WATER-UPTAKE BY ROOTS OF MAIZE AND SUNFLOWER AFFECTS THE RADIAL TRANSPORT OF ABSCISIC-ACID AND ITS CONCENTRATION IN THE XYLEM

The radial movement of cis-abscisic acid (ABA) has been investigated i n young excised roots of Zea mays L. and Helianthus annuus I,. which w ere 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 eve n cross the endodermis. Depending on the ABA concentration of the medi um (range: 5-500 nM) and in the root apoplast, the solvent-drag compon ent of the flow of ABA counteracted the dilution of ABA in the xylem c aused by transpirational water how. Acidification of the rhizosphere a nd of the root apoplast increased the apoplastic transport component. In sunflower, the apoplastic flow of ABA was significantly weaker than in maize roots. This was also indicated by the larger apparent reflec tion coefficient (sigma(ABA)) of sunflower roots for ABA (sunflower: s igma(ABA) = 0.97 +/- 0.02, n = 6 roots; maize: sigma(ABA) = 0.68 +/- 0 .06, n = 6 roots; +/-SD). For both species, sigma(ABA) was smaller tha n unity. Root reflection coefficients were affected 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 th e permeation barrier in the root, the reflection coefficient estimated from solvent drag is also complex. Since unstirred layers affected th e absolute value of the reflection coefficient, sigma(ABA) has been te rmed 'apparent'. It is concluded that the pH and ABA concentration of the soil solution as well as the transpiration rate (suction force) mo dify the intensity of the root-to-shoot signal which is influenced by an apoplastic bypass flow of ABA. The latter may be substantially affe cted by the existence of Casparian bands in the exodermis, which were lacking in the roots studied in this paper.