cl, which might be attributed to the up-regulation of several antioxidant transcripts. Altogether, these results illustrated the involvement of heme-HO1 system in ammonium tolerance by enhancing antioxidant defence, which may improve plant tolerance to excess ammonium fertilizer.

Effects of ammonium to nitrate ratio on growth, nitrogen metabolism, photosynthetic efficiency and bioactive phytochemical production of Prunella vulgaris

CONTEXT

Prunella vulgaris L (Labiatae) is commonly used as a traditional medicinal herb in some Asian and Europe countries. To date, few studies have been conducted to determine the influence of [Formula: see text] - N/[Formula: see text] - N ratio on growth, physiological development, and bioactive phytochemical accumulation in hydroponically grown P. vulgaris.

OBJECTIVE

The current study was conducted to evaluate the effect of five [Formula: see text] - N/[Formula: see text] - N ratios on growth, nitrogen metabolism, photosynthetic efficiency, and bioactive phytochemical production in P. vulgaris.

MATERIALS AND METHODS

Hydroponically cultivated P. vulgaris were fertilized with five [Formula: see text] - N/[Formula: see text] - N ratios in a greenhouse for 85 d. Dried weight of root, stem, leaf and spica, leaf area, photosynthetic efficiency, activities of nitrate reductase (NR), glutamine synthetase (GS), and the concentrations of N, soluble protein, and free amino acids in the leaves, as well as the contents of rosmarinic acid (RA), ursolic acid (UA), and oleanolic acid (OA) in the spicas were measured.

RESULTS

Both [Formula: see text] - N and [Formula: see text] - N as the sole source of nitrogen had inhibitory effects on P. vulgaris growth. P. vulgaris fertilized with the 25/75 ([Formula: see text] - N/NO3 - N) ratio had the highest leaf area, photosynthetic rate, and chlorophyll content. The 25/75 ([Formula: see text]/[Formula: see text]) ratio increased the spica biomass by 1828%, nitrate-reductase (NR) activity by 98%, and soluble protein concentration by 29.45% compared with the 100/0 ([Formula: see text]/[Formula: see text]) treatment. Additionally, 25 [Formula: see text] - N/75 NO3 - N resulted in the highest contents of RA and total flavonoids as well as relatively high contents of UA and OA; therefore, this ratio had the highest yield of RA, UA, OA, and total flavonoids in spicas.

DISCUSSION AND CONCLUSION

The use of 25 [Formula: see text] - N/75 [Formula: see text] - N is recommended to improve biomass production and medicinal quality of P. vulgaris.

Physiological integration modifies δ15N in the clonal plant Fragaria vesca, suggesting preferential transport of nitrogen to water-stressed offspring

BACKGROUND AND AIMS

One of the most striking attributes of clonal plants is their capacity for physiological integration, which enables movement of essential resources between connected ramets. This study investigated the capacity of physiological integration to buffer differences in resource availability experienced by ramets of the clonal wild strawberry plant, Fragaria vesca. Specifically, a study was made of the responses of connected and severed offspring ramets growing in environments with different water availability conditions (well watered or water stressed) and nitrogen forms (nitrate or ammonium).

METHODS

The experimental design consisted of three factors, 'integration' (connected, severed) 'water status' (well watered, water stressed) and 'nitrogen form' (nitrate, ammonium), applied in a pot experiment. The effects of physiological integration were studied by analysing photochemical efficiency, leaf spectral reflectance, photosynthesis and carbon and nitrogen isotope discrimination, the last of which has been neglected in previous studies.

KEY RESULTS

Physiological integration buffered the stress caused by water deprivation. As a consequence, survival was improved in water-stressed offspring ramets that remained connected to their parent plants. The nitrogen isotope composition (δ(15)N) values in the connected water-stressed ramets were similar to those in ramets in the ammonium treatment; however, δ(15)N values in connected well-watered ramets were similar to those in the nitrate treatment. The results also demonstrated the benefit of integration for offspring ramets in terms of photochemical activity and photosynthesis.

CONCLUSIONS

This is the first study in which carbon and nitrogen isotopic discrimination has been used to detect physiological integration in clonal plants. The results for nitrogen isotope composition represent the first evidence of preferential transport of a specific form of nitrogen to compensate for stressful conditions experienced by a member clone. Water consumption was lower in plants supplied with ammonium than in plants supplied with nitrate, and therefore preferential transport of ammonium from parents to water-stressed offspring could potentially optimize the water use of the whole clone.

Disruption of the mitochondrial alternative oxidase (AOX) and uncoupling protein (UCP) alters rates of foliar nitrate and carbon assimilation in Arabidopsis thaliana

Under high light, the rates of photosynthetic CO2 assimilation can be influenced by reductant consumed by both foliar nitrate assimilation and mitochondrial alternative electron transport (mAET). Additionally, nitrate assimilation is dependent on reductant and carbon skeletons generated from both the chloroplast and mitochondria. However, it remains unclear how nitrate assimilation and mAET coordinate and contribute to photosynthesis. Here, hydroponically grown Arabidopsis thaliana T-DNA insertional mutants for alternative oxidase (AOX1A) and uncoupling protein (UCP1) fed either NO3 (-) or NH4 (+) were used to determine (i) the response of NO3 (-) uptake and assimilation to the disruption of mAET, and (ii) the interaction of N source (NO3 (-) versus NH4 (+)) and mAET on photosynthetic CO2 assimilation and electron transport. The results showed that foliar NO3 (-) assimilation was enhanced in both aox1a and ucp1 compared with the wild-type, suggesting that foliar NO3 (-) assimilation is probably driven by a decreased capacity of mAET and an increase in reductant within the cytosol. Wild-type plants had also higher rates of net CO2 assimilation (A net) and quantum yield of PSII (ϕPSII) under NO3 (-) feeding compared with NH4 (+) feeding. Additionally, under NO3 (-) feeding, A net and ϕPSII were decreased in aox1a and ucp1 compared with the wild type; however, under NH4 (+) they were not significantly different between genotypes. This indicates that NO3 (-) assimilation and mAET are both important to maintain optimal rates of photosynthesis, probably in regulating reductant accumulation and over-reduction of the chloroplastic electron transport chain. These results highlight the importance of mAET in partitioning energy between foliar nitrogen and carbon assimilation.

Effects of high atmospheric CO2 concentration on root hydraulic conductivity of conifers depend on species identity and inorganic nitrogen source

We examined root hydraulic conductivity (L(p)) responses of one-year-old seedlings of four conifers to the combined effects of elevated CO2 and inorganic nitrogen (N) sources. We found marked interspecific differences in L(p) responses to high CO2 ranging from a 37% increase in P. abies to a 27% decrease in P. menziesii, but these effects depended on N source. The results indicate that CO2 effects on root water transport may be coupled to leaf area responses under nitrate (NO(3)(-)), but not ammonium (NH(4)(+)) dominated soils. To our knowledge, this is the first study that highlights the role of inorganic N source and species identity as critical factors that determine plant hydraulic responses to rising atmospheric CO2 levels. The results have important implications for understanding root biology in a changing climate and for models designed to predict feedbacks between rising atmospheric CO2, N deposition, and ecohydrology.

Phytochemical changes induced by different nitrogen supply forms and radiation levels in two leafy Brassica species

The effect of three different nitrogen (N) supply forms differing in their ammonium-to-nitrate (NH(4):NO(3)) ratio (100% NH(4), 50% NH(4) + 50% NO(3), 100% NO(3)) under three different levels of daily photosynthetic active radiation (PAR) (low, 5.0; medium, 6.8; high, 9.0 mol m(-2) day(-1)) on a range of desirable health-promoting phytochemicals in Brassica rapa subsp. nipposinica var. chinoleifera and Brassica juncea was determined. The 100% NH(4) supply under medium PAR levels led to the highest concentration of glucosinolates based on a low nitrogen/sulfur ratio as well as high levels of carotenoids in the leaves of both Brassica species. However, the 100% NH(4) supply under low and medium PAR levels resulted in low concentrations of flavonoids based on high N concentration in the leaves. Thus, the data provided here have strong implications for crop management strategies aimed at optimizing both the concentration and composition of a range of phytochemicals.

Effects of nitrogen form on growth, CO₂ assimilation, chlorophyll fluorescence, and photosynthetic electron allocation in cucumber and rice plants

Cucumber and rice plants with varying ammonium (NH(4)(+)) sensitivities were used to examine the effects of different nitrogen (N) sources on gas exchange, chlorophyll (Chl) fluorescence quenching, and photosynthetic electron allocation. Compared to nitrate (NO(3)(-))-grown plants, cucumber plants grown under NH(4)(+)-nutrition showed decreased plant growth, net photosynthetic rate, stomatal conductance, intercellular carbon dioxide (CO(2)) level, transpiration rate, maximum photochemical efficiency of photosystem II, and O(2)-independent alternative electron flux, and increased O(2)-dependent alternative electron flux. However, the N source had little effect on gas exchange, Chl a fluorescence parameters, and photosynthetic electron allocation in rice plants, except that NH(4)(+)-grown plants had a higher O(2)-independent alternative electron flux than NO(3)(-)-grown plants. NO(3)(-) reduction activity was rarely detected in leaves of NH(4)(+)-grown cucumber plants, but was high in NH(4)(+)-grown rice plants. These results demonstrate that significant amounts of photosynthetic electron transport were coupled to NO(3)(-) assimilation, an effect more significant in NO(3)(-)-grown plants than in NH(4)(+)-grown plants. Meanwhile, NH(4)(+)-tolerant plants exhibited a higher demand for the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) for NO(3)(-) reduction, regardless of the N form supplied, while NH(4)(+)-sensitive plants had a high water-water cycle activity when NH(4)(+) was supplied as the sole N source.

Light-saturated photosynthetic rate in high-nitrogen rice (Oryza sativa L.) leaves is related to chloroplastic CO2 concentration

To identify the effect of nitrogen (N) nutrition on photosynthetic efficiency and mesophyll conductance of rice seedlings (Oryza sativa L., cv. 'Shanyou 63' hybrid indica China), hydroponic experiments with different concentrations of N were conducted in a greenhouse. Although leaf N concentration on a dry mass basis increased with increasing supply of N, no significant differences in seedling biomass were observed. A higher light-saturated CO(2) assimilation rate (A) with a high concentration of supplied N was associated with a higher carboxylation efficiency (CE), but not a higher apparent quantum yield (alpha). Based on classic photosynthetic models, both the Rubisco content and the ribulose bisphosphate (RuBP) regeneration rate were sufficient for light-saturated photosynthesis in rice seedlings; the estimated chloroplastic CO(2) concentration (C(c)) and mesophyll conductance (g(m)) demonstrated that a low C(c) was the ultimate limiting factor to photosynthetic efficiency with a higher N supply. Due to a greater chloroplast size (i.e. a shorter distance to the plasma membrane) with a higher supply of N, the CO(2) transport resistance in the liquid phase (g(liq)) in high-N leaves was lower than that in low-N leaves, which resulted in higher g(m) and C(c) in high-N leaves. Although CE(A/Ci) was higher with a high supply of N, there were no differences in CE(A/Cc) between plants grown with different concentrations of N, indicating that the carboxylation capacity of Rubisco between plants grown at different N concentrations was constant. The enhanced photosynthetic rate with supply of a high N concentration was attributed to a higher CO(2) concentration in the chloroplasts, related to a higher mesophyll conductance due to an increased chloroplast size.

Effects of citrus and avocado irrigation and nitrogen-form soil amendment on host selection by adult Homalodisca vitripennis (Hemiptera: Cicadellidae)

Host plant water status is thought to influence dispersal of the xylophagous leafhopper Homalodisca vitripennis Germar, especially where plants are grown under high evaporative demand. Preference by adult H. vitripennis for plants grown under different water deficit and nitrogen form fertilization regimens was studied under laboratory conditions. Leafhopper abundance and ovipositional preference were studied on potted 'Washington navel' orange and 'Haas' avocado in cage choice tests, and feeding rate was estimated using excreta produced by insects confined on plants. A similar study compared responses to citrus treated with 1:1 and 26:1 ratios of fertigated nitrate-N to ammonium-N. The insects were more abundant, oviposited, and fed significantly more on surplus-irrigated plants than on plants under moderate continuous deficit irrigation except avocado feeding, which was nearly significant. Plants exposed to drought became less preferred after 3 and 7 d in avocado and citrus, respectively. Citrus xylem fluid tension at this point was estimated at 0.93 MPa. A corresponding pattern of decline in feeding rate was observed on citrus, but on avocado, feeding rate was low overall and not statistically different between treatments. No statistical differences in abundance, oviposition, or feeding were detected on citrus fertigated with 26:1 or 1:1 ratios of nitrate-N to ammonium-N. Feeding occurred diurnally on both plant species. Discussion is provided on the potential deployment of regulated deficit irrigation to manage H. vitripennis movement as part of a multitactic effort to minimize the risk of disease outbreaks from Xylella fastidiosa Wells et al. in southern California agriculture.