Ultrastructural and autoradiographic studies of non-generative cells in the antheridium of Chara vulgaris L. III. Shield cells

Shield cells form the antheridium envelopes. At the first stage of spermatogenesis they grow intensively in the tangential direction, which is stopped during the period of spermatozoid differentiation. The increase in shield cell volumes is associated with the increase in DNA level in the nucleus up to 16-32 C. 3H thymidine incorporation occurs in about 30% of shields at younger developmental stages and lasts until the stage in which 16 celled antheridial filaments predominate. At first stage of spermatogenesis the intensity of 3H leucine incorporation increases as DNA amount in the nuclei increases, reaching the maximum value at the end of this period. During spermiogenesis it gradually decreases. Shield nuclei are characterized by low content of condensed chromatin, the presence of numerous nucleoli with nucleolonema-like structure as well as the occurrence of bands of intranuclear microtubules. It has been suggested that these microtubules are associated with cyclical changes in the shapes of nuclei. During DNA replication the nuclei have the form of flat discs which between successive endoreplication cycles become ring shaped. Peripheral zone of shield cells is compartmentalized through incomplete walls. They support the radial walls of shields increasing the contact surface of plasmalemma with a cell wall. During spermiogenesis the increase in plasmalemma surface results from the growth of shields in the radial direction. The shield cells contain plastids placed close to each other at the inner tangential wall. They are orange in colour and have fully formed system of grana and intergrana thylakoids, like the plastids of the thallus. The number and sizes of the plastoglobules increase as the anteridium develops. Dictiosomes are surrounded with numerous smooth and coated vesicles. Mitochondria exhibit poorly condensed structure. Microbodies adjoining the plastids are sporadically encountered. It has been assumed that changes in structural organization as well as growth character of shield cells constitute the factor regulating the exchange with external environment, determine light spectrum penetrating to the antheridium and the volume of antheridial space.     

The Transport and Metabolism of Urea in Chara australis: III. TWO SPECIFIC TRANSPORT SYSTEMS

Reciprocal competitive inhibition studies were used to showthat N-methyl-urea (NMU), acetamide and urea all compete forbinding to a common transport system, designated system I andthat this system is one of two specific mechanisms transportingurea in Chara. System I binds urea with a Km of about 0–3mmol m-3 and is strongly influenced by metabolic controls. SystemI is active and electrogenic and may be energized by the couplingof urea uptake to an influx of protons. This is the first reportof an electrogenic urea transport system in an alga. The secondspecific mechanism for urea transport, designated system II,binds urea with a relatively low affinity (K_m c. 7–0 mmolm-3) and does not transport NMU to a significant extent. SystemII is less subject to metabolic control than system I and, thoughit may be active, is not electrogenic. Key words: Urea, methylurea, proton cotransport, metabolic control     

Plasmalemma Transport of OH in Chara corallina: III. FURTHER STUDIES ON TRANSPORT SUBSTRATE AND DIRECTIONALITY

The identity of the plasmalemma-transported species that develops the alkaline bands of Chara corallina was investigated. The effect of fusicoccin on the rate of HCO₃⁻ assimilation, and on the time-dependent alkaline band pH buildup following low pH flushing, was found to be small, with no stimulatory effect. Computer simulation of the flushing experiments showed that in the experimental situation the alkaline band transport system was slowed down, rather than speeded up, by low pH flushing. A detailed theoretical examination of the maximum rate of proton production from water showed that measured alkaline band fluxes are too large to be explicable in terms of an H⁺ influx system. The experimental and theoretical results indicate that the plasmalemma transport of OH⁻ ions is responsible for the measured negative external electric potential and alkalinity flux which are associated with the alkaline band phenomenon. Consequently, HCO₃⁻ influx across the characean plasmalemma must be charge-balanced by the efflux of OH⁻ ions.     

Potassium Transport Across the Membranes of Chara: III. EFFECTS OF pH, INHIBITORS AND ILLUMINATION

Smith, J. R., Walker, N. A. and Smith, F. A. 1987. Potassiumtransport across the membranes of Chara. III. Effects of pH,inhibitors and illumination.—J. exp. Bot. 38: 778–787. The effects of several treatments, normally used to inhibitelectrogenic proton transport, upon the potassium permeability(P_k) of the membranes of Chara were examined by means of simultaneousmeasurements of the ⁴²K influx (ⁱⁿ_K) and the membrane electricalconductance (G_m). ⁱⁿ_K, P_K and G_mwere found to be substantiallyunaffected when the external pH (pH?) was varied over the range5?0 to 85. However, when pH? was increased to 11 it was foundthat, although G_m increased considerably, both P_k and ⁱⁿ_K decreasedtypically by an order of magnitude. When cells were placed intotal darkness, P_K decreased substantially only after one dayhad elapsed. For the particular experimental conditions used,the inhibitors DES, NaN₃, and La₃₊ were found to alter P_K, whereasDCCD left P_K substantially unaffected. These results suggestthat care must be taken with some common procedures used toexamine the electrical properties of the electrogenic protonpump. Key words: Potassium, pH, illumination, inhibitors     

Through pore diameter in the cell wall of Chara corallina.

Determination of pore size of the cell wall of Chara corallina has been made by using the polyethylene glycol (PEG) series as the hydrophilic probing molecules. In these experiments, the polydispersity of commercial preparation of PEGs was allowed for. The mass share (gamma(p)) of polyethylene glycol preparation fractions penetrating through the pores was determined using a cellular 'ghost', i.e. fragments of internodal cell walls filled with a 25% solution of non-penetrating PEG 6000 and tied up at the ends. In water, such a 'ghost' developed a hydrostatic pressure close to the cell turgor which persisted for several days. The determination of gamma(p), for polydisperse polyethylene glycols with different average molecular mass (M) was calculated from the degree of pressure restoration after water was replaced by a 5-10% polymer solution. Pressure was recorded using a dynamometer, which measures, in the quasi-isometric mode, the force necessary for the partial compression of the 'ghost' in its small fragment. By utilizing the data on the distribution of PEG 1000, 1450, 2000, and 3350 fractions over molecular mass (M), it was found that gamma(p), for these polyethylene glycols corresponded to the upper limit of ML=800-1100 D (hydrodynamic radius of molecules, r(h)=0.85-1.05 nm). Thus, the effective diameter of the pores in the cell wall of Chara did not exceed 2.1 nm.     

Breakdown phenomena in the Chara membrane

The breakdown phenomenon in the Chara internodal cell was studiedusing the voltage clamp technique. When a slowly hyperpolarizingramp potential pulse was applied to the Chara membrane, thebreakdown occurred with hyperpolarization of about 220 mV. Thebreakdown was observed by less hyperpolarization, if the externalK⁺ concentration was increased. Such a breakdown phenomenonin the Chara membrane was caused principally by a large shiftof the membrane electromotive force toward depolarization. Thisshift frequently exceeded the peak level of the action potential. (Received July 26, 1976; )     

Effects of local anesthetics on the Chara plasmalemma.

The effects of lidocaine, tetracaine, procaine and bupivacaine (less than 1000 microM) on the Chara corallina internodal cell were studied. These local anesthetics depolarized the membrane at rest, while they affected the rising phase and the peak level of action potential not appreciably. Instead, they prolonged the time course of the falling phase of action potential as slowly as the repolarization was imperfect, even after enough lapse beyond the refractory period. Consequently, an action potential appeared to enhance the degree of depolarization at rest. Such a depolarization with stimulus/excitation was named use-dependent depolarization, while the depolarization without excitation, the resting one. The order of the potency of the use-dependent depolarization almost coincided with that of the nerve-blocking potency. During depolarization the change in membrane conductance was not simple. However, the conductance-voltage (Gm-Vm) relationship curve in the presence of local anesthetic suggested that depolarization was due to, not only the decrease in the electrogenic H(+)-pump, but also the increase in the diffusion conductance.