Apoplastic transport of abscisic acid through roots of maize: effect of the exodermis

The exodermal layers that are formed in maize roots during aeroponic cultur e were investigated with respect to the radial transport of cis-abscisic ac id (ABA). The decrease in root hydraulic conductivity (Lp(r)) of aeroponica lly grown roots was stimulated 1.5-fold by ABA (500 nM), reaching Lp(r) val ues 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 f actor of about three as a result of the existence of an exodermis. Thus. du e to the cooperation between water and solute transport the development of the ABA signal in the xylem was not affected. This resulted in unchanged re flection coeffcients for roots grown hydroponically and aeroponically. Desp ite the well-accepted barrier properties of exodermal layers, it is conclud ed that the endodermis was the more effective filter for ABA, Owing to conc entration polarisation effects, ABA may accumulate ill front of the endoder mal laver, a process which. for both roots possessing and lacking an exoder mis, would tend to increase solvent drag and hence ABA movement into the xy lem sap at increased water How (J(Vr)). This may account for the higher ABA concentrations found in the xylem at greater pressure difference.