Humic Acid Alleviates Fe Chlorosis in Graminaceous Plants Through Coordinated Fe-Dependent and Fe-Independent Mechanisms.

Many studies have shown the close relationship between the beneficial action of soil and sedimentary humic acids on the growth of plants cultivated in calcareous soils and their ability to improve Fe plant nutrition. These results have been ascribed to the humic acid (HA) capability to improve Fe solubility and bioavailability. However, other effects more related to a humic acid action on the specific mechanisms activated in roots of plants under Fe deficiency cannot be ruled out. Although this question has been studied in dicotyledonous plants, in graminaceous plants there are no specific studies. Here we investigate the ability of a humic acid extracted from peat (HA) to improve Fe nutrition in wheat plants cultivated under Fe deficient and sufficient conditions. The results show that HA can improve the physiological status of Fe deficient wheat plants by alleviating some of the deleterious consequences of Fe deficiency on plant development and increasing the plant ability to secrete phytosiderophores to the nutrient solution. This action of HA is associated with increases in the Fe-active pool in leaves that might be related to the mobilization of the Fe complexed by HA resulting from the interaction of HA with the phytosiderophores in the nutrient solution. The Fe translocation from the root to the shoot may be favored by the action of trans-Zeatin Riboside (tZR) since the leaf concentration of this phytohormone was enhanced by HA in Fe deficient plants.

Shoot iron status and auxin are involved in iron deficiency-induced phytosiderophores release in wheat.

BACKGROUND: The release of phytosiderephores (PS) to the rhizosphere is the main root response to iron (Fe) deficiency in graminaceous plants. We have investigated the role of the Fe status in the shoot as well as of the signaling pathways controlled by three relevant phytoregulators - indolacetic acid (IAA), ethylene and nitric oxide (NO) - in the regulation of this root response in Fe-starved wheat plants. To this end, the PS accumulation in the nutrient solution and the root expression of the genes encoding the nicotianamine aminotransferase (TaNAAT) and ferritin (TaFER) have been evaluated in plants subjected to different treatments. RESULTS: The application of Fe to leaves of Fe-deficient plants prevented the increase in both PS root release and TaNAAT gene expression thus showing the relevant role of the shoot to root communication in the regulation of PS root release and some steps of PS biosynthesis. Experiments with specific hormone inhibitors showed that while ethylene and NO did not positively regulate Fe-deficiency induced PS root release, auxin plays an essential role in the regulation of this process. Moreover, the application of IAA to Fe-sufficient plants promoted both PS root release and TaNAAT gene expression thus indicating that auxin might be involved in the shoot to root signaling network regulating Fe-deficiency root responses in wheat. CONCLUSIONS: These results therefore indicate that PS root release in Fe-deficient wheat plants is directly modulated by the shoot Fe status through signaling pathways involving, among other possible effectors, auxin.

Efficiency of urease and nitrification inhibitors in reducing ammonia volatilization from diverse nitrogen fertilizers applied to different soil types and wheat straw mulching.

BACKGROUND: Some authors suggest that the absence of tillage in agricultural soils might have an influence on the efficiency of nitrogen applied in the soil surface. In this study we investigate the influence of no-tillage and soil characteristics on the efficiency of a urease inhibitor (N-(n-butyl)thiophosphoric triamide, NBPT) and a nitrification inhibitor (diciandiamide, DCD) in decreasing ammonia volatilization from urea and ammonium nitrate (AN), respectively. RESULTS: The results indicate that ammonia volatilization in soils amended with urea was significantly higher than in those fertilized with AN. Likewise, the main soil factors affecting ammonia volatilization from urea are clay and sand soil contents. While clay impedes ammonia volatilization, sand favours it. The presence of organic residues on soil surface (no-tillage) tends to increase ammonia volatilization from urea, although this fact depended on soil type. The presence of NBPT in urea fertilizer significantly reduced soil ammonia volatilization. This action of NBPT was negatively affected by acid soil pH and favoured by soil clay content. CONCLUSION: The presence of organic residues on soil surface amended with urea increased ammonia volatilization, and was particularly high in sandy compared with clay soils. Application of NBPT reduced ammonia volatilization although its efficiency is reduced in acid soils. Concerning AN fertilization, there were no differences in ammonia volatilization with or without DCD in no-tillage soils.

Structural characteristics of phosphoramide derivatives as urease inhibitors. Requirements for activity.

Taking as a reference the structural characteristics of a set of compounds that act as jack bean ( Canavalia ensiformis) urease inhibitors, namely, phenylphosphorodiamidate (PPD), N- n-butylthiophosphorictriamide (NBPT), and N- n-butylphosphorictriamide (NBPTO), we have studied the structure-activity relationships of a series of phosphoramide derivatives for which the activity as urease inhibitors in both in vitro and in vivo assays is known. Molecular modeling studies were carried out, and the results highlighted the relevance of characteristics such as the presence of intramolecular hydrogen bonds, the volume of the fragment involved in the enzyme interaction, and the degree of conformational freedom as well as the HOMO orbital and atomic orbital contributions to the HOMO orbital, electron density, and PEM distributions on the activity of these compounds as urease inhibitors. These data, along with the preliminary docking study carried out, allow us to propose a union mode to the active site of the enzyme for these compounds.

Design, synthesis, and biological evaluation of phosphoramide derivatives as urease inhibitors.

The design, synthesis, and biological evaluation of phosphoramide derivatives as urease inhibitors to reduce the loss of ammonia has been carried out. Forty phosphorus derivatives were synthesized and their inhibitory activities evaluated against that of jack bean urease. In addition, in vivo assays have been carried out. All of the compounds were characterized by IR, (1)H NMR, MS, and elemental microanalysis. In some cases, detailed molecular modeling studies were carried out, and these highlighted the interaction between the enzyme active center and the compounds and also the characteristics related to their activity as urease inhibitors. According to the IC(50) values for in vitro inhibitory activity, 12 compounds showed values below 1 microM and 8 of them represent improvements of activity in comparison to the commercial urease inhibitor N-n-butylthiophosphorictriamide (NBPT) (100 nM) (AGROTAIN). On the basis of the activity results and the conclusions of the molecular modeling study, a structural model for new potential inhibitors has been defined.

Development and agronomical validation of new fertilizer compositions of high bioavailability and reduced potential nutrient losses.

To optimize the economical cost of each unit of fertilizer applied and to reduce the environmental contamination caused by nutrient losses, the development of highly efficient granulated fertilizers is of great importance. This study proposes a strategy that consists of developing specific fertilizers having nutrient release patterns that are dependent on plant activity in the rhizosphere. This type of fertilizer is named "rhizosphere-controlled fertilizer" (RCF fertilizer). This fertilizer is based on the introduction of an organomineral matrix composed of metal [Mg (Ca is also possible), Zn (Fe and other metals are also possible)]-humic phosphates. The presence of this matrix modifies the nutrient release pattern of the fertilizer. In this way there are two main nutrient fractions: (i) a water-soluble fraction or "starter" fraction and (ii) a "rhizosphere-controlled" fraction insoluble in water but soluble by the action of the rhizospheric acids released by plants and microorganisms. This study shows the chemical and structural characterization of the organomineral matrix, as well as its efficiency in slowing the nutrient release rate of the RCF fertilizer, principally with respect to P and N. It is demonstrated how these properties of the matrix were also reflected in the significant reduction in both ammonia volatilization and N leaching in a pot system consisting of wheat plants cultivated in a calcareous soil and fertilized with a RCF fertilizer.