Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem |
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Authors: | Elenor Freundl Ernst Steudle Wolfram Hartung |
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Institution: | Julius von Sachs Institut für Biowissenschaften der Universit?t Würzburg, Lehrstuhl Botanik I, Julius von Sachs Platz 2, D-97082 Würzburg, Germany, DE Lehrstuhl für Pflanzen?kologie der Universit?t Bayreuth, Universit?tsstra?e 30, D-95440 Bayreuth, Germany, DE
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Abstract: | 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 flow
of ABA counteracted the dilution of ABA in the xylem caused by transpirational water flow. Acidification of the rhizosphere
and 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 reflection coefficient (σABA) of sunflower roots for ABA (sunflower: σABA = 0.97 ± 0.02, n = 6 roots; maize: σABA = 0.68 ± 0.06, n = 6 roots; ±SD). For both species, σABA was smaller than 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 the permeation barrier
in the root, the reflection coefficient estimated from solvent drag is also complex. Since unstirred layers affected the absolute
value of the reflection coefficient, σABA 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 influenced by an apoplastic bypass flow of
ABA. The latter may be substantially affected by the existence of Casparian bands in the exodermis, which were lacking in
the roots studied in this paper.
Received: 25 February 1998 / Accepted: 16 July 1998 |
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Keywords: | :Cis-abscisic acid Helianthus Hydraulic conductivity Reflection coefficient Root Water transport Zea |
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