Plant Physiol. (1981) 68, 169-1740032-0889/81/68/0169/06/$00.50/0

Effects of Abscisic Acid on the Hydraulic Conductance of and theTotal Ion Transport through Phaseolus Root Systems

Received for publication August 25, 1980 and in revised form January 15, 1981

EDWIN L. FiscusUnited States Department ofAgriculture, Science and Education Administration, Agricultural Research, CropsResearch Laboratory, Colorado State University, Fort Collins, Colorado 80523

ABSTRACT

The response of solute and volume fluxes of Phaseolas root systems toapplied AbA was observed under conditions of applied pressure which wereused to enhance the volume flow. The growth regulator elicited threeseparate responses: a transient release of solutes to the xylem which wasresponsible for an initial increase in volume flux; a long term increase inthe total ion flux; and a long term decrease in the hydraulic conductanceof the root systems. The exact response was highly dependent on themagnitude of the pre-AbA volume flux density, the relative contributionsof osmotic and pressure-induced flow, and the applied dosage.

Calculations suggested that the volume flux in naturafly exuding rootsystems is relatively insensitive to changes in the conductance.

Reports of the effects of AbA applications on the root systemsof plants have been varied and, at times, contradictory. One of themost important responses reported is the effect of this hormoneon the hydraulic conductance of the root systems. Because it isapparent that AbA production is enhanced by water stress (1, 2,13, 17, 19-22), any effect that AbA might have on the system'sconductance is of great potential importance to the water balanceof the plant. Unfortunately, AbA also affects the ion transportprocesses of the roots. Since much of the past experimentation hasdealt with excised roots or root systems with little or no hydraulicpressure difference applied, and since the volume flux in thesesystems has been dominated by ion transport, the effects of AbAon the hydraulic conductance have been difficult to determine.Volume and/or ion fluxes through plant roots have been re-

ported to decrease after AbA treatment (5, 16), remain the same(6), or increase (3, 4, 6, 14, 18). Karmoker and Van Steveninck(14) and Pitman and Wellfare (16) attributed the observed changesin volume flux entirely to changes in ion transport rates. However,Glinka and Reinhold (10) and Glinka (11, 12) have concludedthat the major change in volume flux is due to a direct effect onthe hydraulic conductance of the system.

Recently, Markhart et al. (15), using a pressure chamber tech-nique which measures conductance while minimizing osmoticeffects, have shown that the effect ofAbA on soybean root systemsis to reduce the hydraulic conductance about 4 h after the appli-cation of AbA. However, because of the nature of the proceduresthey were unable to determine the short term effects of AbA onvolume and ion fluxes. If the initial response is due to a significantincrease in conductance, then the response should become morepronounced as the hydrostatic pressure difference, and, therefore,the volume flux is increased. This is true because at higher fluxesthe flow rate is much more dependent on the conductance andshows a very low dependence on the ion fluxes.

Here, we present additional evidence to support the hypothesisthat AbA causes a long term decrease in hydraulic conductance aswell as a short term release of ions to the xylem and a long termincrease in the activity of the ion pumps. The results of theexperimental technique presented here also support the idea thatthe short term response to AbA in Phaseolus is not the result of asignificant increase in the hydraulic conductance of the system.

MATERIALS AND METHODS

Bean seeds (Phaseolus vulgaris [L] cv. Ouray) were germinatedin vermiculte for 4 days, then transferred to 25-cm plastic potsfilled with aerated half-strength Hoagland solution, and grown ina greenhouse. Supplemental lighting gave a mean midday fluxdensity of425 ,uE m-2 s-' at the top of the pot. The plants, selectedfor size at the time of the experiment, averaged about 850 cm2projected leaf area as measured with a LI-COR L131001 areameter. The effects of AbA on the ion and volume fluxes weredetermined in a pressure chamber. Each root system was decapi-tated and sealed into a pressure chamber with the cut stumpprotruding through the lid and the roots surrounded by aeratednutrient solution as previously described (9). The chamber wasbrought to the specified pressure and temperature (25 ± 0.25 C)and equilibrated overnight. In the morning, the system waschecked for steady-state conditions, arbitrarily specified aschanges in the volume and ion fluxes of less than ±2% h-1. Thetotal volume flux was measured at regular intervals and wasexpressed as the volume flux per unit leaf area (9), Qpl, as a meansof comparing plants of slightly different sizes. Unpublished datafrom this lab indicate that, for plants of this size grown under ourgreenhouse conditions, the ratio of projected leaf area to root areais approximately 1, so that Gp is approximately the flux densityinto the roots. The electrical conductivity of the exudate wasmeasured and the concentration expressed as KCI equivalentscm-3. The samples were immediately frozen and the freezing pointdepression determined at a later date with a Precision Osmometer.'Having established steady conditions according to the abovecriteria, the AbA, dissolved in 95% ethanol, was added to thesystem through an injection port without decreasing the pressure.The quantity of AbA added was calculated to yield the sameapproximate dose (7.8 x 10-5 g cm-2 leaf area) in each case.

RESULTS AND DISCUSSION

Comparison of the pre-AbA steady state rates for the five rootsystems used (Fig. 1) indicated that collectively they exhibited thesame type of nonlinear pressure-flux relationship that one would

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169

Plant Physiol. Vol. 68, 1981

AP (bars)FIG. 1. Composite data for five root systems at different levels of

hydrostatic pressure. (0), pre-AbA steady state values; (X), post-AbA peakvalues; (@), AbA + 4 h. Line was calculated by equation 3 using the pre-AbA parameters determined in the text.

expect to generate from a single root system by measuring thesteady state volume flux at five different levels of applied hydro-static pressure (Fiscus, 7-9). Presenting the data this way allowsus to treat the five root systems as one composite system that maybe subjected to the type of analysis described by Fiscus (8). Thisanalysis was based on the standard volume flux equation

Jv= LP(AP- afn) (1)

J. is the volume flux in cm' cm-2 s-5, AP is the hydrostatic pressuredifference in bars, AI is the osmotic pressure difference in bars,a is the dimensionless reflection coefficient or osmotic efficiencyfactor, and Lp is the hydraulic conductance in cm' cm2 s' bar-For purposes of convenience we will use Qpl and J. interchange-ably. Although this is not strictly legitimate it will not affect thesense of the following arguments. Since the osmotic difference (orinternal concentration Ci) is a function of both the solute andwater fluxes (Ci = J.8/J0), the following transport equation wasderived from equation 1 by Fiscus (7) to express this fact.

Ju = Lp([P0 - pi] - aRT[C - J8/J]) (2)

where R is the gas constant in cm3 bar degree-' mol-', T is degreesK, C° is the medium concentration in mol cm-3, J. is the soluteflux in mol cm-2 s-', and the superscripts o and i refer to theoutside (medium) and inside (xylem), respectively.

Following the procedures outlined by Fiscus (8), we were ableto calculate for the pre-AbA composite system Lp = 4.05 x 10-6cm3 cm-2 s-1 bar-', effective II° = 0.7 bar, total J, = 8 x 10-12mol cm-2 s-1, and a = 0.99. The value of a was the same whetherit was calculated on the basis of solution conductivity or freezingpoint depression. These values compared favorably with thosepreviously determined for single root systems (8, 9). And, as inprevious papers, we must emphasize that these are area weightedeffective values for entire root systems and should not be construedas representing the properties of any single lipid bilayer.

Since we have determined that a _ 1, and the barrier nearlyideal in this respect, we will as an approximation also assume thatw, the coefficient of solute mobility in the system in mol cm-2 s-'bar-', is approximately 0 with the result that the root barrier willbe possessed of ideal properties. This allows removal of a fromequation I and replacement of the total solute flux with the active

uptake component J8*. Although these changes are in no waynecessary to the following arguments, they do tend to clarify andsimplify them.Making the changes mentioned and rearranging equation 2 to

a soluble form yields

LP(AP -H0) + 4[Lo(n° _ Ap)]2 + 44RTJ8*U- 2 (3)

If we now specify AP, Il°, and T constant, we may write for thetotal differential of the volume flux

dJ,- dLp + dJ) * (4)

Where the relevant derivatives formed from equation 3 are

( \ AP - 11o L (Ho _ Ap)2 + 2RTJS*I ~~+OdLPJ 2 2Vais*

and

(JV) LpRTaJ.* Lp a4

(5)

(6)

where a is the radicand in equation 3. These expressions nowallow us to approximate the changes in Lp and J8* required toproduce any observed changes in J, and obtain an estimate of thesensitivity of the volume flux to changes in each of these param-eters. Thus, we see that

dLp(req'd) = (J) dJ(obvd)

and (7)

dJ8*(reqd) = aj,* dJv(obeerved)

In the case of root pressure exudation we may evaluate thepartial derivatives of J, for AP = 0, T = 298 K, and Lp, J.*, andfl' having the values determined earlier in this paper. The resultsof such evaluations are that dJ0/dLp = 4.14 x 10 bars, and dJ/dJ8* = 2.96 x I04cm3mol-.To put these numbers into proper perspective, we need to

examine the changes in each of the relevant parameters requiredto produce the observed changes in J0(Qpl).

Figure 2A shows the short and intermediate term responses ofa root system where AP = 0, which is typical ofmany data alreadyreported in the literature (3, 4, 6, 14, 18). Generally, the AbAtreatment resulted in a rapid increase in volume flux whichbecame apparent between 5 and 10 min after the AbA application.This initial response of volume flux was characterized by a tran-sient peak (P in Fig. 2A) which was reached 60 to 90 min after thetreatment. After a period of retrenchment lasting about another90 min the volume flux again began to increase relative to thecontrols. The relative rate continued to increase through the nightuntil at 0900 the following morning the rate was over three timesthat of the controls. The transient peak in volume flux was alsoapparent in those systems subjected to a hydrostatic pressuredifference but its magnitude and subsequent changes in volumeflux were very much dependent on the pre-AbA volume flux. Itis this transient peak, as a percentage of the pre-AbA volume flux,which we will call the "peak" in future.

Figure 2B indicates the time course for two control systems, onetreated with the same volume ofethanol as all the AbA treatmentsand the other receiving nothing. The lack of a steady state underthese conditions is due to endogenous diurnal fluctuations in the

170 FISCUS

AbA RESPONSES IN PHASEOLUS ROOTS

0

700)

c

0

U)

0

NO

UZl

CD

38iA 736112

P0

0

8 0 00 0 0 0

4 0

0- 1 I 5 I

B ~

1 0. 0 0 +Ethanol10 0. 0 0 0 0

S *0 *-Ethanol

0 9. jI i16 17 18 19 9

Time of Day ( hours)FIG. 2. A, response of Q, to AbA when no pressure is applied. Data

are the % change over the ethanol controls. P is the peak. B, controls.(0), no ethanol added; (0), + ethanol. Arrows indicate the time oftreatment.

ion pumps. Because of this, steady state models must be invokedwith much care. In any case, it is clear that addition of the ethanolhad no effect on the volume flux in the root systems, since thedifferential between the two controls remained constant.

In the instance of Figure 2A, the volume flux increased approx-imately 100%1o over the controls. Solving equation 3 with the valuespreviously determined and with AP = 0, we find that J.h = 2.56X 10-7 cm3 cm-2 S-1. Since the peak increase was about 100o wemay let dJL = J,. Returning to equations 7 we now find thatdLp(rq'd) = 5.72 x 10-5 and dJs*(reqd) = 8.01 x 10-12, increases of14.1 and 2 times, respectively. Alternatively, we may say that asimple doubling of J8* will produce the same effect on J, as willbe accornplished by an increase in Lp in excess of 14 times.Because the observed change in J4 is large and not of a differentialnature, the resultant estimates of the required changes in J,* andLp will be somewhat erroneous. Although the large difference inapparent effectiveness of these two parameters in controlling J, atAP = 0 does not constitute proof of which one is responding toAbA, it certainly suggests that Lp is less likely to be the responsibleparameter. However, we will see that the general nature of theseestimates and the resultant conclusion is strongly supported by thefollowing experimental data.We now consider the data of Figure 3 for root systems under

different steady levels of hydrostatic pressure and make the fol-lowing assumptions contrary to our previous conclusion: (a) theincrease in volume flux was due primarily to an increase in Lpand (b) the increase in Lp was similar for all the root systemsstudied. We may now calculate from equation 3 how much J,should have increased due to this change when AP = 4.1 bars, thegreatest pressure used in Figure 3. The result would be a peakresponse to 1.93 x 10-4 cm3 cm-2 S-1, also an increase of over 14times. Obviously, no such dramatic increase was recorded. On thecontrary, Figures 2 and 3 clearly show that the percentage peakresponse declined at higher pre-AbA flow rates.The peak of volume flux as a percentage of the pre-AbA flux is

1l

-2

i-

,10-

10-

4.11.700 0000000 00 0 0

+3+ ++I++++++++ +

1.7 .ooo Dooo cb a a0 0000 0

07 xxxxx x xxXXX)OK

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10 12Time of

0 0 0 0 0

o aa o oO

x X X X x

14Day (hours)

16

FIG. 3. Response of Qu to AbA at different levels of applied hydrostaticpressure. Markers indicate the time ofAbA addition. The applied pressure(AP) in each instance is marked on the figure.

I 11CD0

0)

aUC

v

c

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a.40 2 4 6 8 10 12 14

apl (cm3 cm-2s-1 ) 106FIG. 4. Dependence of the peak response on pre-AbA volume flux

(Qp,). The line is peak = -176.7 - 16.77 In Qpl, with r2 = 0.975.

shown in Figure 4. These data force us to the conclusion that themajor part of the peak response of Jv to AbA cannot be due to anincrease in Lp of the system. If indeed Lp had increased signifi-cantly, the dependence of the response on the pre-AbA volumeflux would have been just the opposite of what we observed. Atthis point we feel that the case is well proven that the initialresponse to AbA in Phaseolus root systems does not involve asignificant increase in Lp. However, a doubling of J.*, as wepreviously calculated, was necessary for the observed increase inJ, at AP = 0, would have produced a negligible effect on J4 at thehighest flow rates. Yet, we still noted a peak of over 8%. We musttherefore search elsewhere for an explanation ofthe peak response.

Examination of an additional time course, looking at the soluteas well as volume fluxes will clarify both the peak response andthe sustained response which is apparent at lower flow rates. Thetime course of Figure 5 was obtained at a higher dose of AbA (IX l0-4 g cm-2) than the previous data and is typical of suchresponses. The higher dosage was used to enhance the effect onLp which was not clear at the lower dosages previously used.Figure 5 demonstrates four major effects of AbA on solute andwater transport. The first is the peak response of volume flux. Thesecond is a pulsed release of solutes into the xylem which is

. .-F..

Plant Physiol. Vol. 68, 1981 171

Plant Physiol. Vol. 68, 1981

12 14of Day (hours)

16

FIG. 5. Solute and volume fluxes for a high AbA dose level showingeffects of decline in LP. Arrow indicates time of AbA application. Dose= I x 10-4 g cm-' leaf area; AP = 4.1 bar.

Table I. Comparison of the Absolute Peaks with the Percentage Peaksand Calculation of the Ratios of the Hypothesized Osmotic Counterforce

to AP

OsmoticCounterforce

AP Peak Peak Ratio

(cm3 cm'2 s'1) (0.35b/AP)bars % x 107 x 1OOO 100 1.230.68 59.2 5.35 51.41.70 41.2 16.0 20.62.72 13.1 11.3 12.94.08 8.4 11.3 8.6

correlated with the peak response in volume flux. The third effect,which becomes clear only after the initial release of solutes iscomplete, is a more gradual steady increase in solute flux, Q8, andthe fourth is a decline in LP that takes place after several hours. Itis probably the initial release of solutes which is responsible forthe peak response in volume flux, either because the solutes weredumped at an osmotically active site on the inside of the systemor because they were released from an osmotically active sitewhich was acting in opposition to the volume flux.

Reference to Table I and Figure 1 reveals some data which mayhelp to distinguish between these two alternatives. Table I showsthat although the peak increase, as a percentage of the pre-AbAQPl, bears an inverse relationship with Qpl, the absolute peakincrease (cm3 cm-2 s-') is directly related to Qpl in what appearsto be a hyperbolic manner. Now, returning to Figure 1, we cansee that the peak fluxes shift the overall flow curve to the left. Thedegree of this transient shift is about the same as the differencebetween the effective and real rl (0.3-0.4 bar). Table I shows thatif we calculate the ratio of this shift, taking a value of 0.35 bar, tothe applied pressure, the relationship follows fairly closely thepeak response (%) of Figure 4. This indicates that the peakresponse may actually be due to the temporary removal of anosmotic counterforce which may be located in some intermediatecompartment. Removal of such an osmotic counterforce will havethe same effect on J, as will an equal increase in AP, except thatthe over-all AP- J. curve will be shifted to the left. Also, sincethe peak is transient, we must speculate that over a period of afew hours the solutes in the compartment are replenished.Even though the evidence of Table I and Figure 1 weigh more

heavily on the side of the osmotic counterforce hypothesis, it is

still possible that the peak response stems from a combination ofthe two effects.A third alternative involves both a small transient increase

(about 5%) in Lp combined with the aforementioned release ofsolutes to an osmotically active site. Thus, at low Qpl, the osmoticcomponent would dominate the system and any changes in Lpwould have little effect. At higher Qpl, Lp would dominate and therelease of solutes would have little effect. This would account forboth the flux-dependent increase in the absolute peak and theflux-dependent decrease in the percentage peak. However, the fitof the peak flow rates versus AP (Fig. 1) indicates a slight (-5%)decrease in Lp. Because the peaks do not represent a steady state,this figure may be in doubt. Still, we are inclined to doubt theexplanation involving the increase in Lp inasmuch as it is difficultto think of a mechanism which would increase Lp only to theextent that the over-all flux curve is shifted by the same amountthat the real Il° differs from the effective H1°.The more gradual increase in Q8 is no doubt responsible for the

longer term sustained increase in Qu which is apparent at lowerlevels of applied pressure and especially for AP = 0.

Additional evidence bearing on the AbA effect on LP is alsoclear from Figure 5. Here we see that even though Q. is increasing,Q, is declining which indicates that at these dosages LP actuallydeclines after a period of hours rather than increasing in responseto AbA. The decline in LP is in keeping with the evidence ofMarkhart et al. (15) who showed that AbA treatment resulted ina decline of LP in soybean which became apparent after severalhours.

Reference to Figure 6 will allow us to tie together what we feelis the most likely sequence of events which occurs after theaddition of large doses of AbA at to. Curve A represents thehydraulic conductance LP. The dashed extension represents thecase in which the AbA dosage is inadequate to change LP signifi-cantly, while the solid line represents the case in which LP isdecreased by a larger AbA dose. Curve B is the endogenous ionflux level and the bump in the curve is the transient release ofions to the xylem (Fig. 5). Curve C is the AbA-stimulated increasein ion flux due to the long term increase in ion pump activity, andcurve D is a summation of curves B and C. Curve D representsthe total transport of ions from the xylem and resembles the Q8

C

a

a

HX~~~~~~~~~_ _p

5

J == = =toTm

FIG. 6. Schematic representation of the events following AbA treat-ment at to. A, Lp; B, pulsed release of solutes; C, AbA-stimulated ion pumpactivity; D, B + C; E, volume flux at AP = 0; F, Volume flux at high AP.

24

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DoOO°000000000000000 °Q00 00 0

0 * *........ . ****-***** **a

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.

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172 FISCUS

AbA RESPONSES IN PHASEOLUS ROOTS

data in Figure 5.Curves E and F depict schematically the changes in Qv with (F)

and without (E) applied pressure. The dashed and solid extensionsofcurve F correspond to the extensions ofcurve A. The observableresults in terms of total volume and ion fluxes may be explainedas follows. The initial response is the pulsed release of solutes intothe xylem (B). This initial release of solutes is responsible for thepeak response of volume flux (E and F). Because of the limitedquantity of solute available for the initial pulse, its effects passand the volume flux again begins to decline. The effectiveness ofthe initial solute pulse will be highly dependent on the pre-AbAvolume flux, since at higher volume fluxes the solutes are muchmore rapidly diluted and washed away from the osmotically activesites. Thus, we can account for the inverse relationship betweenthe pre-AbA volume flux and the peak response.

Alternatively, the pulsed release of solutes may only representthe clearing out of some intermediate osmotic compartment whichacts in opposition to the volume flux. Because elimination of thiscounterforce would have the same effect as a fixed increment ofAP, we would see both an increase to a plateau of the absolutepeak (Table I) and a decrease in the percentage peak with pre-AbA Qp,. The former effect results because of the nonlinear natureof the AP: Qp, curve which has a much lower slope at low APvalues. The latter effect results from the fact that the fixed coun-terforce (_0.35b) becomes an increasingly smaller fraction of thetotal driving force at high AP values.The second phase of the response to AbA involves both the

steady rise in Q. (C) and a decline or no change in Lp (A). Hereagain the resultant changes in QU are highly dependent on thelevel of AP. When AP = 0, QV responds primarily to Q., as wepointed out earlier, with the result that QU increases. As we alsoshowed earlier, the system is relatively insensitive to changes in Lpwhen AP = 0 so that the decline in Lp is completely swamped bythe increase in Q8. However, at higher levels of AP, the systembecomes more and more governed by Lp and the effects of Q8 arecompletely swamped by the decline in 4, at adequate AbAdosages, with the result that QU declines during the second phase.We see in Figures 2A and 3 the gradual shift in dependence of thesystems from the one totally dominated by Q8 (AP = 0, Fig. 2A)through the range of decreasing dependence on Q. and increasingdependence on Lp (Fig. 3) to Figure 5 which is dominated by theLp effects.The proposed relationships between Q., 4, and pre-AbA Qu

are also supported by the data of Figure 1 taken 4 h after the AbAtreatment. These data show that at high AP, Qp4 returns morenearly to its original level after 4 h than at low AP. In fact, theincrease in Qpl is sustained at or near the peak levels at low orzero AP.

Examination of the reflection coefficient, a, at AbA + 4 h failedto show any change. Calculating a as before, we found it still at0.99, so that none of the steady state effects we have beenexamining could be explained on the basis of increased convectiveflow of solutes.The data of Markhart et al. (15) on soybeans also showed that

AbA decreased Lp and that the degree of decrease was stronglydependent on the concentration of AbA applied. The apparentlag in the onset of the change in Lp was also consistent with ourdata on Phaseolus (Fig. 5). However, they observed neither thepeak response in Q, nor the changes in Q, which we presentedhere. Since we feel that the peak response is due to a suddenmovement of solutes either to or from an osmotically active site,the presence or degree of that response might be determined bythe solute reserves of the root system. This being the case, speciesdifferences or culture conditions might account for this discrep-

ancy.Cultural practices do not seem to be a major factor because

observations on soybean in this laboratory, where Glycine wasgrown under the same conditions as the Phaseolus, showed thatthe peak response, when present, was much diminished (only afew per cent) even at AbA concentrations as high as 2 x 10-5 M(5.7 x 10-5 g cm-2 leaf area). We saw no evidence of the pulsedrelease of solutes to the xylem. The difference in the transientresponse may be related to the inherent level of activity of theroot ion pumps and to differences in the root's ability to accu-mulate and compartmentalize solutes. Also, we found a to bemuch higher in Phaseolus than in Glycine (8), and this would helpto account for some of the differences in response to AbA treat-ments. Unfortunately, further comparisons which might help toclarify the situation are not possible at this time. Our data are insubstantial agreement with the studies of Karmoker and vanSteveninck (14) on Phaseolus except that they did not observe thetransient peak. Because they took readings only at 3-h intervalsthe transient peak would not be apparent in their data.

Unpublished data from our laboratory indicate that in terms ofresponses to AbA we should be more concerned about the actualdosage than the concentration of the growth regulator. It is nearlyimpossible to compare the responses reported in the literaturebecause it is rare that enough information is given to allowcalculation of the dosage. If we are dealing with specific sites ofaction for each response and if we are dealing with more than oneresponse (i.e. dLp, dL, etc.) we might expect differing responsesdepending on which of the response sites or combinations thereofis above its threshold value, which are saturated, etc. One mightalso expect to observe species differences in these threshold andsaturation values. If the wealth of contradictory data, independentof contradictory interpretations, in the literature is to be put inorder, an extensive systematic comparison of species and theirvarious response magnitudes and thresholds will have to be un-dertaken.

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3. COLLINS JC, AP KERRIGAN 1973 Hormonal control of ion movements in theplant root? In WP Anderson, ed, Liverpool Workshop on Transport in Plants.Academic Press, London

4. COLLINS JC, AP KERRIGAN 1974 The effect of kinetin and abscisic acid on waterand ion transport in isolated maize roots. New Phytol 73: 309-314

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"US Plant Physiol. Vol. 68, 1981

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