The relationships of guard cell ABA content to eight stress-related physiological parameters were determined on intact Vicia faba L. plants that were grown hydroponically with split-root systems. Continuous stress was imposed by the addition of PEG to part of the root system. The water potentials of roots sampled after the addition of PEG were 0.25 MPa lower than the water potentials of other roots of the same plant, which were similar to the roots of untreated plants. The leaflet water

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... ls of plants sampled within 2 h of stress imposition were similar to those of control plants. However, leaf conductance was lower in plants sampled after only 20 min of stress imposition, and the root-and leaflet apoplastic ABA concentrations of these plants were higher than those of untreated plants. As the essence of this report, there was a linear relationship between guard cell ABA content and leaf conductance. Leaflet apoplastic ABA concentrations -150 nM were also linearly related to leaf conductance, but higher leaflet apoplastic ABA concentration did not cause equally large further declines in leaf conductance. It is suggested that evaporation from guard cell walls caused ABA to accumulate in the guard cell apoplast and this pool was saturated at high leaflet apoplastic ABA concentrations.