Phytol. (1994), 128, 49-56

Wound-induced hydraulic signals and stimulus transmission in Mimosa pudica L

BY M.MALONE Horticulture Research International, Wellesbourne, Warwicks, CV35 9EF, UK [Received 23 February 1994; accepted 25 April 1994)

CONTENTS Summary 49 III. Kinetics of hydraulic dispersal in Mimosa 52 I. Introduction 49 IV. Implications for electrical 'signals' 54 II. Wound-induced hydraulic signals in Acknowledgements 55 Mimosa 51 References 55

SUMMARY The mechanism of wound signalling in Mimosa is discussed with a brief historical survey. It is demonstrated that strong wound-induced hydraulic signals occur in Mimosa pudica L., as m many other plants, and that the basipetal mass flows associated with these events could disperse solutes from the wound site at rates of at least 15 mm s"^, and possibly up to 300 mm s '. When such wound-induced 'hydraulic dispersal' is taken into account, Ricca's theory of chemical signalling can explain long-distance transmission in Mimosa. The pattern of long-distance wound signalling in Mimosa is shown to be consistent with such a model. Implications for theories of electrical signalling in Mimosa are discussed and it is concluded, in agreement with some previous workers, that systemic wound-induced electrical phenomena in Mimosa are not travelling signals, but are local responses to the travelling chemical messengers.

Key words: Mimosa pudica, systemic signals, wound signals, hydraulic signals.

and with rapid loss of water and solutes (notably K* INTRODUCTION and Cr) from cells of the motor cortex (Satter, 1989; The remarkable visible responses of Mimosa pudica Sibaoka, 1991). L. (sensitive plant) and related species have attracted Stimulation at one pinna of Mimosa can cause scientific interest for centuries (Houwink, 1935). rapid responses in neighbouring pinnae or in distant Nineteenth century work on Mimosa invoh ed many leaves. Explanations of the nature of the transmitted of the founders of modern botany, including PfefTer, signal include hydraulic (Clark, described in Hooke, Sachs, Haberlandt and Darwin (Weintraub. 1952). ]667;Dutrochet, 1837; Haberlandt, 1914), electrical The leaf of Mimosa is doubly compound. Fol- (Bose, 1928), chemical (Ricca, 1916), and meta- lowing Weintraub (1952) and Esau (1970), the physical theories (noted in MacDougal, 1896). By smallest leaflets are termed pinnules. About 20 pairs the 1930s, the prevailing view vias that the mech- of these pinnules are distributed along a rachis to anism of signalling of non-wound stimuli, such as form a pinna. Three or four pinnae, together with a touch, W'as fundamentally different from that of petiole (including pulvinus) form a leaf. Rapid, signalling of wound stimuli, such as localized burning visible leaf movements are generated by asym- (Snow, 1924; Houwink, 1935; Sibaoka, 1969; metrical turgor changes in cortical cells of the Pickard, 1973). Note that if a plant is knocked or pulvinus (at the base of the petiole) and pulvinule (at shaken vigorously, many leaves may respond almost the base of each pinnule). There are also secondary immediately. In this case, however, the stimulus pulvini at the base of each pinna, but these are more itself is transmitted mechanically throughout the responsive to light than to mechanical stimuli plant and there is no need to invoke endogenous (Weintraub, 1952). The decreases in turgor which long-distance signals (Satter, 1989; Sibaoka, 1991). drive rapid movements are associated with changes Signalling of non-wound stimuli is often relatively in membrane resistance (Samejima & Sibaoka, 1982) slow (perhaps 1 mm s"') and can progress only to 50 M. Malone

neighbouring pinnules or throughout an individual workers of this century appear to have favoured pinna, but not usually to other leaves. This type of Ricca's theory of chemical signalling of wound signalling is reported to pass only through living stimuli (Snow, 1924; Houwink, 1935; Sibaoka, tissue and is associated with characteristic electrical 1969; Pickard, 1973). Nevertheless, there are two activity (Houwink, 1935; Pickard, 1973). By con- major problems with Ricca's theory (Roblin, 1979): trast, long-distance signals of wound stimuli can pass (1) The wound signal can travel much faster than the rapidly (up to 10 mm s"' or more) throughout a leaf, transpiration stream, and (2) in contrast to the between neighbouring leaves, or throughout all transpiration stream, the signal can travel basipetally leaves on the plant. These signals are also associated as well as acropetally. with characteristic electrical activity but, unlike the I argue here that both of these objections are above, they can pass through dead tissue negated by recent analysis of wound-induced hy- (Cunningham, 1895; Ricca, 1916, 1926; Houwink, draulic signals (Malone & Stankovic, 1991 ; Malone, 1935). In addition to these two types of signalling, 1993 a; Malone, Alarcon & Paiumbo, 1994). there are occasional references to a third type (Snow. The problem of basipetal conduction was con- 1924). The evidence for this third type seems scant sidered in some detail by Ricca. Basipetal conduction and largely anecdotal (Pickard, 1973; but see Umrath of the wound signal is always involved when damage & Kasterberger, 1983). to one pinna or pinnule causes responses in other Here, 1 consider only the second of these types of pinnae or leaves. This is because conduction down transmission, i.e. long-distance signalling of wound the petiole of the damaged leaf is basipetal by stimuli. Ricca (1916) did outstanding work on this definition. Since signal transmission over the earliest subject; he worked chiefly with Mimosa spegazzinii, (basipetal) part of the pathway is usually also the but his conclusions apply also to M. pudica. He most rapid (>10mms"^, Houwink, 1935), the showed that the signal would pass through stems statement of Roblin (1979) and others, that 'the which had been severed transversely and re-con- propagation rate is higher in (the) acropetal nected by a water-filled glass tube. He postulated direction' is too general. Some minutes after that a chemical messenger (now known as ' Ricca's wounding, acropetal conduction may continue at a factor') was released from damaged tissue, that it decreasing rate while further basipetal conduction was transported in the xylem, mainly with the does not occur at all. Response times at various transpiration stream, and that it would activate distances from a burnt leaf of Mimosa are shown in tnotor tissue in the pulvini and pulvinules of Figure 1. These response times are analyzed in more undamaged leaves. Ricca also showed that this detail in a later section. The data in Figure 1, and messenger, applied exogenously, could move into other Figures shown here, are representative excised shoots at the same rate as the transpiration examples; each experiment has been repeated at stream. His work was subsequently confirmed in least three times w-ith similar results. other laboratories (Snow, 1924; Houwink, 1935). To account for the basipetal element of trans- Ricca's preliminary investigations into the nature of duction, Haberlandt (cited in Houwink, 1935) con- the chemical messenger showed that it could be sidered the possible driving effect of steam produced extracted and transferred from plant to plant, and by burning. Houwink (1935) mentioned the 'expan- that it could be stored in a dry form (Ricca, 1916). sion of sap upon heating'. Van Sambeek & Pickard The chemical nature of Ricca's factor remains (1976) stated that, on burning, the spread of Ricca's obscure although some possible structures have factor into the rest of the plant ' is facilitated by been published (Kallas, Meier-Augenstein & the disturbance of the transpiration stream which Schildknecht, 1990). Ricca's experiments appeared doubtless results from heating water in the leaf. to eliminate two alternative models of long-distance Ricca's own ideas on basipetal transmission were wound signalling, each of which envisaged the direct more specific than any of these: he 'supposed that participation of living cells: (1) Positive-pressure some of the substance (Ricca's factor) is set free (at hydraulic signalling through a continuous network the damage site) and is sucked in through the vessels of turgid tube cells (Haberlandt, 1914), and (2) by the negative pressure in the wood' (Houwink, electrical transmission involving the membrane 1935). Snow (1924) quoted Ricca as suggesting that activity of living cells (Bose, 1928). '...the burn may set free liquid from the killed cells, In some current literature, it is implied that the which in the case of leaves attached to shoots might electrical model of wound signalling in Mimosa be sucked away to other living parts so that the forms the 'classical' theory. For example, Tinz- water-current m the leaf would, for a short time, set Fiichtmeier & Gradmann (1990) state that 'the back towards the leaf base' (my italics). Ricca (1926) traditional view [is] that the primary process in the noted the then recent Cohesion-Tension theory and rapid conductance of excitation in Mimosa pudica is stated that ' In cases of heating or cutting the tension not hydraulic but electric'. Similar statements are previously existing in water in conducting tracts made by Satter (1989). However, I can find little must disappear owing to the rapid motion of the evidence in the literature to support this view; most liquid'. Ricca's ideas predicted a wound-induced Mimosa hvdraulics 51

30 it probably occurs in any plant in which a significant xylem tension exists prior to wounding. Wound-induced hydraulic signals may be resolved 20 into two long-distance components: a rapid pressure wave, and a slower mass flow of cell sap from the wound site (Malone, 1993 a). In tomato plants, the 10 pressure-wave component is not involved m wound signalling but the mass flow- component is essential for wound signalling, because it disperses elicitors

-10 5,9-

-20 Basal 200 400 600 800 leaves Time to leaf movement (s) Figure I. Stimulus transmission rate in Mimosa pudica following a localized scorch wound. A pinna of one leaf, near the middle of the central axis of the plant, was scorched for 3 s with a Hame, The central axis was about 70 cm in length. The time from wounding until leaf fall Central (reaction of pulvinus) is plotted for leaves at various T3 leaf distances above and below the wound site, on the central axis. Data from three trials on the same plant are plotted, (blanks) and a different svmbol is used for each trial. Figure 2, Systemic increases in leaf thickness following localized scorch wounding in Mimosa pudica. Dis- ' hydraulic signal' some 80 yr before it was first placement transducers were placed on four ]ea\es of the centra] axis of a plant. Transducers T0-T2 were on basal measured (Malone & Stankovic, 1991). Early leaves, and T3 was on a central leaf. Two blank transducers, attempts to test Ricca's ideas on basipetal transport mounted on the same frame but not containing leaves, were met with limited success. Houwink (1935), using a also run. At the time indicated by the vertical line, one pinna primitive heat-pulse technique, presented some of an apical leaf was scorched for 3 s with a flame. Pinnule closure was noted adjacent to transducer T3 (on the same evidence that 'heating causes the sap to move pinna) at the time indicated by the arrow. There was no downwards'. Snow (1924) found that excision of the pinnule closure in leaves under transducers T0-T2. The distal part of a shoot under a solution of eosm dye curves indicate variation in leaf thickness; absolute leaf results in basipetal movement of the dye ' in the same thickness is not shown, and the curves have been offset way as excitation can be conducted downwards'. merely for convenience. Time and vertical scales are indi- cated on the marker, with upward deflection corresponding Ricca (1926) confirmed this finding. These to increasing leaf thickness. observations provide some support for Ricca's theory.

/ Jl2iim II, WOUND-INDIJCED HYDRAULIC SIGN.^LS IN 45 min MIMOSA A direct test of Ricca's theory can be made by tbe application of high-resolution displacement trans- ducers (Malone, 1992). When the pinna of one leaf of a Mimosa plant is scorched with a flame, distant leaves swell up beginning almost immediately (basal leaves in Fig. 2). This indicates that water (cell sap) freed from damaged cells at the wound site, becomes available to relieve xylem tension locally and (hianksj systemically. Moreover, since remote leaves actually swell significantly, there must be a mass flow- of Figure 3. Systemic leaf swelling in Mimosa pudica induced water from the damaged area tow-ards other regions by watering. Water was withheld for 3 d from a pot plant. of the plant (Malone, 1993 a). The induction of these The peat-based soil was then dry but the plant was not hydraulic signals by localized scorching is by no visibly wilted. Transducers were placed on six different leaves. At the time indicated by the vertical line, the soil means unique to Mimosa; it has been measured in a was watered generously. No pinnule closure was observed. diverse range of plants (Boari & Malone, 1993) and Note different scale from that in Figure 2. 4-2 52 M. Malone

throughout the plant, from the wound site (Malone which is soon overtaken by a marked decrease. The et al., 1994 a). In Mimosa, pressure waves are likewise onset of this decrease probably reflects the time of not involved in wound signalling. This is indicated arrival of cell sap travelling with the mass fiow from by the finding that addition of water to the soil of a the wound site. When the solute-rich cell sap reaches drying plant causes rapid swelling of the leaves (with the xylem inside the undamaged tissue, it draws associated systemic increases in pressure) but no water from the healthy cells by an osmotic process, pinnule closure (Fig, 3). Also, distant leaves swell and becomes diluted. Thus, the effect diminishes rapidly after a localized burn, but they do not further from the wound site (Malone, 1993a), The necessarily close (Fig. 2, basal leaves). downturn in leaf thickness will always occur slightly It is considered in the next section whether the later than the arrival of the first solutes. This is second component of the hydraulic signal (the because there will be a lag between solute arri\'al in wound-induced mass How and associated hydraulic the x>'lem and measurable water flux from the dispersal) can account for rapid long-distance wound surrounding tissue. In addition, if xylem conduits signalling in Mimosa. behave as cylindrical tubes with laminar flow , solutes One unusual feature of the transducer recordings in the centre of the stream will move at up to twice from Mimosa is a sudden decrease in the thickness of the rate of the mean (Nobel, 1991). leaves near the wound site which appears a few Therefore, by monitoring leaf thickness at proxi- minutes after the onset of the wound-induced mal positions close to a wound site, an (under-) swelling (arrow on T3 in Fig, 2). In various other estimate of the initial rate of wound-induced ba- species, decreases in leaf thickness are also observed sipetal mass flow can be gained. Such an experiment near a wound site but the decrease is not as marked with Mimosa is depicted in Figure 4. For this as that in Mimosa. The abrupt decrease in Mimosa experiment, the transducer assembly was modified may be associated with the motor reaction of the so that thickness was measured from needles resting pulvinules: the adaxial surfaces of the pulvinules transverseh' across the pinna-rachis, between neigh- collapse when triggered and, if the transducer is bouring pairs of pinnules. The pulvinules themselves partially supported by these surfaces, a sudden were avoided so as to preclude any effects of pulvinar decrease would result. Consistent with this inter- collapse (as m T3, Fig. 2). pretation, sudden decreases in thickness were also The onset of the decrease in thickness at 15 mm monitored as Mimosa leaves closed in response to proximal to the w-ound site was kinetically in- non-wound (touch) stimuli (data not shown). distinguishable from that at 50 mm (Fig. 4). The temporal resolution in these recordings was < 1 s. Therefore, basipetal wound-induced mass flow must III. KINETICS OF HYDR.M'LIC DISPERS.'^L IN have occurred at an initial rate of at least 15 mm s"'. MIMOSA Mimosa leaves are unusual in exhibiting motor In tomato and maize, basipetal mass flow from a activity and in having an apoplastic or laticifer-like wounded leaf tra\ els at an estimated rate of about fluid-filled open tube network (Haberlandt, 1914; lOmms"' (Malone, 1993(7). Soluble elicitors re- Esau, 1970). Pressure-probe measurements indicate leased from damaged cells are swept along with this that this network is highly pressurized (c. 20 bar; flow. This process has been termed "hydraulic Malone, unpublished) and it may be similar to that dispersal' (Malone et al., 19946). It is interesting to described by Buttery & Boatman (1966) in Hevea. note here the statement of Tinz-Fuchtmeier & These unusual features could cause the thickness of Gradmann (1990) that '...excitation spreads over Mimosa leaves to behave differently from that in the {Mimosa) plant with a velocity up to some other plants. This is exemplified by the sudden 10 mm s"'. This is much too fast for the transport of decrease m thickness of the central leaf in Figure 2. a chemical stimulus over distances... through the Therefore, to check whether the results in Figure 4 plant.' In fact, the rate quoted here is exactly the w-ere some peculiarity of Mimosa, the procedure was same as that of hydraulic dispersal of chemicals repeated with tomato plants. The results were through the xyiem, at least m tomato. \irtually identical (not shown) and it is concluded To examine whether wound-induced mass flow in that rapid wound-mduced mass flow occurs in Mimosa is sufficiently rapid to account for wound Mimosa in the same w'ay as in tomato (Malone, signalling, its magnitude can be estimated from the 1993a) and probably many other plants. The results rate of v^uund-induced leaf swelling. This approach in Figure 4 therefore demonstrate that the minimum has been used on tomato (Malone, 1993 a). However, initial velocity of basipetal hydraulic dispersal from a more direct approach is available. Near to a wound a wound site in Mimosa is 15 mm s~'. This (mini- site, the thickness of healthy tissue decreases soon mum) rate is sufficiently rapid to cover most reports after wounding. This is in contrast to the swelling of w-ound-signalling in Mimosa. observed in tissue distant from the wound site. At A maximum velocity for basipetal wound-induced some intermediate position, a transition can be seen: mass flow can also be estimated. Flow in the xylem the wound causes an earlv increase in thickness is usually considered according to the Hagen- Mimosa hydraulics 53

diameter 25//m will be about 300 mm s '. This value is only 10-20 "o of that required to transgress Reynold's number (Nobel, 1991) and laminar flow is therefore probably retained under these conditions. These estimated maximum velocities of mass fiow (and therefore of chemical signalling by hydraulic 12/7 m dispersal) rival or exceed even the fastest reports of signal transmission in plants (17 cms"', Sibaoka, min 1966; 20 cms"', Oda & Linstead, 1975; 31 cms ', Umrath & Kastberger, 1983). The maximum fiow velocities could be approached only transiently, and immediately after a wound. Thereafter, the flow rate will decay as the hydraulic capacity of the xylem and tissue become progressively fllled. In addition, the radial pathway for entry of water into the xylem at the wound site will introduce further resistances which retard the fiow. In Mimosa, a clean cut through one petiole, in air. causes rapid pulvinar reaction in near and distant Proximal leaves (similar to those in Fig. 1), i.e. cuts are very effective inducers of long-distance wound signals Figure 4. High-resolution measurement of changing (Snow, 1924). In tomato plants, single clean cuts thickness close to a wound site in Mimosa pudica. Rachis through the petiole release very little fiuid and thickness was monitored at four positions along an individual pinna, using displacement transducers modified therefore induce negligible hydraulic signals from those in Figure 2. One blank transducer was also (Malone et al., 19946). This implies that the remote run. At the time indicated by the vertical line, three effects of cut wounds in Mimosa must be signalled pinnules at the distal tip of the pinna were scorched with other than by hydraulic means. However, Mimosa a flame. Transducer TO monitored racbis tbickness at 15 mm from tbe wounded pinnules, Tbe distances (mm) of petioles contain large pressurized laticifer-like com- other transducers from the wounded pinnules, are partments (see above). This is e\'ident from the indicated in parentheses on tbe figure. Tbe racbis was spontaneous exudation of liquid which occurs from 67 mm in length. Arrows on tbe figure indicate times of both cut surfaces after a petiole is severed two mock scorcb wounds: for tbese the flame was (Haberlandt, 1914; Weintraub, 1952). Several //I approacbed near, but not toucbing, tbe apical pinnules. Note faster time scale tban in previous figures. may be exuded from each of the two cut surfaces within seconds after severing a petiole. Clearly, this exudate fluid will be available to the xylem to fuel hydraulic signals from cut surface. Thus, hydraulic Poiseuille law for pressure-driven flow through dispersal can operate m the case of cut wounds m smooth tubes (Xobel, 1991). This model usually Mimosa, but not in tomato or other plants which do provides a reasonable prediction of xylem con- not exhibit spontaneous exudation of fluid. ductivity, often to within a factor of two (Frensch & The wound-induced hydraulic signal is transient. Steudle, 1989; Darlington & Dixon, 1991). In the However, any solutes which it delivers as far as the petiole of Mimosa, there are xylem vessels of up to xylem of the stem can be subsequently transported 25 p,m in diameter (Fleurat-Lessard & Roblin, 1982). with the transpiration stream. Thus, there should be In a transpiring plant the pressure of water inside two successive phases of xylem-borne movement of these vessels might be — 5 to — 10 bar (atmospheric Ricca's factor from a damage site in Mimosa. The = + 1), even in well-watered soil. On burning, water first phase is driven by the wound-induced hydraulic is released from cells at the wound site, and this signal while the second is driven by transpiration: water becomes available, at atmospheric pressure, to Phase I. Rapid flow (initial rate 5= 15 mm s"') in the xylem. From the Hagen-Poiseuille law, the both basipetal and acropetal directions from the average velocity of flow (J,,) will be given by: wound site. This fiow is reflected in the systemic r'^AP swelling of leaves and other tissues, which follows a (1) localized wound. From the kinetics of leaf swelling in Figure 2, this phase appears to exhibit a half time of where r = vessel radius, AP = pressure gradient, about 2 min in Mimosa, as in other species (Boari & f/ = viscosity of fiuid, / = length of tube. Malone, 1993). The relative magnitudes of the Taking viscosity as 10""* bar s and, given the above acropetal and basipetal components of this flow will pressure gradient (6 bar) imposed uniformly along a depend on the corresponding distribution, about the Mimosa petiole of length 40 mm. Equation 1 predicts wound site, of the accessible hydraulic capacitance of that the mean flow velocity through vessels of the tissues ofthe plant (Malone, 1993 a). 'Accessible' 54 M. Malone is used here to denote capacitance with a short half followed by a slower, solely acropetal phase (= time. This is a necessary caveat because there might Phase II). This is further evidence that wound be tissues, such as fruit (Jones, Lakso & Syvertsen, signalling in Mimosa involves hydraulic dispersal. 1985) and others (Morse, 1990), which exhibit only It is concluded that, when wound-induced hy- slow pressure equilibration with the xylem. Such draulic events are taken into account, Ricca's theory tissues could have a large total capacitance but they is entirely adequate to explain long-distance wound might be of minor relevance for short-term flow. In signalling in Mimosa. The evidence that rapid the case of a burn applied to a leaf, almost all the basipetal mass flows are induced by wounding (Figs accessible hydraulic capacitance of the plant will lie 2,4) eliminates difliculties regarding the high initial proximal to the wound site and, therefore, almost all velocity, and the basipetal component, of stimulus of the wound-induced mass flow will be basipetal, at transmission. It follows that wound-induced leaf least until it reaches the stem. At the stem, the mass movements in Mimosa provide a visual indication of flow will split into basipetal and acropetal com- the progress of hydraulic dispersal of chemical ponents, again depending on the distribution of the elicitors from wound sites. The same wound- plant's accessible capacitance about the insertion of signalling mechanism appears to be involved in the petiole of the damaged leaf. Conversely, if the systemic induction of defence responses in tomato scorch wound was applied not to a leaf, but to a basal (Malone et aL, 1994a) and possibly other plants. region of the stem, most flow in phase I would be The visible reactions of Mimosa may thus provide a acropetal. Phase 1 ends when water is no longer convenient model system for tbe study of a long- freely available at the wound site or, in the case of a distance wound-signalling mechanism common to large wound, when the plant's hydraulic capacitance many plants. has been saturated. Phase II. Slow (perhaps < 1 mm s~^), mainly IV. IMPLICATIONS FOR ELECTRICAL acropetal flow driven by transpiration. On 'SIGNALS' wounding, transpiration will continue from all leaves of the plant, although it may be reduced transiently It is argued above that long-distance wound- (Van Sambeek & Pickard, 1976; Wildon et aL, 1992; signalling in Mimosa occurs by bydraulic dispersal of Malone & Stankovic, unpublished). This reduction chemical messengers. However, electrical activity is could result from hydropassive closure of stomata often associated with the wound signal in Mimosa, as caused by systemic swelling of epidermal cells in many other plants (references in Pickard, 1973; (Malone, 1993 a). As phase I flow decays, the normal Malone & Stankovic, 1991). This association has led pattern of transpiration-driven acropetal flow will some workers to conclude that the wound signal is a once again become established. This will carry, again self-propagating electrical phenomenon analogous to by mass flow, any fluid and solutes which have the nerve impulse, or to cell-to-cell 'epithelial' entered the xylem of the stem during phase I. Flow conduction, in animals (Bose, 1928; Wildon et aL, during phase II will be predominantly acropetal. 1992), Others have favoured the electrical mode! However, even transpiration-driven flow need not but, noting the passage of wound signals across dead necessarily be acropetal: shortly after the wound, tissue (which cannot sustain cell-to-cell electrical when fluid is freely available to the xylem at the transmission) have considered a hybrid hypothesis. wound site, this site will replace the root medium as This postulates that the major signal is electrical but, the most accessible source of water for the plant. at dead zones, an accompanying chemical signal Thus, transiently, transpiration from healthy leaves intervenes, which traverses the dead zone and re- will tend to draw water hasipetally down the petiole triggers the electrical signal in living tissue at the of the wounded leaf, and possibly also down tbe stem other side (Sibaoka. 1966; Pickard, 1973; Roblin, towards the nodes of more basal leaves. From there 1979). There seems little to commend this hybrid the flow would once again become acropetal. interpretation. A sitnpler interpretation is that tbe Basipetal flow driven by transpiration is likely to be travelling wound signal is entirely chemical, and that important only when relatively large amounts of the electrical activity observed in living tissue reflects water are released at the wound site - enough to some local response to the travelling chemical signal. saturate much of tbe plant's hydraulic capacity According to this model, the electrical activity does during phase 1. and with fluid to spare. More not itself travel at all, instead, its apparent pro- prolonged basipetal flow^ driven by transpiration, gression marks the underlying movement of the can occur from sites of submerged-excision of chemical signal. Clearly. Ricca's factor can trigger portions of leaf (Malone, 19936). dramatic changes in membrane properties in cells of The two phases of flow described above are the pulvinus of Mimosa. Perhaps this chemical, or reflected in the two phases of wound-stimulus another component of the cocktail released from transmission which occur in Mimosa (Fig. 1): an damaged cells, can affect the membrane properties of early rapid phase of transmission in both acropetal cells neighbouring the xylem, as it passes, and thus and (especially) basipetal directions (= Phase 1). is generate 'travelling' electrical events. The most Mimosa hydraulics 55 prominent electrical activity triggered by wounding pinna. Furthermore, some 'non-wound' stimuli (the 'variation potential') is often interpreted in this (such as sudden cooling or DC current) may affect way (Houwink, 1935; Sibaoka, 1966). Pickard (1973) cell membranes directly and cause local release of states that 'the variation potential, like the w^ound water, solutes, or both, from the treated tissue. Such hormone of Ricca, moves with the transpiration treatments will be tantamount to wounding. stream in the xylem...the wound substance causes With possible exceptions m some carnivorous local electrical changes as it leaks into living cells in plants (Williams & Pickard 1972), electrical activity the vicinity of the tracheids and vessels, perhaps by has not yet been demonstrated to convey any long- depolarising them'. The same interpretation can distance signal in higher-plant systems, despite many account for observations of electrically 'conducting' claims to the contrary (e.g. Roblin, 1979; Simons, or 'excitable' cells adjacent to the vascular tissue of 1981; Satter, 1989; Fromm, 1991; Sibaoka, 1991; Mimosa (Samejima & Sibaoka, 1982). In addition, a Wildon et al., 1992). Indeed, three of the favourite marked electrical event occurs at the pulvinus during paradigms for electrical transmission are better pulvinar movement in Mimosa (Samejima & explained in terms of travelling hydraulic (or Sibaoka, 1980). This is usually termed an action hydraulic dispersal) signals, namely: (1) Long- potential, and it is implied that this event triggers distance wound-signalling in Mimosa (this paper), pulvinar movement. However, this 'action potential' (2) remote effects of wounding on transpiration could well be a result, rather than a cause, of the (Malone, 1993 a), and (3) systemic, wound-induced massive transmembrane ion fluxes associated with defence reactions in tomato (Malone et al,, 1994a). pulvinar movement. If, as postulated by Kumon & Suda (1984), mass-flow channels open in the plasma- membranes of cortical cells when the pulvinus is .•\ C K N O W 1, E D G E M E N T S triggered, the ensuing passive flow of ions must The author is grateful to Dr. D. A. Cookc of Broom's Barn result in rapid depolarization of the membrane Experimental Station for editorial assistance. This work potential, with associated rapid changes in the was funded by AFRC (UK). surface electrical potential. Simply cutting the cells in half would have a similar effect. The finding that REFERENCES the 'action potential' precedes pulvinar movement Boari F, Malone M. 1993. Wound-induced hydraulic signals: by a fraction of a second (Samejima & Sibaoka, 1980) survey of occurrence m a range of species. Journal of may not indicate that it is causal. Eor example, the E.xperimental Botany 44; 741-746. ' action potential' could begin when the first few Bose JC. 1928. Tin motor mechanism nf plants. London: Longmans, (innermost) cells of the motor cortex react, whereas Buttery BR, Boatman SG. 1966. Manometric measurement of significant petiolar movement might develop only turgor pressures in laticifcrous phloem tissues. 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