Illuminated leaf discs of Vicia faba were brought into equilibrium with a series of mannitol solutions. The width of stomatal aperture and the osmotic potential of guard cells and epidermal cells were determined. It was found that the maximal aperture was obtained when epidermal cells were at about incipient plasmolysis and that any increase in their turgor pressure brought about a decrease in stomatal aperture. These findings emphasize the importance of epidermal cells in determining the width of the stomatal pore.

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...

A number of recent observations have indicated a role for abscisic acid in maintaining water balance. Treatment with ABA reduces the transpiration rate (3,5), probably by bringing about stomatal closure (2, 4). Furthermore, water stress leads to a significant increase in ABA concentration in leaves (5, 7). On the other hand, ABA increases the rate of root exudation (6).The present communication reports a marked effect of ABA on the permeability of plant tissues to water. The effect is observable both on Lp, the coefficient of hydraulic conductivity, and on Pd, the coefficient of diffusional permeability to water.The effect on hydraulic conductivity was detected in experiments in which osmotic water flux between cells and medium was followed with time. The equations for net osmotic flux into or out of cells can be written as follows:J, (out) = Lp(-oiri + oe're + P) (2) where JL = volume flow of water; -rx and 1re are the osmotic values for the cell and for the external solution respectively; ai and U-e are the average reflection coefficients for the internal and external solutes; and P is the turgor pressure. Changes in water flux would thus follow from alterations in Lp, in 7ri, in P, or in a. We have earlier (1) defined conditions under which a change in Lp may be distinguished from changes in the other parameters. Figure 1 shows the effect of ABA observed under these defined conditions. Discs excised from the xylem of root storage tissue of Daucus carota L. were washed in running tap water for about 16 hr. In the case of the efflux experiments, they were then immersed for 3 hr in 20 mg/liter ABA or in distilled water, after which they were transferred to 0.45 M mannitol. This concentration is hypotonic to the tissue. Net water efflux was determined by weighing the tissue at intervals. In the case of the influx experiments, the discs were first equilibrated for 3 hr in 0.45 M mannitol with or without 20 mg/liter ABA. They were then transferred to distilled water, and net influx was determined at intervals again by weighing. All experiments reported here were carried out at 27 C. Figure 1 shows that ABA increased the flux of water both into and out of the tissue. This fact rules out the possibility that the change in flux was due to a change in -ri or in P, since it is clear from the reversal of signs in equations 1 and 2 that if a factor affected flux via an alteration in either -xi or P, the effect on influx would have to be the opposite of that on efflux. Moreover, the fact that efflux was measured into hypotonic solution excludes the possibility that ABA treatment was changing a-(1 Pd, the coefficient of diffusional permeability to water, is also raised by ABA treatment. This was detected by measuring the flux of tritiated water between the tissue and the medium. In the experiment sunmmarized in Figure 2, cylinders of carrot tissue were first equilibrated with tritiated water (specific activity 50 ,uc/ml) for 3 hr, with or without added ABA. Each cylinder was then sequentially transferred through a series of...

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...

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