Relationship between changes in the guard cell abscisic-acid content and other stress-related physiological parameters in intact plants

The relationships of guard cell ABA content to eight stress-related physiol ogical 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 roo ts of untreated plants. The leaflet water potentials of plants sampled with in 2 h of stress imposition were similar to those of control plants. Howeve r, leaf conductance was lower in plants sampled after only 20 min of stress imposition, and the root- and leaflet apoplastic ABA concentrations of the se plants were higher than those of untreated plants. As the essence of thi s report, there was a linear relationship between guard cell ABA content an d leaf conductance. Leaflet apoplastic ABA concentrations <150 nM were also linearly related to leaf conductance, but higher leaflet apoplastic ABA co ncentration did not cause equally large further declines in leaf conductanc e. lt is suggested that evaporation from guard cell walls caused ABA to acc umulate in the guard cell apoplast and this pool was saturated at high leaf let apoplastic ABA concentrations.