Rapid Effects of Abscisic Acid on Ion Uptake in Sunflower Roots

The response of solute and volume fluxes of Phaseolas root systems to applied AbA was observed under conditions of applied pressure which were used to enhance the volume flow. The growth regulator elicited three separate responses: a transient release of solutes to the xylem which was responsible for an initial increase in volume flux; a long term increase in the total ion flux; and a long term decrease in the hydraulic conductance of the root systems. The exact response was highly dependent on the magnitude of the pre-AbA volume flux density, the relative contributions of osmotic and pressure-induced flow, and the applied dosage.Calculations suggested that the volume flux in naturafly exuding root systems is relatively insensitive to changes in the conductance.Reports of the effects of AbA applications on the root systems of plants have been varied and, at times, contradictory. One of the most important responses reported is the effect of this hormone on the hydraulic conductance of the root systems. Because it is apparent that AbA production is enhanced by water stress (1,2,13,17,(19)(20)(21)(22), any effect that AbA might have on the system's conductance is of great potential importance to the water balance of the plant. Unfortunately, AbA also affects the ion transport processes of the roots. Since much of the past experimentation has dealt with excised roots or root systems with little or no hydraulic pressure difference applied, and since the volume flux in these systems has been dominated by ion transport, the effects of AbA on the hydraulic conductance have been difficult to determine.Volume and/or ion fluxes through plant roots have been reported to decrease after AbA treatment (5, 16), remain the same (6), or increase (3,4,6,14,18). Karmoker and Van Steveninck (14) and Pitman and Wellfare (16) attributed the observed changes in volume flux entirely to changes in ion transport rates. However, Glinka and Reinhold (10) and Glinka (11, 12) have concluded that the major change in volume flux is due to a direct effect on the hydraulic conductance of the system.Recently, Markhart et al. (15), using a pressure chamber technique which measures conductance while minimizing osmotic effects, have shown that the effect of AbA on soybean root systems is to reduce the hydraulic conductance about 4 h after the application of AbA. However, because of the nature of the procedures they were unable to determine the short term effects of AbA on volume and ion fluxes. If the initial response is due to a significant increase in conductance, then the response should become more pronounced as the hydrostatic pressure difference, and, therefore, the volume flux is increased. This is true because at higher fluxes the flow rate is much more dependent on the conductance and shows a very low dependence on the ion fluxes.Here, we present additional evidence to support the hypothesis that AbA causes a long term decrease in hydraulic conductance as well as a short term release of ions to the xylem and a long term increase in t...

Section: Resultssupporting

confidence: 64%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Effects of Abscisic Acid on the Hydraulic Conductance of and the Total Ion Transport through Phaseolus Root Systems

Fiscus

1981

“…Most recently, ABA was reported to have no effect on osmotically driven water flow in excised sunflower roots (4). Pitman and Wellfare (18) found dramatic decreases in barley root exudation rates treated with ABA, but concluded that this was due to an inhibition of active salt transport and not hydraulic conductivity.…”

mentioning

confidence: 99%

Effect of Abscisic Acid on Root Hydraulic Conductivity

Markhart¹,

Fiscus²,

Naylor³

et al. 1979

Reports of the effects of abscisic acid (ABA) on ion and water fluxes have been contradictory. Some of the confusion seems due to the interaction of ion and water transport across membranes. In these experiments root systems were subjected to hydrostatic pressures up to 5.0 bars to enable measurement of root conductance that was independent of measurement of osmotic potentials or ion fluxes.ABA between 5 x 10-I molar and 2 x 10' molar resulted in a decrease in the conductance of the soybean root systems as compared with the controls. ABA treatment also eliminated the discontinuity in the Arrhenius plot of total flow versus reciprocal temperature at constant pressure. The results suggest that ABA acts at the membrane that is rate-limiting to water flow directly, or by altering metabolism that in turn affects the membrane.The dramatic increase in ABA content in stressed plants (20,23) (12) found that ABA stimulated ion uptake in Phaseolus roots, whereas Erlandsson and co-workers (4) observed a decrease in "Rb uptake in sunflower roots.Cram and Pitman (3) reported that ABA did not change the hydraulic conductivity of maize or barley roots, but Glinka and Reinhold (9, 10) reported that ABA increased both the diffusional permeability and hydraulic conductivity of roots to water flow under a pressure of 0.8 bar. Most recently, ABA was reported to have no effect on osmotically driven water flow in excised sunflower roots (4). Pitman and Wellfare (18) found dramatic decreases in barley root exudation rates treated with ABA, but concluded that this was due to an inhibition of active salt transport and not hydraulic conductivity. ' Research supported by National Science Foundation Grants PCM76-11 142-AO1-2 to Dr. P. J. Kramer and DEB77-15845 These contradictions could be due to the difficulty in evaluating a complex transport system as a root. The standing osmotic gradient (I) and the removal of ions from the ascending xylem water (13) make determination of the osmotic driving force for water flow difficult. At low hydrostatic pressures the interaction between osmotic and hydrostatic driving forces makes it impossible to assess whether a change in flow rate is due to a change in hydraulic conductivity or ion movement (6). The analytical technique of Fiscus (6) provides a method of measuring the hydraulic conductance (L)5 of a root system independent of ion transport.The experiments reported here use applied hydrostatic pressures to determine the effect of ABA on water flux through root systems, to determine if changes in flux are due to changes in L and to determine the nature of the changes ABA induces in the ratelimiting barrier to water flux. MATERIALS AND METHODSCulture conditions and basic experimental procedures have been previously described (15). Soybean plants were grown in a half-strength Hoagland nutrient solution under a 14-h photoperiod and a thermoperiod of 28/23 C. Root systems of decapitated plants were sealed in a 10.3-liter pressure chamber filled with fresh nutrient solution. Hydrostatic pres...

“…Promotive (2,5,6,8,14) as well as inhibitory (3,4,12) effects, however, have been reported with the use of ABA, depending on its concentration, the plant species, the growing conditions, and the temperature (1 1). It was recently shown that in some cases the change in volume flow (increase or decrease) results entirely from corresponding changes in the ion transport (8,13).…”

mentioning

confidence: 99%

Abscisic Acid Promotes Both Volume Flow and Ion Release to the Xylem in Sunflower Roots

Glinka¹

1980

102

The effect of abscisic acid (ABA) on the exudation process in excised sunflower root was investigated. A promotion of both ion flux and volume exudation rate was observed. Cutting off the external supply of ions to the roots markedly increased the magnitude of the ABA effect. The promotive effect of ABA on exudation rate was extremely rapid (less than 6 minutes) and seemed to be biphasic. Reasons are given for relating the ABA effect to two separate actions, namely: (a) on water permeability, and (b) on the release of ions both from vacuoles to the cytoplasm and from symplasm to the xylem. Kinetin inhibited movement of ions to the xylem and its effect did not appear to be related to that of the ABA.ABA is known to affect both volume exudation and ion movement to the xylem in excised roots, phenomena believed to have a bearing, respectively, on water permeability and on ion transport, either from the medium to the symplasm or from the symplasm to the xylem (10).Promotive (2,5,6,8,14) as well as inhibitory (3, 4, 12) effects, however, have been reported with the use of ABA, depending on its concentration, the plant species, the growing conditions, and the temperature (1 1). It was recently shown that in some cases the change in volume flow (increase or decrease) results entirely from corresponding changes in the ion transport (8,13).In a previous paper (5), I showed that in sunflower root systems, the hydraulic conductivity was increased markedly by ABA. The present paper investigates the effect of this hormone on the release of ions to the xylem in sunflower roots and its relation to the increased water flow. To distinguish between the effect of ABA on the release of ions to the xylem and its possible effect on transport into the root cells, the external supply of ions to the roots was cut off. Apart from increasing water permeability, ABA also promotes ion movement by acting on ion release both from vacuoles to cytoplasm as well as from symplasm to the xylem. MATERIALS AND METHODSFour-week-old sunflower plants (Helianthus annuus L.) grown in a growth chamber were used throughout. Seeds were germinated in Vermiculite and the 1-week-old seedlings transferred individually to 1-liter plastic jars containing half-strength Hoagland solution. The nutrient solution was renewed every 4th day and on the day before each experiment. The temperature in the chamber was 25 C day/18 C night. The photoperiod was 14 h (0500-1900) and the light intensity was 280,iE m-2s-1. Because of the circadian fluctuations in the rate of exudation (15), the experiments were started routinely at 7 AM. Throughout the experiments the jars were kept at 25 C and the solutions aerated continuously. The plants were decapitated 1 cm above the transition zone and pieces of tightly fitting rubber tubing were affixed to the cut stumps.At selected time intervals the exuding sap was collected by means ofa syringe and subjected to the appropriate measurements. For rapid determination of volume flow, a 0.2-ml pipette was connected to the rubber tube...