The radial movement of cis-abscisic acid (ABA) has been investigated in young excised roots of Zea mays L. and Helianthus annuus L. which were grown hydroponically. In addition to the symplastic path, ABA was largely translocated across the root apoplast by solvent drag with the water in the transpiration stream. On the apoplastic path ABA may even cross the endodermis. Depending on the ABA concentration of the medium (range: 5±500 nM) and in the root apoplast, the solvent-drag component of the¯ow of ABA counteracted the dilution of ABA in the xylem caused by transpirational water¯ow. Acidi®cation of the rhizosphere and of the root apoplast increased the apoplastic transport component. In sun¯ower, the apoplastic¯ow of ABA was signi®cantly weaker than in maize roots. This was also indicated by the larger apparent re¯ection coecient (r ABA ) of sun¯ower roots for ABA (sun¯ow-er: r ABA 0.97 0.02, n 6 roots; maize: r ABA 0.68 0.06, n 6 roots; SD). For both species, r ABA was smaller than unity. Root re¯ection coecients were aected by factors such as pH, ABA concentration of the medium, and by the suction force applied to excised root systems. Due to the complex composite structure of the permeation barrier in the root, the re¯ection coecient estimated from solvent drag is also complex. Since unstirred layers aected the absolute value of the re¯ection coecient, r ABA has been termed apparent'. It is concluded that the pH and ABA concentration of the soil solution as well as the transpiration rate (suction force) modify the intensity of the root-to-shoot signal which is in¯uenced by an apoplastic bypass¯ow of ABA. The latter may be substantially aected by the existence of Casparian bands in the exodermis, which were lacking in the roots studied in this paper. X J ABA aJ V J ABA radial¯ow of ABA across the root in mol m A2 s A1 ). Hence, an increase in transpiration will cause a dilution of ABA in the xylem.Abbreviations and symbols: ABA cis-abscisic acid; C ABA O ABA concentration in the medium; C ABA X ABA concentration in the xylem; J ABA ABA¯ow per unit root surface area; J V volume¯ow per unit root surface area; Lp r hydraulic conductivity of the root; P ABA S permeability of the root for ABA; PTS trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate; r ABA apparent re¯ection coecient of the root for ABA Correspondence to: W. Hartung;

The exodermal layers that are formed in maize roots during aeroponic culture were investigated with respect to the radial transport of cis-abscisic acid (ABA). The decrease in root hydraulic conductivity (Lp(r)) of aeroponically grown roots was stimulated 1.5-fold by ABA (500 nM), reaching Lp(r) values of roots lacking an exodermis. Similar to water, the radial flow of ABA through roots (J(ABA)) and ABA uptake into root tissue were reduced by a factor of about three as a result of the existence of an exodermis. Thus, due to the cooperation between water and solute transport the development of the ABA signal in the xylem was not affected. This resulted in unchanged reflection coeffcients for roots grown hydroponically and aeroponically. Despite the well-accepted barrier properties of exodermal layers, it is concluded that the endodermis was the more effective filter for ABA. Owing to concentration polarisation effects, ABA may accumulate in front of the endodermal layer, a process which, for both roots possessing and lacking an exodermis, would tend to increase solvent drag and hence ABA movement into the xylem sap at increased water flow (J(Vr)). This may account for the higher ABA concentrations found in the xylem at greater pressure difference.