Although efforts have been made to sample microorganisms from polar regions and to investigate a few of the properties that facilitate survival at freezing or subzero temperatures, soil communities that overwinter in areas exposed to alternate freezing and thawing caused by Foehn or Chinook winds have been largely overlooked. We designed and constructed a cryocycler to automatically subject soil cultures to alternating freeze-thaw cycles. After 48 freeze-thaw cycles, control Escherichia coli and Pseudomonas chlororaphis isolates were no longer viable. Mixed cultures derived from soil samples collected from a Chinook zone showed that the population complexity and viability were reduced after 48 cycles. However, when bacteria that were still viable after the freeze-thaw treatments were used to obtain selected cultures, these cultures proved to be >1,000-fold more freeze-thaw tolerant than the original consortium. Single-colony isolates obtained from survivors after an additional 48 freeze-thaw cycles were putatively identified by 16S RNA gene fragment sequencing. Five different genera were recognized, and one of the cultures, Chryseobacterium sp. strain C14, inhibited ice recrystallization, a property characteristic of antifreeze proteins that prevents the growth of large, potentially damaging ice crystals at temperatures close to the melting temperature. This strain was also notable since cell-free medium derived from cultures of it appeared to enhance the multiple freeze-thaw survival of another isolate, Enterococcus sp. strain C8. The results of this study and the development of a cryocycler should allow further investigations into the biochemical and soil community adaptations to the rigors of a Chinook environment.

A novel, pulse-modulated spectroscopic system for measuring fractional leghemoglobin oxygenation and infected cell 02 concentration (0) in intact attached nodules of soybean (Glycine max) is described. The system is noninvasive and uses a pulsed (1000 Hertz) light-emitting diode coupled to an optical fiber to illuminate the nodule with light at 660 nanometer. A second optical fiber receives a portion of the light reflected from the nodule and directs this to a photodiode. A lock-in amplifier measures only the signal from the photodiode which is in phase with the pulsed light from the light-emitting diode, and the voltage output from the amplifier, proportional to reflectance, is used to calculate fractional leghemoglobin oxygenation and the nanomolar concentration of free 02 in the infected cells of the nodule (0 Studies with a variety of legumes have suggested that the diffusion barrier in the nodule inner cortex is variable (11,12,22,27) perhaps through regulation of the relative proportion of water and gas which occupies the intercellular spaces in this tissue layer (12). In many treatments in which nitrogenase activity is inhibited, the resistance to gas diffusion into the nodule is greater than that in nodules of untreated plants (23,26), suggesting that treatments inhibitory to nodule metabolism and nitrogenase activity act by increasing diffusion resistance and thereby decrease the availability of 02 within the infected cells. In this paper, we test this hypothesis using a novel pulse-modulated spectroscopic technique to measure the infected cell 02 concentration ofsoybean nodules in which nitrogenase activity has been inhibited by different treatments.N2 fixation in legume nodules is adversely affected by a wide range of environmental and physiological conditions. For example, drought ( 14,18,25),23), low temperature (14,25), and various treatments which disrupt phloem supply to nodules (9,23,24) are all known to inhibit nitrogenase activity. In treatments which reduce phloem supply to nodules, the mechanism of nitrogenase inhibition has been attributed to a reduction in the availability of carbohydrate, which would presumably reduce the pool size of reductant required by nitrogenase. However, studies have shown that these inhibitions of nitrogenase activity can be partially or fully recovered by increases in rhizosphere PO2 (9,23), indicating that nitrogenase is limited by 02 supply (and thereby ATP availability) rather than reductant availability.

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Abstract. Positron emission tomography (PET) has been utilized to obtain dynamic images of long distance nutrient translocation in plants. Positron emitting 18F, produced by a Van de Graaff accelerator using the reaction 18O(p,n)18F, was fed in solution to excised stems of Glycine max positioned vertically in a large‐aperture PET detector system. Images of tracer activity were recorded with a time resolution of 0.5 min and a spatial resolution of 4 mm. Maximum tracer activities at stem sites were obtained within 3 min of the pulse feed. A model is presented enabling evaluation of regional values for tracer flow, tracer binding, flow speed and flow volume. Analysis of data for one stem position yielded a flow volume of 2.1mm3 min−1 and a flow speed of 36cm min−1. Comparison with the distribution of 14C‐inulin, which was simultaneously fed to the cut stems, indicates the 18F is suitable for use as an apoplastic tracer; 92% of the tracer activity accumulated in the leaves. The fraction of 18F that remained bound was most concentrated at stem nodal regions, an observation consistent with the existence of transfer cells at these sites. Advantages and limitations of PET applied to plant physiological investigations are discussed.

The passage of the alkali metals Li, Na, K, Rb, and Cs through saturated phosphatidylcholine membranes has been measured using particle induced gamma ray and x-ray emission to observe the ions. Simultaneous measurements of these five cations has not been possible with more traditional methods involving ion specific electrodes or radioactive tracers. To the authors’ knowledge this is the first time this technique has been used in lipid bilayer research. The ion leakage was determined for an incubation period of 30 min at several temperatures which spanned the bilayer melting temperature. The dependence of this permeability on temperature, acyl chain length, and ion size is described theoretically in terms of the density fluctuations in the bilayer. A leakage rate which depends on the mass of the ion was observed. This dependence shows that the heavier the ion the faster it diffuses out of the vesicle. The effect of this selectivity is shown to be most pronounced near the melting temperature of the lipid. Moreover, this size dependence suggests that the permeating entity is a hydrated ion, rather than a naked ion or a neutral, bound ion pair.