Action Potentials in Lupinus angustifolius L. Shoots: III. DETERMINATION OF THE REFRACTORY PERIODS

The refractory periods for action potentials (AP) were investigatedin Lupinus shoots by application of pairs of electrical stimuli(d.c). The response (AP) for the second stimulus is characterized.The second response differs in amplitude, range, and propagationvelocity from the first, depending on the strength of the stimuliand the time interval between them. The absolute refractoryperiod determined on this basis lasts 12 min, and the relativeone from 12 to about 90 min. The character of the refractoryphenomenon is similar in the case of nerves, only the processesin Lupinus are 10³ to 10⁵ times slower. The possibility of transmission of excitation from cell to cellis discussed.     

Transnodal Transport of 14C in Nitella flexilis: I. TANDEM CELLS WITHOUT APPLIED PRESSURE GRADIENTS

Using NaH¹⁴CO₃ (0·01-0·03 mol m^–3) fed to5·0 mm of Nitella flexilis in artificial pond water at22–25°C, we have found that uptake in 5 min or in1 h varies with the streaming rate of the fed cell and is reducedby anoxia over the uptake position. We have also found thatthe %¹⁴C transported across the node between two internodalcells in tandem is highly sensitive to the streaming rate ineach cell, to the physiological state of the cells (summer versuswinter), to the method of feeding (5 min versus 1 h) and tocovering the node, particularly with winter cells or summercells preconditioned for 3 h in the dark and run in the dark.Transnodal transport by plasmodesmata is sensitive to anoxiaand seems to be at least partly active. Key words: Node, plasmodesmata, anoxia, active transport, influx     

Transnodal Transport of 14C in Nitella flexilis: II. TANDEM CELLS WITH APPLIED PRESSURE GRADIENTS

Using carefully standardized test conditions and tandem pairsof cells of Nitella flexilis, the influx of ¹⁴C (added as H¹⁴CO₃^–and transnodal transport were studied under various pressuregradients applied across the node up to P=± 2·5bar. When mannitol was used as the osmoticum, influx was foundto increase only when the mannitol solution was around the cellproximal to the feed. ¹⁴C was transported across the node tothe distal cell, probably as ¹⁴C-photosynthate products, evenagainst a pressure gradient of 2 bars or more, as was ³⁶Cl^–and ³²P. The % transported in general decreased with increasingP, whether assisted or opposed by added pressure. It was unchangedby the presence of mannitol at the node and was essentiallythe same whether the pressure gradient was produced by directpressure or by use of the osmoticum. Transnodal transport of¹⁴C products is almost certainly via plasmodesmata and appearsto be largely by an active mechanism. In absolute amounts itis the same whether the pressure gradient assists or opposesflow. Valving is evident at the node, increasing the resistanceto transport as the pressure gradient increases whether ¹⁴C(asHCO₃^–), ⁴²K⁺ (as KCl), ³⁶Cl^– (as NaCl) or ³²P (asNa₃PO₄) are used to detect it. The mechanism of movement ofK⁺ across the node differs from that of photosynthate products. Key words: Node, plasmodesmata, pressure gradients, active transport     

Extreme tolerance to environmental stress of sexual and parthenogenetic resting eggs of Eucypris virens (Crustacea,Ostracoda)

1. The freshwater ostracod (Ostracoda), Eucypris virens, is commonly found in European temporary pools, where its long‐term persistence completely relies on the build‐up of resting egg banks. Extreme tolerance of dormant eggs and seeds is widely assumed, but freshwater ostracod eggs are relatively poorly studied. The study of ostracod resting eggs is of particular relevance as it may yield the key to understanding the distribution of the sexes in many species capable of both sexual and asexual reproduction. 2. We assessed the tolerance of dried resting eggs produced by females originating from three populations with males and three all‐female E. virens populations. Hatching time and success was compared between control eggs and eggs exposed to one of seven ecologically relevant stressors: digestive enzymes, high salinity, deep freezing, hydration, UV‐B radiation, hypoxia and insecticide treatment. 3. None of the stressors reduced significantly the viability of either sexual or asexual eggs. When compared with the reproductive mode–specific controls, exposure to UV‐B radiation had a mild impact on the survival of sexual and asexual eggs (?16.8 and ?22.4%, respectively), but this was only significant for asexual eggs. These results point to an extreme tolerance of E. virens resting eggs and have important implications for the ecology and evolution of the species. 4. The timing of hatching was not affected by the stress treatment, except for UV‐B radiation. A marginally significant delay in hatching response was observed for UV‐B‐radiated eggs when compared to the overall mean, but this treatment effect was absent when compared with the reproductive mode–specific controls. 5. The populations with males produced eggs that hatched on average earlier (?1.5 days at 17 °C) and were more successful (+26%) than asexual eggs. Due to the limited number of populations and the population‐specific origin and age of the eggs, the possibility due to the differences in age and origin of the resting eggs, or to variations in local conditions, cannot be ruled out.     

Action Potentials in Lupinus angustifolius L. Shoots: II. DETERMINATION OF THE STRENGTH-DURATION RELATION AND THE ALL-OR-NOTHING LAW

In Lupinus shoots an electrical stimulus (d.c.) produces a potentialwave analogous to the action potential wave (AP) in stimulatedsimple plant cells or in nerves. The method used (Paszewskiand Zawadzki, 1973a, b) is similar to those applied in neurophysiologyin research on the excitability of nerves. In the present paper the strength-duration relation (Eqn. (1))of the excitation in Lupinusitalic has been used to calculatethe values of the rheobase, chronaxie, and useful time of pulse.The rheobase, as a threshold value of the stimulus, is examinedin terms of the applicability of the all-or-nothing law. The results suggest that the origin and propagation of AP inplants and in nerves may occur in a similar way. The analogybetween the impulse propagation in nerve trunks and in Lupinusshoots is discussed.     

The assimilation efficiency of Asellus aquaticus L. (Crustacea, Isopoda)

(1) Comparative studies of the ash-ratio and gravimetric methods of determining assimilation efficiency in Asellus aquaticus showed that the differential use of minerals by this species rendered the ash-ratio method unreliable. Results obtained by the gravimetric method were therefore employed in the analysis of further experiments. (2) The assimilation efficiencies estimated for A. aquaticus ranged between 26 and 44%, and varied according to population density and reproductive condition. (3) Individual winter males had a significantly lower assimilation efficiency (26%) than grouped animals (35%), but the assimilation efficiency of summer males (33%) did not differ significantly from that of winter males at the same density. It is concluded that density affects assimilation efficiency in A. aquaticus. (4) The assimilation efficiency of summer males (33 %) is significantly different from that of summer females (non-ovigerous, 41 %; ovigerous, 44%). A mean assimilation efficiency of 40% is proposed for the summer period whereas an overall annual mean of 30% is suggested. (5) Despite the various assimilation efficiencies reported within any one season consumption rate per unit weight is fairly constant (winter, 0·04–0·05 cal/24 h; summer, 0·36–0·38 cal/24 h) and it is suggested that the different assimilation rates are a mechanism whereby the additional energy and material requirements of females for breeding can be met without increasing food intake.     

Action Potentials in Lupinus angustifolius L. Shoots: IV. APPLICATION OF THERMAL STIMULI AND INVESTIGATIONS ON THE CONDUCTION PATHWAYS OF THE EXCITATION

The character of action potentials induced by thermal or electricalstimulation and their blocking by removal of a ring of tissuefrom the shoot as well as the spread of these potentials inthe Lupinus shoot were investigated. The appearance of anothertype of excitation was noted as a graded response occurringlocally. This potential was termed graded potential (GP). Itappears in tissues in which action potentials (AP) are not observed. It is proposed that two interrelated types of reaction to stimulioccur in various types of plant tissue.     

Structural Differences in Unbleached and Mildly‐Bleached Synthetic Tyrosine‐Derived Melanins Identified by Scanning Probe Microscopies

Using scanning tunneling microscopy (STM), we have imaged two types of mildly‐bleached, synthetic tyrosine‐derived melanins for comparison with the unbleached melanin from which they were prepared. These mildly‐bleached melanins were generated by mild oxidation of the unbleached melanin, using either basic hydrogen peroxide or air/light. The unbleached melanin, and two mildly‐bleached melanins, were independently deposited from very dilute tetrahydrofuran (THF) solutions onto highly oriented pyrolytic graphite (HOPG) substrate for STM imaging. Lateral dimensions (23 Å, average of two directions) of structures from each of the three samples showed no differences. However, structures from both mildly‐bleached melanins showed similar dramatic decreases (from ～15 Å to ～5 Å) in their STM‐measured apparent heights, compared with structures from the unbleached melanin sample. These STM observations are compatible with structural models for unbleached and mildly‐bleached melanins, incorporating a three‐dimensional structure for unbleached melanin composed of multi‐layered, Π–Π‐stacked, carboxylic and amino variants of polyaromatic polymeric sheets. The STM‐observed decrease in apparent heights after mild oxidation, which we associate with a change in stack height, has been confirmed by experiments using tapping mode atomic force microscopy (TM‐AFM) for the unbleached and mildly‐hydrogen‐peroxide‐bleached melanins (from ～14 Å to ～6 Å). In these TM‐AFM experiments, the melanins were deposited directly onto magnesium cation‐treated glass substrates in contact with methanolic solutions of each of the melanins. We interpret our mild‐bleaching results as an oxidative conversion of the multi‐layered, stacked sheets of mainly carboxylic and amino variants of polyquinhydrone‐like moieties, to largely de‐stacked, mildly‐bleached melanin sheets. These oxidized and, hence, electron‐deficient sheets should not readily form multi‐layered, Π–Π interacting stacks, but instead appear to be either single‐layer polyquinone sheets or, at most, double‐layer polyquinhydrone sheets. The effects of such de‐stacking on in vivo melanin photoprotection, and structural similarities between melanin derived from natural sources and the synthetic melanin samples used in this work are discussed.     

Photosynthetic activities of cadmium-treated tomato plants

Tomato plants (Lycopersicum esculentum Mill. cv. Moneymaker) grown on nutrient medium containing cadmium exhibit reduced net photosynthesis and reduced contents of chlorophyll and accessory pigments. In chloroplasts isolated from cadmiumtreated plants photosystem II activity, as measured by 2,6-dichlorophenolindophenol photoreduction, and photosystem II + I activity (H₂O → methyl viologen) were both inhibited to about 60%. When 1,5-diphenylcarbazide was used as artificial electron donor, no significant cadmium effect was observed. Photosystem I activity was not affected by cadmium. The fine structure of chloroplasts in cadmium-treated plants was degenerated, similarly to senescence response. The principal symptom of cadmium action was the occurrence of large plastoglobules and a disorganization of the lamellar structure, mainly grana stacks. Transfer of cadmium-treated plants into a medium with increased manganese level caused grana stacking and restoration of photosystem II activity.