Effects of Abscisic Acid and of Hydrostatic Pressure Gradient on Water Movement through Excised Sunflower Roots

Abstract

ABSTRACIThe effect of abscisic acid on the exudation rte from decapitated roots of sunflower plants (felianthus annuus L.) was investigated in the presence and absence of an imposed hydrostatic pressure gradient. The n tude of the abscisic acid effect was constant even when suctions up to 60 cm Hg were applied to the cut stamps.When roots were bathed in a THO-labeled nutrient solution, the course of the appearance of radioactivity in the exudate, expressed as a function of exudate volume, was not affected by abscisic add treatment but was strongly speeded up by applying suction. The implications of those findings with regard to the water pathway through the root and the location of the abscisic add effect are discussed.It is well established that ABA causes a considerable increase in exudation rate from decapitated roots (4, 5, 10), a phenomenon which was attributed to an increase in hydraulic conductivity of the roots. However, these findings do not indicate unequivocally that ABA has an identical effect, if any, in an intact root of a transpiring plant. While exudation experiments are carried out ordinarily when the driving force for water flow is solely an osmotic potential gradient, in the intact plant, on the other hand, an important fraction of the driving force for moving water through the root is a hydrostatic pressure gradient formed by the transpiration pull.The nonequivalence of these two components on water flux was clearly demonstrated (7). It was suggested that hydrostatic pressure gradient causes either changes in water permeability or changes in the root's absorbing area and in the water pathway inside it (1, 2, 7).It was of interest, if so, to test the ABA effect on water flux through the root when a gradient in hydrostatic pressure was imposed, a state which may be comparable to that of a transpiring plant.In addition, the ABA effect was used to obtain further information on the relationship between the nature of the driving force for water flux and its pathway through the root. MATERIALS AND METHODSTwenty-five-day-old sunflower plants (Helianthus annuus L.) grown in a greenhouse were used throughout these experiments. Seeds were germinated in vermiculite and seedlings, 1 week old, were transferred individually to 1-liter plastic jars containing 0.5 strength Hoagland solution. The nutrient solution was renewed every 4th day. The temperature in the greenhouse ranged from a maximum of 30 C at noon to a minimum of 18 C before sunrise. The photoperiod was 14 hr (0500-1900). Noon light intensity was not less than 30,000 lux.The plants were transferred to the laboratory the evening before an experiment. The jars were kept in a thermostated bath at 27 C and the nutrient solution was aerated continuously. These conditions were maintained throughout the experiment. At 8 AM, the nutrient solution was renewed, and the plants were decapitated. The cut stumps were connected with a short, tightly fitting rubber tube to a graduated pipette. The exudation rate was determined by observing the movement of the ex...

The role of ABA and the transpiration stream in the regulation of the osmotic water permeability of leaf cells

“…Thirty years ago, Tal and Imber (19) and Glinka and Reinhold (33) reported that the application of ABA to the root medium of decapitated plants increases the exudation rate from the cut stump. Since that time, a number of studies have been published on this subject (16,17,20,34). Stress conditions such as flooding, chilling, and salt cause a decrease in the hydraulic conductivity of the root system that may be ameliorated by preadaptation or ABA treatment.…”

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

“…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).…”

“…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).…”

Amelioration of Chilling-Induced Water Stress by Abscisic Acid-Induced Changes in Root Hydraulic Conductance

“…Treatments of carrot root tissue or whole sunflower root systems with ABA have been reported to increase the hydraulic conductivity of the tissues (6,7). More recent experiments with Glycine max L. Merr.…”