The vegetative nitrogen response of sorghum lines containing different alleles for nitrate reductase and glutamate synthase

Improving the nitrogen (N) responsiveness of crops is crucial for food security and environmental sustainability, and breeding N use efficient (NUE) crops has to exploit genetic variation for this complex trait. We used reverse genetics to examine allelic variation in two N metabolism genes. In silico analysis of the genomes of 44 genetically diverse sorghum genotypes identified a nitrate reductase and a glutamate synthase gene that were under balancing selection in improved sorghum cultivars. We hypothesised that these genes are a potential source of differences in NUE, and selected parents and progeny of nested association mapping populations with different allelic combinations for these genes. Allelic variation was sourced from African (Macia) and Indian (ICSV754) genotypes that had been incorporated into the Australian elite parent R931945-2-2. Nine genotypes were grown for 30 days in a glasshouse and supplied with continuous limiting or replete N, or replete N for 27 days followed by 3 days of N starvation. Biomass, total N and nitrate contents were quantified together with gene expressions in leaves, stems and roots. Limiting N supply universally resulted in less shoot and root growth, increased root weight ratio and reduced tissue nitrate and total N concentrations. None of the tested genotypes exceeded growth or NUE of the elite parent R931945-2-2 indicating that the allelic combinations did not confer an advantage during early vegetative growth. Thus, the next steps for ascertaining potential effects on NUE include growing plants to maturity. We conclude that reverse genetics that take advantage of rapidly expanding genomic databases enable a systematic approach for developing N-efficient crops.     

The ability of several high arctic plant species to utilize nitrate nitrogen under field conditions

The ability to utilize NO _inf3 ^sup– in seven high arctic plant species from Truelove Lowland, Devon Island, Canada was investigated, using an in vivo assay of maximum potential nitrate reductase (NR) activity and applications of ¹⁵N. Plant species were selected on the basis of being characteristic of nutrient-poor and nutrient-rich habitats. In all species leaves were the dominant site of NR activity. Root NR activity was negligible in all species except Saxifraga cernua. NO _inf3 ^sup– availability per se did not appear to limit NR activity of the species typically found on nutrient-poor sites (Dryas integrifolia, Saxifraga oppositifolia, and Salix arctica), or in Cerastium alpinum, as leaf NR activities remained low, even after NO _inf3 ^sup– addition. ¹⁵NO _inf3 ^sup– uptake was limited in D. integrifolia and Salix arctica. However, the lack of field induction of NR activity in C. alpinum and Saxifraga oppositifolia was not due to restricted nitrate uptake, as ¹⁵NO _inf3 ^sup– labelled NO _inf3 ^sup– entered the roots and shoots of both species. Leaf NR activity rates were low in three of the species typical of nutrient-rich habitats (O. digyna, P. radicatum and Saxifraga cernua), sampled from a site containing low soil NO _inf3 ^sup– . Additions of NO _inf3 ^sup– significantly increased leaf NR activity in these latter species, suggesting that potential NR activity was limited by the availability of NO _inf3 ^sup– . ¹⁵N labelled NO _inf3 ^sup– was taken up by O. digyna. P. radicatum and Saxifraga cernua. Although two species (D. integrifolia and Salix arctica) showed little utilization of NO _inf3 ^sup– , we concluded that five of the seven selected high arctic plant species (C. alpinum, O. digyna, P. radicatum, Saxifraga cernua and Saxifraga oppositifolia) do have the potential to utilize NO _inf3 ^sup– as a nitrogen source under field conditions, with the highest potential to utilize NO _inf3 ^sup– occurring in three of the species typically found on fertile habitats.     

Total soil nitrogen and nitrogen mineralization

Summary The mineralization capacity of 24 different soils was determined from incubation experiments. Relatively rapid mineralization and nitrification was found with soils from cultivated land, and pastures, but soils under natural vegetative covers of conifers and hardwoods were mostly ammonifying. A close relationship could be established between the total nitrogen content of the soil and the amount of mineral nitrogen formed during incubation. Important connections could also be shown between the available nitrogen contents at different times during the incubation period; these suggest that the incubation period can be considerably shortened.     

Long-term comparative study of soil nitrate test,Gilat plant indicator method and wheat nitrogen uptake

A long-term comparison between two routine soil nitrogen tests, soil nitrate versus plant indicator method, was performed on the Negev Desert loessial soil in Israel. The Gilat plant indicator method was found to be a better method to reflect the soil nitrogen availability for wheat under field conditions. It was found that 15 to 38 kg ha^-1 of NO₃-N, measured by nitrate soil test, for each 30 cm soil increment, is not available for plant uptake. This plant unavailable NO₃-N background in the soil cannot be leached by repeated irrigation cycles of 100 mm each, or by heavy rains.     

Milling plant and soil material in plastic tubes over-estimates carbon and under-estimates nitrogen concentrations

Background and aims

Milling of plant and soil material in plastic tubes, such as microcentrifuge tubes, over-estimates carbon (C) and under-estimates nitrogen (N) concentrations due to the introduction of polypropylene into milled samples, as identified using Fourier-transform infra-red spectroscopy.

Methods and results

This study compares C and N concentrations of roots and soil milled in microcentrifuge tubes versus stainless steel containers, demonstrating that a longer milling time, greater milling intensity, smaller sample size and inclusion of abrasive sample material all increase polypropylene contamination from plastic tubes leading to overestimation of C concentrations by up to 8 % (0.08 g?g^?1).

Conclusions

Erroneous estimations of C and N, and other analytes, must be assumed after milling in plastic tubes and milling methods should be adapted to minimise such error.     

A laboratory-scale pulper for leafy plant material

A machine is described that, makes, from 2 to 3 kg samples of leaf, a pulp comparable to that made by the large-scale equipment used in leaf protein extraction. It is therefore suitable for use in agronomic experiments on leaf protein yield.     

A comprehensive differential proteomic study of nitrate deprivation in Arabidopsis reveals complex regulatory networks of plant nitrogen responses

Nitrogen (N) is an important nutrient and signal for plant growth and development. However, to date, our knowledge of how plants sense and transduce the N signals is very limited. To better understand the molecular mechanisms of plant N responses, we took two-dimensional gel-based proteomic and phosphoproteomic approaches to profile the proteins with abundance and phosphorylation state changes during nitrate deprivation and recovery in the model plant Arabidopsis thaliana. After 7-day-old seedlings were N-deprived for up to 48 h followed by 24 h recovery, a total of 170 and 38 proteins were identified with significant changes in abundance and phosphorylation state, respectively. Bioinformatic analyses implicate these proteins in diverse cellular processes including N and protein metabolisms, photosynthesis, cytoskeleton, redox homeostasis, and signal transduction. Functional studies of the selected nitrate-responsive proteins indicate that the proteasome regulatory subunit RPT5a and the cytoskeleton protein Tubulin alpha-6 (TUA6) play important roles in plant nitrate responses by regulating plant N use efficiency (NUE) and low nitrate-induced anthocyanin biosynthesis, respectively. In conclusion, our study provides novel insights into plant responses to nitrate at the proteome level, which are expected to be highly useful for dissecting the N response pathways in higher plants and for improving plant NUE.     

DNA barcoding of medicinal plant material for identification

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Arbuscular mycorrhizal fungi can transfer substantial amounts of nitrogen to their host plant from organic material

Nitrogen (N) capture by arbuscular mycorrhizal (AM) fungi from organic material is a recently discovered phenomenon. This study investigated the ability of two Glomus species to transfer N from organic material to host plants and examined whether the ability to capture N is related to fungal hyphal growth. Experimental microcosms had two compartments; these contained either a single plant of Plantago lanceolata inoculated with Glomus hoi or Glomus intraradices, or a patch of dried shoot material labelled with (15)N and (13)carbon (C). In one treatment, hyphae, but not roots, were allowed access to the patch; in the other treatment, access by both hyphae and roots was prevented. When allowed, fungi proliferated in the patch and captured N but not C, although G. intraradices transferred more N than G. hoi to the plant. Plants colonized with G. intraradices had a higher concentration of N than controls. Up to one-third of the patch N was captured by the AM fungi and transferred to the plant, while c. 20% of plant N may have been patch derived. These findings indicate that uptake from organic N could be important in AM symbiosis for both plant and fungal partners and that some AM fungi may acquire inorganic N from organic sources.     

Using near-infrared reflectance spectroscopy to predict carbon, nitrogen and phosphorus content in heterogeneous plant material

The aim of this study was to produce calibration equations between near-infrared reflectance (NIR) spectra and the concentrations of carbon, nitrogen, and phosphorus in heterogeneous material: from living needles to litter in Pinus halepensis stands subjected to prescribed burnings. The aim was to determine whether calibrations should be conducted within each stage in the transformation of needles (local calibrations), giving relationships that were accurate but valid only for each particular stage, or whether it was possible to integrate the various forms of variation in needles (global calibrations) while retaining an acceptable accuracy. A principal component analysis calculated from the sample spectral data was used to distinguish three different sets, each sharing spectral characteristics and corresponding to three categories of needle: needles collected on the pines (N), falling needles (F), and litter (L), and each containing samples collected from the burnt sites and a control site. Samples representative of all the forms of variation in spectral properties were selected from within each category and their carbon, nitrogen, and phosphorus concentrations were measured using standard wet chemistry methods; these constituted the calibration sets n, f, and l. Calibrations were produced between the nutrient concentrations and the NIR spectra of the calibration sets n, f, and l and the grouped sets (n+f, f+l, n+f+l). The results of local calibrations made from each individual category showed that the carbon, nitrogen, and phosphorus concentrations were accurately predictable by NIR spectra. The global calibrations made by lumping together several categories were valid for a wider range of concentrations and for spectrally heterogeneous materials and in most cases were just as accurate as the local calibrations produced from each individual category. Received: 2 March 1998 / Accepted: 19 November 1998     

Manometric method for determining bicarbonate content in plant leaves.

A method is described for manometric determination of bicarbonate (as little as 0.25 μmoles) in leaves. Conditions such as optimum pH and inactivation of carbonic anhydrase are introduced to stabilize the bicarbonate. Crude leaf extracts are purified with chloroform and Darco G-60. An ionic mixture is separated from macromolecular compounds by elution from a column of Sephadex G-25. Carbon dioxide is released by acidifying the ionic mixture and is determined with a respirometer.     

Simulation of the decomposition and nitrogen mineralization of aboveground plant material in two unfertilized grassland ecosystems

A simple model of the decomposition and nitrogen mineralization of plant material from two unfertilized grassland ecosystems has been developed, with only the proportion of leaves and stems in the original material, the initial nitrogen contents of these plant parts and temperature as input data. The model simulates carbon losses from stems and leaves, using a double exponential decay function, with the temperature sum as independent variable. Mineralization of nitrogen is not calculated via microbial growth rates, but simulated on the basis of the carbon utilization efficiency of the microorganisms and the critical C/N ratio, i.e. the C/N ratio of the litter at which the microbial demand for nitrogen is met exactly. The parameter values for leaching fractions of carbon and nitrogen, relative decay rates, microbial carbon utilization efficiencies and critical C/N ratios were derived from a litter bag experiment with 12 litter types (species) including both green and dead materials, carried out in two unfertilized grassland ecosystems differing in production level. The model was evaluated using a cross-validation method, in which one species was omitted from the parametrization procedure, and its decomposition and mineralization were predicted by the resulting model. In general there was good agreement between the observed and predicted amounts of carbon and nitrogen remaining for all green litter types/species, but carbon and nitrogen dynamics in the dead material of Festuca rubra were poorly predicted. This disparity has been attributed to the proportion of leaves in the material of Festuca rubra (95%) being far beyond the range of leaf proportions in the three litter types the calibration set consisted of (8–35%). When the data of all litter types were used to determine the model parameters, good agreement was obtained between measured and simulated values for the changes in nitrogen and carbon in all litter types of both the green and dead material series. Optimization yielded parameter values for microbial carbon utilization efficiencies of 0.30 for microorganisms associated with green litter and 0.35 for those associated with dead litter. The critical C/N ratios for green and dead material were found to be 29 and 36, respectively.     

A standardized method for determining buffering capacity of plant foliage

Summary A method for determining the buffering capacity (B.C.) of foliage extracts was standardized and evaluated. Sources of variations (biological, field and laboratory) were identified. These variations were reflected in inter-specific differences, seasonal fluctuations, age of the foliage and duration and the conditions of storage of the extracts. Procedures have been recommended to eliminate or minimize sources of variations (other than inherent specific) by standardizing the field sampling, laboratory processing and methods, and calculations of the buffering capacity. Plants such as lichens, known to be sensitive to air pollutants, had very low B.C. whereas species of intermediate sensitivity such as balsam fir had higher B.C. The B.C. being inherited and significantly different among species, has potential for its use in indexing the relative sensitivity of species to air pollutants especially in areas where large numbers of species are to be compared.