Plant Physiol. (1987) 83, 33-380032-0889/87/83/0033/06/$0 1.00/0

Effect on Root Growth of Endogenous and Applied IAA andABAA CRITICAL REEXAMINATION

Received for publication April 23, 1986 and in revised form August 14, 1986

PAUL-EMILE PILET* AND MARTIAL SAUGYInstitute ofPlant Biology and Physiology ofthe University ofLausanne, 1015 Lausanne, Switzerland

ABSTRACT

Applications of indole-3yl-acetic acid (IAA) and abscisic acid (ABA)were done on two-day-old intact maize (cv LG 11) roots. The effect ofthe treatment on the root growth depends on their initial elongation rate.The slow growing roots were all inhibited by exogenous IAA and ABAat any concentrations used whereas for the fast growing roots theirelongation was promoted by these two hormones at low concentrations.Quantitative analyses of endogenous IAA and ABA were performed usingthe gas chromatography-mass spectrometry technique. Detection andquantification of endogenous IAA and ABA were done on the zone of theroot implicated in elongation. These techniques were achieved by electronimpact on the IAA-Me-heptafluorobutyryl derivative and by negative ionchemical ionization with NH3 on the ABA-Me ester derivative. A negativecorrelation between the growth and the endogenous content of these twohormones was obtained. ABA presented a larger range of endogenouslevel than IAA on the whole population of roots tested. When usingapplied IAA and ABA at different concentrations the same differentiatingeffect on the growth was observed. This allowed us to conclude that foridentical concentrations, IAA has a more powerful effect on root elonga-tion than ABA. Present results are discussed in relation to previous datarelated to the role of IAA and ABA in the growth and gravireaction ofmaize roots.

Root elongation is controlled by several endogenous regulators(2, 3, 24) whose interaction appears essential (19). Many pub-lished results indicate that, at least in the elongating part of theroot, some growth inhibitors produced in the cap move from thetip to the base (8, 15, 16). The preferential movement ofABA isbasipetal (6, 10, 17) while that of IAA is acropetal (2, 9, 14).Exogenous IAA usually inhibits root growth (19, 22, 30) as doesapplied ABA (18, 20). But in some cases IAA (13, 22) and ABA(12, 20) could promote root elongation. The use of GC-MStechnique has made it possible to observe unequivocally thatIAA (5, 7, 26) and ABA (25) are present in maize roots. Thistechnique has revealed that there is a significant relation betweenthe endogenous level of IAA, root elongation (22) and gravitro-pism (4, 29). The same technique has also indicated similarrelations with ABA content in roots (21, 28).The aim of the present work was to analyze the level of

endogenous IAA and ABA in maize roots to understand bettersome controversial data obtained on the growth effect of thesehormones when applied. It was necessary to correlate for identicalmaterial which means growth in the same conditions, informa-tion about the endogenous hormone content and the concentra-tion effect of applied hormone on root growth. The reason that

we insisted on identical material was that, unfortunately, manyreports have used different material for testing both elongationand for analyzing hormone content. Furthermore, in the rootswhich we have used, it is the zone 2.5 to 5.00 mm behind thetip which is most relevant to total axial growth. It is in this zonethat the hormone status of the root has been analyzed.

MATERIALS AND METHODS

Preparation of the Material. Plant Material. Selected caryopsesof Zea mays (cv LG. 11, Assoc. suisse des Selectionneurs, Lau-sanne, Switzerland) were germinated in the dark (21 ± 1°C) aspreviously described (17). Intact seedlings with rectilinear pri-mary roots were used.Immersion Experiments. Seedlings with primary roots 10.0 +

0.5 mm in length were first placed in a vertical position on plasticframes (17), in temperature-controlled boxes (dark, 22 + 1°C) inwhich humid atmosphere (90 ± 5%) was maintained. After 4 h,the elongation that had occurred in that period was measuredand two groups of roots were selected: roots having a low (about0.37 mm-h-') or a high (about 0.81 mm-h-') growth rate. Theseroots (still attached to their caryopsis) were immersed verticallyfor 30 min or 1 h in a buffered solution (3,3-dimethylglutaricacid at 1mM: pH 6.0 ± 0.1) containing IAA or ABA. Afterwashing with this buffer, roots were maintained vertically inhumid air and darkness as before. For all the assays, seedlingswere mounted on the same plastic frame. After 6 h, their growthrate was again measured. Each mean value is given with its SE;significant differences between means were assessed by the t test.Materialfor the Hormone Analysis. Intact seedlings with rec-

tilinear primary roots (15 ± 0.5 mm) were placed vertically for8 h on special frames in the dark as done previously (22). Growthmeasurements were made using photocopied films and a digitiz-ing pad (Hi Pad, Houston Instruments Austin, TX) interfacedwith a microcomputer (ABC 80, Luxor, Motala, Sweden) (23).After photographing at both zero time and 8 h later, the elonga-tion zone of each root, located 2.5 to 5.0 mm from the tip (32),was excised and stored (-80°C). The segments of the selectedgrowth classes mentioned above were pooled.IAA and ABA Determination. Extraction of IAA and ABA.

The method of extraction of IAA has been previously described(1, 20, 26) and that ofABA also (21, 28) (Fig. 1).GC-MS Quantifcation. The GC-MS system consisted of a

Hewlett-Packard 5985 A GC-MS (Hewlett Packard) equippedwith the NCI' option. The same system was used and describedin previous papers (22, 28). Splitless injections of 3 M1 were madeonto a SE 54 WCOT fused silica capillary column (25 m long,

'Abbreviations: NCI, negative ions chemical ionization; HFB, hepta-fluorobutyryl; HFBI, heptafluorobutyryl-imidazole.

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Plant Physiol. Vol. 83, 1987

G IEXTRACTION|PLANT MATERIAL: FREEZE DRIED"20ng

+5mr BUFFER (pH :4.0)0--/ +INTERNAL STANDARD

GRINDING

PARTITIONI +

CENTRIFUGATION

AGUEOUS PHASE DISCARDED

20 ng D6 -ABA

4 ml CHLOROFORM

ORGANIC PHASE EVAPORATEDf TO DRYNESS f

+ 0.1 ml MeOH + 01ml MeOHI .~WI METHYDLATZOM N E -

r----- WITH DIAZOMETHANE----jABA-Me ESTER

I

IGC-MS QUANTIFICATIONI(Fg. 3)

FIG. 2. Reconstructed SIM traces of IAA; 3 ju injection of an extractof 50 elongation zones (2.5-5 mm from the root tip). Peaks 1 and 2 arethe traces of the ,B-cleavage fragment ion of the nonlabeled and labeledcompounds, respectively; peaks 3 and 4 represent the traces of the M+ion of the same products. MS conditions being: ionization potential 79eV, ion source 200C. GC conditions were: injector 250C, GC oven50C programmed at 25°C/min to 220C, He approximately 1.5 ml/min.

®ISECOND DERIVATIZATIONIHFBI [2h 85 C]IAA-Me-HFB DERIVATIVE

I

(IGC-MS QUANTIFICATION (Fig.2)FIG. 1. Summary ofthe essential steps for isolation and quantification

of IAA and ABA.

0.3 mm i.d.). The detection of the IAA-Me-HFB derivative wasdone with normal EI option. The detection of the ABA-Mesamples were performed with NCI option using ammonia asreactant -gas. Quantification was undertaken using the stableisotope internal standard dilution method (1) and the selectedion monitoring program of the GC-MS.The GC-MS Conditionsfor IAA-Me-HFB Detection. GC con-

ditions were: injector 250C, GC oven 50C programmed at 25Cmin-' to 220C, He approximately 1.5 ml min-'. MS conditionswere: ionization potential 70 eV, ion source 200C. For IAAmeasurements, the values of m/z of the base peak and themolecular ion, respectively (326 and 385 for IAA-methyl-HFB;331 and 390 for 2H5-methyl HFB) were monitored with a circleperiod of 80 ms (Fig. 2). Using these two peaks for the quantifi-cation of endogenous IAA, the estimations are precise. It mustbe noted that the choice of this HFB-derivative gives a veryspecific detection, better than the IAA methylester derivative.The GC-MS ConditionsforABA-Me Detection. GC conditions

were: injector 250C GC oven 100C for 1 min then programmedat 25C min-' to 240C, He approximately 1.5 ml min-'. MSconditions were: ionization potential 150 eV with 1.5 mampsemission, and source pressure: 10-4 Torr. In the ion sourcevolume, the pressure of ammonia was approximately 0.5 Torrin order to obtain the highest signal. For ABA analysis, the valuesof the molecular ion and the first fragmented ion, respectively(278 and 260 for ABA-Methyl, 284 and 266 for hexadeuteratedABA-methyl) were monitored with a circle period of 80 ms (Fig.3).

Calibration Curves. From 0 to 20 ng of either IAA or ABAusing 30 ng of each deuterated internal standard are fitted withthe equation of a straight line with coefficient of regression betterthan 0.995 (Fig. 4). The content of both phytohormones in the

FIG. 3. Reconstructed SIM traces of extracts prepared from 30 apicalsegments (0-2.5 mm) from maize roots whose mean growth rate was

0.359 mm/h; 20 ng hexadeuterated ABA was added at the beginning ofthe analytical procedure. Peak I endogenous and peak 2 added deuteratedABA, as their Me-ester derivative. Peaks 3 and 4 are molecular ion peaksfor, respectively, endogenous (3) and deuterated (4) ABA as their Me-ester derivative. Peak 5 (for each channel): endogenous (260 or 276) anddeuterated (266 or 284) trans-ABA-Me-ester derivative. MS conditionsbeing: ionization potential 70 eV; source temperature 200C, source

pressure 10' Torr; pressure of ammonia in the source 0.5 Torr. GCconditions were: injector 250°C; GC oven 100°C for 1 min then pro-grammed at 25°C min-' to 240°C; He approximately 1.5 ml-min-'.

extracts has been calculated from their respective calibrationcurves. The limit detection after extraction and purification liesaround 10 pg injected for IAA-Me-HFB and 1 pg for ABA-Mewith a signal to noise ratio better than 3 when injecting thesequantities (27, 28). The variation of each determination wasfound to be less than 10% within the range of concentrationused. The final error of 10 to 15% observed on the results reflectsmore the heterogeneity of the biological material than the repro-ducibility ofthe assay. The results for endogenous IAA and ABAquantification presented here, are based on the measurement ofthe most abundant ion (326-331 for IAA; 278-284 for ABA).The other masses were used for confirmation of identity andamounts which were not significantly different.

+ 20ng D5 -IAA

+4ml EtOAc

IAA -MeESTER

Et OAc I HEXANE 2:1

PILET AND SAUGY34

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MAIZE ROOT GROWTH: IAA AND ABA

0.6

0

4c

0.4

LAA(326/3133

**'ABA(2781284)

02

I I I I

0 5 10 5 20CONCENTRATION of [AA or ABA IN ng

FIG. 4. Calibration curve for the quantification of IAA and ABA.The ratio of m/z 326 to m/z 331 (for IAA-Me-HFB) and m/z 278 to m/z 284 (for ABA-Me) were used to compute the endogenous levels ofIAAand ABA, respectively.

RESULTS AND DISCUSSION

Many experiments have already investigated the relation be-tween different concentrations of applied hormones and growth(for reviews, see Refs. 3 and 18 for ABA; 19, 30, 31 for IAA).The inhibitory action of these two hormones has been generallydemonstrated (6, 30). One of the most common techniques usedto apply hormones to intact roots (attached to their caryopsis) isby immersing them in solution.Growth Effect of Applied IAA and ABA. The reactivity of

growing roots to applied hormones depends on several factorsthat are not often discussed (19). For instance, the initial growthrate was found to be an essential parameter upon which theeffect of applied IAA depends strongly (22). This parameter hasbeen analyzed for both IAA and ABA treatments. Roots of 10.0

0.5 mm length were first kept (humid air, dark) in a verticalposition for 4 h. Data about their growth rates are given in TableI. The elongation of the slow growing roots is inhibited by bothIAA and ABA, the effect being more pronounced the greater theconcentration of applied hormones. In contrast, the fast growingroots were stimulated by IAA and ABA when applied at a lowconcentration (5.10-9 M), while at a higher concentration (1.10-6M) both hormones inhibited the growth rate. This may indicate,as discussed later, that the endogenous content of both IAA andABA is not the same for slow and fast growing roots. It must benoticed that the mean elongation rate after 1 h immersion inbuffer (no IAA, no ABA) was promoted for the slow growingroots. Such growth promotion was reported in previous work(20) and could be explained at least by the release of a largeamount of hormones in the buffer. This release must be higher

for slow growing roots than for fast growing ones which are notstimulated by such treatment.

Clear though these results are, their interpretation is not easy.Obviously other factors that affect growth need to be known.For example, the possible variation in uptake of IAA and ABAalong the root axis is not known. The possibility is not excludedthat the absorption of applied hormones may change for thedifferent zones of immersed roots. Moreover, the endogenousIAA and ABA is released into the buffer by immersion (20) withmore ABA than IAA being lost. It is also certain that ifIAA andABA are released from the root then other important substancesmust also be released and the whole osmotic equilibrium of theroot could be affected. It is also clear that the hormone taken upwill be metabolized and redistributed during the 6 h period.Nevertheless, this pulse method of giving hormones offered theadvantage not to change the hydric and the endogenous hor-mones equilibrium as much as a treatment which continued forthe entire growth period.Endogenous Level of Hormones in Root Elongation. To see if

the mechanism of growth control by applied hormones reallyreflects the endogenous regulation of root elongation, a largepopulation of roots was used. Such a sample includes individualswith a range of different growth rates (Fig. 5). These data wereobtained for roots whose ABA content was then determined.Similar values have been found previously in roots whose IAAlevel was analyzed (22).On the basis of these results, the population was divided into

six (IAA) or nine (ABA) growth rate classes in order to quantifythe endogenous IAA and ABA in the elongation zone (2.5-5.00mm from the tip) (Fig. 6). A clear relation between the endoge-nous level of these hormones and the elongation rate can beseen. It can also be observed that both for IAA and ABA, thehigher was the level, the lower was the growth rate. In theelongation zone of the fast growing roots, the endogenous levelwas almost similar for the two hormones (expressed in ng per 30segments, corresponding to one sample). In the slow growingroots, more IAA was found in the elongation zone, and the ABAlevel was dramatically increased compared to the fast growingroots. The shape of the curve (endogenous ABA) indicates thata small difference in the growth of the slowly growing rootscorresponds to a large change in the ABA level, but on the otherhand rapidly growing roots all contain relatively small amountsof ABA. This attests to the variability of the endogenous ABAlevel in maize roots grown under similar conditions, and to thedifference between IAA and ABA in their relation with growth.Root Growth and Endogenous IAA and ABA Content. The

next experiment was done to compare the effect of applied

Table I. Absolute (GR in mm ± sE/h) and Relative (%o) Growth Rate ofIntact Maize Roots Attached to TheirCaryopsis and Maintained Vertically in Humid Air (Dark, 22 ± 1°C)

Roots (initial length: 10.0 ± 0.5 mm) were kept 4 h and then selected according to their mean elongationrate (IGRa). Slow and fast growing roots were immersed 1 h in a buffer solution (3,3-DGA; pH 6.0 ± 0.1) withIAA or ABA, and after washing, kept a further 6 h for the growth rate measurement (FGRb). Experimentswere repeated, for each concentration tested, two to four times. Total number of roots tested in parentheses.

Two Groups of Selected RootsTreatment

(1 h immersion) IGR: 0.37 ± 0.05 IGR: 0.81 ± 0.09(278) (341)FGR %C FGR %C

No IAA, no ABA 0.53 ± 0.06 (63) 0 0.89 ± 0.11 (74) 0IAA (5.10-9 M) 0.41 ± 0.05 (54) -22.6 1.12 ± 0.08 (62) +25.8IAA (L.I0- M) 0.27 ± 0.03 (53) -49.1 0.67 ± 0.07 (66) -24.7ABA (5.10-9 M) 0.44 ± 0.07 (48) -17.0 1.10 ± 0.09 (64) +23.6ABA (.1.0 M) 0.32 ± 0.05 (41) -39.6 0.75 ± 0.08 (63) -15.7

a IGR, initial mean growth rate measured during the first 4 h. b FGR, final mean growth rate measuredduring the 6 h after 1 h immersion. c %, -, Inhibition; +, stimulation.

35

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~~~~~~~~~~~~~~~~~PlantPhysiol. Vol. 83, 1987

160

140

120

s0

60

40

20

In

ort-i II IIiI-I I I I I02 CA 08g 1I 1.2 1.4

OROWTH ARE IN PER h

FIG. 5. Changes in the elongation rate (in mm per h) of maize (cv

LG 1) vertical roots attached to their caryopsis. The number of individ-

ual roots (total oftested roots: 840) given per 14 classes. Roots maintained

in the dark (90 ± 5% humidity: 21 ± VQC

20 r

16

z

X 12

I,)

0 48

4

42

4

L IAA

.

0.2 0.4 0.6 I81. 1.2

GROWTH RATE HN mm PER h

FIG;. 6. Variation in the endogenous IAA and ABA level (in ng IAA

and ABA SE/30 segments) as a function of the elongation rate during

an 8 h period (in mm ± SE/h). The levels are give,n for the elongation

zone (2.5-5 mm) segments. For preparation of material, see "Materials

and Methods."

hormones at different concentrations and the effect of endoge-nous hormones on root growth. The distal ± mm of vertical

primary roots 12 ± mm was immersed in buffered hormone

solution. Following washing in the buffer, they were kept for 6 h

in the dark (19 ± 0.50C) (Fig. 7). It is clear that the effect of IAA

on the elongation of roots is stronger than that of ABA. TreatingwithI10-8 toIl0' m IAA caused root elongation to decrease from

about 7 mm in 6 h to 2.5 mm in 6 h. With the same concentration

of ABA, the decrease was only from 6 mm for 6 h (10-8 m) to 4

mm (10-' m). We tried to express the action of endogenous IAA

and ABA on the growth in the same way as in Figure 7 for

exogenous hormones. For that, we calculated the results of the

hormone content expressed on the fresh weight to a molar

concentration and the growth of the different groups tested (Fig.

7 ~~IAA ABA

a111. IIIzz0 34

C io41 t-16 1io4 C 16o1 16-6 yo-4CONCENTRATION IN M

FIG. 7. Elongation (mm ± SE) o'f intact primary roots pretreated 30±5 min in buffered solution (3,3-dimethyl glutaric acid at lO-1 m: pH

6.0 ± 0.1) containing IAA (A) or ABA (B) at different concentrations (inm). Roots of 12 ± 1 mm (A, B) in length (at zero time) were partly (thedistal 10 ± 1 mm) immersed in a vertical position. Following washing ina buffer, they were kept for 6 h in the dark (19.0 ± O.50C). Data are themean of five experiments, each with 40 ±5 roots.

1.21.1 0

1.0 * 0Y1.140-O.199X

0.9- ~~0 Y-aO.W6-0.07sX

aw0.7 "

E 0.6 \ AB

Z_0.5

~0.3 IAA"0 2

-7.0 -6.5 -6.0CONCENTRATION IN LOG M

FIG. 8. Growth (in mm/h) expressed as a function of endogenousconcentration of hormones (IAA and ABA). The endogenous concentra-tion (in m) is obtained by using the level of endogenous hormonesexpressed per kg of fresh weight (estimated to a liter of water). It is, infact, an approximate concentration for all the material without regardfor compartmentation.

5) was reported as a function of this "endogenous concentration"(Fig. 8). This concept ofendogenous concentration does not takeinto account the compartmentation of the hormone. For us, itis a theoretical concept allowing us to -make some comparisonwith the effects of applied hormones. We ignore here the heter-ogeneous distribution of the IAA in the elongation zone (29).The first very important remark t'o make in regard to Figure 8 isthe range of 'endogenous concentration' with which the rootsgrow. Endogenous ABA is 'working' between iO-' and 10-6 Mand IAA between IO-' and 4.3. lO0-'. This spectrum of concen-trations is quite different from that used with applied hormones.We could understand here that the treatments did not correspondat all to the action of the hormones provided by the plant. Thiscould indicate that the compartment of endogenous hormonesand that of applied hormones are surely not the same. If theywere similar, quite a large part of applied hormones could have

36 PILET AND SAUGY

I I I I I I I

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MAIZE ROOT GROWTH: IAA AND ABA

an effect on growth, and this is not the case. However, even ifthey are not working at the same amounts, the comparativeeffects of IAA and ABA seem to work in the same sense.Endogenous IAA is more powerful than ABA, the latter couldserve as a 'fine tuning' regulator of growth. Looking at theseresults, we tried to understand the promoting effect of lowconcentration of applied IAA or ABA on fast growing roots(Table I) and on the whole population (Fig. 7). It has beendemonstrated (22) that the growth of a part of fast growing rootpopulation is promoted by application of a high concentration(1 0- M) of IAA. In that case, the hypothesis of suboptimal levelof IAA for fast growing roots was made (22). In fact, it is difficultto decide what is the real mechanism ofstimulation ofthe growthwithout knowing exchanges of hormones (endogenous and ex-ogenous) occurring between the root and the immersion me-dium. We could imagine that all of the fast roots have a subop-timal content of IAA and ABA which allows a stimulation of thegrowth by application of a low concentration of these hormoneswhile only a part of these roots are stimulated by higher concen-trations. Furthermore, Figure 7 showed that the whole popula-tion exhibited a slight stimulation at 10-8 M. This stimulation iscertainly a mean of the effect on those roots which are enhancedin their growth by this concentration as well as including someslow growing roots which are inhibited (Table I) by such treat-ment.The promoting effect due to both IAA and ABA at 10`8 M

could be the result of the combination of two processes: therelease of endogenous hormones in buffer and the growth en-hancement caused by the uptake ofa small amount ofhormones.In light of these data, a new examination of the redistribution ofthe hormones in gravireacting maize roots has to be done:gravireaction is a consequence of a growth change and it hasbeen demonstrated that endogenous ABA (1 1, 21) and endoge-nous IAA (29) are implicated in the gravireaction of maize roots.The difference in the hormone level between the upper and

the lower parts of gravireacting roots cut into two halves wasevident for ABA. The ratio upper half/lower halfwas 44/56 (21);it was not the case for IAA (1 1, 29). The high IAA level in thestele reduces any difference between the two halves. By elimi-nating the stele and testing the IAA level within the upper andlower cortex, it was possible to demonstrate an asymmetricdistribution (22.2% of the total IAA in the upper cortex against24.8% in the lower cortex). This distribution is much smallerthan that of ABA and this could be explained by reference toFigure 6. The mean elongation of the whole population of rootsis 0.60 ± 0.05 mm/h. For any change in the elongation rate nearthis value, the change in the ABA level is much higher than thechange in the IAA level of the roots. This could explain why thisdistribution in IAA is hard to detect, although it exists.

CONCLUSIONSWe conclude by presenting some critical comments relative to

the present observations.It is necessary to correlate, for material grown in similar

conditions of culture, information about the endogenous hor-mone level. Working on an entire population of roots all grownin similar conditions, it is possible to correlate endogenoushormone content and root elongation. The variability of growthrate within a population is sufficient to test this correlation.The zone 2.5 to 5.00 mm from the tip is primarily concerned

in axial growth. Changes in this part of the root characterize thehormonal effects on the growth. Therefore, it is important toanalyze in this zone the implicated hormones to better under-stand their role and their implication in the regulation of elon-gation.Endogenous IAA and ABA play a crucial role in the growth

of maize roots. But we do not know, at the moment, how IAA

and ABA act in the regulation of growth. Some synergisticalinteractions between these two hormones cannot be excluded.They could be similar to that reported (17) for root curvaturecaused by applied ABA on the apical cut end and enhanced byIAA given on the basal cut section of maize root segments.We can compare the effect ofexogenous applied hormones on

roots and the role of endogenous ones. In the case of IAA andABA, they are working in the same sense. But this comparisonmust be manipulated carefully. The spectrum of concentrationis generally different, and with applied hormones, the mode ofapplication could be very important. Furthermore, we do notknow which compartment is touched by the applied hormonesand which part of them is metabolized, conjugated, or simplynot used in the process of growth.Nothing is really known about the correspondence between

the amounts found by GC-MS and the level of the hormonesnecessary for growth. An observation of the endogenous metab-olism flux and in general of the turnover of the hormones couldbring a better understanding of the problem.

It has been demonstrated here that endogenous IAA and ABAare certainly playing a role in the growth of roots reflected bysome observations on applied IAA and ABA on roots having agiven growth rate. IAA seems to be more powerful than ABAand endogenous hormones are working in a smaller concentra-tion spectrum than exogenous hormones.

Acknowledgments-We thank Dr L. Rivier and Dr G. Mayor for technicalassistance and helpful discussion and Dr P. W. Barlow for critically reading thepresent manuscript.

LITERATURE CITED

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2. AUDUS U 1975 Geotropism in roots. In JG Torrey, DT Clarkson, eds, TheDevelopment and Function of Roots. Academic Press, London, pp 327-363

3. AUDUS U 1983 Abscisic acid in root growth and geotropism. In FT Addicott,ed, Abscisic Acid. Praeger, New York, pp 421-477

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10. HARTUNG W 1976 The basipetal transport of [2-'4C]abscisic acid in roots ofintact seedlings of runner beans and its significance for root geotropism.Planta 128: 59-62

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14. PILET PE 1964 Auxin transport in roots. Nature 204: 560-56115. PILET PE 1971 Root cap and georeaction. Nature 233: 115-11616. PILET PE 1972 Root cap and root growth. Planta 106: 169-17117. PILET PE 1977 Growth inhibitors in growing and geostimulated maize roots.

In PE Pilet, ed, Plant Growth Regulation. Springer, Berlin, pp 115-12818. PILET PE, A CHANSON 1981 Effect of abscisic acid on maize root growth. A

critical examination. Plant Sci Lett 21: 99-10619. PILET PE, MC ELLIOTT 1981 Some aspects of the control of root growth and

georeaction: the involvement of indoleacetic acid and abscisic acid. PlantPhysiol 67: 1047-1050

20. PILET PE, JE REBEAUD 1983 Effect of abscisic acid on growth and indol-3-acetic acid levels in maize roots. Plant Sci lett 31: 117-122

21. PILET PE, L RIvIER 1981 Abscisic acid distribution in horizontal maize rootsegments. Planta 153: 453-458

22. PILEr PE, M SAUGY 1985 Effect of applied and endogenous IAA on maize

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38 PILET A}~root growth. Planta 164: 254-258

23. PILET PE, JM VERSEL, G MAYOR 1983 Growth distribution and pH patternalong maize roots. Planta 158: 398-402

24. REINHOLD L 1978 Phytohormones and the orientation of growth. In DSLetham, PB Goodwin. TJV Higgins eds, Phytohormones and Related Com-pounds, Vol 2. Elsevier. Amsterdam, pp 251-289

25. RIVIER L, H MILON, PE PILET 1977 Gas chromatography-mass spectrometricdeterminations ofabscisic acid levels in the caps and the apex ofmaize roots.Planta 134: 23-27

26. RIVIER L. PE PILET 1974 Indolyl-3-acetic acid in cap and apex of maize roots:identification and quantification by mass fragmentography. Planta 120: 107-112

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4D SAUGY PlantPhysiol. Vol. 83, 1987

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