Combination of micro suction cups and time-domain reflectometry to measure osmotic potential gradients between bulk soil and rhizosphere at high resolution in time and space

A prerequisite to investigate the importance of osmotic potential (Ψ_o) in relation to matric potential (Ψ_m) in the soil for water uptake is the existence of a method that measures the temporal and spatial dynamics of Ψ_o in the vicinity of roots. One method for measuring Ψ_oin situ is the collection of soil solution with micro suction cups, the spatial resolution of which is suitable for rhizosphere studies. A major drawback of soil solution sampling is the disturbance of soil solution equilibrium, which makes frequent measurements impossible, so another method is required to provide information on the temporal dynamics of Ψ_o. The time‐domain reflectometry (TDR) technique might be suitable as the signal attenuation (σ) shows a close linear correlation with the salt concentration for a known soil water content. The temporal resolution of the TDR technique is high and the measurement has no impact on soil solution equilibrium. However, the spatial resolution of the TDR technique is too coarse to be used on its own in rhizosphere studies. We used a combination of TDR (fine temporal resolution) and micro suction cups (fine spatial resolution) to measure Ψ_o in a model system with Zea mays grown in quartz substrates. Osmotic potential changed continuously with time, and a steep gradient between bulk soil and the root compartment developed during the 39‐day growing period. The steepest gradient measured over a distance of 6 mm across the nylon net, separating the bulk soil from the root compartment, was ?365 kPa. The combination of both methods made it possible to extend the time interval between micro suction cup samplings and thus minimize the impact of sampling on soil solution equilibrium. Problems of separate calibration were avoided by calibrating the TDR measurements against the results obtained with the micro suction cups within the same experiment.     

Andosols and soils with andic properties in the German soil taxonomy

The presence of soils with andic properties on German territory has been suspected for decades and there are numerous reports of sites where they may potentially occur. Andic properties, however, are not adequately represented by the German soil‐classification system. The German taxonomic category “Lockerbraunerde” has not been revised or reconciled with international taxonomic categories since the year 1957, when it was initially proposed. With this review, we show that there are true Andosols of both the silandic (allophane‐containing) and the aluandic (Al‐Humus‐dominated) type in Germany and that their properties differ substantially from other soils which merely exhibit low bulk density. By (1) comparing soil carbon storage between some German Andosols, Chernozems, and nonandic Cambisols with particularly low bulk density and (2) elucidation of the differential pedogenetic pathways leading to Andosol formation, we further demonstrate that Andosols are important objects of study in research issues of contemporary interest. We propose that appropriate measures be taken to lay the foundations for the protection and conservation of these soils, because they are valuable as archives of natural history and provide opportunities to study unique soil processes.     

RNA-dependent cysteine biosynthesis in archaea

Several methanogenic archaea lack cysteinyl-transfer RNA (tRNA) synthetase (CysRS), the essential enzyme that provides Cys-tRNA(Cys) for translation in most organisms. Partial purification of the corresponding activity from Methanocaldococcus jannaschii indicated that tRNA(Cys) becomes acylated with O-phosphoserine (Sep) but not with cysteine. Further analyses identified a class II-type O-phosphoseryl-tRNA synthetase (SepRS) and Sep-tRNA:Cys-tRNA synthase (SepCysS). SepRS specifically forms Sep-tRNA(Cys), which is then converted to Cys-tRNA(Cys) by SepCysS. Comparative genomic analyses suggest that this pathway, encoded in all organisms lacking CysRS, can also act as the sole route for cysteine biosynthesis. This was proven for Methanococcus maripaludis, where deletion of the SepRS-encoding gene resulted in cysteine auxotrophy. As the conversions of Sep-tRNA to Cys-tRNA or to selenocysteinyl-tRNA are chemically analogous, the catalytic activity of SepCysS provides a means by which both cysteine and selenocysteine may have originally been added to the genetic code.     

ISOLATION OF TRICHOMONAS VAGINALIS WITH PENICILLIN

Exposure of Trichomonas vaginalis in vaginal discharge for 60 hours to 5,000-10,000 units of penicillin in 10.0 ml of a medium containing cysteine (0.15 per cent.), peptone, liver infusion, maltose and human serum was adequate to destroy the associated bacteria. Seven strains were isolated from seven women without a failure. This technic widens the field of investigation by offering a means of obtaining additional strains of bacteria-free Trichomonas vaginalis for comparative study. Efforts to isolate Trichomonas vaginalis by such laborious methods as washing, micropipetting and migration have all failed to yield bacteria-free cultures at this and other laboratories. The adherence of bacteria to the trichomonads, which appear to have sticky surfaces, and the relatively slow speed with which these protozoa swim are probably in large measure responsible for failure. Further trials by these methods now seem unnecessary. In the cases reported above the success of the penicillin method may be attributed to several factors. Resistance of Trichomonas vaginalis to penicillin in the concentrations reported above, the absence from the specimens of vaginal discharge of bacteria resistant to penicillin in these same concentrations, and the destruction of bacteria adherent to the sticky surfaces of the trichomonads perhaps played a large part in assuring successful isolation of the protozoa.