Z. Hejnowicz

Oriented movement of statoliths studied in a reduced gravitational field during parabolic flights of rockets.

During five rocket flights (TEXUS 18, 19, 21, 23 and 25), experiments were performed to investigate the behaviour of statoliths in rhizoids of the green alga Chara globularia Thuill. and in statocytes of cress (Lepidium sativum L.) roots, when the gravitational field changed to approx. 10−4 · g (i.e. microgravity) during the parabolic flight (lasting for 301–390 s) of the rockets. The position of statoliths was only slightly influenced by the conditions during launch, e.g. vibration, acceleration and rotation of the rocket. Within approx. 6 min of microgravity conditions the shape of the statolith complex in the rhizoids changed from a transversely oriented lens into a longitudinally oriented spindle. The center of the statolith complex moved approx. 14 μm and 3.6 μm in rhizoids and root statocytes, respectively, in the opposite direction to the originally acting gravity vector. The kinetics of statolith displacement in rhizoids demonstrate that the velocity was nearly constant under microgravity whereas it decreased remarkably after inversion of rhizoids on Earth. It can be concluded that on Earth the position of statoliths in both rhizoids and root statocytes depends on the balance of two forces, i.e. the gravitational force and the counteracting force mediated by microfilaments.

Tip growth: Patterns of growth rate and stress in the Chara rhizoid

Summary The displacement of resin spheres attached to the growing Chara rhizoid was measured by means of time-lapse photography. The distribution of elemental extension rate in meridional and latitudinal direction as well as the tensional stress in both directions were calculated. The maximum of the extension rate coincides with the minimum of stress.

Gravity induced changes in intracellular potentials in statocytes of cress roots.

Two glass microelectrodes were inserted from opposite sides of the root cap into statocytes of Lepidium sativum L. immersed in medium with or without cytochalasin D (CD). Intracellular potentials (Eis) of statocytes were measured with reference to an earthed electrode in the bathing solution. In the absence of CD, Ei values were −160 ±2 mV (n = 52) in vertical roots. During the recording of EiS, the roots were tilted from the vertical by 45° so that in a tilted root one electrode was on the upper side and the other on the lower side; after 5 min the roots were returned to the vertical. At approximately 64 s after tilting (lasting 5–15 s) there was a transient lowering of Ei (more negative) by an average of 4.7 mV on both the upper and lower sides (n = 52). In some cases, this decrease in Ei was preceded by a transitory increase. Returning the roots to the vertical resulted in a response similar to that obtained by tilting. In roots treated with CD at a concentration of 3 μM for 1 h, the initial Ei was -145 ±2 mV (n = 43), and the lowering of Ei on position change (tilting or returning) was smaller (2.0 mV) in some statocytes (n = 50) and higher (8.1 mV) in others (n = 14) compared to control roots (without and with DMSO). A higher concentration (10 μM) of CD and longer treatment (2 h) further reduced the decrease in Ei (1.1 mV) on position change (n = 26). The observed effects of CD support the hypothesis that statoliths in statocytes are anchored by actin filaments to the plasma membrane and/or to the cortical endoplasmic reticulum. Movement of statoliths during the first step of graviperception may lead to stress changes in actin filaments, affecting the transmembrane potential and also the Ei.

Propagated waves induced by gradients of physiological factors within plasmodia ofPhysarum polycephalum

Plasmodia ofPhysarum polycephalum were analyzed with the aid of cinematography and the infrared reflection technique for characterization of the phase behavior of their oscillating contraction activity, with special emphasis placed on the effects of temperature gradients. In response to temperature gradients, phase gradients were documented cinematographically as well as by infrared registration. A quantitative evaluation of the cinematographically recorded phenomena was carried out with the aid of streak photography. The phase gradient is directed across the region of the temperature gradient with a delay in phase toward the colder side. The correspondingly generated waves are as short as 1 mm and are propagated toward the colder region. A comparison of these waves with the known “flickering” phenomena in cinematographic films reveals a common nature of both.

Propagated fluctuations of the electric potential in the apoplasm of Lepidium sativum L. roots.

The electric potential on the surface of the Lepidium sativum L. root apex was recorded by means of six non-polarizable electrodes. Nonevoked fluctuations of the potential with amplitudes below 0.1 mV were observed. The fluctuations could be reversibly inhibited either by ether vapor or by anoxia caused by N2. They did not occur in killed roots. Cross-correlation analysis of the fluctuations from six electrodes located one above another along the 3-mm apical region showed a pattern of time delay which indicates that the fluctuations may be the consequence of signals propagated in the root with a velocity of 3–9 mm · s−1 in a basipetal direction from the root cap. We hypothesize that the fluctuations are due to signals of an unknown nature propagated along an intrasymplasmic continuous system, the “symreticulum”, composed of the cortical ER of individual cells and desmotubules passing through the plasmodesmata.

Cyclopiazonic acid disturbs the regulation of cytosolic calcium when repetitive action potentials are evoked in Dionaea traps

Evoking of action potentials (APs) in the trap of Dionaea muscipula Ellis at intervals shorter than 20 s caused a gradual decrease in the amplitude of the APs. At longer intervals the amplitude was constant. The calcium ionophore A23187 (1 μM) caused a considerable decrease of AP amplitude. Pretreatment of a segment of the Dionaea trap with cyclopiazonic acid (CPA), which is a specific inhibitor of the Ca2+-ATPase in the sarcoplasmic seticulum of animal cells and in ER vesicles isolated from plant cells, only slightly affected the amplitude when APs were evoked every 10 min; however, it caused a considerable decrease in the amplitude when the stimulation was repeated every 2 min. Assuming that APs increase the concentration of cytosolic Ca2+ and the amplitude of AP depends on the gradient of Ca2+ across the plasma membrane, the effect of CPA on the AP amplitude indicates that CPA inhibits the sequestration of Ca2+ in Dionaea cells.

Tissue stresses and their graviresponsive changes in stems of Reynoutria japonica Houtt.

Summary The internodes of Reynoutria (Japanese knotweed) form hollow cylinders in which the epidermis and the underlying collenchyma are under tissue stress (TS) of a tensile type, while most of the compressive TS occurs in the thin-walled peripheral pith. To fulfil static equilibrium, the forces that generate these stresses have equal magnitude, in this case 0.23 N per mm of circumference in vertical stems as measured by restretching tissue peels consisting of epidermis and collenchyma. The tensile TS in the collenchyma and the epidermis amounted to 3 · 106N · m−2, nearly 5 times higher than the average osmotic pressure of cell sap, while the magnitude of the compressive TS in the remaining tissue was on the average 20 times lower. The tensile TS underwent relaxation on the lower side of gravireacting stems. This was demonstrated by a smaller bending of a strip slit from the lower side, and also by measurements of the force required to restretch the peels from this side. In horizontally positioned stems, the tensile force on the lower side decreased by a factor of 3, while the force on the upper side remained unaltered. Relaxation of the tensile TS started in the collenchyma. This relaxation on the lower side of a tilted stem is the first mechanical event occurring during a graviresponse. It causes asymmetry in distribution of TSs in the stem, which brings about a net bending moment.

The pattern of acropetal and basipetal cytoplasmic streaming velocities in Chara rhizoids and protonemata, and gravity effect on the pattern as measured by laser-Doppler-velocimetry

Abstract. The spatial pattern of acropetal and basipetal cytoplasmic streaming velocities has been studied by laser-Doppler-velocimetry (LDV) in the positively gravitropic (downward growing) rhizoids of Chara globularis Thuill. and for the first time in the negatively gravitropic (upward growing) protonemata. The LDV method proved to be precise and yielded reproducible results even when tiny differences in velocities were measured. In the apical parts of the streaming regions of both cell types, acropetal streaming was faster than basipetal streaming. Starting at the apical reversal point of streaming, the velocity increased basipetally with the distance from that point and became fairly constant close to the basal reversal point; subsequently, the velocity decreased slightly acropetally as the apical reversal point was again approached. There was no change in velocity at the basal reversal point. However, at the apical reversal point there was an abrupt decrease in velocity. The pattern of the ratio of acropetal to basipetal streaming velocity (VR) was a function of the relative distance of the site of measurement from the apical reversal point rather than a function of the absolute distance. Upon inversion of the rhizoids, the VR decreased on average by 3.8% (±0.4%), indicating that the effect of gravity on the streaming velocity was merely physical and without a physiological amplification. Rhizoids that had developed on the slowly rotating horizontal axis of a clinostat, and had never experienced a constant gravity vector, were similar to normally grown rhizoids with respect to VR pattern. In protonemata, the VR pattern was not significantly different from that in rhizoids although the direction of growth was inverse. In rhizoids, oryzalin caused the polar organization of the cell to disappear and nullified the differences in streaming velocities, and cytochalasin D decreased the velocity of basipetal streaming slightly more than that of acropetal streaming. Cyclopiazonic acid, known as an inhibitor of the Ca2+-ATPase of the endoplasmic reticulum, also reduced the streaming velocities in rhizoids, but had slightly more effect on the acropetal stream. It is possible that the endogenous difference in streaming velocities in both rhizoids and protonemata is caused by differences in the cytoskeletal organization of the opposing streams and/or loading of inhibitors (like Ca2+) from the apical/subapical zone into the basipetally streaming endoplasm.

Gravitropic response of the stem of Reynoutria. Isolated strips in different positions in relation to the gravity vector

Summary To study the graviresponse of isolated sectors of a stem, the elongating hollow internodes of Reynoutria were slit into at least six strips, and pairs of them were sandwiched between glass plates with the epidermis oriented oppositely. After mounting, the strips were cut at the plate edges, so their elongation could be determined from the extensions beyond the edges. Exposition of the specimens in different positions to gravity resulted in extensions of the strips in the following decreasing order: horizontal flat with the epidermis facing downward, vertical, horizontal flat facing upward. The extensions in inverted vertical position or clinostatted around the longitudinal axis did not differ from that in normal vertical position. Strips oriented similarly as the lateral sides in a horizontal stem bent in their tangential plane, convex at the bottom. Strips devoid of the outer tissue (OT = epidermis + collenchyma) developed exceptional length increments independently of their position, which indicates that the OT restrains the elongation of the inner tissue (IT containing mostly the parenchyma of peripheral pith). The OT isolated from the IT did not elongate at all. The lack of the tensile tissue stress in the isolated OT should be taken into account in this respect; such a stress acts on the OT in an intact strip and drives its elongation there. When intact strips were freed after being kept between glass plates for 3 h, they bent in the radial plane due to tissue stresses. The curvature of this bending was lower in downward racing strips than in upward facing strips, which was the same as in strips freshly slit. This indicates that the tissue stresses were partly released in the downward facing strips during their enhanced elongation. We conclude that angular sectors of a stem, as small as ┛ circumference, are self-governing with respect to gravity.