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Summary The sodium-induced relative drop of the membrane potential Δ E ′ E A in corn coleoptile cells depends on the ionic strength as well as on the calcium ion content of the exchange medium. The drops follow linear logarithmic curves. The same situation is true for the potassium-induced drops. From these experimental findings a formula is derived which allows to calculate directly the relative sodium permeability PNa: PK. This permeability correlates with the sodium and potassium ion concentrations of the medium and maintains at the same level, if proper calcium ion concentrations (0.1 to 1.5 mM) are present. Preincubation of the coleoptiles for 16 h with 10−5 M indoleacetic acid led to a decrease of the relative sodium permeability due to an increased potassium permeability because the sodium-induced drop of the membrane potential remains unaffected.

Summary The membrane potentials of corn coleoptile cells are measured during the alteration of the potassium ion concentration within the medium. The relative change Δ E E A of the membrane potential is logarithmic-linear in respect to the ionic strength of the medium as well as to the calcium ion content. From these findings equations are derived which express the membrane potentials as functions of both calcium and potassium ion concentrations of the medium. It is suggested from these equations that low concentrations of calcium (up to 0.2 mM) cause a hyperpolarization followed by a depolarization at higher calcium levels (0.2—6 mM). Furthermore, rising concentrations of calcium ions within the medium depress the potassium-induced drop of the membrane potential, especially at low potassium ion concentrations (up to 10 mM).

Summary According to the Hodgkin-Horowicz equation originally derived from an electrical analogues of the membrane, the potential drop Δ E induced by a sudden change of the external potassium ion concentration Δ K should be linear with respect to the concentration term In(l + Δ K/K 01 ). Analyzing 24 experimental investigations taken from the literature we found that in most cases the above relation does not hold. Taking into account the ionic strength of the medium a modified voltage-concentration relation is derived containing besides others the transport number TK of the Hodgkin-Horowicz equation. All the data taken from the literature agree very well with this modified relation. Some consequences and predictions are discussed.

Summary By common glass microelectrode technique the membrane potential of corn coleoptile cells is measured during both slowly increasing (1 °C min -1 ) and suddenly raising temperature. Furthermore the response on heat pulses at various basic temperature (19-35 °C) is followed. Raising the temperature leads to an initial increase of the potential which is followed by a steep decrease and a subsequent restoration of the membrane potential. The results show that the membrane potential is only transiently affected by heat probably due to a temperature-dependent (ion transport?) system adjusting the membrane potential at a certain level.

Summary The membrane potential of corn coleoptile cells measured by the microelectrode technique is markedly affected by rapid temperature drops but only slightly influenced by slowly decreasing temperature. The passive component of the membrane potential measured in KCN-treated cells (0.5 mM) depends in a non-linear way upon temperature. The electrogenic part of the membrane potential, calculated from the differences in between the total and the diffusion potential, increases with decreasing temperature in slowly cooled cells. Neglecting the possibility of a negative activation energy of the electrogenic pumps our observation may be explained by cold-induced changes of the lipid composition of the membrane enhancing the electrogenic pump activity.

Summary Using glass microelectrodes the influence of enhanced external ion ( K+ or Na+) concentrations on membrane potential in dwarf corn coleoptile cells is measured. From that results a modified form of the Hodgkin-Horowicz-equation, containing both the relative ion conductivity Tj and the ionic strength of the medium, is derived. Making use of this modified equation the passive and energy-dependent parts of the Tj’s are estimated from experiments in presence or absence of 0.5 mM KCN, respectively. It is found, that indoleacetic acid affects an electrogenic ion transport system, probably a proton pump, whereas gibberellic acid strongly influences the passive ion conductivity.

Summary The cell potential of corn (Zea mays L.) coleoptile cells is decreased within a few seconds with increasing concentrations of potassium and sodium ions in the external solution. The effects of these ions upon the membrane potential is reduced by both IAA and calcium ions. Both calcium ions and IAA together are less active than calcium ions alone. We conclude that calcium ions as well as IAA act at the plasma membrane.

Summary The potassium-induced drop of the membrane potential of corn coleoptile cells is used to calculate the membrane potential as affected by calcium ions, gibberellic acid, indole acetic acid, and dwarfism. The influence of calcium ions upon the membrane potential is different in tall and dwarf plant species. This difference is diminished by the growth regulating substances. Whether calcium ions decrease or increase the membrane potential of dwarf plant cells depends on the potassium ion concentration. In contrast, the membrane potential of normal tall plant cells is decreased at all potassium ion concentrations used.

Summary The cation content of maize seedlings (Zea mays L.) has been investigated by flame-photometry comparing dwarfs (d1dd) and tall plants (d1NN). Dwarf caryopsis, coleoptiles and primary leaves show a higher calcium content and a lower potassium content per fresh weight as well as per dry weight if compared with the respective organs of the tall form. The intracellular distribution of calcium in coleoptiles and primary leaves does not differ between dwarfs and tall plants. Application of 45Ca to the seedlings shows that the higher 45Ca content of coleoptiles and leaves and the altered pattern of 45Ca distribution between the organs can be explained as a result of the restricted growth of the dwarfs. The higher Ca++ content seems not to be the “reason” of the dwarfism.

Summary 1. Discs of the leaf lamina or segments of the leaf petiole of Nicotiana rustica accumulate nicotine from a 10‒3 molar solution reaching an accumulation quotient of about 30. 2. Nicotine-free leaves are obtained from grafts with Lycopersicon esculentum as the stock and Nicotiana rustica as the scion. These leaves do not differ from normal leaves in their velocity and capacity of nicotine uptake. 3. The accumulation of nicotine is higher in the marginal zone of the leaf discs or in the cells near the cut surface of the leaf petioles, as compared with the central zone. 4. The more powerful accumulation in the marginal zone is not caused by a better supply of nicotine from the solution. Higher accumulation in the marginal zone appears likewise if this zone is not in direct contact with the solution. Nicotine is supplied at the opposite side of the discs or the petioles in this experiment and it has to pass the central zone before it can be accumulated in the marginal zone. 5. Respiration inhibitors (Na-azide 10‒3 molar and DNP 10‒4 molar), darkness, and DCMU have no or slight influence on the accumulation and on the development of the accumulating potency in the marginal zone. Inhibition occurs with high concentrations of DNP (10‒3 molar). 6. Salts inhibit the nicotine accumulation, especially in the marginal zone. Alkali-kations (Na+ and K+) and Ca++ act in the same direction but they differ in concentration efficiency. 7. Phosphate buffer inhibits over the whole pH-range tested in accord with the salt effect. Comparing different pH-values, inhibition increases with lower pH. 8. The higher accumulation of nicotine in the marginal zone is not caused by an acidification of the cell sap, leading to an ion trap mechanism. 9. Light- and electronmicroscopic autoradiography show predominant accumulation of applied radioactive nicotine in the cell walls especially in the xylem and in the marginal zone as well as in chloroplasts. Diffuse accumulation in the vacuole is low. 10. In the chloroplasts, vesiculation occurs under the combined action of nicotine and light.

Summary 1. Slices of the leaf petiole of Nicotiana rustica L. accumulate nicotine from a 10-3 molar solution. The kinetics of this process are composite. A rapid uptake proceeds during the first 15 minutes. During the following 5—6 hours the kinetics are of first order. This part of the uptake process is characterized by the experiments. During this period of time the main quantity of nicotine is accumulated. The energy of activation of the process is ca. 6 kcal/moles. This agrees with a process of passive permeation. 2. Uptake of nicotine is inhibited by salts (KCl, NaCI, 50 mM/I). Salts reduce the capacity for nicotine accumulation. Neither the uptake kinetics nor the energy of activation of the process are changed during the time period, characterized by first order kinetics. 3. A parallelism is described between inhibition of nicotine uptake and a depression of the negative cell membrane potential. This parallelism has been found under the action of salts or of dinitrophenol and azide.

Summary The plasma membrane permeability ratio (P Na : P K ) of dwarf corn coleoptile cells is decreased by 10 −5 M gibberellic acid (GA 3 ) indicating a direct action at the membrane. The effect of calcium ions on the permeability ratio, different in both dwarf and tall coleoptile cells, is affected by GA 3 in dwarf plant species.

Summary The proton efflux from Begonia leaf cells is only slightly enhanced by increasing temperature up to a critical (~ 50 °C) temperature. Above 50 °C the proton efflux suddenly increases reaching about 10 times higher values. This indicates a remarkable thermal stability of the tonoplast and/or plasmalemma, which is, however, influenced by external ion concentrations.

Summary IAA does not act on the membrane potential of cells of exised coleoptiles of dwarf corn (Zea mays L. dIdd) in a incubation medium containing only 1,0 mM KCl and 0,1 mM NaCI. Addition of sucrose (30 mM) lowers the cell potential from about - 133 m V to about - 120 m V. This decrease can be influenced by IAA. 10 −5 to 10 −7 M IAA prevent the decrease of the potential, higher concentration (10 −4 M) increase the depolarizing action of sucrose. Gibberellic acid (GA3) has a similar but less pronounced effect.

Summary Calcium ions cause decrease or increase of the membrane potential of dwarf corn coleoptile cells dependent on the potassium ion concentration of the bathing medium. The Na + : K + permeability ratio, however, is not affected by calcium ions but by the monovalent ion concentration (Nelles, in press). Thus, the different effect of calcium ions on the membrane potential at various potassium ion concentrations seems to be determined by the permeability ratio and not vice versa .

Summary Increasing concentrations of potassium ions (1 to 10 mM) in the medium lower the membrane potential of coleoptile cells. A smaller decrease is obtained after preincubation with IAA (10 −5 M).