Description: Background and aims Modern maize breeding has increased maize yields worldwide. The changes in above-ground traits accompanying yield improvement are well-known, but less information is available as to the effect of modern plant breeding on changes in maize root traits. Methods Root growth, nitrogen uptake, dry matter accumulation and yield formation of six maize hybrids released from 1973 to 2000 in China were compared. Experiments were conducted under low and high nitrogen supply in a black soil in Northeast China in 2010 and 2011. Results While nitrogen accumulation, dry matter production and yield formation have been increased, modern maize breeding in China since 1990 has reduced root length density in the topsoil without much effect on root growth in the deeper soil. The efficiency of roots in acquiring N has increased so as to match the requirement of N accumulation for plant growth and yield formation. The responses of root growth, nitrogen and dry matter accumulation, and grain yield to low-N stress were similar in the more modern hybrids as in the older ones. Conclusions Modern maize breeding has constitutively changed root and shoot growth and plant productivity without producing any specific enhancement in root responsiveness to soil N availability.

Description: Background and aims Drivers of ecosystem dynamics that are under human influence range from local, land-management decisions to global processes such as warming temperatures and N deposition. The goal of this study was to understand how multiple, potentially interacting factors influence net primary production, N mineralization, and water and soil CO 2 fluxes. Methods Here I report on a three-year experiment that manipulated air temperature using ITEX passive warming cones and N deposition in a mountain meadow ecosystems that were historically grazed or protected from grazing. Results The strongest and most consistent effect was due to the legacy of grazing, with previously grazed sites having lower primary production, lower soil respiration rates, lower soil moisture, and lower soil C and N stocks than historically ungrazed sites. Warming increased soil respiration, but the effect was transient, and decreased over the 3-year study. Nitrogen addition increased primary production in the second and third year of the experiment but had no significant effect on soil respiration. The effect of historical grazing on primary production was approximately double the effect of N addition. Temperature and N deposition rarely interacted except for increasing N availability during the warm, wet growing season of 2004. Conclusions These findings indicate that the legacies of land use, with their influence on plant community composition and hydrologic processes, are locally more important than short-term step changes in temperature and nutrient availability.

Description: Aims Nitrous oxide (N 2 O) emissions from pastoral agriculture are considered to originate from the soil as a consequence of microbial activity during soil nitrification and denitrification. However, recent studies have identified the plant canopy as a potentially significant source of N 2 O emissions to the atmosphere. Understanding the extent and mechanisms of plant emissions may provide new mitigation opportunities as current options only target soil microbial processes. Methods We developed an experimental apparatus and protocol to partition N 2 O emissions between the leaves of grasses and the soil and measured emissions from ten common grass species found in New Zealand pastures. Results The chamber design enabled us to identify measurable changes in N 2 O concentration over a period of 1 h and to distinguish a range of emissions from 0.001 to 0.25 mg N 2 O-N/m 2 leaf area/h. There was a 10-fold variation among species; Holcus lanataus , Lolium perenne and Paspalum dilatatum had the highest leaf N 2 O emissions and Poa annua the lowest. Conclusions Grasses do emit N 2 O from their leaves and the rate that this occurs varies among grass species. The emission does not appear to arise from formation of N 2 O in plant leaves but more likely reflects transport of N 2 O from the soil. Differences in emission rates appear to arise from a plant influence on the rate of formation of N 2 O in the soil rather than the rate of transportation through the plant.

Description: Background and aims The total concentration of dissolved organic carbon (DOC) has often been observed to correlate positively with soil microbial respiration. The aim was to explain the correlation with the properties of dissolved organic matter (DOM). Methods A dataset from previously published papers was gathered together and subjected to multivariate analyses. Samples were collected from five tree species experiments in Finland. The degradability of DOM was assessed by measuring bacterial and fungal growth in DOM. The chemical properties of DOM were assessed by XAD resin fractionation and molecular weight. Soil microbial activity was assessed as C and N mineralization and microbial biomass. Results Both low and high molecular weight compounds, as well as hydrophilic neutral compounds, seemed to be relatively easily degradable. In contrast to our presupposition, easily degradable DOM seemed to be less abundant in soil where variables describing microbial activity were higher. Birch soil with higher microbial biomass N seemed to contain less easily degradable DOM than spruce soil. Conclusion We suggest that DOM collected and characterized at a certain point reflects more the accumulation of refractory compounds following high microbial activity than the easily degradable compounds that microbes would be using when measured.

Description: Aims Heavy-metal tolerance is a widespread phenotype in bacteria, particularly occurring in strains isolated from heavy-metal contaminated sites. Concerning nickel tolerance, the nre system is one of the most common. An ortholog of the nreB gene is present in the alfalfa symbiont Sinorhizobium meliloti also, which stir the attention on its functional role in such Ni-sensitive species and on the evolutionary relationships with Ni-resistant strain orthologs. Methods Phylogenetic reconstruction and comparative genomics were performed to analyze the phylogenetic relationships of nreB orthologs, as well aa nreB deletion mutant S. meliloti strain was constructed and subjected to phenotypic analysis. Results Phylogenetic analysis of nreB genes indicated horizontal gene transfer events, possibly mediated via mobile genetic elements. Phenotype Microarray, biochemical and symbiotic analyses of the deletion mutant strain (Δ nreB ) showed that in S. meliloti nreB is involved in the tolerance to several stresses other than Ni (mainly urea and copper), possibly partially mediated through the modulation of urease and hydrogenase activities. Conclusions Obtained results allowed us to speculate that nreB is a highly mobile gene cassette, spread in the bacterial phylogenetic tree via many HGT events, which could have been recruited to confer nickel-tolerance in strains thriving in contaminated environments, by small changes linked to its basic functions (e.g. modulation of urease and hydrogenase activity).

Description: Background and aims Recent studies suggest that root border cells function in defense of the root by an extracellular DNA-based trapping mechanism similar to that described in mammalian white blood cells. Genes controlling the specialized properties of border cells as they detach from the root tip therefore are of interest. Methods mRNA differential display was used to identify Brd13 , a sequence expressed in border cells but not other root tissues. RNase protection and mRNA Northern blot analyses, and reporter gene expression under the control of the Brd13 promoter in transgenic hairy roots were used to confirm localized expression. Phenotype analysis of transgenic hairy roots expressing Brd13 antisense mRNA was carried out. Results Brd13 was expressed constitutively in border cells but not in leaves, stems, or roots without border cells. The predicted protein shares sequence similarity with flavin-binding proteins. Transgenic hairy roots expressing Brd13 antisense mRNA exhibited abnormal growth and morphology. Conclusions We report here that altered expression of a putative flavin-binding protein in border cells resulted in altered root development. Flavin-binding proteins play key roles in development, defense, and local auxin biosynthesis. The Brd13 gene and its promoter may be useful in creating defined changes in root development and defense.

Description: Background and aims Nitrogen (N 2 ) fixation in feather moss-cyanobacteria associations is a major source of N for boreal ecosystems. However, mosses experience significant shifts in their moisture status due to daily and yearly fluctuations in sunlight, temperature and precipitation. While the effects of drying and rewetting on nutrient leaching and photosynthesis in mosses have been studied, no attempt has been made to assess the consequences for N 2 fixation in feather mosses. Methods We conducted an experiment in which we dried (3 day at 28 °C; 〈9 % of field moisture) and rewetted samples of the feather moss Pleurozium schreberi (Brid.) Mitt. that is colonized by N 2 -fixing-cyanobacteria to assess the influence on N 2 fixation. Further, we tested how long it takes for N 2 fixation to recover from a drying-rewetting cycle. In addition, we assessed how N 2 fixation changes with incubation time with acetylene (2–65 h). Results A drying period of 3 days almost completely eliminated N 2 fixation (〈0.2 μmol m −2  h −1 ) in the moss. However, rates slowly recovered after rewetting, reaching N 2 fixation levels of moist (non-water stressed) moss 5 days after rewetting. Nitrogen fixation increased significantly with incubation time with acetylene (0 μmol m −2  h −1 at 2 h vs. 26 μmol m −2  h −1 at 65 h incubation). Conclusions Although N 2 fixation recommenced upon rewetting, the recovery was slow. Thus, recurrent drying and rewetting cycles could reduce total N 2 fixation in moss-cyanobacteria associations over time, leading to reduced total N input to the system.

Description: Backgrounds and aims Mung bean ( Vigna radiata (L.) R. Wilczek), a widely cultivated pulse crops in India, experiences severe drought stress during the cultivation period. Apart from the conventional plant breeding and transgenic approaches, the application of plant growth-promoting rhizobacteria (PGPR) has always been a promising approach to improve abiotic stress tolerance in crop plants. The aim of the present study was to investigate the role of mung bean rhizosphere-associated Pseudomonas aeruginosa GGRJ21 strain on drought stress alleviation in the host plant. Methods Fluorescent pseudomonads were isolated from mung bean rhizosphere by employing a culture-dependent approach. The role of osmotic stress tolerant P. aeruginosa GGRJ21 on drought stress alleviation in host plants was further examined in both the green house and field conditions. Results An elevated production of reactive oxygen species scavenging enzymes and cellular osmolytes; increased root length, shoot length, dry weight, relative water content; and a stronger upregulation of three drought stress-responsive genes, i.e., dehydration-responsive element binding protein ( DREB2A ), catalase ( CAT1 ), and dehydrin ( DHN ) were observed in GGRJ21 inoculated plants in comparison with the uninoculated control plants tested under drought conditions. The field experimental data show an increase in biomass and better growth and development in inoculated and stressed plants when compared with untreated and stressed plants. Conclusion P. aeruginosa GGRJ21 strain was found to elicit water stress tolerance in mung bean plants by accelerating the accumulation of inherent levels of antioxidant enzymes, cell osmolytes, and consistently expediting the upregulation of stress responsive genes in PGPR-treated plants under water stress conditions.

Description: Aims We studied how the availability of carbon affects the decomposition of soil organic matter (SOM) in the presence of living root system compared to bare SOM treatments without roots. Methods We measured the effect of living Pinus sylvestris L. root systems on SOM C and N budgets, on the age of carbon in CO 2 efflux, and on SOM protease enzyme activity. We examined the possible explanatory factors for increased SOM mineralization such as plant biomass, photosynthesis, microbial C, and protease enzyme activity in the soil. Results The age of carbon respired during the decomposition of soil organic matter was significantly older in the presence of living root systems than in treatments lacking roots. If plants were present, glucose added directly in the SOM accelerated the C and N loss from the bulk material and decreased the rate of photosynthesis. Conclusions We conclude that the priming effect of SOM decomposition was affected by the presence of living root system. Our results stress the importance of including the plant–soil interactions in the SOM decomposition models used in climate change studies.

Description: Background and aims Reduced tillage is widely used in maintaining sustainable agricultural systems. Early growth of wheat ( Triticum aestivum L.) is often impaired in the high strength soils typical of no-till. This may reflect the intrinsically reduced vigour of commercial wheat varieties. We investigated how genotypes selected for greater early vigour performed relative to a broader range of wheat genotypes for shoot and root growth in cultivated and no-till conditions. Methods We assessed a range of cereal germplasm varying for early growth (establishment and leaf area development) under contrasting tillage conditions in a very favourable season, and then the performance of a selected subset validated in repacked soil cores in controlled environments. We measured above- and belowground growth, and parameters associated with adaptation to increasing soil strength. Results High strength soil reduced early shoot and root growth. There were no effects on plant number at emergence and coleoptile length was unimportant. Increased soil strength reduced early growth of all genotypes including genetically vigourous wheats. However, the ranking for vigour was maintained with high strength soil suggesting breeding lines and populations containing genes for greater early growth may still be beneficial in selection for improved performance in no-till. Genotypic increases in specific leaf area and leaf breadth were both associated with genetically greater seedling leaf area and biomass, and potential for greater root growth in no-till. Conclusions Early growth of all wheats was reduced with no till and the harder, high strength soil associated with this tillage system. Genetic variation was large in no-till and cultivated soils alike. In high strength and no-till soils, the relationship with shoot and root vigour was positive indicating selection for greater early growth in breeding programs may increase leaf area and improve root growth.