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

Abstract

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...

Overproduction of Abscisic Acid in Tomato Increases Transpiration Efficiency and Root Hydraulic Conductivity and Influences Leaf Expansion

“…Mean osmolarities of the hydroponic media in the two experiments were 54.1 and 52.0 mOsm. Lp r was calculated as (Glinka, 1980):…”

“…Exogenously applied ABA is known to increase root hydraulic conductivity (Lp r ) both at the organ and the cellular level (Glinka, 1980;Hose et al, 2000), and this is at least partly explained by an up-regulation of aquaporin genes (Zhu et al, 2005). In agreement with this, ABA-deficient mutants are known to have decreased Lp r (Tal and Nevo, 1973).…”

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

“…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.…”

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.…”

“…Our results obtained with a pretreatment of exogenous ABA prior to cooling of the root zone of B. rapa are clearly in agreement with most of the results previously obtained. Thus ABA was found to increase root hydraulic conductivity in certain species such as bean (Karmorker & van Steveninck, 1978 ;Pardossi et al, 1992) and maize (Collins & Morgan, 1980), and ion flows in the root of sunflower (Glinka, 1980) and pear tree (Andersen & Proebsting, 1984).…”