The effect of an intermittent water-table gradient on soil and xylem nitrate in cotton

Summary Cotton (Gossypium hirsutum cv Deltapine 61) was grown in a sloping plot of soil in the field to examine the effect of a gradient of water-table depth on soil nitrate availability and plant uptake during two periods of the growing season. Before the water-table was imposed NO₃ was less concentrated at the lower end of the sloping plot. This was attributed to slow denitrification at microsites within the soil at the lower end which was wetter than further up the plot. At flooding NO₃ disappeared only slowly due to a carbon substrate limitation to denitrification in the soil. This loss occurred primarily in areas where the water-table was high and oxygen concentration in the soil solution was low. Plant NO₃ uptake, assessed by measuring the concentration in the xylem, parallelled the distribution of NO₃ in the soil solution. Under high water-tables xylem NO₃ levels fell but it was not possible to say whether this was due to impaired root function or to the reduced concentration of NO₃ observed in the soil solution. At intermediate water-table depths where soil NO₃ availability remained high xylem NO₃ concentration fell relative to the well drained control plants, suggesting that flooding had damaged the root system.Manufactured by Merck. Mention of commerical names does not imply endorsement by either CSIRO or USDA.     

Application of the Hodgkin-Huxley equations to an electric field model for interaction between excitable cells

We previously described a model for the electrical transfer of excitation from one cell to the next which utilized the electric potential generated in the junctional cleft between the cells. Low-resistance connections between the cells were not used in the model, and it was assumed that the junctional membranes were excitable. This model was analyzed for the static case without capacitances and for the dynamic case in which capacitances were part of the circuit elements. For simplicity, the Na⁺ resistance (R_Na), after a threshold potential was exceeded, was allowed to decrease exponentially (to 1% of its initial value) within 0·25–1·0 ms, and possible changes in the K⁺ resistance were ignored. In this paper, we have incorporated the Hodgkin-Huxley equations into the operation of the lumped membrane units for the electrical equivalent circuit of the cell membrane. The parameters varied are the membrane capacitances, resistances, maximum Na⁺ conductance ($\text{g}{}_{\mathrm{<mtext>Na</mtext>}}$), and the radial cleft resistance (R_jc). We demonstrated that our model worked very well, i.e. the successful transfer of action potentials was achieved, with the membrane units following Hodgkin-Huxley dynamics for changes in g_Na and g_K. The calculations indicate that transmission is facilitated when the junctional units have a higher $\text{g}{}_{\mathrm{<mtext>Na</mtext>}}$ and a lower capacitance and when R_jc is elevated. Lowering the resistance of the junctional membrane units several fold, relative to the surface membrane units, also facilitated transmission; however, the absolute resistance of the junctional membrane was still well above the maximum value that would allow sufficient local-circuit current to flow to effect transmission. Thus, the electric field model provides an alternative means of cell-to-cell propagation between myocardial cells which is electrical in nature but does not require the presence of low-resistance connections between cells.     

Experiments of the amplification of optical activity

By way of investigating possible mechanisms for the abiotic amplification of small enantiomeric excesses (e.e.'s) in almost racemic mixtures of amino acid enantiomers, we have undertaken a quantitative study of the base-initiated partial polymerization of leucine and valineN-carboxy-anhydride (NCA) enantiomer mixtures containing known excesses of both theR- andS-forms. Polymerization to the extent ofca. 50% of leucine NCA having an 8–70% e.e. of either theR- orS-enantiomer led to an e.e. enhancement in the polymer, which contained a 12–84% e.e. of that enantiomer which predominated in the original monomer NCA. A corresponding decrease in the e.e. of the initially predominant enantiomer was noted in the unpolymerized residue from each reaction. Polymerization to the extent of 25–50% of mixtures of valine NCA enantiomers containing a 12–13% e.e. of eitherR- orS-isomer led to polymers showing a 7–8%decrease in the e.e. of the initially predominant enantiomer, and to an increase of its e.e. in the unpolymerized residue. These divergent results, the latter of which is quite novel, are compared with earlier qualitative results in the literature and are discussed briefly from the viewpoint of both mechanism and the amplification of optical activity.A portion of this material was presented at a symposium on The Origins of Optical Activity in Nature, Chemical Institute of Canada, University of British Columbia, Vancouver, B.C., June 5, 1979.     

Determination of oestrogen monoglucuronides by the Brown method

In the Brown (Brown, 1955; Brown, Bulbrook & Greenwood, 1957) procedure for the determination of urinary oestrogens, losses may occur during the hydrolysis of the oestrogen conjugates and during the purification of the oestrogens thus formed. Losses during the latter stages were measured previously by adding free oestrogens to the hydrolysed urine. In the present study losses during acid and enzyme hydrolysis were measured by adding synthetic oestrogen monoglucuronides to urine.     

Effect of Intraluminal pH on the Absorption of Pteroylmonoglutamic Acid

The administration of pteroylmonoglutamic acid and either sodium bicarbonate or phenytoin sodium in normal subjects and epileptics results in lower serum levels of folic acid than when administered alone. Bicarbonate administration was shown to produce a more alkaline pH in the jejunum, while an alkaline pH was found in some epileptics on phenytoin therapy. The physicochemical mechanisms whereby folic acid absorption is reduced in an alkaline medium are considered, and the implications on folate absorption in other disease states are discussed. The mechanism by which phenytoin sodium alters the intrajejunal milieu is obscure.     

Do Microtubules Control Growth in Tropism? Experiments with Maize Coleoptiles

A recently proposed hypothesis [Nick et al. (1990) Planta 181:162] suggests that, in maize coleoptiles, tropistic curvaturemight be caused by a stimulus-induced trans-organ gradient overthe orientation of cortical microtubules adjacent to the outercell wall of the outer epidermis. This gradient, in turn, iscontrolled by a light-induced redistribution of auxin. The hypothesiswas tested by following the behaviour of microtubules for variouslight stimuli using indirect immuno-fiuorescence in epidermalstrips as assay. Analysis of gravitropic straightening, nasticcurvature on the horizontal clinostat, effects of tonic irradiationwith red and/or blue light, and experiments involving opposinglight pulses demonstrate that bending direction and microtubuleorientation gradients are not as closely linked as predicted:Considerable bending can be produced without detectable gradientsof microtubule orientation, and conspicuous gradients of microtubuleorientation are not necessarily expressed as corresponding curvature.Thus, a monocausal relationship between microtubules and tropismis excluded. Furthermore, a comparison of tonic light effectson microtubules to earlier studies into the impact of lightupon auxin content indicate that the relationship between auxinand microtubules might be more complex than hitherto assumed.It is concluded that, at least in maize coleoptiles, growthcan be regulated by various mechanisms, and that microtubules,although somehow related to tropism, are probably not the causeof the fast tropistic responses. (Received April 26, 1991; Accepted July 17, 1991)