Rapid accumulation of free proline was found in barley leaves under conditions of osmotic stress at the level of -8.3 bars, but not before 9 days of its duration. The threshold value of osmotic stress for the start of a rapid accumulation of proline was about -6 bars. The excised leaves of barley subjected to water stress by wilting accumulated free proline more rapidly. In the course of 24 hours free proline content increased about 30-fold, while the water content in the leaves decreased 3-fold only. We found also that excised barley leaves with full turgor treated with abscisic acid (10<sup>-4</sup> M) accumulated free proline as early as after 3 hours and subsequently after 24 hours free proline content increased 10-fold.
The Mechanism of Abscisic Acid-induced Proline Accumulation in Barley Leaves