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

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Abstract
When leaf blades of fully expanded 2nd leaves of barley (cv. Prior) were excised and incubated with the cut end in a 20 mg/l solution of abscisic acid, they accumulated proline at the rate of about 1 .mu.mol/h per gram fresh weight after a 3-4 h lag. This accumulation occurred reproducibly only if leaves were pretreated by placing the cut end in a solution consisting of 50 mM sucrose and 1 mM glutamate. Treated leaves were taken from plants which had been in the light for 24 h. 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 themetabolic 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.