A Comparative Study on the Effects of H2S and SO2 Fumigation on the Growth and Accumulation of Sulphate and Sulphydryl Compounds in Trifolium pratense L., Glycine max Merr. and Phaseolus vulgaris L.

Maas, F. M., De Kok, L. J., Peters, J. L. and Kuiper, P. J.C. 1987. A comparative study on the effects of H₂S and SO₂ fumigationon the growth and accumulation of sulphate and sulphydryl compoundsin Trifolium pratense L., Glycine max Merr. and Phaseolus vulgarisL.—J. exp. Bot. 38: 1459-1469. The effects of 0—25 mm³ dm³ H₂S and SO₂ on growth andsulphur content of shoots of Trifolium pratense, Glycine maxand Phaseolus vulgaris were studied. After 2 weeks of fumigationthe yield of T. pralense was reduced by 32% by H₂S, but notaffected by SO₂. Yield of G. max was not affected by H₂S, butreduced by 20% by SO₂, whereas that of P. vulgaris was increasedby 11% by H₂S and not affected by SO₂. Increases in sulphydrylcontent were already observed after 24 h of exposure to H₂Sand SO₂ in all plants. The increase was greatest in T. pratenseand smallest in P. vulgaris and, except for T. pratense, alwaysgreater in the H₂S-exposed than the SO₂-exposed plants. Oneday of exposure resulted in an increase in sulphate contentonly in the SO₂-fumigated plants, with the highest accumulationin T. pratense and the lowest in P. vulgaris. After 2 weeksan increase in sulphate content was also observed in the H₂S-exposedplants. This increase was also highest in T. pratense and lowestin P. vulgaris. Transpiration rate was not affected by a 24 h exposure to H₂Sor SO₂ and was highest in T. pratense, intermediate in G. maxand lowest in P. vulgaris. The order of theoretical rates of deposition of H₂S and SO₂correlated with the observed increases in sulphydryl contentduring the first 24 h of exposure in both H₂S and SO₂-fumigatedplants and with the increase in sulphate content in the SO₂-exposedplants. The increases in sulphydryl content were only 8% ofthe theoretical H₂S and SO₂-deposition fluxes, whereas sulphateaccumulation accounted for at least 57% of the theoretical SO₂-depositionflux. Key words: Air pollution, clover, French bean, Glutathione, Soybean, sulphur metabolism.     

An empirical analysis of the factors contributing to 20-year decrease in soil pH in an old-field plantation of loblolly pine

The pH of weak-acid solutions is controlled by acid concentration (HA + A^–), the degree of acid dissociation (A^–/HA), and the strength of the acids present (pK_a). We developed an empirical approach that allows the relative importance of each of these factors to be estimated for soils. This empirical model was applied to soils collected from an old-field plantation of loblolly pine (Pinus taeda L.) at 5 and 25 years of age. During this period, soil pH dropped by 0.3 to 0.8 units, and extractable calcium, magnesium and potassium declined by 20 to 80%. The empirical model indicates that the decline in pH resulted largely from the reduction in base saturation of the exchange complex. However, the average acid strength of the exchange complex decreased during the 20 years, preventing a greater decline of perhaps 0.1 to 0.2 units in the observed pH. The rate of decrease in the acid neutralizing capacity to pH 3.5 was about 1.3 kmol_c/ha annually, while the increase in base neutralizing capacity was about 2.7 and 1.6 kmol_c/ha annually to pH 5.5 and 8.2, respectively. Extractable alkali and alkaline earth cations declined by about 2.2 kmol_c/ha annually, matched by the rate of increase in aluminium. These changes demonstrated the dynamic nature of poorly buffered soils, and indicated that changes in soil acidity may be expected over a period of decades (especially following changes in land-use).     

Protein engineering of disulfide bonds in subtilisin BPN'

Five single-disulfide mutants were studied in subtilisin BPN', a cysteine-free, secreted serine protease from Bacillus amyloliquefaciens. The disulfides were engineered between residues 26-232, 29-119, 36-210, 41-80, and 148-243. These bonds connected a variety of secondary structural elements, located in buried or exposed positions at least 10 A from the catalytic Ser-221, and linked residues that were separated by 39 up to 206 amino acids. All disulfide bonds formed in the enzyme when the expressed protein was secreted from Bacillus subtilis, and the disulfides had only minor effects on the enzyme kinetics. Although these disulfide bonds varied by over 50-fold in their equilibrium constants for reduction with dithiothreitol, there was no correlation between the strength of the disulfide bond and the stability it imparted to the enzyme to irreversible inactivation. In some cases, the disulfide-bonded protein was stabilized greatly relative to its reduced counterpart. However, no disulfide mutant was substantially more stable than wild-type subtilisin BPN'. Some of these results can be rationalized by destabilizing effects of the cysteine mutations that disrupt interactions present in the folded enzyme structure. It is also possible that the rate of irreversible inactivation depends upon the kinetics and not the thermodynamics of unfolding and so the entropically stabilizing effect expected from a disulfide bond may not apply.     

Site-specific inhibition of EcoRI restriction/modification enzymes by a DNA triple helix.

The ability of oligopyrimidines to inhibit, through triple helix formation, the specific protein-DNA interactions of the EcoRI restriction and modification enzymes (EcoRI and MEcoRI) with their recognition sequence (GAATTC) was studied. The oligonucleotides (CTT)4 and (CTT)8 formed triplexes in plasmids at (GAA)n repeats containing EcoRI sites. Cleavage and methylation of EcoRI sites within these sequences were specifically inhibited by the oligonucleotides, whereas an EcoRI site adjacent to a (GAA)n sequence was inhibited much less. Also, other EcoRI sites within the plasmid, or in exogenously added lambda DNA, were not inhibited. These results demonstrate the potential of using triplex-forming oligonucleotides to block protein-DNA interactions at specific sites, and thus this technique may be useful in chromosome mapping and in the modulation of gene expression.     

Postural dynamics of walking in humans

The dynamics of postural control in human biped locomotion were studied using(1) a model, and(2) experimentally applied impulsive force disturbances. The model was planar, and contained five rigid segments, articulating at frictionless pin joints. The model was used to identify joint torque combinations which would successfully correct for an impulsive force disturbance applied at different points in the walking cycle. The simulation results suggested that(1) early responses (within 80ms) can be effective in compensating for impulsive disturbances,(2) the same strategies which successfully counteract similar disturbances during quiet standing are also effective in certain phases of the walking cycle,(3) modifications in the response strategies are needed to accomodate differences in the dynamics over the stride cycle, and(4) the swing leg is ineffective in compensating for disturbances in the short term. These model predictions were tested experimentally. Subject responses to an impulsive force disturbance applied during walking were studied. The electromyographic results generally support the model predictions.     

Water Balance in Cucumis Plants, Measured by Nuclear Magnetic Resonance, I

In this paper we demonstrate the study of plant water balanceby the non-invasive measurement of tissue water content andwater flow using proton nuclear magnetic resonance (NMR). Sapvelocity and flux were measured independently in the presenceof an excess of stationary tissue water. The instrumentationdescribed allows automated and unattended measurement of flow-and water content-variables in a well-defined region of theplant over periods of several days, with a time resolution betweensuccessive measurements of c. 5 s. Using this apparatus theeffect of changes in light intensity (day/night rhythm) andrelative humidity on stem tissue water content as well as onthe velocity and flux of xylem sap in the stem were investigatedin a cucumber plant. The results are in agreement with predictionsfrom a simple model for plant water balance, which is basedon water potential, flow rate and resistance to flow. As longas only transpiration is varied, flow rate and water content(or potential) are affected in opposite ways as demonstratedin this paper. In contrast, the model predicts that changesin uptake (resulting from changes in, for example, root resistance)will induce changes in water content and flow in the same direction.An experimental verification of this prediction is given ina subsequent paper, where, in addition, the NMR results arecompared to those obtained with a dendrometer. Key words: Water balance model, Cucumis sativus L., flow, water content, NMR, water balance measurement