The involvement of protein kinases and protein phosphatases in signal transduction during neurotransmitter-induced stimulation of root water-pumping activity

In detached roots of etiolated maize (Zea mays L.) seedlings, neurotransmitters, adrenalin and noradrenalin, stimulated exudation by increasing the root pressure due to activation of its metabolic component. In these treatments, the osmotic pressure of the exudate was somewhat reduced. In contrast, a temperature coefficient Q₁₀ was increased, which as in accordance with the increase of the absolute value of the metabolic component and its proportion in the total root pressure. To obtain some information about transmitting the signals induced by adrenalin and noradrenalin action on water transport, we used two inhibitors of the most important and universal elements of signaling pathways, staurosporine (the inhibitor of protein kinases) and okadaic acid (the inhibitor of protein phosphatases). In control roots, staurosporine markedly slowed and okadaic acid accelerated exudation. In the presence of staurosporine in the incubation medium, a stimulatory effect of both neurotransmitters was completely abolished and the rate of exudation became even below the control value. Okadaic acid exerted an opposite action: it augmented markedly stimulatory effects of both neurotrasmitters. The data obtained indicated the involvement of protein kinases and protein phosphatases in transduction of signals induced by adrenalin and noradrenalin, which stimulated root water-pumping activity.     

A model for radial water transport across plant roots

Summary A model based on the canal theory (Katou andFurumoto 1986 a, b) is proposed for the absorption of solute and water at the root periphery. The present canal model in the periphery and the model which was previously proposed for the exudation in the stele (Katou et al. 1987), are organized into a model for radial transport across excised plant roots, in the light of anatomical and physiological knowledge of maize roots. The canal equations for both canals are numerically solved to give quite a good explanation for the observed exudation of maize roots. It is found that the regulation of solute transport has a primary importance in the regulation of water transport across excised roots. The internal cell pressure of the symplast adjusts the water absorption at the root periphery to the water secretion into the vessels. There seems no need for this explanation of the radial water transport across roots to assume cell membranes with low reflection coefficient or variable water permeability. It would seem that the apoplast wall layers play a crucial role in metabolic control of water transport in roots as well as in hypocotyls.Abbreviations J _s ^ex* the theoretically estimated rate of solute exudation per unit surface area of model maize roots - J that of volume exudation per unit surface area of model maize roots - the reflection coefficient of the cell membrane against solutes     

A model for water transport in the stele of plant roots

Summary The mechanism of water movement across roots is, as yet, not well understood. Some workable black box theories have already been proposed. They, however, assumed unrealistic cell membranes with low values of , or were based on a poor anatomical knowledge of roots. The role of root stele in solute and water transport seems to be especially uncertain. An attempted explanation of the nature of root exudation and root pressure by applying the apoplast canal theory (Katou andFurumoto 1986 a, b) to transport in the root stele is given. The canal equations are solved for boundary conditions based on anatomical and physiological knowledge of the root stele. It is found that the symplast cell membrane, cell wall and net solute transport into the wall apoplast are the essential constituents of the canal system. Numerical analysis shows that the canal system enables the coupled transport of solutes and water into a xylem vessel, and the development of root pressure beyond the level predicted by the osmotic potential difference between the ambient medium and the exudate. Observations on root exudation and root pressure previously reported seem to be explained quite well. It is concluded that the movement of water in the root stele although apparently active is essentially osmotic.Abbreviations J _v ^ex volume exudation per root surface - J₀ non-osmotic exudation - L^r overall radial hydraulic conductivity of an excised root - reflection coefficient - C_s difference in the osmotic concentration between the bathing medium and the exudate - R gas constant - T absolute temperature - C_K molar concentration of K⁺ - C_Cl molar concentration of Cl^– - C_j molar concentration of ion species j - P_j membrane permeability of ion j - z_j valence of ion j - F Faraday constant - V_ix intracellular electric potential with reference to the canal     

Der Einfluss verschiedener Salze und verschiedener Inhibitoren auf den Wurzeldruck von Zea mays

The effect of different salts and inhibitors on the root pressure of Zea mays.— The influence of various salt solutions and inhibitors on the exudation rate has been investigated with young excised primary roots of Zea mays. The following results were obtained. — The effect of chlorides on the exudation rate was higher than the effect of sulphates K⁺ and Na⁺ effected higher flux rates than Ca²⁺ and Mg²⁺ The highest exudation rate was obtained with KCl. — In comparison to an isotonic Lutrol-solution (a liquid condensed polyethylenoxid) a 0.5525 molar KCl-solution, applicated on the root stump, increased the exudation rate considerably. — Metabolic inhibitors and anaerobic conditions decreased the exudation rate. — Experiments, concerning the influence of metabolic inhibitors on the exudation and on the Rb-uptake showed a highly significant positive correlation (r =+0.72***) between the exudation rates and the Rb-concentrations in the exudates. The Rb-accumulation in the root tissue was not correlated to the exudation rate. — The experimental data agree with the concept of a transversal water transport in the root tissue, effected by osmotic forces. The root pressure is based on the osmotic gradient between the xylem sap and the outer solution. This gradient is built up by the metabolic secretion of ions into the xylem sap. It is supposed that the transversal water transport in the roots mainly goes through the free space of the cortex.     

The influence of temperature on the exudation of xylem sap from detached root systems of rye (Secale cereale) and barley (Hordeum vulgare)

Summary Roots of intact plants of rye and barley which had been growing at 20° were cooled for 12–72 h at 8–14° C while the shoots were kept at 20°. The roots were then excised and placed in solutions at temperatures ranging from 2.5–22.5° C. The rate of exudation of xylem sap and the chemical composition and osmotic potential of the sap were measured and compared with controls which had been kept at 20° C during the pretreatment period. Pre-cooling increased the fluxes of K⁺, Ca²⁺ and H₂PO ₄ ^- into the xylem sap of both species by factors of two to three; the total volume of exudate rose by larger factors. Thus the concentrations of these ions were lower in the sap exuding from cooled roots than in that from controls. Measurements of the osmotic potential of the sap from barley roots indicated that the osmotic driving force in cooled and control roots was similar even though flow in the former was much greater.The enhancement of exudation was shown to be dependent on the duration and the temperature experienced by the roots during pretreatment, and was lost rapidly when roots of intact plants were returned to 20°.Analysis of the temperature coefficients for exudation and Arrhenius plots revealed very distinct changes in the activation energy for exudation above and below a transition temperature. In control plants of barley and rye this temperature was around 10° C, but in cooled roots of rye there was a significant shift in the transition temperature to 5° C. Activation energies for exudation of control and cooled roots above or below the transition temperature were broadly similar, thus pre-cooling roots did not alter the temperature sensitivity of exudation but merely its rate at a given temperature.The results are discussed in relation to active ion transport, membrane fluidity and the resistance of the root to water flow.Abbreviation ABA abscisic acid     

Effects of kinetin and root tip removal on exudation and potassium (rubidium) transport in roots of honey locust

Exudation, (86)Rb transport, and water permeability were examined in excised roots of honey locust (Gleditsia triacanthos L.) treated by removing the tip 2 mm (tip-cut 2 mm) or tip 8 mm of the root, or by adding kinetin, or by both treatments. Tip removal increased the rate of exudation. Kinetin, 5 x 10(-6)m, inhibited exudation and Rb transport in tip-cut 2-mm roots; the inhibition was reversible. Kinetin inhibition of exudation was initially associated with lower K(Rb) transport and later with decreases in both ion transport and water permeability. Exudation was also inhibited at 10(-10) to 10(-7)m kinetin. Exudation from roots with intact tips was not altered by kinetin until after about 24 hours. Light during the exudation period had no significant (95%) influence on rate of exudation during the first 24 hours whether root tips were cut or kinetin applied.The results suggest the involvement of the root tip in regulating exudation in other parts of the root. This regulation might occur through cytokinin control of water permeability and the rate of ion transport.     

Radial salt transport in corn roots

Primary roots of solution-grown, 5-day-old or 6-day-old seedlings of corn (Zea mays L.) 10 to 14 cm in length were used to study radial salt transport. Measurements were made of the volume of root pressure exudation, salt concentration of the exudate, and rate of salt movement into the xylem exudate. The ³²P uptake, O₂ consumption, and dehydrogenase activity of the root cortex and stele also were studied.

These roots produced copious root pressure exudate containing 4 to 10 times the concentration of ³²P in the external solution. Freshly separated stele from 5-day-old roots accumulated ³²P as rapidly as the cortex from which it was separated and the stele of intact roots also accumulated ³²P. Separated stele has a higher oxygen uptake than cortex. It also shows strong dehydrogenase activity with the tetrazolium test. The high oxygen consumption, ³²P uptake and strong dehydrogenase activity indicate that the cells of the stele probably play a direct role in salt transport.

These data raise doubts concerning theories of radial salt transport into the xylem based on the assumption that the stele is unable to accumulate salt vigorously.

     

Effects of HgCl2 on principal parameters of exudation from maize detached root system

To elucidate the role of aquaporins in the control of the root pressure, we tested the effects of HgCl₂ (aquaporin blocker) at concentrations from 10^?8 to 10^?2 M on the exudation rate (J _w). Experiments were performed with detached roots of 5–7-day-old etiolated maize (Zea mays L.) seedlings. HgCl₂ suppressed exudation by 50–70% at the concentration of 2 × 10^?5 M. At the concentration of 2.5 × 10^?4 M, HgCl₂ reduced J _w during first 20–40 min, but in 2 h, it activated exudation by ten and more times. In this case, the root and osmotic pressures of the exudates increased by 1.5 times. The hydraulic conductance reduced approximately by 30% during first 30 min and increased severalfold in 2 h. The temperature coefficient (Q₁₀) of J _w attained 14 in 2 h. At the concentration of 10^?2 M, HgCl₂-induced acceleration of exudation was replaced by its inhibition essentially immediately. We suggested that a driving force for HgCl₂-induced stimulation of the J _w might be an increase in the osmotic component of the root pressure or the involvement of its nonosmotic (so-called metabolic) component. The results allow a supposition that aquaporins are involved in the control of water transport in the root.     

A chamber for applying pressure to roots of intact plants

A chamber was designed to apply up to 20 bars pressure to roots of intact plants. The unique features of this chamber are a split top arrangement to permit enclosing roots of intact plants within the chamber, a circulation coil to control temperature of rooting media, and a valve arrangement to permit changing solution without disturbing the plant. Changes in transpiration in response to changes in the pressure applied to roots of intact pepper plants illustrate one use of the equipment. Well watered plants at low light (0.05 langley/min) were observed to exude water from the leaf margins when 5 bars pressure was applied to the roots. When roots were cut off, a 1 bar pressure caused exudation. Plants with cooled roots or plants in dry soil did not exude water when as much as 6 bars pressure was applied. Transient response of transpiration rates to sudden application and release of pressure was observed in pepper and bean plants but not in rhododendron. The magnitude of this transient response was highly dependent upon light intensity and CO₂ concentration in the aerial environment.     

A Correlation between Structure and Function in the Root of Zea mays

The exudation rates of fluid and potassium ions from isolatedmaize roots were determined before and after excision of certainlengths of root tip. The results of this study suggest thatexcised maize roots possess the ability to absorb potassium(and presumably chloride) ions and concomitant amounts of waterover a considerable distance (10 cm) from the tip. Moreover,the observed power of absorption of ions and water into thetranslocatory pathway decreases in passing from the tip towardsthe base of the root. Both light and electron microscope techniques were used to examinethe anatomy of primary roots similar to those used in the physiologicalexperiments. The principal observation was that the xylem vesselsnear the root tip contain membrane-bounded cytoplasm with organelles.The number of mature xylem vessels, i.e. without cytoplasm,progressively increased in transverse sections cut from 1 to10 cm from the root tip; above 10 cm from the root tip all ofthe xylem vessels were found to be completely mature. It isevident that prima facie a connexion exists between this singleaspect of root anatomy and fluid exudation from excised roots. The uptake of tritiated water by roots and its transport intoexudates was examined. These data were analysed on the assumptionthat the exchange of external labelled water with the exudatewas achieved by the fluid exudation itself; this analysis indicatedthat an operational volume, similar to that of the total xylemvolume within the root, must become labelled during the formationof the exudate.     

Water Pumping Activity of the Root System in the Process of Cross-Adaptation of Sunflower Plants to Hyperthermia and Water Deficiency

To elucidate the mechanisms of cross-adaptation, we investigated the effect of heat shock (HS, for 2 h at 45°C) on leaf tolerance to overheating and exudation by roots detached from 25–30-day old sunflower (Helianthus annuus L.) plants. It was demonstrated that preheating enhanced considerably leaf tolerance and activated root exudation, especially under water deficiency produced by plant transfer to the hypertonic NaCl solution (17 mM). Under water deficiency conditions, the roots of HS-treated plants pumped water against the osmotic pressure (OP) gradient between the exudate and the external solution. Therefore, we concluded that this pumping was realized due to a metabolic (non-osmotic) constituent of root pressure. In the roots of plants that were not treated with HS, the OP gradient became positive. This fact implies that the HS-pretreatment of plants retarded the penetration of sodium and chlorine ions into roots. The data obtained demonstrate that HS induced a cross-adaptation of plants to high temperature and water deficiency. Such cross-adaptation involves, as an important component, an acceleration of water metabolism, including an enhanced water pumping activity of root system.     

Transpiration integrated model for root ion absorption under salinized condition

Salinization of crop fields is a pressing matter for sustainable agriculture under desertification and is largely attributed to root absorptive functions of the major crops such as maize. The rates of water and ion absorption of intact root system of maize plants were measured under the salinized condition, and the salt absorptive function of maize roots was analyzed by applying different two kinetic models of root ion absorption (i.e. the concentration dependent model and the transpiration integrated model). The absorption rates for salinization ions (Na⁺, Cl^?, Ca²⁺ and Mg²⁺) were found to depend on ion mass flow through roots driven by the transpiration, and therefore the transpiration integrated model represented more accurately rates of root ion absorption. The root absorption of salinization ions was characterized quantitatively by two model parameters of Q′_max and K′_M involved in the transpiration integrated model, which are considered to relate to the potential absorbing power and the ion affinity of transport proteins on root cell membranes, respectively.     

The Effect of Abscisic Acid on the Uptake and Distribution of Ions in Intact Seedlings of Phaseolus vulgaris cv. Redland Pioneer

Abscisic acid (ABA) was found to increase the accumulation of ³⁶Cl, total Cl, ²²Na and total Na⁺ in roots of intact bean seedlings. After an initial promotion. ABA inhibited longdistance transport of these ions from the root to the shoot. However, it consistently inhibited both uptake and transport of ⁴²K and total K⁺ in intact bean seedlings. A promotion of net ³⁶Cl influx (ψ_oc) and its accumulation in the root (Q*_v) concomitant decrease in transport index (long-distance transport as percentage of total influx) showed that ABA stimulates -³⁶Cl transport at the tonoplast. It inhibited H⁴ extrusion and net ⁸⁶Rb influx which agrees with a cation exchange theory K⁺/Rb⁺ transport.     

The arbuscular mycorrhizal symbiosis regulates aquaporins activity and improves root cell water permeability in maize plants subjected to water stress

Studies have suggested that increased root hydraulic conductivity in mycorrhizal roots could be the result of increased cell‐to‐cell water flux via aquaporins. This study aimed to elucidate if the key effect of the regulation of maize aquaporins by the arbuscular mycorrhizal (AM) symbiosis is the enhancement of root cell water transport capacity. Thus, water permeability coefficient (P_f) and cell hydraulic conductivity (L_pc) were measured in root protoplast and intact cortex cells of AM and non‐AM plants subjected or not to water stress. Results showed that cells from droughted‐AM roots maintained P_f and L_pc values of nonstressed plants, whereas in non‐AM roots, these values declined drastically as a consequence of water deficit. Interestingly, the phosphorylation status of PIP2 aquaporins increased in AM plants subjected to water deficit, and P_f values higher than 12 μm s^?1 were found only in protoplasts from AM roots, revealing the higher water permeability of AM root cells. In parallel, the AM symbiosis increased stomatal conductance, net photosynthesis, and related parameters, showing a higher photosynthetic capacity in these plants. This study demonstrates a better performance of AM root cells in water transport under water deficit, which is connected to the shoot physiological performance in terms of photosynthetic capacity.     

Nod factors activate both heterotrimeric and monomeric G-proteins in <Emphasis Type="Italic">Vigna unguiculata</Emphasis> (L.) Walp

Nod factors are lipo-chito-oligosaccharides secreted by rhizobia that initiate many responses in the root hairs of the legume hosts, culminating in deformed hairs. The heterotrimeric G-protein agonists mastoparan, Mas7, melittin, compound 48/80 and cholera toxin provoke root hair deformation, whereas the heterotrimeric G-protein antagonist pertussis toxin inhibits mastoparan and Nod factor NodNGR[S]- (from Rhizobiumsp. NGR234) induced root hair deformation. Another heterotrimeric G-protein antagonist, isotetrandrine, only inhibited root hair deformation provoked by mastoparan and melittin. These results support the notion that G-proteins are implicated in Nod factor signalling. To study the role of G-proteins at a biochemical level, we examined the GTP-binding profiles of root microsomal membrane fractions isolated from the nodulation competent zone of Vigna unguiculata(L.) Walp. GTP competitively bound to the microsomal membrane fractions labelled with [(35)S]GTPgammaS, yielding a two-site displacement curve with displacement constants ( K(i)) of 0.58 micro M and 0.16 mM. Competition with either ATP or GDP revealed a one-site displacement curve with K(i) of 4.4 and 29 micro M, respectively, whereas ADP and UTP were ineffective competitors. The GTP-binding profiles of microsomal membrane fractions isolated from roots pretreated with either NodNGR[S] or the four-sugar, N- N'- N"- N'"-tetracetylchitotetraose (TACT) backbone of Nod factors were significantly altered compared with control microsomal fractions. To identify candidate proteins, membrane proteins were separated by SDS-PAGE and electrotransferred to nitrocellulose. GTP overlay experiments revealed that membrane fractions isolated from roots pretreated with NodNGR[S] or TACT contained two proteins (28 kDa and 25 kDa) with a higher affinity for GTPgammaS than control membrane fractions. Western analysis demonstrated that membranes from the pretreated roots contained more of another protein (~55 kDa) recognised by Galpha(common) antisera. These results provide pharmacological and biochemical evidence supporting the contention that G-proteins are involved in Nod factor signalling and, importantly, implicate monomeric G-proteins in this process.     

Response of root respiration and root exudation to alterations in root C supply and demand in wheat

Rising atmospheric CO₂ concentrations have highlighted the importance of being able to understand and predict C fluxes in plant-soil systems. We investigated the responses of the two fluxes contributing to below-ground efflux of plant root-dependent CO₂, root respiration and rhizomicrobial respiration of root exudates. Wheat (Triticum aestivum L., var. Consort) plants were grown in hydroponics at 20°C, pulse-labelled with ¹⁴CO₂ and subjected to two regimes of temperature and light (12 h photoperiod or darkness at either 15°C or 25°C), to alter plant C supply and demand. Root respiration was increased by temperature with a Q ₁₀ of 1.6. Root exudation was, in itself, unaltered by temperature, however, it was reduced when C supply to the roots was reduced and demand for C for respiration was increased by elevated temperature. The rate of exudation responded much more rapidly to the restriction of C input than did respiration and was approximately four times more sensitive to the decline in C supply than respiration. Although temporal responses of exudation and respiration were treatment dependent, at the end of the experimental period (2 days) the relative proportion of C lost by the two processes was conserved despite differences in the magnitude of total root C loss. Approximately 77% of total C and 67% of ¹⁴C lost from roots was accounted for by root respiration. The ratio of exudate specific activity to CO₂ specific activity converged to a common value for all treatments of 2, suggesting that exudates and respired CO₂were not composed of C of the same age. The results suggest that the contributions of root and rhizomicrobial respiration to root-dependent below-ground respiration are conserved and highlight the dangers in estimating short-term respiration and exudation only from measurements of labelled C. The differences in responses over time and in the age of C lost may ultimately prove useful in improving estimates of root and rhizomicrobial respiration.     

Effects of defoliation and atmospheric CO2 depletion on nitrate acquisition,and exudation of organic compounds by roots of Festuca rubra

The aim of this study was to investigate the physiological basis of increased root exudation from Festuca rubra, in response to defoliation. The hypothesis, that assimilate supply to roots is a key determinant of the response of root exudation to defoliation was tested by imposing CO₂-deplete (< 50 mol mol^–1) atmospheres to F. rubra. This was done as a non-destructive means of preventing supply of new assimilate to roots of intact and defoliated plants. F. rubra was grown in axenic sand systems, with defoliation and CO₂-depletion treatments applied to plants at 14 and 35 days after planting. Root exudation and NO₃ ^– uptake were quantified throughout, and post-treatment uptake and allocation of N were determined from the distribution of ¹⁵N label, supplied as ¹⁵NO₃ ^–. Defoliation of F. rubra resulted in significantly (P <0.01) increased root exudation, CO₂-depletion did not result in increased exudation from plants of either age. When treatments were applied to F. rubra after 14 days, defoliation and CO₂-depletion each reduced NO₃ ^– uptake significantly (P <0.05). However, in older plants, uptake of NO₃ ^– was less sensitive to defoliation and CO₂-depletion. The results indicate that increased root exudation following defoliation is not related directly to reduced assimilate supply to roots. This was evident from the lack of effect of CO₂-depletion on root exudation, and the absence of correlation between root-C efflux and the rate of NO₃ ^– uptake. The physiological basis of increased exudation following defoliation remains uncertain, but may be dependent on physical damage, either directly or as a consequence of systemic responses to wounding.     

Short-term responses of Brassica rapa plants to low root temperature: Effects on nitrate uptake and its translocation to the shoot

Brassica rapa L. plants were grown hydroponically for 5 or 6 weeks at 20°C and then half batches of plants were transferred to tanks in which the root temperature was lowered decrementally over 1 h to 7°C. Changes in nitrate uptake rate (NUR) and nitrate transfer from roots were studied in relation to transpiration and root pressure xylem exudation flow rates over a 48- or 72-h period. The response of plants following the root temperature decrease was biphasic. During phase 1, NUR and water and solute flow rates through the root decreased sharply. Coping mechanisms came into operation during phase 2, and tended to offset the effects of low temperature. The 3-h cold-treated roots exhibited a very low NUR but 48-h cold-treated roots partly recovered their ability to absorb nitrate. Transpiration rate decreased more slowly (during 24 h) than both root xylem exudation and parameters of root conductivity (during 6 h). Beyond these respective times, transpiration rate was balanced while root xylem exudation clearly increased, but without returning to the level of control plants. Nitrate transfer to the root xylem was strongly and rapidly affected by low root temperature, but the subsequent readjustment was such that no or little difference compared with the control was apparent after 48 h. Water and solute flows were strongly decreased when nitrate was replaced by chloride in the culture solution during exudation sampling. The major role of nitrate in root hydraulic conductivity and root xylem exudation is discussed.     

Radial Turgor and Osmotic Pressure Profiles in Intact and Excised Roots of Aster tripolium: Pressure Probe Measurements and Nuclear Magnetic Resonance-Imaging Analysis

High-resolution nuclear magnetic resonance images (using very short spin-echo times of 3.8 milliseconds) of cross-sections of excised roots of the halophyte Aster tripolium showed radial cell strands separated by air-filled spaces. Radial insertion of the pressure probe (along the cell strands) into roots of intact plants revealed a marked increase of the turgor pressure from the outermost to the sixth cortical layer (from about 0.1-0.6 megapascals). Corresponding measurements of intracellular osmotic pressure in individual cortical cells (by means of a nanoliter osmometer) showed an osmotic pressure gradient of equal magnitude to the turgor pressure. Neither gradient changed significantly when the plants were grown in, or exposed for 1 hour to, media of high salinity. Differences were recorded in the ability of salts and nonelectrolytes to penetrate the apoplast in the root. The reflection coefficients of the cortical cells were approximately 1 for all the solutes tested. Excision of the root from the stem resulted in a collapse of the turgor and osmotic pressure gradients. After about 15 to 30 minutes, the turgor pressure throughout the cortex attained an intermediate (quasistationary) level of about 0.3 megapascals. This value agreed well with the osmotic value deduced from plasmolysis experiments on excised root segments. These and other data provided conclusions about the driving forces for water and solute transport in the roots and about the function of the air-filled radial spaces in water transport. They also showed that excised roots may be artifactual systems.     

Effect of gibberellic acid on K+ (Rb+) uptake and transport in sunflower roots

Four-week-old sunflower plants ( Helianthus annuus L. cv. Halcón), grown in different nutrient solutions, were used to study the effects of gibberellic acid (GA₃) on K⁺ (Rb⁺) uptake by roots or transport to the shoot. Gibberellic acid application to the nutrient solution did not affect the exudation process of excised roots. When GA₃ was sprayed on leaves 2 to 6 days before excising the roots, the rate of exudation and the K⁺ flux increased. When the exudation study was done keeping the roots in a nutrient solution in which Rb⁺ replaced K⁺, the GA₃ effects were evident also on Rb⁺ uptake and transport. In intact plants, GA₃ increased the Rb⁺ transported to the shoot but did not affect Rb⁺ accumulation in the root. It is suggested that these GA₃ effects can be explained if it is assumed that GA₃ acts on the transport of ions to the xylem vessels.