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

Abstract

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...

Penetration of Soybean Root Systems by Abscisic Acid Isomers

“…The observed decrease in exudation rate due to ABA treatment is consistent with previously published reports and related to a decrease in the hydraulic conductance due to the ABA treatment (5,13 The appearance of ABA in the root exudate demonstrates that considerable quantities of the hormone will be carried by the transpiration stream to the shoot if ABA is added to the irrigation water. 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.…”

“…Exogenously applied as well as endogenously produced ABA hardened plants to exposure to low temperatures (18,19). ABA is known to cause stomatal closure (16,20) and has been reported to alter root permeability to water (5,(7)(8)(9)13). In experiments where ABA is added to the irrigation water (18), it is often unclear whether the hormone acts on the root or is transported to the shoot and directly effects the stomata.…”

Diurnal Changes in Volume and Solute Transport Coefficients of Phaseolus Roots

“…1 B). Newman (20) (5,6,10). As a mathematical convenience we will now use the term 7r* to indicate the difference between the actual 7r0 and Wre (Fig.…”

“…Diurnal fluctuations in root conductance (alternatively resistance) to water flow have also been demonstrated (2, 21, 23). However, we have shown (4)(5)(6)9) that changes in solute transport can manifest themselves as conductance changes when, in fact, the root hydraulic conductance coefficient may not change at all. In most of the above studies where conditions were adequately controlled, it was possible to demonstrate that the periodicity was endogenous but could be controlled, altered, or made to disappear by proper manipulation of the day-night cycle.…”

Effects of synthetic plant growth retardants and abscisic acid on root functions of Brassica rapa plants exposed to low root‐zone temperature

“…Thus, ABA has been implicated in plant response to environmental stress, and therefore to low temperature stress (Chandler & Robertson, 1994). As with direct effects of temperature, ABA responses have also been ascribed to root hydraulic conductivity (Collins & Morgan, 1980 ;Glinka, 1980 ;Fiscus, 1981 ;Bassirirad & Radin, 1992), ion flow (Karmorker & van Steveninck, 1978 ;Pitman & Wellfare, 1978), or both. Although various species have been used to assess the effect of root-applied ABA on water and ion fluxes, the results have often been contradictory (Andersen & Proebsting, 1984), and little research has been specially devoted to nitrate uptake and transfer.…”