miethods rapi(llv plicate(l · mide soluition lost these ions most rapi(llv from the apical region...

PLANT PHYSIOLOGY

phate, malate, lactate, and fumarate. Propyl phos-phate was beneficial for apple but not for corn.

The assistance to these investigations of Dr.Joseph Seiberlich, of the New Hampshire Engineer-ing Experiment Station, in synthesis of certain of theorganic compounds of magnesium used, of Mr. G. P.Percival, in pH tests of solutions, and of 'Mr.Howard Moulton for the care of some of the cul-tures, is gratefully acknowledged.

LITERATURE CITED1. ARNON, D. I., FRATZKE, W. E., and JOHNSON, C. M.

Hydrogen ion concentration in ielation to absorp-tion of inorganic ntrtlients by higlheriplants. PlantPhysiol. 17: 515-524. 1942.

2. ARNON, D. I. and JOHNSON, C. M. Influence oflhydrogen ion concentration on the growth ofhigher plants under controlled conditions. PlantPhvsiol. 17: 525-539. 1942.

3. BOYNTON, D. Magnesium nutr ition of apple trees.Soil Sci. 63: 53-58. 1947.

4. DAVIS, J. F. and MCCALL, W. W. Occurrence of mag-nesium deficiency in celery on the organic soils ofMichigan. Michigan Agr. Exp. Sta. Quart. Bull.35: 324-329. 1953.

5. HILL, H. and JOHNSTON, F. B. Magnesium deficiencyof apple trees in sand culture and in commercialorchards. Sci. Agr. 20: 516-525. 1940.

6. LoTT, W. L. Magnesium deficiency in muscadinegrape vines. Proc. Amer. Soc. Hort. Sci. 60: 123-131. 1952.

7. MULDER, D. Magnesium deficiency in fruit trees onsandy soils and clay soils in Holln(l. Plant andSoil 2: 145-157. 1950.

8. SCOTT, L. E. and ScoTT, D. H. Response of grapevines to soil and spray applications of magnesiumsulfate. Proc. Amer. Soc. Hort. Sci. 57: 53-58.1951.

9. ScoTr, L. E. and ScoTT, D. H. Further observationson the response of grape vines to soil and sprayap)plications of magnesium suilfate. Pr oc. Amer.Soc. Hort. Sci. 60: 117-122. 1952.

TRANSLOCATION OF RADIOACTIVE ISOTOPES FROM VARIOUSREGIONS OF ROOTS OF BARLEY SEEDLINGS12

HERMAN H. WIEBE3 AND PAUL, J. KRAMERDEPARTMENT OF BOTANY. DUKE USNIVERSITY, DuRHAM, 'NORTH CAROLINA

Altlhoughl muchl attention has been giv-en to theuptake of minerals by roots it is not known fromwhich regions of the root most of the translocation ofminerals to the shoot occurs. The fact that mineralsare accumulated freely in a certain region of the rootdoes not prove that they are translocated out of thatregion equally freelv. In fact it miglht be expectedthat those regions w-hich acecumulate minerals mostrapidlyv would release them most slowly. The evi-dence on this problem is limited and contradictory.Prevot and Steward (18) found the greatest accumu-lation of bromide ion in the apical region of excisedbarley roots, and Steward et al (22) found a similargra(luation of accumulation of rubidium andl bromidein attached barley roots. The latter investigatorsalso found that attached barley roots which weretransferred to water after 24 hours in rubidiium bro-mide soluition lost these ions most rapi(llv from theapical region in wlhichl accumulation had been greatest.Lundegardh (14), on the other hand, founcd by analy-sis of the exuding sap that the greatest translocationof nitrate occurred from the basal region of wheatroots. He suggested that nitrate was rapidly ab-sorbed by all regions, but more nitrate was reducedin the younger regions, leaving less avallable fortranslocation from near the apex.

'Received October 12, 1953.2 The work on which this paper is based was largely

financed by contract No. At-(4-1)-1031 with the UnitedStates Atomic Energy Commission.

3 Present address: Department of Botany, IUtah StateAgricuiltuiral College, Logan, Utah.

Some of our previous work (12) indicated that,heavy accumulation of radioactive phosphorus occursseverall centimeters behind the root tip and that it isreadily translocated from this region to other parts ofthe plant. A series of experiments wzas therefore per-formed to studcy the relative amotunts of several ra-dioactive isotopes absorbed at various distances be-hin(d the root tip and translocated to other parts ofthe seedling. Use of radioactive isotopes made itpossible to observe movement of very small amountsof material ovrer short periods of time with relative

MIETHODSIsoToPEs USED: The radioactive isotopes used

were P32 Rb86, 1131, S35, Ca45, and Sr90. Monova-lent cations and anions are commonly used in studiesof nineral accumulation because it is believed thatthex (lo not generally enter into organic compounds.As a result the study of accumulation is not com-plicate(l by metabolic removal of the ions from thevacuolar solution. Rb86 and I131 were selected asrepresentative monovalent ions. They probably re-main in the vacuolar sap in ionic form. The absorp-tion of Rb appears to occur on the same sites as ab-sorption of K, and iodide appears to be absorbed atthe same sites as bromides and chlorides (5, 6). Alsothese elements, if initially present in the plant at all,occulr only in trace amounts; hence absorption of theradioactive isotope is a good measure of absorption ofthe element. This is not necessarily true of p32,Ca45, or S35 becauise these isotopes could conceivably

342

Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

WIEBE AND KRAMER-TRANSLOCATION FROM ROOTS

enter the plant in exchange for stable isotopes of therespective elements initially present in the plants.The rubidium and iodide ions could not enter in ex-change for stable isotopes of the same elements, al-though they might enter in exchange for ions of otherelements already present (5, 6).

The isotopes were supplied to the plants in fullstrength Hoagland's solution because it was believedthat if the isotopes were supplied as tracers in theusual concentration of stable isotopes they would givea better indication of mineral absorption and trans-location than if supplied in extremely dilute, carrier-free solution where surface precipitation and ex-change are relatively more important.

The basic nutrient solution consisted of 5 mili-moles of KNO3, 5 millimoles of Ca(NO3)2, 2 milli-moles of MgSO4, and 1 millimole of KH2PO4 per 1.Sufficient quantities of the various isotopes wereadded to this base solution to give an activity of 2to 3 mc per 1. The isotopes were added in the fol-lowing chemical forms: p32 as KH2PO4, Rb86 asRbCl, I131 as KI, S35 as MgSO4, Ca45 as CaCl2, andSr90 as SrCl2. One millimole of stable KI was addedper 1 of the I131 solution and the Rb86 solution con-tained about 18 millimoles of carrier RbCl per 1. Noinert carrier was added to the Sr90 solution.

CULTURE OF BARLEY SEEDLINGS: The Sacramentovariety of barley was used in these experiments. Theseedlings were grown by a method modified from thatdescribed by MIachlis (15). The seeds were soakedin water for 15 minutes, the hulls removed, and themoist seeds placed in a flask where they were keptmoist for a day by repeated rinsing with water, andat the same time they were well aerated. The germi-nating seeds were then transferred to glass grids soarranged in 700-ml cylinders that the coleorhizas werein contact with dilute nutrient solution. The dilutenutrient solution consisted of the basic nutrient solu-tion described earlier diluted with 1000 volumes ofwater. The cylinders were painted black on the out-side to reduce illumination of the roots. The nutrientsolution was aerated continuously and was changedabout 15 times a day bNy slow gravity flow from areservoir. The seedlings were grown either in a lab-oratory window or in a constant temperature room at24 to 260 C with 12 hours of illumination daily fromfluorescent and mazda lamps. The seedlings devel-oped long, uniform, unbranched roots in 5 days afterthe initial soaking of the seeds.

In our earlier experiments on absorption of P32(12) many irregularities in absorption were observedwhich appeared to be related to variations in com-position and aeration of the nutrient solution.Growth of seedlings in flowing nutrient solutiongreatly reduced this variation. Use of a dilute nu-trient solution produced seedlings with a low saltcontent, yet avoided the abnormal growth sometimesobserved in distilled water.

METHOD OF SUPPLYING ISOTOPES: An absorptioncell (fig 1) was developed for supplying the isotopesolution to limited regions of individual roots undernormal conditions for mineral absorption and growth.

These cells were suggested by the circulation vesselsof Lundegardh (13) and the potometers of Hayward,Blair and Skaling (8). The glass portion consisted ofa tube 10 mm in diameter with a side arm. The tubewas drawn out and bent around so that the narrowend faced the side arm. This end and the side armwere connected by pure gum rubber tubing, 3 mm indiameter, designed to hold a portion of the root. Ahole was burned through both walls of the rubbertubing by using a hot nichrome wire of the samediameter as the roots. The holes were then connected

FIG. 1. Diagram of the absorption cell used in sup-plying radioactive isotopes to localized regions of roots.A segment of the selected root 3 mm in length was in-closed in soft rubber tubing through which the isotopeswere circulated while the other roots were immersed inflowing non-radioactive nutrient solution.

by a slit. Any portion of the root could be fittedinto this tubing. Warm, almost liquid lanolin wasused to seal the junction between root and rubber.A nichrome wire holder held the grain of the seedlingin place. The solution containing the radioactiveisotope was then placed in the absorption cell, andthe entire apparatus was immersed in the diluteflowing solution. Gentle tapping usually dislodged airbubbles trapped inside the rubber tubing.

Air bubbling through the solution in the large armof the absorption cell both aerated and circulated thesolution. The level of the isotope solution was usu-ally kept 0.5 inch below the level of the external cul-

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PLANT PHYSIOLOGY

ture solution so that if leaks developed, the flowwould be inward where it could be detected by arise in the level of the solution. In actual practiceleakage occurred in less than 10 % of the experi-ments. The external culture solution also was aer-ated and was changed at the rate of 10 volumes perhour to keep contamination by leakage or leachingfrom the roots to a minimum. Contamination of theexternal solution was low as indicated by the smallamount of radioactivity in control plants which wereplaced in the solution with the non-treated roots ofplants supplied with isotopes through the absorptioncells.

The experiments with absorption cells were con-ducted at 25° C and were of 6 hours duration except-ing some preliminary experiments which ran for 12hours. In repetition of the experiments plants wereselected at random from the same lot of seedlings.The fact that roots often grew as much as 3 or 4 mmwhile in the absorption cells indicated that neitherthe mechanical manipulation nor the radiation dur-ing the relatively short experimental- period washarmful. The plants were treated when they were 5to 6 days old. At this age the seminal roots were 10cm long and were growing rapidly. As roots becomeolder the growth rate decreases and the different tis-sues mature more nearly to the apex (7, p. 491, 10).Treatment therefore occurred at the time of maxi-mum longitudinal separation of the different anatomi-cal regions of the roots.

MEASUREMENT OF TRANSLOCATION: Immediatelyafter treatment the plants were removed from theabsorption cells and rinsed in three changes of thediluted nutrient solution. They were then placed be-tween layers of glass cloth and dried in an oven at950 C. Glass cloth was used because the roots donot adhere to it. Autoradiographs were prepared inthe usual manner, using Kodak Blue Brand X-rayfilm. Goodyear Pliofilm was placed between theplants and the film, except for the low energy iso-topes, C45 and S35, which were placed in direct con-tact with the film. Plants containing no radioactivitywere exposed at the same time to detect the effect onthe film of any chemical compound found in barleyplants (4). No such effects were noted.

After preparation of the autoradiographs theplants were cut up and counted for radioactivity.The portions of the treated root within 2 mm of theregion of isotope supply were counted as one sample.The apical and basal portions of the treated rootwere counted separately as were the non-treatedroots, grain, and leaves from each plant. Roots fromthe control plants were also counted. The countswere corrected for background radiation and for con-tamination of the external solution as determined bythe control plants.

Plants supplied with P32, Rb86, I131, and Sr90were counted directly with a thin-end-window Geigertube and scaler. Self absorption of radiation fromthese isotopes is not great with dry organic materialof small diameter such as these barley roots and waspresumed to be quite uniform since the roots increasevery little in diameter over their entire length. Ma-

terial containing Ca45 was ashed in an electric furnaceat 500° C, and the dry powdery ash was cemented tothe dish with a drop of dilute collodion. Materialcontaining S35 was digested directly in glass countingdishes or in 1-inch watch glasses. The roots werecovered with about 0.5 ml of concentrated HNO3 for12 hours after which several drops of H202 wereadded. After about 3 more hours the samples wereslowly evaporated to dryness under infra-red lamps.The Ca45 and S35 samples were counted in a window-less flow counter.

RESULTS AND DISCUSSIONFigure 2 shows photographs of autoradiographs

obtained in a preliminary experiment using 5-day-oldplants and supplying the p32 for 12 hours instead ofthe usual 6 hours. On these autoradiographs theamount of p32 present is approximately proportionalto the blackness and width of the image. In plant Athe isotope was supplied about 5 cm from the root

B

.N.

Pp3Z

FIG. 2. Translocation of P3 supplied to roots ofbarley seedlings at the points indicated by the arrows.

When supplied 5 cm behind the tip (left) it was trans-located throughout the seedling, but little translocationoccurred when it was supplied to the root tip (right).

apex. Large quantities were translocated to thegrain and leaf (the upper portion of the leaf lies tothe right of the grain) and to the tips of the non-treated roots as well as to the tip of the treated root.Plant B was supplied with P32 at the root apex. Asomewhat larger quantity of p32 was absorbed thanby plant A, but so little was translocated upwardthat it was scarcely detectible in the other parts ofthis seedling. The slightly greater P32 absorption ofplant B may have been due to the fact that moresurface was exposed, since the tip of this root grewseveral mm while it was in the absorption cell. Auto-radiographs of later replications were similar to theone pictured.

In further experiments four general regions wereselected for the placement of the absorption cells.The first was the terminal 3-mm portion of the rootwhich comprises the meristem and some elongatingcells. The second region was about 6 to 9 mm fromthe apex, representing the region in which the proto-xylem may be mature. In the third region, 20 to 30mm from the apex, the metaxylem is mature and the

344



TABLE ITRANSLOCATION OF ISOTOPES FROM VARIOUS REGIONS OF THE ROOT. VALUES ARE COUNTS PER SECOND AND INDICATETHE AMOUNT OF ISOTOPE PRESENT IN THE REGION TO WHICH IT WAS SUPPLIED AND THE AMOUNT TRANSLOCATED TO

OTHER PARTS OF THE PLANT

ISOTOPE ......................No. OF REPLICATIONS ..........DISTANCE OF SUPPLY FROM ROOT

TIP IN MM .................

Apical region of treated root ..

Treated region * ..............Base, treated root ............Non-treated roots ............Leaf .........................Grain ........................Total translocated ............Total absorbed ...............%ho translocated ..............Standard deviation ...........

ISOTOPE ......................No. OF REPLICATIONS..........DISTANCE OF SUPPLY FROM ROOT

TIP IN MM .................

Apical region of treated rootTreated region * ..............Base, treated root ............Non-treated roots ............Leaf .........................Grain ........................Total translocated ............Total absorbed ...............%o translocated ...............Standard deviation ...........

PHOSPHORUS'3 3 3

0-5

166.48.39.93.72.24

2.28168.76

1.351.03

RUBIDIUM'2 4 33

6-9 28-31 82-85

* . .. .

146.922.964.355.45.79

13.54159.97

8.474.70

13.9979.846.514.8625.092.66

49.11142.9434.415.8

36.3665.60

.8212.7519.222.37

34.14137.1024.95.4

IODINE13'3 3 3 3

0-4 7-10 25-28 69-72

..... ..... 17.23

101.20.59.28.18.00

1.05102.30

1.030.7

103.7626.167.993.583.27

41.00144.8028.318.3

81.1016.579.075.032.32

32.99114.1228.913.5

44.89.99

10234212.83

18.2680.4122.72.4

4

0-4 69 20-23 58-61

.... .... .... 14.4721.28 40.44 42.85 23.12

.44 5.25 1.88 .10

.49 .82 3.98 2.50

.00 .37 .94 .96

.02 .31 .56 .34

.94 6.75 7.36 3.9022.22 47.18 50.21 41.484.24 14.3 14.7 9.44.5 6.0 4.4 6.4

SULFUR'2 2 2 2

0-4 7-10 23-26 59-62

.... .... .... 8.8532.02 21.91 14.75 18.76

.10 .26 .23 .25

.18 .40 1.03 1.62

.22 .08 .09 .18

.11 .46 .62 .74

.61 1.20 1.97 2.7936.63 23.11 16.72 30.401.7 5.2 11.8 9.2.9 2.3 4.3 3.0

* The treated region has a 2 cm segment including the portion of the treated root enclosed in the absorption cell.

endodermis is developed but passage cells remain.The fourth region was between 55 and 70 mm fromthe apex where the endodermis forms a complete cyl-inder, uninterrupted except by lateral root primordia.

Results of counts of the various parts of the seed-lings are summarized in tables I and II. There wasrelatively little translocation of any of the isotopesfrom the terminal 4 mm of the root and maximumtranslocation generally occurred from the regionabout 10 to 30 mm above the apex. Upward trans-location from basal portions of roots was generally

TABLE IISUMMARY OF DATA ON TRANSLOCATION OF RADIOACTIVEISOTOPES FROM VA.RIOUS REGIONS OF ROOTS. EACH VALUEREPRESENTS THE AMOUNT TRANSLOCATED AS A PERCENTAGEOF THE TOTAL AmOUNT ABSORBED AND IS THE AVERAGE OF

Two TO FOUR REPLICATIONS

APPROXIMATE DISTANCE % OF ISOTOPE TRANSLOCATEDFROM ROOT TIP AT UPWARDWHICH ISOTOPEWAS SUPPLIED P." Rb" I1S SW

0-4mm 1.3 42 1.0 1.77-10 mm 8.5 14.3 28.3 5.2

27-30 mm 34.4 14.7 28.9 11.857-0 mm 24.9 9.4 22.7 9.2

less than from the middle, but much greater thanfrom the apex.

In some plants large quantities of isotope accumu-lated in the basal regions of the treated roots, pre-sumably because it was absorbed from the xylemstream. This undoubtedly reduced the amountreaching the shoot and untreated roots. For ex-ample, when 1131 or Rb" was supplied about 8 mmfrom the apex, larger quantities of these isotopeswere found in the basal portions of the treated rootthan in the grain, leaf, or untreated roots. Theamount of upward translocation was defined as thequantity of an isotope that had moved more than 2cm up the treated root from the point of supply andincludes all that found in the grain, leaf, and non-treated roots. Downward translocation in the treatedroot was defined as the quantity of isotope whichmoved more than 2 cm toward the apex from thepoint of supply.

The length of the treated root had little effect onthe amount translocated upward. In several pre-liminary experiments roots of different lengths wereselected and the absorption cell was placed at astandard distance from the grain while its distancefrom the root apex depended on the length of theroot. Examination of autoradiographs revealed that

345


PLANT PHYSIOLOGY

tsrl31

41-3 ~,-S 5

4FIG. 3. Autoradiographs showing, from left to right,

the pattern of downward translocation of R', IP, S8,and Ca' in roots of barley seedlings. The upper arrowsindicate the points of supply, the lower arrows indicatethe location of the root tips.

the amount of upward translocation was related tothe distance of the supply from the root apex ratherthan the distance to the grain.

Most of the isotopes were readily translocated tothe leaves as well as to the non-treated roots. Rela-tively less reached the grain and almost all of theradioactivity recorded for the grain was in the stem-root junction which usually was not separated fromthe grain before counting. In a few cases the storagetissue of the grain was counted separately and foundto contain very little radioactivity. p32 and I131were most readily translocated while smaller propor-tions of the absorbed Rb8ff and S35 were translocatedto the leaves and non-treated roots. The quantitiesof Ca45 translocated were small and difficult to detectwith certainty above the slight contamination of theexternal solution. It appeared, however, that Ca45,like the other isotopes studied, was more rapidlytranslocated from the mature regions of the rootsthan from the root tips. In the one experiment withSr90 no measurable translocation occurred more than20 mm above or below the point of supply.

All of the isotopes moved downward in thetreated roots, but the pattern varied somewhat forthe different isotopes (fig 3). P32 and S35 moved allthe way to the root tip and usually accumulatedthere. I131 and the cations generally did not moveto the root tip but stopped, often rather abruptly,at a distance of 3 to 15 mm from the apex. Oftenthe isotopes accumulated in this region. No explana-tion of this effect is apparent.

The results of this experiment differ from thoseobtained by Steward, Prevot, and Harrison (22) byanother method. They reported that the terminalcm of roots of intact barley plants absorbed RbBrmost rapidly and also lost this salt most rapidly bytranslocation to other parts of the seedling when the

roots were transferred to distilled water. They alsoreported that their roots ceased elongation and thetips became hypertrophied during the experimentswhich lasted 66 to 72 hours. It is quite possible thatduring this period of time xylem vessels were ma-tured almost to the tips of the roots, resulting in in-creased translocation away from the tips. Some Rbapparently was also lost to the external solution intheir experiments; hence it is impossible to determinethe relative importance of translocation and loss byleaching in removal of salt from any particular re-gion of the root.

MOVEMENT OF ISOTOPES THROUGH A KILLED SEG-MENT OF THE ROOT: A few experiments were con-ducted to determine the tissue in which the isotopesmove through the roots. A segment of the intactroot was killed by directing a jet of steam on it for20 seconds. Since upward movement of the isotopeswas reduced by only about 30 %, it seemed likelythat the isotopes moved upward in the xylem.Steaming either above or below the region of p32 en-try reduced upward movement by 30 %.

The effect of steam girdling on downward trans-location was more variable. In these experimentsroots were steamed about 1 cm below (toward theapex) the point of p32 application (fig 4). Steamingalmost completely prevented all movement of p32downward into the apical cm of the root. Evensteam girdling above the region of P32 entry reducedmovement into the terminal centimeter by over 50 %.

//..P32.p32

FIG. 4. Autoradiographs showing the pattern of down-ward translocation of P' in roots which were girdled bysteaming above the point of isotope supply (left), belowthe point of supply (center), with an untreated controlroot (right).

346

.1..-i.



It is quite possible that downward translocation ofP occurs in association witlh organic compounds.Girdling between the source of organic compounds inthe grain and the region of P supply would reducethe movement of organic compounds past this regionand, as a result, might easily reduce downward trans-location of p32. A "remote" effect on translocationhas been reported by B6hning, Swanson, and Linck(3). They found that chilling the hypocotyl of beanplants apparently reduced translocation of carbo-hydrates from leaves to the stem apex.

Four complete steam girdling experiments were

carried out with p32 and one each with S35, Rb86, andI131. In all of these, transfer into the apical cm ofthe root was negligible, but in over half of the experi-ments there was as much downward transfer of theisotopes past the steamed region as in the roots ofunsteamed control plants. It is not known how, or

through what tissue this downward movement oc-

curred. Since the distance was only a few cm, it mayhave diffused downward. The steamed segment was

about 8 mm in length, and the killing might haveincreased the permeability of the cells.

THE ROLE OF THE ENDODERMIS: It has been sug-gested by several investigators that the endodermispresents a serious barrier to the entrance of solutes(1, 17, 18, 20). Examination of cross sections of 6-day-old barley roots showed that the inner walls ofall endodermal cells, including passage cells, were

thickened at distances of more than 5 cm above theroot apex. Prevot and Steward (18) found the same

situation in the roots they studied. Nevertheless, theresults of these experiments show that the endoder-mis is not an effective barrier to the inward move-

ment of various ions. Neither does it prevent theentrance of water, because considerable water ab-sorption has been observed at distances of several cm

above the apex (8, 14, 19, 21).REDUCED TRANSLOCATION FROM THE ROOT APEX:

There are several possible explanations for the rela-tively small amount of translocation from the apicalportion of the root. Since growing cells can accumu-

late large amounts of mineral salts, it is possible thatall of the absorbed isotope is held by the growingcells. The ability of meristematic cells to accumulateP32 and S35 from other parts of the plant is illus-trated in figures 3c and 4c. In these experiments theroot tips contained more p32 and S35 than the regionsjust back of the tips. Others have reported on theability of meristematic and growing tissues to ac-

cumulate p32 from the rest of the plant (2, 16).Lundegardh (14) has presented data which sub-

stantiate this possibility. In studies on exudationfrom segments of wheat roots he found larger quanti-ties of nitrate in the exudate from basal segmentsthan from apical segments. He suggested that thisoccurred because the nitrate, as potassium nitrate,was absorbed as rapidly in the apical as in the basalsegments of the roots but the nitrate was morerapidly reduced in the younger parts of the root and,therefore, less of it was available to be translocatedfrom that region. It is probable that phosphate andsulfate are also bound in organic compounds more

rapidly near the root tip, and that for this reasonsmaller quantities of these salts are available fortranslocation from the tip. Our experiments withisotopes unfortunately, do not distinguish between or-ganic and inorganic combinations of the radioactiveisotopes.

The incorporation of sulfates and phosphates inorganic compounds might partially explain the failureof these salts to move out of the root apex. Otherexplanations, however, must be advanced for the re-tention of iodide and rubidium ions in the apex be-cause thev are believed to remain in ionic form inthe cells. In this connection it was noticed that usu-ally the latter two isotopes did not move completelvto the apex in the absorption cell studies (see fig 3).In contrast, P32 and S35 invariably became more con-centrated in the apex of the root than in immediatelyadjacent regions (figs 3c, 4c) demonstrating that somesubstances can move down to the apex. Since theapical region does not readily absorb iodide and ru-bidium ions from the rest of the root, it is unlikelythat it would prevent upward movement of theseions.

A more probable explanation for the very slighttranslocation from the apex is based on the observa-tion that the xylem is not matured completely to theroot tip. Heimsch (10) has reported that in un-branched, growing roots of Sacramento variety ofbarley, the same variety used in these experiments,the protoxylem matures 0.5 to 1.0 cm from the apexand the metaxylem matures more than 0.8 to 2.0 cmfrom the apex. Since the roots used in these experi-ments were young and growing rapidly, it is probablethat the xylem was usually not mature within 0.5 to1.0 cm of the apex. In older or more slowly growingroots, the xylem is differentiated nearer the apex (10,7). Below the region of mature vessels upwardtranslocation would appear to be by relatively slowdiffusion.

This explanation assumes that upward transloca-tion of mineral salts is mainly in the xylem, an as-sumption for which there is considerable experi-mental evidence (2, 23) and which is supported bythe steam girdling experiments reported earlier in thispaper. Although direct experimental evidence islacking, the absence of mature xylem vessels in theapical region could account adequately for the in-significant translocation from this region.

Such an explanation of the limited translocationof minerals from the meristematic region agrees withwhat is known about water intake. Most investi-gators agree that little water is absorbed through theroot tips and maximum water intake occurs throughthe region of the root where the xylem is maturing(8, 9, 21), which also is the region from which maxi-mum translocation of the various ions occurred in ourexperiments. As the upward translocation of ionsappears to occur in the xylem it is not surprising thatthere is a close correlation between the regions ofmaximum water and mineral absorption. Hylmo(11) has recently presented convincing evidence ofincreased absorption of CaCl2 by pea plants accom-panying increased absorption of water.

347


PLANT PHYSIOLOGY

In conclusion it appears that although the meri-stematic regions of roots accumulate large amounts ofminerals, relatively little is translocated from this re-gion to other parts of the plant. The greatestamount of translocation of minerals to the shoot ap-parently occurs from a region several cm behind theroot tip where the xylem is fully matured.

SUMMARYThe translocation of radioactive isotopes from

various regions of the root was studied in attachedroots of barley seedlings. The isotopes used werep32 S35, Ca45, Rb86, 1I31, and SrO. Absorption cellswere developed by which the isotope solution couldbe supplied to any 3 mm region of a root while theremainder of that root and the other roots of theseedling were kept in nonradioactive nutrient solu-tion. The absorption cells were attached at the tipabout 10 mm, 20 to 30 mm, and 55 to 70 mm behindthe tip.

Although the tips absorbed the various ions freely,very little upward translocation of any of them oc-curred from the terminal 5 mm region of the roots.Greatest translocation occurred from the region 30mm back of the root tip. Translocation from theregion more than 50 mm above the tip ivas somewhatlower, but much greater than from the apical region.Upward translocation of Ca and Sr was very limited.

All isotopes moved downward in the treated roots.P3' and S35 moved down to the tips of the roots andaccumulated there. Rb86, Ca45, Sr90, and I131 moveddown also, but stopped several mm short of the roottips. N

Killing a narrow segment of root tissue with steamreduced movement of p32 both upward and down-wvard, whether the steamed segment was above or be-low the point where phosphorus was supplied. Kill-ing tissue in one part of the root seems to affecttranslocation elsewhere in the root as well as throughthe dead tissue.

The results of these experiments suggest that mosttranslocation of minerals to the shoots occurs from aregion of the roots several cm behind the root tips.This seems to coincide approximately with the regionof maximum water absorption.

Part of the work was done while the senior authorwas at North Carolina State College of Agricultureand Engineering. Grateful acknowledgement is madeof the cooperation of Drs. D. B. Anderson, H. T.Scofield, and N. S. Hale of that institution in provid-ing space and equipment, and for their helpful sug-gestions. MIr. Charles Averre assisted in the labora-tory work.

LITERATURE CITED1. ARNOLD, A. Uber den Functionsmechanismus der

Endodermiszellen der Wurzeln. Protoplasma 41:189-211. 1952.

2. BIDDULPH, 0. The translocation of minerals inplants. In: Mineral Nutrition of Plants, E. Truog,

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