Carmen González-Murua

Dicyandiamide and 3,4-dimethyl pyrazole phosphate decrease N2O emissions from grassland but dicyandiamide produces deleterious effects in clover.

The application of nitrogen fertilisers leads to different ecological problems such as nitrate leaching and the release of nitrogenous gases. N2O is a gas involved in global warming, therefore, agricultural soils can be regarded as a source of global warming. Soil N2O production comes from both the nitrification and denitrification processes. From an ecological viewpoint, using nitrification inhibitors with ammonium based fertilisers may be a potential management strategy to lower the fluxes of N2O, thus decreasing its undesirable effect. In this study, the nitrification inhibitors (NIs) dicyandiamide (DCD) and 3,4-dimethyl pyrazole phosphate (DMPP) have been evaluated as management tools to mitigate N2O emissions from mineral fertilisation and slurry application in grassland systems (experiments 1 and 2), and to assess the phytotoxic effect of these inhibitors per se on clover (experiment 3). Both nitrification inhibitors acted in maintaining soil nitrogen (N) in ammonium form, decreasing cumulative N2O emissions. DCD, but not DMPP, produced phytotoxic effects and yield reduction in white clover. A nutrient imbalance, which led to a senescence process visually observed as chlorosis and necrosis at the border of the leaves, was noted.

Zea mays L. amylacea from the Lluta Valley (Arica-Chile) tolerates salinity stress when high levels of boron are available

Elevated levels of boron occurring naturally in soil or irrigation waters are detrimental to many crops grown in agricultural regions of the world. If such levels of boron are accompanied by conditions of excessive salinity, as occurs in the Lluta valley in Northern Chile, the consequences can be drastic for crops. A variety of sweet corn from this valley (Zea mays L. amylacea) has arisen as a consequence of practiced seed selection, suggesting that it is extremely tolerant to high salt and boron levels. In the present study, seeds ofZea mays L. amylacea were collected in order to study their physiological mechanisms of tolerance to high levels of NaCl and boron. Concentrations of 100 and 430 mM NaCl and 20 and 40 mg kg−1 boron were imposed as treatments. The plants did not exhibit symptoms of toxicity to either NaCl and boron during the 20 days of treatment. Na+ accumulation was substantial in roots, while boron was translocated to leaves. Boron alleviated the negative effect of salinity on tissue K+ and maintained membrane integrity. The higher values of water potential seem to be related to the capacity of this ecotype to maintain a better relative water content in leaves. Despite the fact that boron enhanced slightly the effect of salinity on CO2 assimilation, no effect on photochemical parameters was observed in this ecotype. Osmotic adjustment allows this ecotype to survive in high saline soils; however the presence of boron makes this strategy unnecessary since boron contributed to the maintenance of cell wall elasticity.

Effect of N-(n-butyl) thiophosphoric triamide and 3,4 dimethylpyrazole phosphate on gaseous emissions from grasslands under different soil water contents.

The intensification of grassland systems is leading to serious environmental risks due to the large input of nitrogen (N) in fertilizers and the subsequent gaseous losses. Addition of nitrification inhibitors (NI) or urease activity inhibitors to fertilizers could reduce these losses to the atmosphere. In the present study, the effects of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) and the urease activity inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) were evaluated on NH3, N2O, NO, and CO2 emissions. Ammonium sulphate nitrate (ASN), urea and cattle slurry were applied at a rate of 70 kg N ha(-1) to a mixed clover-ryegrass sward in the Basque Country (northern Spain) under different soil water contents. NH3 and NO emissions were determined by photoacoustic and chemiluminescence respectively using an open chamber technique while N2O and CO2 emissions were measured by photoacoustic using a closed chamber technique. When the water filled pore space (WFPS) was under 60%, the application of NBPT reduced NO emissions a 34% on urea and an 18% on slurry, and the application of DMPP reduced them a 2% on ASN and a 4% on slurry. No significant effect was observed on NH3 losses. When WFPS was over 60%, no effect could be observed on NO and N2O emissions after the application of both inhibitors, but NH3 losses were reduced a 31% by NBPT when applied with the slurry. Carbon dioxide emissions were unaffected by the use of DMPP or NBPT at any soil water content. Neither grassland yield nor herbage N concentration were influenced by the application of both inhibitors.

Nitrification and denitrification derived N2O production from a grassland soil under application of DCD and Actilith F2

The relative contribution of nitrification and denitrification to N2O production was investigated by means of soil incubations with acetylene in a mixed clover/ryegrass sown sward 5 days after application of a mineral fertiliser (calcium ammonium nitrate) or an organic one (cattle slurry) with and without the addition of the nitrification inhibitor dicyandiamide (DCD) and the commercial slurry additive Actilith-F2. At this time, maximum field N2O emissions were taking place. N2O production by the slurry amended soil was twice as high as that of the mineral amended one. N2O came in a greater proportion from nitrification rather than from denitrification in the slurry treatment, while for the mineral fertilisation most N2O came from denitrification. The addition of DCD to slurry produced a decrease in N2O production both from nitrification and denitrification. No reduction in N2O losses was observed from addition of DCD to the mineral fertilisation, although DCD resulted effective in reducing the nitrification rate by 53% both in the slurry and the mineral fertilisation. Actilith F2 induced a high nitrification rate and N2O production from denitrification was reduced while that from nitrification was not.

Denitrification losses from a natural grassland in the Basque Country under organic and inorganic fertilization

Denitrification losses from a poorly drained clayey loamy soil under natural pasture were measured over a two-year period using the acetylene inhibition technique. Plots received two different applications of fertilizer as calcium ammonium nitrate or cow slurry (a total of 145–290 kg N ha−1 in 1991 and 120–240 kg in 1992). In the first year, N losses in the mineral treatments were about 4 times greater than losses in the slurry treatments. In the second year losses in the slurry treatments increased in such a way that losses in the higher slurry application became similar to those for the two mineral treatments. Soil nitrate was the factor producing differences between treatments. In this way, N mineralization in periods between fertilizations coinciding with high soil water contents was responsible in the second year for the increase in N losses in the slurry treatments. Denitrification rates greater than 0.1 kg N ha−1 day−1 occurred at soil water contents > 33 % (air filled porosity < 26 %) and soil nitrate contents > 1 mg N kg−1 dry soil. Spring and autumn were the seasons of highest risk of denitrification because of N fertilizations coinciding with periods of soil saturation with water. Winter losses were low, but this is a period when there is a risk of denitrification in wetter seasons, particularly for a slurry application management.

Temporal study of the effect of phosphinothricin on the activity of glutamine synthetase, glutamate dehydrogenase and nitrate reductase in Medicago sativa L.

Summary In a preliminary work we showed that when plants were sprayed with several doses of glufosinate, GS activity of the leaf was reduced by 50% after 48 h of application of 250 µM herbicide. Enzyme inhibition was accompained by a dramatic accumulation of ammonia. In this paper we analyze the time-course effect of phosphinothricin on nitrogen metabolism. Our results show that GS activity is the first process affected; more than 50% activity reduction is observed after 2 h of a 1,000 µM treatment, whereby ammonia values 400% higher than control were reached. NRase did not change until 24 h of assay, whereas protein content was reduced after 48 h. GDH activity was enhanced, but only 24 h after starting the treatment. These results indicate that the main target of PPT action is on GS activity, and other processes are long-term modified. GDH could, perhaps, reassimilate some of the ammonia produced but its activity is not able to completely prevent, the injury caused by phosphinothricin.

Physiological consequences of continuous, sublethal imazethapyr supply to pea plants

Summary Imazethapyr (IM) is a herbicide that inhibits the branched-chain amino acid (BCAA) biosynthesis through the specific inhibition of acetolactate synthase activity. This herbicide acts very slowly and several weeks are required for complete plant death. From the BCAA biosynthesis inhibition to the growth inhibition and plant death, the processes involved are not fully understood. Starvation for BCAAs and/or starvation for carbohydrates in sinks. have been proposed as part of the death mechanisms. In this study, a permanent acetolactate synthase inhibition is used in order to (1) determine whether the growth inhibition effects can be attributed to a reduction in BCAA content and/or to starvation of carbohydrates; and (2) to analyse the physiological changes induced. Sublethal doses of IM were continuously supplied in the nutrient solution of nodulated pea plants. These conditions led to a significant decline in plant growth. The herbicide also caused a decline in nodule initiation, but had little effect on nodule development. However, plants were not nitrogen-limited and net photosynthesis was only slightly affected at the higher herbicide concentration. Total soluble sugars and starch were accumulated in both leaves and roots following herbicide supply. These results were also found in non-nodulated, nitrate-fed plants. In relation with a likely BCAA starvation, a significant increase was observed in the free amino acid pool, with a marked imbalance among different amino acids, although among BCAAs, only valine pool declined as a consequence of IM supply. It is concluded that acetolactate synthase inhibition by continuous, sublethal IM supply does not induce carbohydrate or a specific BCAA starvation in pea plants.

Depletion of the heaviest stable N isotope is associated with NH4+/NH3 toxicity in NH4+-fed plants

BackgroundIn plants, nitrate (NO3-) nutrition gives rise to a natural N isotopic signature (δ15N), which correlates with the δ15N of the N source. However, little is known about the relationship between the δ15N of the N source and the 14N/15N fractionation in plants under ammonium (NH4+) nutrition. When NH4+ is the major N source, the two forms, NH4+ and NH3, are present in the nutrient solution. There is a 1.025 thermodynamic isotope effect between NH3 (g) and NH4+ (aq) which drives to a different δ15N. Nine plant species with different NH4+-sensitivities were cultured hydroponically with NO3- or NH4+ as the sole N sources, and plant growth and δ15N were determined. Short-term NH4+/NH3 uptake experiments at pH 6.0 and 9.0 (which favours NH3 form) were carried out in order to support and substantiate our hypothesis. N source fractionation throughout the whole plant was interpreted on the basis of the relative transport of NH4+ and NH3.ResultsSeveral NO3--fed plants were consistently enriched in 15N, whereas plants under NH4+ nutrition were depleted of 15N. It was shown that more sensitive plants to NH4+ toxicity were the most depleted in 15N. In parallel, N-deficient pea and spinach plants fed with 15NH4+ showed an increased level of NH3 uptake at alkaline pH that was related to the 15N depletion of the plant. Tolerant to NH4+ pea plants or sensitive spinach plants showed similar trend on 15N depletion while slight differences in the time kinetics were observed during the initial stages. The use of RbNO3 as control discarded that the differences observed arise from pH detrimental effects.ConclusionsThis article proposes that the negative values of δ15N in NH4+-fed plants are originated from NH3 uptake by plants. Moreover, this depletion of the heavier N isotope is proportional to the NH4+/NH3 toxicity in plants species. Therefore, we hypothesise that the low affinity transport system for NH4+ may have two components: one that transports N in the molecular form and is associated with fractionation and another that transports N in the ionic form and is not associated with fractionation.

Effect of Phosphinothricin (Glufosinate) on Activities of Glutamine Synthetase and Glutamate Dehydrogenase in Medicago sativa L.

Summary The effect of the glutamate analogue phosphinothricin on enzymes of nitrogen assimilation was studied as well as the accumulation of ammonia and nitrate after treatment of lucerne plants with this herbicide. Glutamine synthetase (GS) was assayed with several phosphinothricin concentrations. When plants were sprayed with 250, 500 and 1,000 μM of the herbicide, GS activity of the leaf was reduced by 50% at 250 μM. Phosphinothricin caused a high ammonia accumulation in the tissues, reaching values of 70 μmol ammonia/g dry weight after treatment with 1,000 μM phosphinothricin. Glutamate dehydrogenase (GDH) activity increased ca. 40 % and nitrate reductase activity (NRase) was strongly decreased. Neither nitrate content nor proteolytic activity were altered.

Boric acid and salinity effects on maize roots. Response of aquaporins ZmPIP1 and ZmPIP2, and plasma membrane H+‐ATPase, in relation to water and nutrient uptake

Under saline conditions, an optimal cell water balance, possibly mediated by aquaporins, is important to maintain the whole-plant water status. Furthermore, excessive accumulation of boric acid in the soil solution can be observed in saline soils. In this work, the interaction between salinity and excess boron with respect to the root hydraulic conductance (L(0)), abundance of aquaporins (ZmPIP1 and ZmPIP2), ATPase activity and root sap nutrient content, in the highly boron- and salt-tolerant Zea mays L. cv. amylacea, was evaluated. A downregulation of root ZmPIP1 and ZmPIP2 aquaporin contents were observed in NaCl-treated plants in agreement with the L(0) measurements. However, in the H3BO3-treated plants differences in the ZmPIP1 and ZmPIP2 abundance were observed. The ATPase activity was related directly to the amount of ATPase protein and Na+ concentration in the roots, for which an increase in NaCl- and H3BO3+ NaCl-treated plants was observed with respect to untreated and H3BO3-treated plants. Although nutrient imbalance may result from the effect of salinity or H3BO3 alone, an ameliorative effect was observed when both treatments were applied together. In conclusion, our results suggest that under salt stress, the activity of specific membrane components can be influenced directly by boric acid, regulating the functions of certain aquaporin isoforms and ATPase as possible components of the salinity tolerance mechanism.