There have been several reports of an increase in ABA in water-stressed tissue (12,14,16,20,23,24 The psychrometers were of the Peltier-type (15,21). The walls of the chambers were coated with melted and resolidified petrolatum (1) to prevent adsorption of water. Readings were taken after a 3-hr equilibration in a water bath controlled to 0.001 C.RESULTS AND DISCUSSION Figure 2 shows ABA content of leaves and leaf water potential as functions of time after the onset of a high evaporative demand. In the first phase, lasting 8 hr, ABA remained at low levels in both species, while the water potential was either constant at -9 atm in A. artemisiifolia L. or decreased from -4 to -8 atm in A. trifida L. After 8 hr, ABA began to increase in both species, as the water potential of A. artemisiifolia L. began to decrease sharply and the water potential of A. trifida L. continued its more gradual decline. Differences in rates of drying both between species and among plants of the same species are partially due to differences in plant size, which resulted in differential rates of exhaustion of soil moisture. Figure 3 shows the relationship between ABA and leaf water potential under the conditions of these experiments. In both species, ABA concentration increased sharply over a narrow range of leaf water potentials. Thus there appears to be a water potential threshold at which ABA begins to increase, while over a range of high water potentials the endogenous ABA concentration remains nearly constant. Slight differences in ABA at these high water potentials probably reflect the limits of accuracy of chromatography for low ABA concentrations. The increase in ABA at the threshold water potential is so abrupt that a reduction in leaf water potential on only 1 atm might cause a significant increase in ABA from its baseline concentration. A comparison of the two species shows a remarkable similarity in the water potential (-10 to -12 atm) at which ABA synthesis seems to increase.There is a lag between the occurrence of a given water po-125 www.plantphysiol.org on May 11, 2018 -Published by Downloaded from