Effect of fusicoccin and gibberellic acid on primary dormant Avena fatua caryopses

Fusicoccin induced germination in dormant and partially afterripened dormant caryopses of Avena fatua L. The rate of caryopsis germination was slower and final percentage germination lower in the highly dormant inbred line M73 at a given concentration of fusicoccin than in the dormant caryopses of line AN265. Gibberellic acid was more effective than fusicoccin in breaking dormancy in both lines. Promotion of germination of dormant caryopses by fusicoccin was inhibited by a 6-day pretreatment with (2-chloroethyl)trimethylammonium chloride.
The basal rate of proton efflux from embryos isolated from dormant and fully afterripened line AN265 caryopses was similar. Addition of fusicoccin increased the rate of proton efflux from the isolated embryos of dormant and afterripened caryopses by nearly 400%. Gibberellic acid had no effect on the rate of proton extrusion. The uptake of ⁸⁶Rb⁺ in dormant and afterripened A. fatua embryos was similar after a 2 h uptake period. The addition of fusicoccin to the medium doubled the uptake of ⁸⁶Rb⁴ by dormant and afterripened embryos. Gibberelleic acid had no effect on the uptake of ⁸⁶Rb⁺ by isolated embryos from either dormant or afterripened caryopses. The experimental results indicate that gibberellic acid is more versatile in its action than fusicoccin, and gibberellic acid may facilitate dormant A. fatua caryopsis germination by stimulating mechanisms other than the direct H⁺ efflux and K⁺ uptake at the membrane level.     

Nonstructural carbohydrates in dormant and afterripened wild oat caryopses

Nonstructural carbohydrates were determined in both embryo and endosperm of dormant (nongerminating) and afterripened (germinating) intact caryopses of wild oat ( Avena fatua L.). No changes in endosperm starch or soluble sugar were observed at the onset of germination (18 h). No changes in glucose, fructose, sucrose or starch within dormant or afterripened embryos correlated with onset of visual germination. In afterripened embryos, depletion of raffinose (18 h), stachyose (18 h) and galactose (24 h) was correlated with germination. In contrast, raffinose-family oligosaccharide levels in dormant embryos remained constant for 7 days following imbibition. Germination of isolated dormant embryos on 88 m M galactose-containing media was accompanied by decreased endogenous levels of raffinose and stachyose. Isolated embryos from dormant caryopses incorporated ¹⁴C from ¹⁴C-fructose into both raffinose and stachyose during 24 h of imbibition. In contrast, no ¹⁴C incorporation into stachyose was observed in embryos from afterripened caryopses. No ¹⁴C incorporation into raffinose was observed at 18 and 24 h. When in vitro activities of α galactosidase were measured, no temporal differences between dormant or afterripened caryopses were detected in either embryo or endosperm tissue. Although the mechanism associated with differences in utilization of raffinose and stachyose is yet unidentified, alterations in raffinose-family oligosaccharide metabolism in the embryo appear to be a unique prerequisite for afterripening-induced germination.     

The Effect of Potassium Nitrate on the Reduction of Phytophthora Stem Rot Disease of Soybeans, the Growth Rate and Zoospore Release of Phytophthora sojae

The effects of potassium nitrate (KNO₃) application on Phytophthora stem rot disease reduction of Glycine max (L.) Merr. cvs. Chusei-Hikarikuro and Sachiyutaka, and mycelium growth and zoospore release of a Phytophthora sojae isolate were investigated under laboratory conditions. The application of 4–30 m m KNO₃ prior to inoculation greatly reduced incidence of disease in the two soybean cultivars. Although a concentration of 20–30 m m KNO₃ led to a slight decrease in the growth rate of the PJ-H30 isolate on PDA medium, no significant relationship was observed between inhibition of the growth rate and disease reduction on application of 0.4–10 m m KNO₃. Disease suppression recorded in laboratory experiments using pathogen mycelium was due to the response of plant tissues rather than a direct inhibition of pathogen hyphal growth by the application of KNO₃. The extent of disease reduction was related to increased potassium concentration in plants of the two cultivars (except for some cases involving cv. Sachiyutaka), suggesting that differences existed between the two cultivars in terms of the effect of KNO₃ application on disease suppression. Scanning electron microscopic observation with fresh samples indicated marked accumulation of potassium at the penetration-stopping sites of P. sojae in the cortex layer of soybean plants treated with 30 m m KNO₃, compared with the non-treated control plants. The presence of 0.4–30 m m KNO₃ decreased the release of zoospores. These results suggest the possibility of applying a solution containing 20–30 m m of KNO₃ to decrease the incidence of disease in agricultural fields by the response of plant tissues to KNO₃.     

Induction of vivipary in Avena fatua

An investigation was conducted under controlled conditions to determine whether treatments designed to maximize the availability of water during seed development could induce viviparous germination in wild oats ( Avena fatua L.). Panicles of three genetic lines, which differed in their degree of dormancy, were kept in darkness at ca 100% RH and 20±1°C and were either supplied with water through the cut end of the rachis or left attached to the plant which was exposed to light. In the non-dormant line, germination of both primary and secondary caryopses on excised panicles increased with their stage of development when treated, i.e., 0, 5 and 10 days after anthesis. Germination of primary caryopses varied between 70 and 80% and was similar on both isolated and attached panicles treated at 10 and 5 days after anthesis, respectively. The percentage germination was considerably lower in all treatments of the two dormant lines and was inversely related to the genetically determined difference in their degree of dormancy. In these dormant lines germination was significantly lower on the intact plant than on the detached panicles. Water potential measurements suggested that this difference may be due partly to the transpiration-induced negative ψ_xyin the stem which may contribute to the inhibition of embryo growth and thus to the prevention of viparous germination.     

Increase in glutamate synthase (NADH) activity in maize seedlings in response to nitrate and ammonium nitrogen

Roots and leaves of Zea mays L. cv. Ganga Safed-2 seedlings grown with nutrient solution containing either 10 m M KNO₃ or NH₄Cl or 5 m M NH₄NO₃ had considerably higher glutamate synthase (NADH, EC 1.4.1.14) activity than the corresponding organs from seedlings grown without any nitrogen. The supply of inorganic nitrogen for a short time, i.e. 3 h, to roots and leaves excised from seedlings grown without nitrogen also increased the enzyme activity in these organs. This increase was more pronounced with nitrate than with ammonium nitrogen. When excised roots and leaves from NH₄NO₃-grown seedlings were incubated in a minus nitrogen medium for 24 h, the enzyme activity declined considerably. This decline was inhibited to some extent by nitrogen, especially by nitrate. Inorganic nitrogen prevented similarly the decline in in vitro enzyme activity during 24 h storage at 25°C, more regularly for the root than for the leaf enzyme. The experiments demonstrate the role of inorganic nitrogen in the regulation of glutamate synthase activity.     

Effect of nitrate on nitrogen fixation and nodule carbohydrate and organic acid concentrations in pea mutants deficient in nitrate reductase

Nitrate inhibits symbiotic N₂ fixation and a number of hypotheses concerned with NO₃⁻ assimilation have been suggested to explain this inhibition. These hypotheses were tested using a pea ( Pisum sativum L. cv. Juneau) with normal nitrate reductase NR; (EC 1,6,6,4) activity and two mutants of cv. Juneau, A317 and A334, with impaired NR activity. The plants were inoculated with three strains of Rhizobium leguminosarum and grown for 3 weeks in N-free medium, followed by 1 week in medium supplemented with 0, 5 or 10 m M KNO₃ before harvesting. NO₃ was taken up at comparable rates by the parent and the mutants and accumulated in leaf and stem tissue of the latter. Acetylene reduction rates were inhibited similarly in both the parent and mutants in the presence of KNO₃ but there were differences among rhizobial strains. Starch concentration of the nodules decreased by 46% in the presence of KNO₃ and there were differences among rhizobial strains but not among pea genotypes. Malate and succinate accumulated in nodules in the presence of KNO₃. These data are not consistent with the photosynthate deprivation hypothesis as a primary mechanism for NO₃ inhibition of N₂ fixation since NO₃ affected the nodule carbohydrate composition of all three pea genotypes in a similar manner. The lack of correlation between NR activity and NO₃ inhibition of N₂ fixation suggests that NO₃ assimilation may be only indirectly involved in the inhibition phenomenon.     

Dormancy-associated embryonic mRNAs and proteins in imbibing Avena fatua caryopses

The mechanisms controlling seed dormancy maintenance and release are not understood. To characterize the molecular events accompanying dormancy release, two-dimensional gel electrophoresis was used to monitor changes in soluble proteins and in vitro translation products of embryonic mRNA populations during imbibition of dormant and nondormant (after-ripened) Avena fatua L. caryopses. No differences were observed between in vitro translation products of mRNA extracted from dry dormant and nondormant embryos. However, the expression patterns of several imbibition- and germination-associated mRNAs were temporally modulated during the first 24 h of imbibition. Two dormancy-associated mRNAs, represented by polypeptides D₁ and D₂, were differentially overexpressed in dormant embryos after 3 h of imbibition. mRNA levels for D₁ and D₂ were about 8- and 3-fold higher, respectively, in dormant embryos than in nondormant embryos after 3 h of imbibition. Overexpression of D₁ continued through 12 h of imbibition, while expression of both mRNAs fell to low and equivalent amounts in dormant and nondormant embryos after 24 h. Similar dormancy-associated changes in two soluble proteins were observed during imbibition. The results demonstrate that steady-state levels of specific mRNAs and proteins change during early imbibition of dormant and nondormant A. fatua embryos and indicate that these changes may be associated with differential gene expression responsible for the maintenance of dormancy.     

Water uptake and distribution in non-dormant and dormant wild oat (Avena fatua L.) caryopses

The influence of seed coat modification and light quality onwater uptake and distribution in caryopses of dormant and non-dormantlines of wild oat (Avena fatua L.) was determined using NMRmicroimaging. Non-dormant seeds absorbed water more rapidlythan dormant seeds during imbibition on distilled water. Thiseffect was detected first in the embryo-scutellar region (8h) and later in the proximal endosperm (12 h). Cutting the testaand pericarp close to the embryo or scarification with KOH promotedrapid embryo/scutellum hydration and germination. Cutting atthe middle part of the caryopsis did not enhance embryo hydrationnor did it greatly improve germination. The sensitivity of waterdistribution to the phytochrome germination effect was examined.Significant differences in imbibitional water uptake by embryos-scutellumtissue were detected by 18 h following red-light (germinationpromoter) compared with far-red (germination inhibitor) treatment.The results indicated that both the rate and the sequence ofembryo/scutellum hydration were important in initiating germinationin dormant seeds. A refinement of the model that describes waterimbibition in wild oat seeds during the early stages of germinationis discussed. Key words: Water uptake, water distribution, Avena fatua, seed coat modification, light quality, dormant and non-dormant seeds     

Factors affecting the induction and release of secondary dormancy in Kalanchoë seeds

Abstract. Several short daily R irradiations are required from the first day of incubation on water to induce germination of Kalanchoë seeds. When the same light treatment is given after a prolonged dark incubation period at 20°C, secondary dormancy prevents germination. Factors controlling the induction and breaking of secondary dormancy have been investigated. The induction of secondary dormancy is very temperature dependent. Locally puncturing the seed coat strongly delays it. Secondary dormancy is not induced in the presence of GA₃ during the first 10 d of dark incubation, although this growth substance cannot induce dark germination. Prolonged or cyclic daily R irradiations can relieve secondary dormancy of seeds kept on water, even after a dark period of 20 d. A 24 h treatment at 4°C restores responsiveness to short R exposures of slightly secondarily dormant seeds. The synergism between GA₃ and Pfr in non-dormant Kalanchoë seeds, leading to high effectiveness of even one short FR irradiation, still occurs in seeds made secondarily dormant before transfer to GA₃, but more R or FR irradiations, in combination with GA₃, are required for the release of secondary dormancy. A combination of red light and 6-benzyl-aminopurine is ineffective in removing dormancy.     

Loss of ionic and osmotic regulation in Chara internodal cells in concentrated KCl solutions

Intact internodal cells of Chara are known to maintain their osmotic pressures at constant levels in artificial pond water at room temperature. Cell fragments with osmotic pressures higher and those cell fragments with osmotic pressures lower than the original, both of which are prepared from intact internodal cells using transcellular osmosis and ligation with threads, can also return their osmotic pressures to the original level within a week in artificial pond water. These regulatory phenomena are realized mainly by extrusion of K⁺ and Cl⁻ in the cytoplasm and/or vacuole or by absorption of K⁺ and Cl⁻ from the external solution. According to the electrochemical potential difference calculated for K⁺ between the vacuole and the external solution, the cells should be able to maintain these regulatory functions even in 50–100 m M KCl+ 1 m M CaCl₂ solutions. However, novel phenomena were observed when they were immersed in such concentrated KCl solutions. To maintain electroneutrality, their osmotic pressures increased up to ca l MPa in 2 days due to absorption of K⁺ and Cl⁻ and many gradually died over time. Ionic and osmotic reguratory functions of Chara cells were lost when they were immersed in 50–100 m M K-salt solutions containing 1 m M Ca²⁺.     

SALT-DEPENDENT PROPERTIES OF HATCHING ENZYME FROM EMBRYOS OF THE SEA URCHIN, HEMICENTROTUS PULCHERRIMUS

The effect of salts on hatching enzyme and protease from the embryo of the sea urchin, Hemicentrotus pulcherrimus , was investigated. The culture medium containing hatching enzyme secreted from the hatched blastula was dialyzed against Tris-HCl (pH 8.0) with or without salts. Both hatching enzyme and protease were activated and stabilized by CaCL₂, NaCI and KCI, while inhibited by MgCI₂. Protease activity was maximal at about 0.25 M NaCI. KCI, NH₄, CI and LiCI. Maximal activity of hatching enzyme was obtained at 0.5 M NaCl, KCI and NH₄ CI, while activity was inhibited by any concentration of LiC1. Among monovalcnt cations, the order of activation was NaCI, KCI > NH₄Cl. The activity of hatching enzyme was stabilized by dialysis against 1 M NaCI or KCI in the presence of CaCl.₂, but was rapidly lost by dialysis against lower concentrations of salts. Reactivation of hatching enzyme was not achieved by redialysis against I M NaCI. On the other hand, protease was reactivated by I M NaCI or KCI. From these results, hatching enzyme of the sea urchin may be called a moderate halophilic enzyme. It was assumed that at least two enzymes exist in the crude enzyme preparation and that they may have different functions.     

Nitrate reductase activity in nodules of pea inoculated with hydrogenase positive and hydrogenase negative strains of Rhizobium leguminosarum

The nitrate reductase (NR, EC 1.6.6.1) activity in root nodules formed by hydrogenase positive (Hup⁺) and hydrogenase negative (Hup⁻) Rhizobium leguminosarum strains was examined in symbioses with the pea cultivar Alaska ( Pisum sativum L.), Rates of activity were determined by the in vivo assay in nodules from plants that were only N₂-dependent or grown in the presence of 2 m M KNO₃. The rates varied widely among strains, regardless of the Hup phenotype of the R. leguminosarum strain used for inoculation, but the overall results indicated that nodules formed by Hup⁻ strains accumulated more nitrite in the incubation medium than did those with Hup⁻ phenotypes. Total plant dry weight and reduced nitrogen content of pea plants grown in the presence of 2 m M KNO₃ and inoculated with single Hup⁺ and Hup⁻ R. leguminosarum strains were statistically different among some strains. These observations suggest that the possible advantages derived from the presence of the Hup system on whole plant growth may be counteracted by the higher rates of NR activity in the Hup⁻ strains in the R. leguminosarum -pea symbiosis.     

Assimilation of nitrate and ammonium by the Scots pine (Pinus sylvestris) seedling under conditions of high nitrogen supply

Seedlings of Scots pine ( Pinus sylvestris L.) were grown on perlite for 21 days under controlled conditions. Apart from the water control, KNO₃ (15 m M ), (NH₄)₂SO₄ (7.5 m M ), and NH₄NO₃ (15 m M ) were offered to study the effects of a high nitrogen supply on nitrogen assimilation. In some experiments 1.3 m M potassium was added to the basic ammonium solutions. In labelling studies nitrate and ammonium were 2.3 atom%¹⁵N-enriched. It was found that over the 21-day period approximately three times more ammonium-N was taken up than nitrate-N. However, nitrate and ammonium, applied simultaneously, were taken up to the same extent as if they were applied separately (additivity). The presence of K⁺ in the medium did not affect N-uptake. Among the soluble N-containing compounds nitrate, ammonium and 8 amino acids were quantified. It was found that assimilation of nitrate can cope with the uptake of NO⁻₃ under all circumstances. Neither free nitrate nor ammonium or amino acids accumulated to an extent exceeding the values of water-grown seedlings. On the other hand, in case of high ammonium supply considerably more nitrogen was taken up than could be incorporated into nonsoluble N-containing substance ('protein'). The remaining nitrogen was found to accumulate in intermediary storage pools (free NH₄⁺, glutamine, asparagine, arginine). Part of this accumulated N could be incorporated into protein when potassium was offered in the nutrient solution. It is concluded that potassium is a requirement for a high rate of protein synthesis not only in crop plants but also in conifers.     

The rice coleoptile: an example of anaerobic nitrate assimilation

Nitrate present in rice caryopses can be reduced to ammonium and the ammonium subsequently assimilated by the coleoptile during anaerobic germination. All the enzymes of nitrate reduction and ammonia assimilation are present in the coleoptile. The supply of ¹⁵NO₃ confirms that the nitrate nitrogen is anaerobically incorporated into amino acids. Under anoxia, nitrate and nitrite reductase activities are increased in the coleoptile by exogenous nitrate. The importance of nitrate utilization during the anaerobic germination of rice caryopses is discussed.     

Effect of different nitrogen nutrients on the viability, protein synthesis and tannin production of Scots pine callus

The effect of two different media on the growth, metabolism and viability of Scots pine ( Pinus sylvestris ) callus cultures was studied. Inorganic nitrogen in the culture media (modified MS) was in the form of either KNO₃ or NH₄NO₃. The cultures were started from buds of mature Scots pine. Growth was poor on the medium with KNO₃, but this compound had a noticeable effect on the metabolism of the callus, which was reflected in alterations in protein and polyphenol synthesis and the pH of the culture medium. Although the fresh mass, water content and viability of the callus decreased when KNO₃ was the exclusive inorganic nitrogen nutrient, protein synthesis was more abundant. Electrophoretic analyses indicated alterations in the patterns of soluble proteins and purified glycoproteins. Phenylalanine ammonia-lyase (EC 4.3.1.5) activities were high in all the calluses, and concentrations of condensed tannins and their precursors, catechins, were higher than in intact buds. The role of inorganic nitrogen nutrition in the deterioration of tissues is discussed on the basis of the effect of ammonium on the metabolism of pine callus.     

Adenosine A2a Receptor-Mediated Modulation of Striatal [3H]GABA and [3H]Acetylcholine Release

Abstract: The ability of adenosine agonists to modulate K⁺-evoked 4D†-[³H]aminobutyric acid ([³H]GABA) and acetylcholine (ACh) release from rat striatal synaptosomes was investigated. The A_2a receptor-selective agonist CGS 21680 inhibited Ca²⁺-dependent [³H]GABA release evoked by 15 m M KCI with a maximal inhibition of 29 ± 4% (IC₅₀ of ∼4 ± 10 ⁻¹² M ). The relative order of potency of three agonists was CGS 21680 ± 5'- N -ethylcarboxamidoadenosine > R-phenylisopropyladenosine (R-PIA), with the inhibition being blocked by A_2a receptor-selective antagonists (CP 66,713 and CGS 15943A) but not by the A₁-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). When release of [³H]GABA was evoked by 30 mM KCI, no significant inhibition was observed. In contrast, CGS 21680 stimulated the release of [³H]ACh evoked by 30 m M KCI, with a maximal stimulation of 26 ± 5% (IC₅₀ of ∼10⁻¹¹ M ). This effect was blocked by CP 66,713 but not by DPCPX. The A₁ agonist R -PIA inhibited [³H]ACh release, an effect blocked by DPCPX. It is concluded that adenosine A_2a receptors are present on both GABAergic and cholinergic striatal nerve terminals where they inhibit and stimulate transmitter release, respectively. Key Words : GABA—Acetylcholine—Adenosine receptors—Striatum.     

ATP-dependent H+-pumping of a low-density fraction of pea stem microsomes

A low-density fraction of pea ( Pisum sativum L. cv. Alaska) stem microsomes, obtained from a discontinuous sucrose gradient, possessed an H⁺-ATPase able to generate a proton gradient and an electrical potential. The proton pumping was insensitive to monovalent cations, to vanadate and oligomycin, required a permeant anion and was inhibited by nitrate, N, N'-dicyclohexylcarbodiimide and diethylstilbestrol. The H⁺-ATPase had a pH optimum around 6.0–6.5 and was saturable with respect to the substrate Tris-ATP (K_m≅ 0.4 m M ). Ca²⁺ (0.05–1 m M ) induced a dissipation of the ATP-generated δpH without affecting ATPase activity. At physiological concentrations (1–5 m M ), nitrate caused an initial slight increase of the ATP-generated proton gradient followed by a complete dissipation after 2–3 min. The dissipating effect was not caused by inhibition of ATPase activity, since ATP prevented the nitrate-induced collapse of δpH. On the other hand, ATPase activity, evaluated as release of P_i, was not inhibited by concentrations lower than 20 m M KNO₃. These results indicate that nitrate entered the vesicles in response to an electrical potential and then could exit in symport with protons, while Ca²⁺ entered in exchange for protons (antiport).     

Physiological function of water channels as affected by salinity in roots of paprika pepper

The sap flow (Jv) and the osmotic pressure-dependent hydraulic conductance (L₀) of detached exuding root systems from paprika pepper plants (cv. Albar) were measured. Plants stressed with NaCl (30 m M ) and with six times the macronutrients of the Hoagland nutrient solution (6×HNS) were compared with controls grown in complete Hoagland nutrient solution. Jv of +NaCl and +6×HNS plants decreased markedly, but recovered to values similar to those of controls after removal of the treatments. Hydraulic conductance L₀ was always less in NaCl plants than in controls and 6×HNS. A total increase in the ion concentration of the xylem (except Na⁺ and Cl⁻) was observed with both treatments. In control and 6×HNS plants, HgCl₂ treatment (50 μ M ) caused a sharp decline in L₀ to values similar to those of NaCl-stressed roots, but were restored by treating with 5 m M dithiothreitol (DTT). However, in NaCl roots only a slight effect of Hg²⁺ and DTT was observed. In each treatment, there was no difference in the flux of K⁺ into the xylem after HgCl₂ and DTT application. The results suggest that NaCl decreased L₀ of the roots by reducing either the activity or abundance of Hg-sensitive water channels. The putative reduction in water-channel function of NaCl-treated plants did not seem to be due to the osmotic effect.     

The physiological basis of seed dormancy in Arena fatua V. Action of ethanol and other organic compounds

Ethanol (50-200 mM )induced germination in four genetically pure dormant lines of Avena fatua L. The sensitivity to this treatment was moderate immediately after harvest and increased steadily during six months of after-ripening. This sensitivity to ethanol was detectable much earlier during after-ripening than with two other germination promoters, NaN³, and NaNO³, Because ethanol can overcome dormancy in freshly harvested caryopses, the mode of action of ethanol in these caryopses apparently differs from that of the two other promoters, azide and nitrate. Nevertheless, it is clear that induction of germination by the three promoters is fully gibberellin-dependent since in each case this response can be blocked by the administration of 2-chlorocthyl trimethylammonium chloride, an inhibitor of gibberellin biosynthesis. Short-term incubation treatments with ethanol were relatively more effective than continuous treatments. These brief treatments were most effective when presented near the beginning of seed imbibition. Among other organic compounds tested only acetaldehyde significantly promoted germination in all lines tested. Propan-1-ol, butan-l-ol, chloral hydrate, procaine, methanol and chloroform were marginally effective on the least dormant lines, while ether, formaldehyde, acetone and ethyl acetate were ineffective. The mode of action of ethanol in overcoming dormancy in both freshly harvested and partly after-ripened caryopses is discussed and the possible role as a metabolic substrate or anaesthetic is indicated.     

EFFECTS OF MEDIA OF DIFFERENT IONIC COMPOSITION ON THE ACTIVATION POTENTIAL OF ANURAN EGG CELLS

Activation potentials of the eggs of two anuran species, Bufo vulgaris formosus and Rana rugosa , were measured in media of different ionic composition, and the mechanism of their generation is discussed. The same trends of ionic effects upon the activation potential were consistently obtained in both species.
The membrane potential of the unactivated eggs was negative with respect to tap water, Ringer's solution, and the media described below except isotonic KCl and KNO₃, in which the potentials were nearly zero or sometimes slightly positive. Upon activation induced by pricking in tap water, isotonic Na₂SO₄, or 42 mM or 63 mM buffered sodium phosphate solution, depolarization followed by reversal of the membrane potential took place, associated with a decrease in the effective resistance. Then the potential gradually decreased and returned to the initial value.
On the other hand, in Ringer's solution, or in isotonic NaBr, NaNO₃, KCl or KNO₃, the activation potential occurred in a hyperpolarizing direction, associated with a decrease in the effective resistance.
These results suggest that the ooplasmic membrane of the anuran egg surface during activation is selectively permeable to NO₃⁻ and Br⁻ as well as Cl⁻, but not to SO₄⁻ and phosphate ions, or alternatively, that the activation potential is due to this selective permeability and to the concentration gradients of these monovalent anions across the ooplasmic membrane.