Abscisic Acid Transport Coefficients of Phaseolus Root Systems

Abstract

Diffusive and convective transport coefficients of Phaseolus vulgaris L. cv. Ouray root systems for abscisic acid for (ABA) were measured. The convective coefficient (reflection coefficient or osmotic efficiency factor) a was determined to be 0.% for ABA while the diffusive coefficient, c, was found to be 1.44 x 10-11 mole per square centimeter per second per bar.Steady-state concentrations of ABA in the root system exudates were not achieved until at least three hours after the applications suggesting either a slow saturation of binding sites or equilibration with tissues surrounding the xylem.Much of the methodology for determining the mode of action of ABA in the whole plant or in organs depends upon a supply of exogenous ABA. ABA has been applied to plants or plant parts in a wide variety of ways. One especially useful mode of application, which allows the plant to remain intact, is adding the ABA to the root medium of hydroponically grown plants. A L13 1002 area meter. The chamber was brought to the specified pressure and temperture (25 ± 0.25°C) and equilibrated overnight. In the morning, the system was checked for steady-state conditions, arbitrarily specified as changes in the volume and solute fluxes of less than ± 2% h-'. The total volume flux was measured at regular intervals and was expressed as the total volume flux per unit leaf area Qp,,3 as a means of comparing plants of different sizes (2). Unpublished data from this laboratory show that the ratio of projected leaf area to root surface area is constant and approximately 1 so that Qpl will approximate the volume and solute flux densities through the roots. The electrical conductivity of the exudates was measured and the concentration expressed as equivalents of KCI cm 3. The samples were immediately frozen for future ABA analysis and for osmotic pressure determinations by freezing point depression. Having established steady conditions according to the above criteria, the ABA (Sigma ± cis-trans), dissolved in 95% ethanol, was added to the system through an injection port without decreasing the pressure. The quantity of ABA added was calculated to yield the same approximate dose (1.5 x l0-7 mol cm-2 leaf area) in each case.ABA Analysis. Analyses of ABA in the exudates and nutrient solutions were conducted using HPLC. A [L-Porasil column was used in a Waters Associates system with a model 6000A pump operating at 3,000 p.s.i. The flow rate was 2.0 ml min-' and a UV detector was used operating at 254 nm. The eluting solvent was prepared by shaking a mixture of 350 ml acetonitrile and 700 ml chloroform with 300 ml 0.17 N acetic acid. The organic layer was separated and run through a micropore filter prior to use. With these conditions and solvent, the ± cis-trans ABA had a retention time of 4.0 minutes.One-ml samples of nutrient solution or exudate were brought to about pH 2 by the addition of 4 drops of 1 M HCI. The solutions were then extracted 3 times with 1.5 ml ethyl acetate. The ethyl acetate layer was evaporated to dryness in vacuo at ...

Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem

“…In part, our ®nding of a substantial solvent drag of ABA is in line with the results of Fiscus (1982) and of Markhart (1982) who measured transport coecients of ABA in bean root systems. These authors pressurised excised root systems that were sitting in nutrient solution containing ABA at micromolar concentrations.…”

“…This should have caused large ABA concentrations in the root apoplast. Despite the high re¯ection coecient, the ABA¯ow density in the experiments of Fiscus (1982) and of Markhart (1982) was larger by 3 orders of magnitude than those measured in this paper. Therefore, Fiscus and Markhart could not completely exclude a substantial contribution of the diusional and/or active component of ABA¯ow.…”

“…Thus, there may be considerable dierences between species. The transport coecients reported by Fiscus (1982) and by Markhart (1982) were obtained in the presence of external ABA concentrations which were larger by 2±3 orders of magnitude than in the present study. They also refer to large volume¯ows.…”

“…However, from these data no conclusions about the transport path could be drawn. Fiscus (1982) and Markhart (1982) have used excised root systems of bean (Phaseolus vulgaris L.) to work out transport coecients of ABA [permeability coecients (P ABA S ) and re¯ection coecients (r ABA s )] from pressure-perfusion experiments. The P ABA S was rather high.…”

Penetration of Soybean Root Systems by Abscisic Acid Isomers

“…During the 6 h required for steady-state exudate ABA levels to be achieved, a volume of exudate 8 to 12 times the volume of the root system has been forced through the root system. This is similar to what Fiscus found for Phaseolus root systems (6). This suggests that either specific or nonspecific binding sites are gradually saturated or that metabolism of the hormone takes place preventing simple transfer through the root.…”