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The Mechanism of Abscisic Acid-induced Proline Accumulation in Barley Leaves

Department of Botany, Iowa State University, Ames, Iowa 50011

When leaf blades of fully expanded second leaves of barley (cv. Prior) were excised and incubated with the cut end in a 20 milligram per liter solution of abscisic acid, they accumulated proline at the rate of about 1 micromole per hour per gram fresh weight after a 3- to 4-hour lag. This accumulation occurred reproducibly only if leaves were pretreated by placing the cut end in a solution consisting of 50 millimolar sucrose and 1 millimolar glutamate. Treated leaves were taken from plants which had been in the light for 24 hours.

Abscisic acid caused a stimulation of proline synthesis from glutamic acid. Proline oxidation rates were similar in leaves incubated in abscisic acid and in water even though the proline level in abscisic acid-treated leaves was 2.5 times the level in the water-treated controls. The incorporation of proline into protein was not affected by abscisic acid.

These results are interpreted to indicate that the metabolic cause of abscisic acid-induced proline accumulation is a stimulation of proline synthesis from glutamic acid. Inhibition of the utilization of proline by oxidation and protein synthesis does not contribute to proline accumulation the way it does in drought-stressed leaves.