44PLANT PHYSIOLOGY

11. HAGEN, C. E. AND H. T. HOPKINS. 1955. Ionicspecies in orthophosphate absorption by barley roots.Plant Physiol. 30: 193-99.

12. HAGEN, C. E., J. E. LEGGETT, AND P. C. JACKSONN.1957. The sites of orthophosphate uptake by barleyroots. Proc. Natl. Acadl. Sci. 43: 496-506.

13. HODGES, T. K. A-ND Y. \-AADIA. 1964. Uptake andtransport of radiochloride anid tritiated water byvarious zones of onioni roots of differenit salt status.Plant Physiol. 39: 104-8.

14. HODGEs, T. K. AND Y. VAADIA. 1964. Chlorideuptake and transport in roots of different salt status.Plant PhNvsiol. 39: 109-14.

15. HOFSTEE, B. H. J. 1952. On the evaluation of the

constants Vm and Kmn in enzymiie reactioIns. Science116: 329-31.

16. JACKSON, P. C. A.Ni) H. R. ADI);AIS. 1963. Cationi-anion balance (luring potassium anid sodium absorp-tioni by barley roots. J. Geni. Physiol. 46: 369-86.

17. l. EGGETT, J. E. 1961. Entry of phospbate inito yeastcells. Plant Physiol. 36: 277-84.

18. LFL(G(GETT, J. F. \N) E. EPSTEIN. 1956. Kinetics ofsulfate absorptioni by barley roots. Planit Phbsiol.31: 222-26.

19. NOGGLE, J. C. ANDNM. F. FRIEI). 1960. A kinetic anal-ysis of phosphate absorptioni by excised roots bvmillet, barley, and(I alfalfa. Soil Sci. Soc. A\m1.Proc. 24: 33-35.

Effect of Water Movement on Ion Movementinto the Xylem of Tomato Roots 1,

William Lopushinsky4Department of Botany, Duke University, Durham, North Carolina

It is generally accep)te(l that anl increase in tran-spiration can. in some circumstances, cause ain in-crease in tbe rate of salt uptake by plants ( 17 ). How-ever, there is consi(lerable disagreement as to howtranspiration causes increase(l salt uptake. AManywNorkers regard salt absorption as involving an activetransport process in which salt transport into the rootxylem iS (lependent upoIn energy release(d in respira-tion (7. 17, 18). Accor(ling to this vie-, watermlovemiienit through roots affects salt uptake indirectlvby redlucing the salt concentration in the root xylem,thereby accelerating the inward movenment of salt byactive transport. Other investigators have concludeedthat ions can mlove through roots wvith Imcass flow ofwater. Accor(ling to their viewv, salt movement intothe root xyleml of rapidly transpiring plants involvesmass flow as xvell as active transport (12, 13).

Estimates of the amount of salt moved by the 2processes vary considerably. Hy-lmno (8), workingwvith pea plants, reporte(l that mlore thain 75 % of thetotal uptake of Ca andl Cl ions could occur (lirectly bynmass flowvwitlh water, whereas Brouwer (6'). investi-gating RI) anId Cl uptake by Vicia fab(a Plants, C'onl-

1 Received Sept. 3, 1963.2 This work was supported by funds from the United

States Atomic Energy Commission (Contract AT-(40-1) -1827).

3 A prelimiinary report (f this work has appeared inNNature, 192: 994, 1961.

4 Present address: Forest Hydrology Laboratory,U nited States Forest Service, \NVenatchee, \Vashingtoin.

clu(le(d that only approxinmately 15 (vwas moved pas-sivelv wvith wvater.

In the present paper, experiments are (lescribe(lin w-hich wrater wvas nmoved through (letoppe(l rootsystems by increasing the hy(lrostatic pressure of thesolution surroun(ding the roots. Using this metho(dwith tomiiato root svsten is, Jackson an(l Weatherlev(10) found that al)l)lication of a pressure of 2 atmiicaused approximiiatel- a fourfoldl increase in the rateof K movement into the root xylenm. They conclu(le(lthat the increase xvas cause(d in part by an increase inpermeability of the root cells to K ions. Jensen (11)foun(d that wvhen vacuum was applie(d to the stumps ofletoppedl tomato roots, NO., uptake increasedl w ithincrease in wvater uptake but not in prol)ortion tow-ater uptake.

In the present experiments, the effect of wvatermovement on the rate of movemient of P32, Ca45, an(dtotal salts was stu(ie(l. Particularly, an effort wasmla(le to determine accurately what fraction of thetotal salt monved into the root xy-lemi was associate(dw-ith water movement.

Materials and Methods

Tomuato plants (Lvcopcirsici nii escutlen turnii Mill.var. MaIarglobe), grow-ni in full strength Hoagland'ssolution in the greenhouse, were usedl in the experi-ments. Solutions wvere aerate(l continuously and(lchanged every 5 days. After 3 to 4 weeks in nutrielntsoluti'on. the plants wi ere 40 to 50 clml in height,

494

LOPUSHINSKY-WATER AND ION MOVEMENT

weighed 40 to 110 g, and had large, extensivelybranched root systems. Plants were 7 to 9 weeksold when used.

Plants were conditioned overnight before eachexperiment by immersing the roots for 16 hours in aradioactive or nonradioactive pre-experimental solu-tion identical to the particular experimental solutionto be used. The next morning the plants were de-topped and the stumps sealed into the lid of a pres-sure chamber in such a way that the roots could beimmersed in the experimental solution and subjectedto hydrostatic pressure while the stumps projectedthrough the lid of the chamber. Stem tissues exter-nal to the stele were removed from the stumps abovethe seal so that measurements were confined to solu-tions exuding mainly from the xylem. During theexperiments, the experimental solution was aeratedcontinuously and pressures were closely regulated.

Iron as Geigy Sequestrene 330 Fe was present inthe culture solutions but was omitted from the pre-experimental and experimental solutions which, in allexperiments, were half-strength Hoagland's solutions.The radioactive tracers used were p32 and Ca45 at aconcentration of 0.5 mc/liter. P32 was obtained asH3PO4 and Ca45 as CaCl., each in weak HCI solu-tion, from Oak Ridge National Laboratory. Isotopeactivity was determined by placing a measuredamount of solution on disks of filter paper and count-ing the dried disks with a thin end-window GM tube.Rates of salt movement into the root xylem were de-termined by measuring the volume and salt concen-tration of exudate collected from the stumps. Plantswere conditioned and experiments carried out in aconstant temperature room at 25 ± 1°. Most ex-periments were carried out with 6 plants, and theresults shown are average values. The results shownin figures 2, 3, and 7 are average values for 3 plants.

Experiments and Results

Effect of Pressure on Water Movement. Afterconditioning overnight, root systems were enclosedin the chamber and subjected to successive incrementsof pressure from 5 to 40 lb/in2. Exudation was mea-sured for 15 minutes at each pressure. Root pres-sure exudation was measured for 15 minutes beforeand after each pressure application. Half of the rootsystems were placed in the chamber in Hoagland'ssolution containing 10-3 M NaN3, and subjected topressure under the same conditions as the controlroots. The rate of water movement through the con-trol roots increased with increasing pressure from 0to 40 lb/in2, but not as a linear function of pressureexcept at pressures above 15 lb/in2 (fig 1A). Theshape of the curve suggests that the permeability ofthe roots to water increased with increasing pressureup to 15 lb and then reniained constant with furtherincrease in pressure. The change in slope from cur-vilinear to straight line at 15 lb suggests that, at pres-sures above 15 lb, water was moving through all ofthe pathway available for water flow through theroots.

A~~~~~

5-S Couir 1.08 1~~~~~~~~~~~~~.

J _ a =w2-~~~~~~~~~00.6-

w~ ~~~~~ O

*Auide ~~0.2-0 10 20 3 40 0 2 3

PRESSURE (LBS/SO. I) TIME (HOURS)

FIG. 1. A, Effect of pressure on rate of water move-ment through control and 10-3 M sodium azide-treatedtomato roots. Azide-treated roots were preconditionedwith azide for 1 hour before the experiment. B, Rate ofroot pressure exudation, from control and azide-treatedtomato roots, ocurring during the experiment shown inpart A.

The rate of water movement through azide-treatedroots increased more slowly over the same pressurerange and, at 40 lb/in2, was only 11 % of the ratethrough the control roots. Thus azide drasticallyreduced the permeability of the roots to water.

The rate of root pressure exudation from controlroots decreased during the experiment (fig 1B).This was found later to be a time effect which alsooccurred in roots not subjected to pressure. The rateof root pressure exudation from azide-treated rootswas much less than that from control roots. The rateof exudation increased gradually, reaching a maxi-mum 1.5 hours after the start of the experiment, andthen decreased during the remainder of the experi-ment.

Effect of Time on p32 Content of Exudate. Inthe first experiment conducted with P32, roots weresubjected to successive increments of pressure from5 to 40 lb. The rate of P32 movement into the xylemdecreased with increasing pressure up to 20 lb andthen increased with increasing pressure above 20 lb.The amount of p32 in root pressure exudate also de-creased rapidly during the first half of the experi-ment, suggesting a time effect which probably was re-sponsible for the initial decline in the rate of p32movement under pressure. To study this effect moreclosely, changes in total P32 content of exudate withtime, after detopping, were followed (fig 2).

During the first 4 hours after detopping, the totalamount of P32 in the root pressure exudate increasedslightly and then decreased greatly. The P32 con-tent of the root pressure exudate continued to de-crease during the remainder of the experiment but ata slower rate. Since salts are known to accumulateto high levels in the xylenm sap of slowly transpiringplants (18), the rapid decline in total P32 content ofexudate after detopping probably occurred becauseexcess accumulated P32 was washed out of the xylem

495

PLANT PHYSIOLOGY

12400-

zPressure Appied

10 Total P-32 inExWsed

loo 20

P-32 MOvIedby Prewsse

0 2 4 6 8 10 12 14 16

TIME (HOURS)

FIG. 2. Effect of time and pressure on total p32 inroot pressure exudate and in exudate collected duringperiods of applied pressure. Values 5 to 40 indicate lbof pressure applied. P32 moved by pressure represents thefraction of the total p32 which was associated with watermovement under applied pressure. Root pressure exudatewas collected every 30 minutes before pressure applicationswere begun. After the beginning of pressure application,roots were conditioned for 30 minutes at each appliedpressure and each 0 lb pressure before collection ofexudate. Exudate was collected for 15 minutes at eachpressure and for 15 minutes before and after each pres-sure application.

z0

I

(I)

Z xW-j

Z aO in

IZcn 2

OZ

CL0

0x _.

elements by water moving upward through the xylem.Successive periods of increasing pressure apparentlyhad no effect on the p32 content of root pressureexudate measured before and after each pressure ap-plication. The total amount of P32 moved duringpressure applications decreased with increasing pres-sure from 5 to 20 lb, perhaps because the p32 contentof root pressure exudate was still decreasing at aconsiderable rate. Total p32 increased slightly at 30lb pressure and remained unchanged at 40 lb. Theadditional amount of p32 moved into the root xylem asa result of applied pressure remained essentially con-stant from 5 to 20 lb and then increased slightly athigher pressures.

Changes in p32 concentration of the exudate withtime were also followed (fig 3). During pressureapplications, the p32 concentration of exudate initiallydecreased and then became relatively constant. P32

concentration decreased more at 40 lb than at 10 lband also became c3nstant more quickly at the higherpressure, probably because the greater amount ofwater moved through the roots at the higher pres-sure removed p32 from the root xylem more quickly.After pressure was released, the p32 concentration ofroot pressure exudate increased rapidly, bec'omingrelatively constant after approximately 20 minutes.The P32 concentration of root pressure exudate de-creased during the experiment, probably reflectingthe time eftect discussed earlier. The results empha-size the importance of allowing sufficient time for the

3TI-ME (HOURS)

FIG. 3. Effect of time and pressure on P32 concentration of root pressure exudate (small dashed lines), and exudatecollected during periods of applied pressure (solid lines). Applied pressure was maintained constant at 10 and 40 lb/in2. Exudate was collected for 5 minutes at intervals during the periods of applied pressure and no pressure.

496

LOPUSHINSKY-WATER AND ION MOVEMENT

160

K)

'0

z

Cl)

I-z

Cl

n

0~CL)

oI

140

1201

100

801

601_

40F_

201

0 10 20 30PRESSURE (LBS./SQ. IN.) PRESSURE (LBS./SQ. IN.)

FIG. 4. A, Effect of pressure on rates of water and p32 movement into the xylem of tomato roots. p32 concentra-tions (dashed lines) are in count/min/ml. B, Effect of pressure on rate of p32 movement. p32 in root pressure exudaterepresents the amount of p32 moved into the root xylem in the absence of applied pressure. The value for p32 in rootpressure exudate shown for each pressure is really an average of measurements taken before and after that pressure

application. p32 moved by pressure represents the iraction of the total p32 which was associated with water move-

ment under applied pressure.

ion concentration of the xylem exudate to attainequilibrium before collecting samples.

Effect of Pressutre on P32, Ca45, and Total SaltContents of Exudates. An attempt was made tominimize the time effects which influenced the resultsof earlier experiments. Roots were conditioned be-fore each experiment to remove excess accumulatedsalts from the xylem, and sufficient time was allowedfor the concentration of the xylem sap to reach a

steady state before samples of exudate were collectel.In the experiment with p32, roots were subjected

to a pressure of 5 lb for 1.5 hours before the experi-ment. During the experiment, roots were conditionedfor 1.5 hours at 5 lb and 0.5 hour at all other appliedand 0 lb pressures. The results shown in figure 3indicated that these times should permit the concen-

tration of the xylem sap to reach a steady state. Exu-date was collected for 15 minutes at each pressure androot pressure exudate was collected for 30 minutes be-fore and after each pressure application.

The results are shown in figure 4A where total P32

is the product of the volume and P32 concentration ofthe exudate. As the rate of water movement throughthe roots increased, the P32 concentration of the exu-

date decreased. The total amount of P32 moved into

the root xylem increased with increasing pressure andrate of water movement, except from 10 to 15 lb whereit decreased.

The amount of p32 in root pressure exudate dur-ing the experiment and the effect of pressure on rateof P32 movement are shown in figure 4 B. Pressureincreased the rate of P32 movement into the rootxylem as shown by the fact that the total amount ofp32 in exudate obtained under applied pressure was

always greater than the amount in root pressure exu-

date. The additional amount of P32 moved into thexylem as a result of applied pressure increased withincreasing pressure, except from 10 to 15 lb where itdecreased. At 30 lb pressure, the total amount of P3'in the exudate was 2.0 times the amount in root pres-

sure exudate.A similar experiment was carried out with Ca45

(fig 5). Roots were conditioned and subjected topressure as described above for P32 (fig 4), exceptthat roots were conditioned for 1.5 hours at eachapplied pressure. It was assumed that these timeswould be sufficient to permit the concentration of thexylem sap to reach a steady state. Exudate was col-lected for 15 minutes at each applied pressure. Rootpressure exudate was collected for 30 minutes.

(00fI-

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P-32Con,c.of ExI I8*P-'32 Conc. of External Solution

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497

I I

PLANT PHYSIOLOGY

AI ~~~~~II

C-CnofEtnOl Solution _Ca 5 -C.o -f00

Volumeof/

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Ca-45 Conc.of Exudate- -Q..

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120 120

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40 z 4000

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Ca-45 Conc, ofExternalSSoi__-~~~~~~~~I-

lF

h

Total Ca-45

o.. -_ .-- C4a5A Moved by Prsure

Co-45 inRo Pressure ExudateL ~ ~ ~~~II I

10 20 30

PRESSURE (LBS./SQ. IN.)FIG. 5. Effect of pressure on rates of water and Ca45

explanation of the relationship of the curves, see figure 4.

As water movement through the roots increasedwith increasing pressure, the Ca45 concentration ofthe exudate decreased. At the same time, the totalamount of Ca45 moved into the root xylem remainedessentially constant.

The amount of Ca45 in root pressure exudate dur-ing the experiment and the effect of pressure are

shown in figure 5B. Application of pressure defi-nitely increased the amount of Ca45 moved into theroot xylem; however, the additional amount of Ca45moved under pressure remained relatively constantover most of the pressure range. At 30 lb pressure,the total amount of Ca45 in the exudate was 2.2 timesthe amount of Ca45 in the root pressure exudate.

In view of the differences obtained with p32 andCa45, it is interesting to note the effect of pressure on

the movement of total salts into the xylem. Beforethe experiment, roots were conditioned for 2 hours at0 lb pressure. During the experiment, roots were

conditioned for 1.5 hours at 5 lb and 0.5 hours at allother applied and 0 lb pressures as in the experimentwith P32. It was assumed that these times would besufficient to permit attainment of concentration equi-librium in the xylem sap. Exudate was collected for30 minutes at each pressure. Root pressure exudatewas collected for 60 minutes. Salt concentration ofthe exudate was determined by measuring its electri-cal conductivity with a conductivity bridge calibratedwith KC1 solutions.

PRESSURE (LBS./SQ. IN.)movement into the xylem of tomato roots. For detailed

The results, shown in figure 6, agree in generalwith those obtained for p32. As water movementthrough the roots increased, the salt concentration ofthe exudate decreased. Total salt increased with in-creasing pressure, except in the pressure range of 10to 20 lb where it remained relatively constant. Totalsalt in root pressure exudate and the effect of pressure

are shown in figure 6 B. Application of pressure in-creased the rate of salt movement into the root xylem,with more total salt occurring in the exudate duringpressure applications than in the absence of pressure.

As in the case of P32, the adclitional amount of saltmoved by pressure increase(d with increasing pressure,

except from 10 to 20 lb where relatively little increaseoccurred. At 30 lb, the total amount of salt in theexudate was 2.4 times the amount of salt in root pres-

sure exudate.Effect of Sodiuml Azide. A sufficient amount of

NaN3 was added to P32-labeled Hoagland's solutionin the chamber to produce a concentration of 10-3 MNaN3 while a constant pressure was maintained. Theamount of water moved through the roots under pres-

sure initially decreased after the addition of azide(fig 7). One hour after the addition of azide, therate of water movement was only 9 % of the rate be-fore treatment. Water movement remained at thislow rate for 4 hours after treatment was starte(d andthen began to increase.

The total amount of P-2 moved into the root xvlem

C.

6_

4F

I-

I-

0

0

w

U.

0

-J

0

0

M. * rw alj lntJ .

498

7l

2 _

I 1[1L

LOPUSHINSKY-WATER AND ION MOVEMENT

0.6

z

.0

I-

-J

4Ctcn

05

0.4[

0.3[

0.11

0PRESSURE (LBSs/SQ. N.)

10 20PRESSURE (LBS./SQs IN.)

FIG. 6. Effect of pressure on rates of water and total salt movement into th- xylem of tomato roots. Salt con-

centrations are in mg/ml. For detailed explanation of the relationship of the curves, see figure 4.

X CoToa iN~~~~~~~~~~~~~

4000

CL. /P-32C0C. ELtVflbIOP3Cone.

TIME (HOURS)

FIG. 7. Effect of 10-3 M sodium azide on rates of waterand p32 movement into the xylem of tomato roots. Pres-sure was maintained constant at 30 lb/in2. p32 concen-trations (dashed lines) are in count/min/ml. pH was ad-justed with HC2 to 4.9 at the time azide was added.

initially decreased after the addition of azide and, onehour after treatment was begun, was only 12 % ofthe amount moved before treatment. The rate of p32movement remained at this low level for 4 hours andthen began to increase. Two hours after treatmentwith azide, the p32 concentration of the exudate in-creased to a level exceeding the p32 concentration ofthe external solution. Four hours after the additionof azide, the p32 concentration of the exudate was the

same as that of the external solution, suggesting thatthe roots had lost their differential permeability tophosphate ions. Quite similar results were obtainedin an experiment with Ca45 in which nitrogen was

bubbled through the experimental solution while rootswere maintained under constant pressure.

Discussion

The results of the above experiments show thatwhen water is moved through roots by increasing thehydrostatic pressure of the external solution, the rateof ion movement into the root xylem increases but notin proportion to the increase in rate of water move-

ment. At 30 lb/in2, 50 % or more of the salt trans-ported into the root xylem was moved as a result ofapplied pressure. The remaining fraction was movedinto the xylem by active transport.

The way in which pressure causes increased ionmovement is not clear. One explanation, suggestedby Broyer and Hoagland (7), is that water move-

ment through roots reduces the salt concentration ofthe xylem sap, thereby stimulating active transport ofions into the xylem. Also, Bernstein and Niemen (2)have pointed out that a more rapid movement of waterthan salts through roots should result in an increasein salt concentration in the root cortex. These 2 fac-tors operating together, an increase in concentrationin the cortex plus a decrease in the xylem, would

*-N

of1-

Co

0

IE-

0

w

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0

DI I I

Total _Salt

/ Salt Conc. of External-,oz ~~Solution

. Salt in Root°.Pressure Exudate

..... 0 -

alt Movedb Pressure

30

n.H. 11

499

0.7~

0.21

PLANT PHYSIOLOGY

steepeni the concentration gra(lient from cortex toxylenm and perhaps accelerate ion transport into thexylem.

An examination of figures 4A and 6A shows thatwhile a decrease in exuclate concentration might ex-plain the initial increase in rate of salt movement atlow pressures, it cannot explain the subsequent in-crease whiclh occurredl at higher pressures where onlya slight reduction in concentration occurred. Thisincrease in rate of salt miiovenment at higher pressureswas found in all experiments, except in the case ofCa4, and suggests that the permeability of the rootsto ioOis increasedl wlhen pressures of miore than 15 to20 11) were applied. An increase in permeability ofroot cells to ions, brought about by increased suctiontension in the xyleml, has beeni reporte(d by Brouwer(4). MIore recently, Jackson and WVeatherley (10)found( thalt externally applied pressure increased therate )f K miiov-emiient into the xylem of detoppecl to-niato roots even whlen the concenltration of K in thexyleml sap increasecl at the samne tinle. They con-clude(d that the increase(d rate of K nmovenment underpressure mlust result, at least partly, fromii an increasein permleability of the roots to ions.

As mlenltionled earlier, the slhape of the curve forrate of wvater miiovement under pressure (fig 1A) sug-gests that pressure also increasedl the permeability ofthe roots to Nvater. 'Mees and Weatherley (14) alsofound( that externally applied pressure increased thepermeability of detopped tomato roots to water, andBrouwer (5) found an increase in water permeabilityin roots of intact broa(l bean plants with anl increas-ing (liffusion pressure gradient across themii. Brouwerattributed the increased permeability to a decrease inturgi(lity of the root cells. According to Hylmo (9),increase in hydlrostatic pressure causes flow of waterthrouglh smialler and smlialler pores in the water path-way until all available pore space is being utilized.Thereafter, the rate of wvater mlovemiietnt is propor-tional to pressure in accordance with l'oiseuille's law.

In the present experiments, no increase in rate athigher pressures x-as noted xwith Ca . Ca, however,is known to be less mobile in plant roots than P (3,19), and at the higlher pressures, Ca movement mayhave been less responsive to permeability changesthan wvas Pimovement.

Wlhether the increased rate of ion movement underpressure is the result of mass flow, increased diffusion,or increased active transport is not known. How-ever, a more general explanation for the over-all in-crease in rate of salt movement can be suggested.That is, perhaps pressure increases the rate of saltmovement indirectly in 2 ways: A) at all pressuresby increasing the rate of water movenment through theroots, thereby creating a steeper salt concentrationgradient from the cortex to the xylem which possiblyaccelerates the rate of salt movement into the xylem,and B) at higher pressures by increasing the perme-ability of the root cells to ions. Thus at low rates ofwater movement, salt nmovement into the root xylemoccurs primarily by active transport. In view of theincrease in ioIn permeability whiclh seems to occur at

higher pressures, the possibility exists that at highrates of water movement, some ion movement mayoccur by diffusion and mass flow in addition to thatoccurring by active transport.

The question also arises wlhetlher the increase(lrate of salt movement under pressure represents in-creased salt movement from the nmedium or increased"tissue exudation" (1) from sites in the root tissues.According to Jackson and Weatlherley (10), all of theincreased K movement under pressure in their experi-ments could be accountedl for as increasedl tissue exu-dation from the root cells. In the present experi-ments, however, when roots conditioned overnightwith P32 were transferred to nonradioactive Hoag-land's solution and subjected to pressure, less P32appeared in the exudate tlhani when roots were trans-ferred to a solution contaiining P32. At 5 lb pressure,60 % of the total P32 moved into the xylenm could beaccounted for as tissue exudation. At 40 lb, tissueexudation amounted to only 30 % of the total, theremaining fraction presumably entering fromii the ex-ternal solution. Thus, at least at higher pressures aiiolrates of water movement, muclh of the salt transportedlinto the xylem entered dlirectly fronm the externalmedium.

In all of the experiments, as the rate of watermovement through roots increased with increasinogpressure, the salt concentration of the exudlate (le-crease(d to a small percentage (11 %4 or less) of theexternal solution. This indicates the existence of abarrier in the roots, possibly the endodermis, xvhiclprevents free movement of ions into the xylem.

The initial effect of treatment with NaN3 wa's toreduce the rate of both water and P32 imovement intothe xylem. The effect on water moveim-ent is con-sistent with results obtained by nutmerous investiga-tors with various respiration inhibitors (14, 15, 16).In one experiment, the rate of water movementthrough roots un(ler pressure was reduced almost in-stantaneously (2 mmin) after addition of azicle, suggest-ing that azide had directly affected the permeabilityof the root cells to water.

The decrease in rate of P32 moveimient may be ex-plained in several ways. First, azide is a respirationinhibitor and probably reduced the rate of activetransport of P32 into the xylem. Second, azide mayhave caused a decrease in the permeability of the rootcells to p32 as was found to occur for water. Third,by decreasing the rate of water movement, azide mayhave reduced the amount of P32imoved into the xylen3in response to changes in the concentration gradient.Finally, if any movement of P32 occurred directly withmass flow of water, reduction in water movementwould also result in reduced P32 movemenIit. It seemsmost likely that a combination of these factors wasinvolved.

Prolonged exposure to azide resulted in an in-crease in the rate of water and P32 movement, indicat-ing that azide had begun to damage the root cells,thereby increasing their permeability to water andP32. It is interesting to note that although the exu-date concentration exceeded the external solution con-

500

LOPUSHINSKY-WATER AND ION MOVEMENT

centration 2 hours after addition of azide, indicatingleakage of accumulated p32 from the root cells, therate of water movement did not increase until 5 hoursafter treatment. This suggests that azide increasedthe permeability of the roots to p32 before increasingtheir permeability to water. The drastic effect ofazide on the permeability of the roots to water andp32 indicates that the major resistance to both waterand salt movement is located in the protoplasm of theroot cells.

Summary

The relationship betveen water movement andsalt movement through tomato roots was studied.Water movement through detopped root systems wasincreased by increasing the hydrostatic pressure ofthe solution surrounding the roots. The rate of saltmovement into the root xylem was determined bymeasuring the volume and salt concentration of exu-

c!ate collected from the stumps. Comparisons were

made of amounts of salt moved into the root xylemwhen water was moved under pressure with amountsmoved by active transport in the absence of externalpressure.

Application of pressure increased the amount ofp32, Ca45, and total salts moved into the root xylem,but not in proportion to the increased rates of watermovement. At 30 lb/in2, the amount of ions movedinto the root xylem was 2.0 to 2.4 times the amountmoved by active transport in the absence of pressure.

The salt concentration of exudates obtained underpressure usually was less than the concentration ofthe external solution.

Treatment of roots with 10-3 M sodium azideinitially reduced the rates of both water and p32 move-

ment to about 10 % of the control rates. Longer ex-

posure to sodium azide resulted in an increase in therates of both water and P32 movement.

Acknowledgment

Thanks are due to Professor Paul J. Kramer, whosuggested this problem, for his help and advice throughoutthe course of the investigation.

Literature Cited

1. ANDEL, 0. M. VAN. 1953. The influence of saltson the exudation of tomato plants. Acta Botan.Neerl. 2: 445-521.

2. BERNSTEIN, L. AND R. H. NIEMAN. 1960. Apparentfree space of plant roots. Plant Physiol. 35: 589-98.

3. BIDDULPH, O., S. BIDDULPH, R. CORY, AND H.KOONTZ. 1958. Circulation patterns for phos-phorus, sulfur, and calcium in the bean plant. PlantPliysiol. 33: 293-300.

4. BROUWER, R. 1953. Transpiration and anion up-take. Proc. Koninkl. Ned. Akad. Wetenschap.C 56: 639-49.

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