Electrophysiological properties of onion guard cells

Intracellular electrical recordings in onion (Allium cepa L.) guard cells show that they maintain a membrane potential difference (MPD), inside negative. The MPD of intact cells averaged -72±29 mV (n=45); MPD of cells partially digested with a cellulolytic enzyme, -39±7 mV (n=65). Evidence indicates that the guard cells have two electrically distinct compartments, presumably delimited by the plasmalemma and tonoplast. Epidermal cells in partially digested preparations also showed MPD that could be either positive (+15±7 mV; n=23) or negative (-15 ±8 mV; n=13). Guard cells exposed to light-dark cycles hyperpolarized in the light and depolarized in the dark. The largest observed voltage changes reached 52 mV during hyperpolarizations and 60 mV during depolarizations. The light responses saturated with roughly exponential kinetics, with the depolarizations exhibiting a slower second phase that might be related to the contracting movements of the guard cells. Initial rates of the responses averaged about 14 mV min^-1 in the dark and about 8 mV min^-1 in the light. The results can be interpreted as electrical correlates of fluctuations in intracellular potassium concentration, as light-induced changes in membrane permeability, or as the photoactivation of an electrogenic proton pump. The last possibility seems to be the simplest interpretation of the data that also provides us with a mechanism driving the ion fluxes associated with stomatal function.     

Restoration of membrane potential in mitochondria deenergized with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP)

The membrane potential (Δψ) of rat liver mitochondria dropped upon addition of carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) but was gradually and fully restored to the original value by the subsequent addition of dithioerythritol. Concomitantly, Ca²⁺ released from mitochondria was reaccumulated and the oxidative phosphorylation process completely recoupled. Neither of these effects has been observed with dinitro-o-cresol or 2,4-dinitrophenol, uncouplers which, unlike FCCP, do not react with thiols. Δψ abolished by FCCP was also restored, though incompletely, by albumin; a prompt and complete restoration was however achieved upon subsequent addition of dithioerythritol. Dithioerythritol also completely and rapidly restored the Δψ decreased by addition of diazene dicarboxylic acid bisdimethylamide (diamide).     

Ion flow through membranes and the resting potential of cells

Summary A knowledge of the relationship between ion flow, both passive and active, ionic concentration, and membrane potential is essential to the understanding of cellular function. The problem has been analyzed on the basis of elementary physical and biophysical principles, providing a theoretical model of current flow and resting potential of cells, including those in epithelia. The model assumes that the permeability of the ion channets is not voltage dependent, but applies to gated channels when the gates are open. Two sources of nonlinearity of the current-voltage relationship are included in the analysis: ionic depletion and accumulation at the channels' mouths, and channel saturation at higher concentrations. The predictions of the model have been quantitative, validated by comparison with experiment, which has been limited to the only two cases in which adequate data was found. Application of the theory to the scala media of the mammalian cochlea has explained the source of its high positive potential and provided estimates of the Na⁺ and K⁺ permeabilities of the membranes of its marginel cess. This analysis provides a theoretically sound alternative to the widely used Goldman equation, the limited validity of which was emphasized by Goldman (D.E. Goldman, 1943,J. Gen. Physiol.27:37–60), as well as its derivatives, including the Goldman-Hodgkin-Katz equation for resting potentials.     

Estimating ecotoxicological risk and impact using indigenous aquatic microbial communities

Emphasis has increased on accuracy in predicting the effect that anthropogenic stress has on natural ecosystems. Although toxicity tests low in environmental realism, such as standardized single species procedures, have been useful in providing a certain degree of protection to human health and the environment, the accuracy of such tests for predicting the effects of anthropogenic activities on complex ecosystems is questionable. The use of indigenous communities of microorganisms to assess the hazard of toxicants in aquatic ecosystems has many advantages. Theoretical and practical aspects of microbial community tests are discussed, particularly in related to widely cited problems in the use of multispecies test systems for predicting hazard. Further standardization of testing protocols using microbial colonization dynamics is advocated on the basis of previous studies, which have shown these parameters to be useful in assessing risk and impact of hazardous substances in aquatic ecosystems.     

Relations between water content, potential and permeability in stems of conifers

Abstract. The influence of sapwood water content on the conductivity of sapwood to water was measured on stem sections of Pinus contorta. A reduction in relative water content from 100 to 90% caused permeability to fall to about 10% of the saturated value.
Pressure–volume curves of branchwood and stem sapwood of Pinus contorta and Picea sitchensis have been analysed to definè the tissue capacitance and the time constant and resistance for water movement between stored water and the functional xylem as functions of tissue water potential. Three phases in water loss were discernible. In the initial phase at high water potentials (> –0.5 MPa), the capacitance was large, the time constant long and the resistance to flow large in comparison with intermediate water potentials (−0.5 to −1.5 MPa). At still lower water potentials (−1.5 to −3.0 MPa), the time constant and resistance declined still further but the capacitance had a tendency to increase again, especially in the stemwood of Sitka spruce. Typical values in the second phase were for the time constant 5 s, for the resistance 4 × 10⁻¹³ N s m⁻⁵ and for the capacitance (change in relative water content per unit change in potential) 1×10⁻¹¹ m³ Pa⁻¹. These parameters define the availability of stored water and are being used in a dynamic model of water transport in trees.     

A respiration-dependent primary sodium extrusion system functioning at alkaline pH in the marine bacterium Vibrioalginolyticus

The membrane potential generated at pH 8.5 by K⁺-depleted and Na⁺-loaded $\text{Vibrio}\text{alginolyticus}$ is not collapsed by proton conductors which, instead, induce the accumulation of protons in equilibrium with the membrane potential. The generation of such a membrane potential and the accumulation of protons are specific to Na⁺-loaded cells at alkaline pH and are dependent on respiration. Extrusion of Na⁺ at pH 8.5 occurs in the presence of proton conductors unless respiration is inhibited while it is abolished by proton conductors at acidic pH. The uptake of α-aminoisobutyric acid, which is driven by the Na⁺-electrochemical gradient, is observed even in the presence of proton conductors at pH 8.5 but not at acidic pH. We conclude that a respiration-dependent primary electrogenic Na⁺ extrusion system is functioning at alkaline pH to generate the proton conductor-insensitive membrane potential and Na⁺ chemical gradient.     

PKC regulation of ion channels: The involvement of PIP2

Ion channels are integral membrane proteins whose gating has been increasingly shown to depend on the presence of the low-abundance membrane phospholipid, phosphatidylinositol (4,5) bisphosphate. The expression and function of ion channels is tightly regulated via protein phosphorylation by specific kinases, including various PKC isoforms. Several channels have further been shown to be regulated by PKC through altered surface expression, probability of channel opening, shifts in voltage dependence of their activation, or changes in inactivation or desensitization. In this review, we survey the impact of phosphorylation of various ion channels by PKC isoforms and examine the dependence of phosphorylated ion channels on phosphatidylinositol (4,5) bisphosphate as a mechanistic endpoint to control channel gating.     

Mycorrhizal colonization of Pinus muricata from resistant propagules after a stand-replacing wildfire

Colonization of mycorrhizal fungi was studied in a Pinus muricata forest on the coast in California, USA, burned by a stand-replacing wildfire in October 1995. Naturally established field seedlings of P. muricata were harvested 1 yr after the fire. The species composition of the mycorrhizal fungi on these field seedlings was dominated by Rhizopogon species, Wilcoxina mikolae and Tomentella sublilacina . Bioassays, set up with soil collected immediately after the fire, were used to determine which mycorrhizal species had colonized the burned area from resistant propagules. The P. muricata seedlings in these bioassays were dominated by suilloid and ascomycetous fungi, the same fungi which dominated the mycorrhizal flora of seedlings in pre-fire bioassays derived from the same forest site, suggesting that resistant propagules were the primary inoculum source for naturally establishing seedlings. Drying of post-fire soil for 1 month raised the number of bioassay seedlings associated with Rhizopogon olivaceotinctus , while the number of bioassay seedlings associated with Rhizopogon ochraceorubens was reduced. Fire appeared to have either stimulated or provided a competitive advantage to R. olivaceotinctus , which increased in abundance on the post-fire bioassay and field seedlings. Soil collected from the burned area was diluted with sterile soil in three different concentrations, and the number and frequency of mycorrhizal taxa on bioassay seedlings decreased with increased dilution. Although precise quantification was not possible, propagules of the Rhizopogon species were much more abundant than those of Tomentella or Wilcoxina species. Differences between the mycorrhizal associates of bioassay seedlings, naturally regenerated seedlings, and different inoculum sources are discussed.     

Greenhouse evaluation of fitness-related reproductive traits in roundup<Superscript>®</Superscript>-tolerant transgenic creeping bentgrass (<Emphasis Type="Italic">Agrostis Stolonifera</Emphasis> L.)

Summary Creeping bentgrass is a very important turfgrass species used extensively on golf course greens, fairways, and tees. One of the challenges of creeping bentgrass management is the control of grassy weeds, most of which respond to herbicides in a similar manner to that of creeping bentgrass. As part of a weed management program for golf courses, Roundup^?-tolerant creeping bentgrass will be simple to employ and more effective in controlling problem weeds than currently available methods. The goal of this research was to evaluate fitness-related reproductive traits in four transgenic creeping bentgrass events modified to express a Roundup^?-tolerant gene, cp4 epsps, to determine if these creeping bentgrass events had gained an unexpected reproductive fitness advantage. We compared transgenic events ASR 333, ASR801 with their nontransformed tissue culture line, C99056L and transgenic events ASR365, ASR368 with their non-transformed tissue culture line, B99061R. Populations of plants from three conventional cultivars were also included for comparison to determine whether significant variations, if present in transgenic events, were novel to the non-transformed organism, Agrostis stolonifera L. Our results showed that none of the four transgenic events surveyed were significantly different from the respective non-transformed tissue culture line plants for the following characteristics: first heading date, anthesis duration, inflorescence length, number of florets per inflorescence, pollen size, and seed-set capacity through open-pollination. One of the transgenic events, ASR333, needed significantly more days for anthesis initiation than the nontransformed tissue culture line, C99056L; while another transgenic event, ASR801, exhibited significantly shorter pollen longevity than plants of the tissue culture line, C99056L. However, ASR801 was not significantly different from the conventional cultivars ‘Penn A-4’ and ‘Penncross’ for pollen longevity. Plants of both transgenic events ASR365 and ASR368 did not differ significantly from plants of the tissue culture line, B99061R, for all characters measured.