Action potentials and variation potentials in sunflower: An analysis of their relationships and distinguishing characteristics

Sunflower plants ( Helianthus annuus L.) were given an electrical stimulus to the stem or a heat (flame)‐wound to a single leaf or a cotyledon. The resulting electrical activity was monitored with extracellular electrodes. An electrical stimulus applied to the stem frequently evoked an action potential (AP), but never a variation potential (VP). In contrast, a heat‐wound applied to a leaf virtually always elicited a VP, which was often accompanied by one or more superimposed spikes (putative APs). The kinetic parameters of the AP and the VP were investigated. The AP appears to propagate without decrement in velocity or magnitude, whereas the VP parameters decrease significantly with distance. The heat stimulus triggered rapid alterations in stem elongation/contraction, which preceded changes in electrical potential, indicating the transmission of a hydraulic signal. Light‐off and light‐on stimuli evoked negative‐ and positive‐going changes in extracellular electrical potential, respectively, corresponding to de‐ and hyper‐polarization of the plasma membrane. Membrane depolarization (extracellularly manifested as a VP) evoked by both the light‐off and heat‐wounding stimuli was able to trigger one or more APs. We interpret these results to suggest that APs are "genuine" electrical signals involving voltage‐gated ion channels or pumps, which can be evoked directly by electrical stimulation or indirectly by changes in membrane potential occurring during the VP or after the light‐off stimulus. In contrast, VPs appear to be a local (non‐transmissible) electrical consequence of the passage of a rapidly transmitted hydraulic signal in the xylem, presumably acting on mechanosensitive ion channels or pumps in adjacent living cells.     

Stochastic simulations on the reliability of action potential propagation in thin axons

It is generally assumed that axons use action potentials (APs) to transmit information fast and reliably to synapses. Yet, the reliability of transmission along fibers below 0.5 μm diameter, such as cortical and cerebellar axons, is unknown. Using detailed models of rodent cortical and squid axons and stochastic simulations, we show how conduction along such thin axons is affected by the probabilistic nature of voltage-gated ion channels (channel noise). We identify four distinct effects that corrupt propagating spike trains in thin axons: spikes were added, deleted, jittered, or split into groups depending upon the temporal pattern of spikes. Additional APs may appear spontaneously; however, APs in general seldom fail (<1%). Spike timing is jittered on the order of milliseconds over distances of millimeters, as conduction velocity fluctuates in two ways. First, variability in the number of Na channels opening in the early rising phase of the AP cause propagation speed to fluctuate gradually. Second, a novel mode of AP propagation (stochastic microsaltatory conduction), where the AP leaps ahead toward spontaneously formed clusters of open Na channels, produces random discrete jumps in spike time reliability. The combined effect of these two mechanisms depends on the pattern of spikes. Our results show that axonal variability is a general problem and should be taken into account when considering both neural coding and the reliability of synaptic transmission in densely connected cortical networks, where small synapses are typically innervated by thin axons. In contrast we find that thicker axons above 0.5 μm diameter are reliable.     

Electrophysiological characterization of tomato hypocotyl putative action potentials induced by cotyledon heating

Young tomato plants ( Lycopersico n esculentum , 8 days old) were given a heat-wound to a cotyledon. The resulting electrical activity at the hypocotyl level was monitored with intracellular microelectrodes. We observed an original pattern of slow wave potentials (SWPs), consisting of 2–3 slow waves, with associated spikes. The electrophysiological study of the SWPs confirms previous conclusions that the SWPs are due to the inhibition of an active component of the membrane potential. The electrophysiological study of the spikes shows that they fit particularities of putative action potentials (APs). They seem to be triggered by the depolarization accompanying the SWPs and thus can appear late during the SWP. An ionic characterization of the spikes by using different extracellular ionic concentrations and channel blockers suggests that anionic channels might be involved, carrying SO₄^2– ions. The channels activity might be down regulated by the calcium released by the vacuole during the SWPs and APs. A better characterization of the nature of these APs could permit the understanding of the information transmission mechanisms in higher plants.     

Comparison of the effects of epibrassinolide and steroidal estrogens on adventitious root growth and early shoot development in mung bean cuttings

Concentrations of 24-epibrassinolide as low as 0.1 μ M consistently inhibited adventitious root formation and elongation in both hypocotyl and epicotyl cuttings from mung bean ( Phaseolus aureus L.). Similar, but less pronounced, inhibitory effects on root elongation were also observed with estrone sulphate and estradiol sulphate. With regards to root number, estrone sulphate enhanced this in both types of cutting, whereas estradiol sulphate was stimulatory in hypocotyl cuttings but inhibitory in epicotyl cuttings. Brassinolide caused a marked stimulation of epicotyl (but not hypocotyl) elongation and a swelling and splitting of the epicotyl in both types of cutting, whereas estrogens varied in their effect from inhibition of epicotyl growth to no effect. Root-applied brassinolide and estrogen sulphates brought about similar morphological abnormalities in shoots viz. epinasty and inrolling of primary leaves and delayed expansion of the first trifoliate leaf.     

Water potentials induced by growth in soybean hypocotyls

Gradients in water potential form the driving force for the movement of water for cell enlargement. In stems, they are oriented radially around the vascular system but should also be present along the stem. To test this possibility, growth, water potential, osmotic potential, and turgor were determined at intervals along the length of dark-grown soybean (Glycine max L. Merr., cv. Wayne) hypocotyls. Transpiration was negligible in the dark, humid conditions, so that all water uptake was for growth. Elongation occurred in the terminal 1.5 centimeters of the hypocotyl. Water potential was −3.5 bars in the elongating region but −0.5 bar in the mature region, both in intact plants and detached tissue. There was a gradual transition between these values that was related to the growth profile along the hypocotyl. Tissue osmotic potentials generally paralleled tissue water potentials, so that turgor was the same throughout the length of the hypocotyl. If the elongating zone was excised, growth ceased immediately. If the elongating zone was excised along with mature tissue, however, growth continued, which confirmed the presence of a water-potential gradient that caused longitudinal water movement from the mature zone to the elongating zone. When the plants were grown in vermiculite having low water potentials, tissue water potentials and osmotic potentials both decreased, so that water potential gradients and turgor remained undiminished. It is concluded that growth-induced water potentials reflect the local activity for cell enlargement and are supported by appropriate osmotic potentials.     

Production of a Lectin in Tissue Cultures of Dolichos biflorus

Callus cultures have been produced from the epicotyl and leaves, hypocotyl, and roots of germinating Dolichos biflorus seeds. These cultures were initiated on media containing 2,4-dichlorophenoxyacetic acid and kinetin, transferred to media with increased amounts of these hormones, and then maintained on hormone-free media. Extracts of these cultures were examined by radioimmunoassays specific for the lectin from the seeds of this plant and for a lectin that is present only in the stems and leaves of the intact plant. Although the seed lectin was not detected in any cultures, the stem and leaf lectin was produced in those cultures grown on the hormone free media. Lectin isolated from these cultures had subunits identical in electrophoretic mobilities to the subunits from the lectin isolated from intact stems and leaves. Levels of this lectin decreased when the cells were transferred back to media containing hormones and increased again upon transfer to the hormone-free media. The absence of exogenous hormones and the production of lectin were also correlated with the rapid growth and greening of the cells. Immunofluorescence and immunocytochemical studies on sections of cultured cells indicated that the stem and leaf lectin is associated with the cytoplasm as well as the cell wall as has been found in previous studies on the subcellular localization of this lectin in the intact plant.     

Characteristics of hook formation by bean seedlings

Explants were isolated from 6-day-old etiolated bean seedlings (Phaseolus vulgaris L. cv. Black Valentine) containing the cotyledons with 4 mm of hypocotyl just below the node and/or the epicotyl. During incubation on distilled water, uneven growth of the hypocotyl or epicotyl occurred resulting in the formation of a hook. The more rapid growth of the side which became convex was not dependent upon the presence of the slower growing concave side. It was concluded that the main axis has an intrinsic capacity for asymmetric growth. The growth leading to hook formation was inhibited by α-naphthaleneacetic acid at concentrations above 0.2 milligram per liter.     

Water Potential and Stomatal Resistance of Sunflower and Soybean Subjected to Water Stress during Various Growth Stages

Plants of two varieties of soybean (Glycine max (L.) Merr.) and two varieties of sunflower (Helianthus annuus L.) were grown in controlled environments and subjected to water stress at various stages of growth. Leaf resistances and leaf water potentials were measured as stress developed. In soybeans the upper leaf surface had a higher resistance than the lower surface at all leaf water potentials and growth stages. Resistance of the upper surface began to increase at a higher water potential and increased more than the resistance of the lower surface. Resistances returned to prestress values 4 days after rewatering. In sunflowers upper and lower leaf surfaces had similar resistances at all water potentials and growth stages. Leaf resistances were higher in sunflower plants stressed before flowering than in those stressed later. Sunflower plants stressed to −16 bars recovered their prestress leaf resistance and water potential a few days after rewatering, but leaves of sunflower plants stressed to −23 bars died. Leaves of soybean and sunflower plants stressed before flowering suffered less injury than those of older plants and sunflowers stressed after flowering suffered more injury than soybeans.     

Development and Distribution of a Lectin from the Stems and Leaves of Dolichos biflorus

The stems and leaves of the Dolichos biflorus plant contain a lectin that cross-reacts with antiserum against the seed lectin. This cross-reactive material (CRM) was followed during early seedling growth, stem elongation, and seed development using a specific radioimmunoassay.

No CRM was detected in developing seeds, but very low levels were found in dormant and imbibed seeds. As germination proceeds, the CRM accumulates at the apex of both etiolated and green seedlings in the epicotyl and leaves. Lower amounts of CRM are found in the cotyledons and hypocotyl, but no CRM was detected in the roots.

The amount of CRM in the first and second stem internodes increases during elongation and gradually declines after the completion of elongation. Approximately 80% of the CRM in the stems of 19-day-old Dolichos biflorus plants is associated with the elongating tissues. These results are discussed with respect to the possible roles of lectins in plants.

     

Responses of intact and excised young bean plants to fusicoccin

The effect of fusicoccin (FC) on the growth of epicotyls and leaves of Phaseolus vulgaris L. intact and excised seedlings has been examined, and several unexpected responses were observed. FC was added either to small wounds on one side of the epicotyl of 10 day old red-light grown seedlings or to the base of shoots excised at the base of the hypocotyl. Plants were kept in either dim red light (4 mol m^-2sec^-1) or bright white light (175 mol m^-2sec^-1) during the FC treatment. FC added to the base of the shoots was found to inhibit leaf expansion in either light condition. At the same time stem elongation was enhanced. The active concentration range was 10^-7–10^-5M. The basal fed FC caused a rapid and severe bending of the epicotyl starting at the base of the elongation zone. The direction of curvature was random, not related to the plane of the cotyledons or the direction of the gravity vector. Application of FC to one side of the epicotyl caused a similar but smaller bending away from the treated side. The bending occurred at either end of the elongation zone in accordance with site of FC application above or anywhere below it along the epicotyl and hypocotyl. It is concluded that the curvature of the epicotyl induced by FC fed in the transpiration stream may either be due to a differential loss of the capacity of cells at the base of the elongation zone to grow in response to FC or as a result of elongation of the first responsive cells encountered by the FC.     

Glutamate induces series of action potentials and a decrease in circumnutation rate in Helianthus annuus

Reports concerning the function of glutamate (Glu) in the electrical and movement phenomena in plants are scarce. Using the method of extracellular measurement, we recorded electrical potential changes in the stem of 3‐week‐old Helianthus annuus L. plants after injection of Glu solution. Simultaneously, circumnutation movements of the stem were measured with the use of time‐lapse images. Injection of Glu solution at millimolar (200, 50, 5 mM) concentrations in the basal part of the stem evoked a series of action potentials (APs). The APs appeared in the site of injection and in different parts of the stem and were propagated acropetally and/or basipetally along the stem. Glu injection also resulted in a transient, approximately 5‐h‐long decrease in the stem circumnutation rate. The APs initiated and propagating in the sunflower stem after Glu injection testify the existence of a Glu perception system in vascular plants and suggest its involvement in electrical, long‐distance signaling. Our experiments also demonstrated that Glu is a factor affecting circumnutation movements.     

Leaf water potentials measured with a pressure chamber

Leaf water potentials were estimated from the sum of the balancing pressure measured with a pressure chamber and the osmotic potential of the xylem sap in leafy shoots or leaves. When leaf water potentials in yew, rhododendron, and sunflower were compared with those measured with a thermocouple psychrometer known to indicate accurate values of leaf water potential, determinations were within ± 2 bars of the psychrometer measurements with sunflower and yew. In rhododendron. water potentials measured with the pressure chamber plus xylem sap were 2.5 bars less negative to 4 bars more negative than psychrometer measurements.

The discrepancies in the rhododendron measurements could be attributed, at least in part, to the filling of tissues other than xylem with xylem sap during measurements with the pressure chamber. It was concluded that, although stem characteristics may affect the measurements, pressure chamber determinations were sufficiently close to psychrometer measurements that the pressure chamber may be used for relative measurements of leaf water potentials, especially in sunflower and yew. For accurate determinations of leaf water potential, however, pressure chamber measurements must be calibrated with a thermocouple psychrometer.

     

Action Potential Energy Efficiency Varies Among Neuron Types in Vertebrates and Invertebrates

The initiation and propagation of action potentials (APs) places high demands on the energetic resources of neural tissue. Each AP forces ATP-driven ion pumps to work harder to restore the ionic concentration gradients, thus consuming more energy. Here, we ask whether the ionic currents underlying the AP can be predicted theoretically from the principle of minimum energy consumption. A long-held supposition that APs are energetically wasteful, based on theoretical analysis of the squid giant axon AP, has recently been overturned by studies that measured the currents contributing to the AP in several mammalian neurons. In the single compartment models studied here, AP energy consumption varies greatly among vertebrate and invertebrate neurons, with several mammalian neuron models using close to the capacitive minimum of energy needed. Strikingly, energy consumption can increase by more than ten-fold simply by changing the overlap of the Na⁺ and K⁺ currents during the AP without changing the APs shape. As a consequence, the height and width of the AP are poor predictors of energy consumption. In the Hodgkin–Huxley model of the squid axon, optimizing the kinetics or number of Na⁺ and K⁺ channels can whittle down the number of ATP molecules needed for each AP by a factor of four. In contrast to the squid AP, the temporal profile of the currents underlying APs of some mammalian neurons are nearly perfectly matched to the optimized properties of ionic conductances so as to minimize the ATP cost.     

3个向日葵品种生长期内水分、蛋白质和总黄酮含量动态变化

本实验研究了3个向日葵品种在生长周期内茎、叶及茎叶混合样的水分、蛋白质和总黄酮含量的动态变化.结果表明:向日葵不同品种、不同生长期以及同一品种的不同部位的水分、蛋白质、总黄酮含量均存在差异.水分含量整体呈现下降趋势,且茎＞茎叶混合＞叶;蛋白质含量为叶＞茎叶混合样＞茎,其中高秆食用葵叶片中蛋白质含量为8.10g/100g...     

Glycophyte salt resistance

By perfusion of entire sunflower stems with NaCl solutions of various concentrations, we studied the phenomenon of sodium decrement, i.e., sodium retaining in the stem and leaf petioles. Such retaining could comprise up to 50–80% of initial sodium concentration. It depended on the rate of perfusion, the length of xylem vessels, and NaCl concentration. When perfusion with 100–500 mM NaCl concentrations (high for glycophytes) lasted for 10–12 days, we did not observe any decrease in the degree of sodium decrement. Simultaneously with sodium decrement, other ions (K⁺ and Ca²⁺) were secreted into the perfusate, thus providing for physiological equilibrating the monosalt solution supplied to the stem base. The high salt concentration in the perfusate induced a decrease in the hydraulic conductance of the vessels. The conclusion is that stressful NaCl solutions attain the shoot meristem and reproductive organs as an “equilibrated” salt solution and at a declined rate of xylem flow. The mechanisms of observed phenomenon of glycophyte salt resistance are discussed, the main of them being related to osmosis-dependent responses of stem living cells and the processes of ion exchange between the cells and xylem vessel content.     

Recovery of photosynthesis in sunflower after a period of low leaf water potential

Photosynthesis was studied in sunflower plants subjected to 1 to 2 days of desiccation and then permitted to recover. The leaf water potential to which leaves returned after rewatering was dependent on the severity of desiccation and the evaporative conditions. Under moderately evaporative conditions, leaf water potential returned to predesiccation levels after 3 to 5 hours when desiccation was slight. Leaf water potentials remained below predesiccation levels for several days after rewatering when leaf water potentials decreased to −13 to −19 bars during desiccation. Leaf water potential showed no sign of recovery when leaf water potentials decreased to −20 bars or below during desiccation. The lack of full recovery of leaf water potential was attributable to increased resistance to water transport in the roots and stem. The resistance ultimately became large enough to result in death of the leaves because net water loss continued even after the soil had been rewatered.     

Relationship of water potential to growth of leaves

A thermocouple psychrometer that measures water potentials of intact leaves was used to study the water potentials at which leaves grow. Water potentials and water uptake during recovery from water deficits were measured simultaneously with leaves of sunflower (Helianthus annuus L.), tomato (Lycopersicon esculentum Mill.), papaya (Carica papaya L.), and Abutilon striatum Dickson. Recovery occurred in 2 phases. The first was associated with elimination of water deficits; the second with cell enlargement. The second phase was characterized by a steady rate of water uptake and a relatively constant leaf water potential. Enlargement was 70% irreversible and could be inhibited by puromycin and actinomycin D. During this time, leaves growing with their petioles in contact with pure water remained at a water potential of —1.5 to —2.5 bars regardless of the length of the experiment. It was not possible to obtain growing leaf tissue with a water potential of zero. It was concluded that leaves are not in equilibrium with the potential of the water which is absorbed during growth. The nonequilibrium is brought about by a resistance to water flow which requires a potential difference of 1.5 to 2.5 bars in order to supply water at the rate necessary for maximum growth.

Leaf growth occurred in sunflower only when leaf water potentials were above —3.5 bars. Sunflower leaves therefore require a minimum turgor for enlargement, in this instance equivalent to a turgor of about 6.5 bars. The high water potentials required for growth favored rapid leaf growth at night and reduced growth during the day.

     

Chloroplast Response to Low Leaf Water Potentials: III. Differing Inhibition of Electron Transport and Photophosphorylation

Cyclic and noncyclic photophosphorylation and electron transport by photosystem 1, photosystem 2, and from water to methyl viologen (“whole chain”) were studied in chloroplasts isolated from sunflower (Helianthus annus L. var Russian Mammoth) leaves that had been desiccated to varying degrees. Electron transport showed considerable inhibition at leaf water potentials of −9 bars when the chloroplasts were exposed to an uncoupler in vitro, and it continued to decline in activity as leaf water potentials decreased. Electron transport by photosystem 2 and coupled electron transport by photosystem 1 and the whole chain were unaffected at leaf water potentials of −10 to −11 bars but became progressively inhibited between leaf water potentials of −11 and −17 bars. A low, stable activity remained at leaf water potentials below −17 bars. In contrast, both types of photophosphorylation were unaffected by leaf water potentials of −10 to −11 bars, but then ultimately became zero at leaf water potentials of −17 bars. Although the chloroplasts isolated from the desiccated leaves were coupled at leaf water potentials of −11 to −12 bars, they became progressively uncoupled as leaf water potentials decreased to −17 bars. Abscisic acid and ribonuclease had no effect on chloroplast photophosphorylation. The results are generally consistent with the idea that chloroplast activity begins to decrease at the same leaf water potentials that cause stomatal closure in sunflower leaves and that chloroplast electron transport begins to limit photosynthesis at leaf water potentials below about −11 bars. However, it suggests that, during severe desiccation, the limitation may shift from electron transport to photophosphorylation.     

Vein recovery from embolism occurs under negative pressure in leaves of sunflower (Helianthus annuus)

Leaf veins undergo cavitation at water potentials (Psi(leaf)) commonly experienced by field-growing plants. Theoretically, embolism reversal should not be possible until xylem pressures rise by several kilopascals of atmospheric pressure, but recent evidence suggests that embolized conduits can be refilled even when surrounded by others at substantial tension (novel refilling). The present study reports 'novel refilling' occurring in leaf veins of sunflower (Helianthus annuus L.) while at Psi(leaf) = -0.33 MPa. Sixty per cent loss of vein hydraulic conductance (K(vein)) was recorded at Psi(leaf) < -0.65 MPa, while stem hydraulic conductance (K(stem)) was unaffected even at Psi(leaf) = -1.1 MPa. Loss of K(vein) was accompanied by stomatal closure. Water-stressed plants (Psi(leaf) = -1.1 MPa) were rehydrated overnight to different target water potentials achieved by using PEG at different concentrations as irrigation medium. K(vein) recovered by 50% at Psi(leaf) = -0.47 MPa and vein refilling was complete at Psi(leaf) = -0.33 MPa, i.e. well below the theoretical limit for conduit refilling (-0.05 MPa as calculated for sunflower minor veins). Mercurials supplied to detached leaves had no effect on the refilling process. Upon rehydration, recovery of K(vein) was not paralleled by recovery of whole-plant hydraulic conductance or leaf conductance to water vapour (g(L)), as a likely consequence of hydraulic failure of other components of the water pathway (root system or extravascular leaf compartments) and/or root-to-leaf chemical signalling. This is the first study providing experimental evidence for 'novel refilling' in a herbaceous dicot and highlighting the importance of this process in the leaf.     

Dynamics of growth and the content of endogenous phytohormones during kidney bean scoto-and photomorphogenesis

The dynamics of growth and the contents of free and bound endogenous IAA, gibberellins (GA), cytokinins (zeatin and its riboside), and ABA in kidney bean plants (Phaseolus vulgaris L., cv. Belozernaya) grown in darkness or in the light was studied. Phytohormones were quantified in 5–15-day-old plants by the ELISA technique. Plant growth and phytohormone content were shown to depend on plant age and the conditions of illumination. During scotomorphogenesis, changes in the biomass and hypocotyl length were highly correlated with the content of GA, whereas during photomorphogeneses, these parameters were correlated with the content of zeatin. In darkness, epicotyl growth displayed a positive correlation with the content of GA, whereas in the light, the correlation was negative. Growth characteristics of the primary leaves were shown to correlate with IAA in darkness and with GA and zeatin in the light. At a low concentration of cytokinins in illuminated leaves, cell divisions occurred, whereas, at the higher cytokinin concentrations, cell expansion occurred. The highest content of GA was characteristic of leaves in the period of growth cessation. ABA accumulated during active leaf and root elongation and biomass increment in the light and during hypocotyl growth in darkness. After plant illumination, the ratio of auxins to cytokinins increased in bean roots and decreased in their epicotyls. Thus, light changed the developmental programs of bean plants, which was manifested in the changed rate and duration of growth of various organs (root, hypocotyl, epicotyl, and leaf). Some mechanisms of light action depended on the contents of IAA, ABA, GA, and cytokinins and the ratios between these phytohormones. Differences between scotonorphogenesis of mono-and dicotyledonous plants are discussed in relation to the levels of phytohormones in them.