Induction and ionic basis of slow wave potentials in seedlings of Pisum sativum L.

Slow wave potentials (SWPs) are transient depolarizations which propagate substantial distances from their point of origin. They were induced in the epidermal cells of pea epicotyls by injurious methods such as root excision and heat treatment, as well as by externally applied defined steps in xylem pressure (Px) in the absence of wounding. The common principle of induction was a rapid increase in Px. Such a stimulus appeared under natural conditions after (i) bending of the epicotyl, (ii) wounding of the epidermis, (iii) rewatering of dehydrated roots, and (iv) embolism. The induced depolarization was not associated with a change in cell input resistance. This result and the ineffectiveness of ion channel blockers point to H(+)-pumps rather than ion channels as the ionic basis of the SWP. Stimuli such as excision, heat treatment and pressure steps, which generate SWPs, caused a transient increase in the fluorescence intensity of epicotyls loaded with the pH-indicator DM-NERF, a 2',7'-dimethyl derivative of rhodol, but not of those loaded with the pH indicator 2',7'bis(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). Matching kinetics of depolarization and pH response identify a transient inactivation of proton pumps in the plasma membrane as the causal mechanism of the SWP. Feeding pump inhibitors to the cut surface of excised epicotyls failed to chemically simulate a SWP; cyanide, azide and 2,4-dinitrophenol caused sustained, local depolarizations which did not propagate. Of all tested substances, only sodium cholate caused a transient and propagating depolarization whose arrival in the growing region of the epicotyl coincided with a transient growth rate reduction.     

Comparison of electric and growth responses to excision in cucumber and pea seedlings. II. Long-distance effects are caused by the release of xylem pressure

Excision of a growing stem causes local wound responses, such as membrane depolarization and growth inhibition, as well as effects at larger distances from the cut. In this study, cucumber hypocotyls were excised 100mm below the hook, so that the growing region was beyond the reach of the wound-induced depolarization (up to 40mm). Even at such a distance, the cut still caused a considerable and rapid drop in the hypocotyl growth rate. This growth response is not a direct wound response because it does not result from the cut-induced depolarization and because it can be simulated by root pressure manipulation (using a pressure chamber). The results indicate that the growth response resulted from the rapid release of the xylem pressure upon excision. To test this conclusion we measured the xylem pressure by connecting a pressure probe to the cut surface of the stem. Xylem pressure (P_x) was found to be +10 to +40kPa in cucumber hypocotyls and -5 to -10 kPa or lower in pea epicotyls. Excision of the cucumber hypocotyl base led to a rapid drop in P_x to negative values, whereas excision in pea led to a rapid rise in P_x to ambient (zero) pressure. These fast and opposite p_x changes parallel the excision-induced changes in growth rate (GR): a decrease in cucumber and a rise in pea. The sign of the endogenous xylem pressure also determined whether excision induced a propagating depolarization in the form of a slow wave potential (SWP). Under normal circumstances pea seedlings generated an SWP upon excision whereas cucumber seedlings failed to do so. When the P_x in cucumber hypocotyls was experimentally inverted to negative values by incubating the cumber roots in solutions of NaCN or n-ethylmaleimide, excision caused a propagating depolarization (SWP). The experiment shows that only hydraulic signals in the form of positive P_x steps are converted into propagating electric SWP signals. These propagating depolarizations might be causally linked to systemic ‘wound’ responses, which occur independently of the short-distance or direct wound responses.     

Rapid alterations in growth rate and electrical potentials upon stem excision in pea seedlings

Excision of the epicotyl base of pea (Pisum sativum L.) seedlings in air results in a fast drop in the growth rate and rapid transient membrane depolarization of the surface cells near the cut. Subsequent immersion of the cut end into solution leads to a rapid, transient rise in the epicotyl growth rate and an acropetally propagating depolarization with an amplitude of about 35 mV and a speed of approx. 1 mm · s^–1. The same result can be achieved directly by excision of the pea epicotyl under water. Shape, amplitude and velocity of the depolarization characterize it as a slow-wave potential. These results indicate that the propagating depolarization is caused by a surge in water uptake. Neither a second surge in water uptake (measured as a rapid increase in growth rate when the cut end was placed in air and then back into solution) nor another cut can produce the depolarization a second time. Cyanide suppresses the electrical signal at the treated position without inhibiting its transmission through this area and its development in untreated parts of the epicotyl. The large depolarization and repolarization which occur in the epidermal and subepidermal cells are not associated with changes in cell input resistance. Both results indicate that it is a transient shut-down of the plasma-membrane proton pump rather than large ion fluxes which is causing the depolarization. We conclude that the slow wave potential is spread in the stem via a hydraulic surge occurring upon relief of the negative xylem pressure after the hydraulic resistance of the root has been removed by excision.Abbreviations and Symbols GR growth rate - P_x xylem pressure - R_in cell input resistance - SWP slow wave potential - V_m membrane potential - V_s surface potential This work was supported by grants to D.J.C. from the National Science Foundation and the U.S. Department of Energy.     

Slow wave potentials in cucumber differ in form and growth effect from those in pea seedlings

A positive hydraulic signal in the form of a xylem pressure step was applied to the roots of intact seedlings of Cucumis sativus L. and Pisum sativum L. Surface electrodes at three positions along the epicotyl/hypocotyl recorded a propagating depolarization which appeared first in the basal, then the central and sometimes the apical electrode positions and fitted the characteristics of a slow wave potential (SWP). This depolarization differed between pea and cucumber. It was transient in cells of pea epicotyls but sustained in cucumber hypocotyls. It was not associated with a change in cell input resistance in pea epicotyls but preceded an increase in the input resistance of cucumber hypocotyl cells. With the increased xylem pressure the growth rate (GR) of cucumber hypocotyls and pea epicotyls underwent a transient increase, peaking after 5 min. If the depolarization reached the growing upper region, it preceded a sustained decrease in the GR of cucumber hypocotyls but only a transient decrease in the GR of pea epicotyls. A temperature jump in the root medium (heat treatment) induced a steep pressure spike in the xylem of the cucumber hypocotyl which showed similar electric and growth effects as the previously applied, non-injurious pressure steps. We suggest that the observed differences in the electric and growth responses between the species were caused by the closure of ion channels in depolarized cells of cucumber but not pea seedlings.     

Decrement and amplification of slow wave potentials during their propagation in<Emphasis Type="Italic"> Helianthus annuus</Emphasis> L. shoots

Slow wave potentials (SWPs) are transitory depolarizations occurring in response to treatments that result in a pressure increase in the xylem conduits (P_x). Here SWPs are induced by excision of the root under water in 40- to 50-cm-tall light-grown sunflower plants in order to determine the effective signal range to a naturally sized pressure signal. The induced slow wave depolarization appears to move up the stem while it is progressively decremented (i.e. the amplitude decreases with increasing distance from the point of excision) with a rate that appears to rise acropetally from 2.5 to 5.5% cm^–1. The decline of the SWP signal, in both amplitude and range, could be experimentally increased (i) when root excision was carried out in air and (ii) when the transpiration of the sunflower shoot was minimized by a preceding removal or coating of the leaves. A further decline of the SW signal was expected to occur when leaves were included in the measured path. However, when the most distant apical electrode was attached to an upper leaf, it showed a considerably larger depolarization than a neighboring stem position. This apparent amplification of the SWP signal is not confined to the leaf blade but includes the petiole as well. The amplification disappeared (i) when the illumination level was lowered to room light, (ii) when the blade was excised either completely or along the remaining midvein and (iii) when the intact leaf blade was submersed in water. These treatments reduce the SWP at the petiole to a small fraction of the signal in the opposite control leaf and specify bright illumination and blade-mediated transpiration as prerequisites of a signal increase that is confined to young, expanding leaves.     

Mannitol inhibits growth of intact cucumber but not pea seedlings by mechanically collapsing the root pressure

The positive xylem pressure (Px) in cucumber hypocotyls is a direct extension of root pressure and therefore depends on the root environment. Solutions of the electrolyte KCl (0-10 osm) reduced the hypocotyl Px transiently (biphasic response), while the Px reduction by mannitol solutions was sustained. The amplitudes of the induced Px reduction depended directly, and the degree of Px restoration after stress release depended indirectly, on the size of the initial positive Px indicating that mannitol released the root pressure by a mechanical rather than osmotic mechanism. Mannitol treatment and other means of root pressure reduction revealed two separate growth responses in the affected cucumber hypocotyls. Only steep Px drops (following root excision or root pressure release in mannitol) directly cause a rapid, transient drop in growth rate (GR). Both rapid and slow (after root incubation in KCN or NEM) decreases in root pressure, however, led to a sustained growth inhibition of cucumber hypocotyls after about 30 min. This delay characterizes the growth response as an indirect consequence of the Px change. Pea seedlings, which lacked root pressure and had a negative Px throughout, showed extremely small changes in epicotyl Px and GR after root incubation in mannitol. It is apparent that the higher sensitivity of cucumber growth to mannitol depended on the presence and release of root pressure.     

Surface potentials and hydraulic signals in wheat leaves following localized wounding by heat

Abstract. Localized burning of a leaf causes a rapid change in apoplastic electrical potential throughout the shoot of wheat seedlings ('variation potential'). It also causes marked increases in turgor pressure in epidermal cells of adjoining leaves. These turgor increases indicate rapid propagation throughout the seedling, of a hydraulic pressure wave from the site of wounding. Evidence is presented that this pressure wave is caused by relief of xylem tension, by water released from damaged cells in the wounded region. It is demonstrated that, in the absence of wounding, pressure waves imposed at the tip of one leaf can travel to neighbouring leaves, and can there induce change in apoplastic electrical potential similar to a 'variation potential'. This indicates that the hydraulic event produced by wounding is the signal responsible for systemic induction of the 'variation potential'. This signal has been termed 'Ricca's factor'. It is suggested that arrival of the hydraulic wave alters leaf water potential and thereby induces stomatal activity. Leaf surface potential may be dominated by electrogenic ion pumping or flux at stomatal cells, and the 'variation potential' may therefore be a reflection of stomatal activity induced by the hydraulic signal.     

Shade-Induced Action Potentials in Helianthus annuus L. Originate Primarily from the Epicotyl

Repeated observations that shading (a drastic reduction in illumination rate) increased the generation of spikes (rapidly reversed depolarizations) in leaves and stems of many cucumber and sunflower plants suggests a phenomenon widespread among plant organs and species. Although shaded leaves occasionally generate spikes and have been suggested to trigger systemic action potentials (APs) in sunflower stems, we never found leaf-generated spikes to propagate out of the leaf and into the stem. On the contrary, our data consistently implicate the epicotyl as the location where most spikes and APs (propagating spikes) originate. Microelectrode studies of light and shading responses in mesophyll cells of leaf strips and in epidermis/cortex cells of epicotyl segments confirm this conclusion and show that spike induction is not confined to intact plants. 90% of the epicotyl-generated APs undergo basipetal propagation to the lower epicotyl, hypocotyl and root. They propagate with an average rate of 2 ± 0.3 mm s⁻¹ and always undergo a large decrement from the hypocotyl to the root. The few epicotyl-derived APs that can be tracked to leaf blades (< 10%) undergo either a large decrement or fail to be transmitted at all. Occasionally (5% of the observations) spikes were be generated in hypocotyl and lower epicotyl that moved towards the upper epicotyl unaltered, decremented, or amplified. This study confirms that plant APs arise to natural, nontraumatic changes. In simultaneous recordings with epicotyl growth, AP generation was found to parallel the acceleration of stem growth under shade. The possible relatedness of both processes must be further investigated.Key Words: Helianthus annuus L., action potentials, natural induction, AP propaqgation, amplification, stem growth, decrement     

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.

     

A method for inducing xylem emboli in situ: experiments with a field-grown tree

A pressure collar, assembled around 25cm sections of 4-year-old willow twigs, was used to examine cavitation events under field conditions. When the air pressure inside the collar was raised to between 1–8 and 2–8MPa, ultrasound acoustic emission signals were triggered which indicated the breaking of water columns in the xylem. The hydraulic conductivity of the twig portion inside the chamber decreased markedly. As a result, water potentials and conductances in leaves at the end of the twig decreased. Similar changes were induced at comparable pressures in detached twigs. The equipment used is described in detail, and evidence is presented that the mechanism of this artificial production of emboli follows the air-seeding principle hypothesized for natural cavitation events.     

New evidence for large negative xylem pressures and their measurement by the pressure chamber method

Pressure probe measurements have been interpreted as showing that xylem pressures below c. –0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below –0.4 MPa. We deduced xylem pressures in water-stressed stem xylem from the following experiment: (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air-seeding). Deduced xylem pressure showed a one-to-one correspondence with pressure chamber measurements for 12 species (woody angiosperms and gymnosperms); data extended to c. –10 MPa. The same correspondence was obtained under field conditions in Betula occidentalis Hook., where pressure differences between air- and water-filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is difficult to explain these results if xylem pressures were above –0.4 MPa, if the pressure chamber was inaccurate, and if cavitation occurred by some mechanism other than air-seeding. A probable reason why the pressure probe does not register large negative pressures is that, just as cavitation within the probe limits its calibration to pressures above c. –0.5 MPa, cavitation limits its measurement range in situ.     

Occurrence of Inverted Water Potential Gradients between Soil and Bean Roots

Measurements with a pressure chamber were made of the xylem water potential of leaves, shoots and roots from bean plants (Pkaseolus vulgaris L. cv. Processor) grown with a 12 hour dark period and natural or artificial light conditions during the day. The water potentials were measured at the end of a dark period and during the light period. Measurements taken at the end of the dark period indicated normal potential gradients within the soil/plant system (leaf < shoot < root < soil), when the matric potential of soil water was relatively high (above ?0.02 bar), and the gradients then also remained normal during the day (natural light). When the soil water potential was ?1 bar or lower in the morning, however, the root xylem water potential was higher than the soil water potential; at very low soil water potentials (< ?4 bar) it remained higher during most of the day. In this case also leaf and shoot xylem water potentials were higher than the soil water potential in the early morning, although decreasing rapidly in daylight. Under artificial light, both leaf and root water potentials were higher than the soil water potential throughout the whole diurnal cycle when the latter potential was below ?4 bar. From measurements of stomatal diffusion resistance, transpiration, relative water content of leaves and of changes in the matric potential of soil water, it was concluded that when the matric potential of soil water was low, water could be taken up by the plant against a water potential gradient. Because leaf xylem water potential was always lower than root xylem water potential, the mechanism involved in the inversion of water potential gradient must be localized in the roots, and probably related to ion uptake. Symbols and abbreviations used in the text: Ψ: Plant water potential (thermocouple psychrometer); Ψx: Xylem water potential (pressure chamber); Ψs: Osmotic potential of xylem sap; Ψm: Matric potential of soil water; RWC: Relative water content.     

Measurement of the water potential of stored potato tubers

A method of measuring the water potential of stored potato tubers (Solanum tuberosum L.) was needed to investigate the relationship of bacterial soft rot in tubers to water potential. Pressure chamber measurements, while useful for tubers with functional stolons, cannot be made on stored tubers. Measurements could be made on excised tissue pieces in a hygrometer chamber and with hygrometers implanted into tubers. We report here our evaluation of these hygrometric methods using a comparison with the pressure chamber on tubers harvested with stolons intact.

In tubers of high water potential, measurements on excised tissue were as much as 0.5 megapascals lower than the pressure chamber, probably due to turgor-driven expansion of the sample when freed from constraints imposed by surrounding tissue. Good agreement (±0.05 megapascals) was found between the implanted hygrometer and the pressure chamber at potentials higher than −0.5 megapascals. At lower water potentials, both hygrometer measurements were higher than the pressure chamber. Respirational heating of the tissue contributed to the increase in the excised tissue samples, but not with the implanted hygrometers because of the hygrometer design. The osmotic pressure balanced the pressure chamber measurement of potential at −0.7 megapascals, but was too small to do so at lower potentials. At most, 25% of this discrepancy can be accounted for by dilution by apoplastic water. We believe that the pressure chamber measurement is too low at low water potentials and that the error is associated with air bubbles in the xylem. At low potentials air emerged from xylem vessels along with sap, and fewer xylem emitted sap as potentials decreased.

     

The transmission of gas pressure to xylem fluid pressure when plants are inside a pressure bomb

In earlier work tobacco leaves were placed in a Scholander-Hammel pressure bomb and the end of the petiole sealed with a pressure transducer in order to measure pressure transmission from the compressed gas (Pg) in the bomb to the xylem fluid (Px). Pressure bomb theory would predict a 1:1 relationship for Pg:Px when tobacco leaves start at a balance pressure of zero. Failure to observe the expected 1:1 relationship has cast doubt on the pressure-bomb technique in the measurement of the xylem pressure of plants. The experimental and theoretical relationship between Px and Pg was investigated in Tsuga canadensis (L) branches and Nicotiana rustica (L) leaves in this paper. It is concluded that the non 1:1 outcome was due to the compression of air bubbles in embolized xylem vessels, evaporation of water from the tissue, and the expansion of the sealed stem segment (or petiole) protruding beyond the seal of the pressure bomb. The expected 1:1 relationship could be obtained when xylem embolism was eliminated and stem expansion prevented. It is argued that the non 1:1 relationship in the positive pressure range does not invalidate the Scholander pressure bomb method of measuring xylem pressure in plants because Px never reaches positive values during the determination of the balance pressure.     

Xylem water transport: is the available evidence consistent with the cohesion theory?

Since its introduction in the late 19th century, the so-called cohesion theory has become widely accepted as explaining the mechanism of the ascent of sap. According to the cohesion theory, the minimum standing vertical xylem tension gradient should be 0·01 MPa m⁻¹. When transpiration is occurring, frictional resistances are expected to make this gradient considerably steeper. The results of numerous pressure chamber measurements reported in the literature are generally regarded as corroborating the cohesion theory. Nevertheless, several reports of pressure chamber measurements in tall trees appear to be incompatible with predictions of the cohesion theory. Furthermore, the pressure chamber is an indirect method for inferring xylem pressure, which, until recently, has not been validated by comparison against a direct method. The xylem pressure probe provides a means of testing the validity of the pressure chamber and other indirect techniques for estimating xylem pressure. We discuss here the results of concurrent measurements made with the pressure chamber and the xylem pressure probe, particularly recent measurements made at the top of a tall tropical tree during the rainy season. These measurements indicate that the pressure chamber often substantially overestimates the tension previously existing in the xylem, especially in the partially dehydrated tissue of droughted plants. We also discuss other evidence obtained from classical and recent approaches for studying water transport. We conclude that the available evidence derived from a wide range of independent approaches warrants a critical reappraisal of tension-driven water transport as the exclusive mechanism of long-distance water transport in plants.     

The myogenic component in distention-induced peristalsis in the guinea pig small intestine

In an in vitro model for distention-induced peristalsis in the guinea pig small intestine, the electrical activity, intraluminal pressure, and outflow of contents were studied simultaneously to search for evidence of myogenic control activity. Intraluminal distention induced periods of nifedipine-sensitive slow wave activity with superimposed action potentials, alternating with periods of quiescence. Slow waves and associated high intraluminal pressure transients propagated aborally, causing outflow of content. In the proximal small intestine, a frequency gradient of distention-induced slow waves was observed, with a frequency of 19 cycles/min in the first 1 cm and 11 cycles/min 10 cm distally. Intracellular recording revealed that the guinea pig small intestinal musculature, in response to carbachol, generated slow waves with superimposed action potentials, both sensitive to nifedipine. These slow waves also exhibited a frequency gradient. In addition, distention and cholinergic stimulation induced high-frequency membrane potential oscillations (~55 cycles/min) that were not associated with distention-induced peristalsis. Continuous distention produced excitation of the musculature, in part neurally mediated, that resulted in periodic occurrence of bursts of distally propagating nifedipine-sensitive slow waves with superimposed action potentials associated with propagating intraluminal pressure waves that caused pulsatile outflow of content at the slow wave frequency.     

EVIDENCE FOR XYLEM CONSTRICTIONS IN THE PRIMARY VASCULATURE OF PSILOPHYTON DAWSONII,AN EMSIAN TRIMEROPHYTE

Serial peels through the branch junctions of Psilophyton dawsonii were examined in an attempt to determine if “hydraulic constrictions” (i.e., a localized decrease in mean diameter of xylem elements) sensu Zimmermann (1983) had occurred during bifurcation. Based on the mean diameters of primary xylem tracheids measured acropetally and basipetally to branch junctions, evidence for xylem constrictions (= localized decrease in mean tracheid diameter) was found within the basalmost portions of four out of six minor axes examined near their attachment to major axes (anisotomous junctions). Xylem constrictions were not detected in branch junctions between axes of equal girth (isotomous junctions). Xylem constrictions were detected within the base of five out of seven junctions between fertile and main axes. The mean diameters of tracheids of the branch trace near its origin are larger both basipetally and acropetally from the constriction. There is no evidence for a localized decrease in the mean diameter of the xylem strand in the region of a constriction. Therefore, xylem constrictions are not the result of epidogenesis. Based on the production of “hydraulic constrictions” in extant plants, which serve to localize the formation of embolisms in lateral branches during water stress, it is speculated that P. dawsonii could protect the vasculature of major axes by a similar anatomical feature.     

Changes in stomatal conductance along grass blades reflect changes in leaf structure

Identifying the consequences of grass blade morphology (long, narrow leaves) on the heterogeneity of gas exchange is fundamental to an understanding of the physiology of this growth form. We examined acropetal changes in anatomy, hydraulic conductivity and rates of gas exchange in five grass species (including C(3) and C(4) functional types). Both stomatal conductance and photosynthesis increased along all grass blades despite constant light availability. Hydraulic efficiency within the xylem remained constant along the leaf, but structural changes outside the xylem changed in concert with stomatal conductance. Stomatal density and stomatal pore index remained constant along grass blades but interveinal distance decreased acropetally resulting in a decreased path length for water movement from vascular bundle to stomate. The increase in stomatal conductance was correlated with the decreased path length through the leaf mesophyll. A strong correlation between the distance from vascular bundles to stomatal pores and stomatal conductance has been identified across species; our results suggest this relationship also exists within individual leaves.     

Use of positive pressures to establish vulnerability curves : further support for the air-seeding hypothesis and implications for pressure-volume analysis

Loss of hydraulic conductivity occurs in stems when the water in xylem conduits is subjected to sufficiently negative pressure. According to the air-seeding hypothesis, this loss of conductivity occurs when air bubbles are sucked into water-filled conduits through micropores adjacent to air spaces in the stem. Results in this study showed that loss of hydraulic conductivity occurred in stem segments pressurized in a pressure chamber while the xylem water was under positive pressure. Vulnerability curves can be defined as a plot of percentage loss of hydraulic conductivity versus the pressure difference between xylem water and the outside air inducing the loss of conductivity. Vulnerability curves were similar whether loss of conductivity was induced by lowering the xylem water pressure or by raising the external air pressure. These results are consistent with the air-seeding hypothesis of how embolisms are nucleated, but not with the nucleation of embolisms at hydrophobic cracks because the latter requires negative xylem water pressure. The results also call into question some basic underlying assumptions used in the determination of components of tissue water potential using “pressure-volume” analysis.     

Distinct roles of electric and hydraulic signals on the reaction of leaf gas exchange upon re-irrigation in Zea mays L

The hypothesis that electric and hydraulic long-distance signals modify photosynthesis and stomatal aperture upon re-irrigation in intact drought-stressed plants was examined. Maize plants (Zea mays L.) were exposed to drought conditions by decreasing the soil water content to 40-50% of field capacity. The decrease in water content resulted in a decline in stomatal conductance to 50-60% of the level in well-watered plants. Re-irrigation of the plants initiated both hydraulic and electric signals, followed by a two-phase response of the net CO2 uptake rate and stomatal conductance of leaves. The transitional first phase (phase 1) is characterized by a rapid decrease in both levels. In the second phase (phase 2), both parameters gradually increase to levels above those of drought-stressed plants. Elimination of either the hydraulic signal by compensatory pressure application to the root system, or of the electric signal by cooling of the leaf blade gave evidence that the two signals (1) propagated independently from each other and (2) triggered the two-phase response in leaf gas exchange. The results provided evidence that the hydraulic signal initiated a hydropassive decrease in stomatal aperture and for the involvement of electric signals in the regulation of photosynthesis of drought-stressed plants.