DO LEAVES CONTRIBUTE TO THE ABSCISIC-ACID PRESENT IN THE XYLEM SAP OF DROUGHTED SUNFLOWER PLANTS

The root systems of 30-d-old sunflower plants were treated with polyethylene glycol (PEG; osmotic potential - 1.0MPa) for 2h, causing mild and transient wilting. Ten minutes before this treatment was applied, half the plants were defoliated. At varying times after the imposition of the PEG 'drought stimulus', the plant stems were cut and the sap exudate was collected and analysed for abscisic acid (ABA), using an ELISA method. When stems were cut 2.25h after the treatments were applied, the ABA concentration in the sap of the controls did not vary with time: the mean concentration was 10.7 +/- 1.0-mu-mol m-3. However, in the treated plants, the first sample contained 78.1 +/- 10.1-mu-mol m-3, decreasing to 13.6 +/- 2.8-mu-mol m-3 over 8.75h. Defoliation did not affect the ABA concentration in the sap. When stems were cut at varying times (up to 25h) after treatment, the PEG treatment again caused an immediate increase in the ABA concentration in the sap, from 20 +/- 1 to 136 +/- 21-mu-mol m-3. However, defoliation reduced this increase, but only in plants sampled 4-25h after treatment. We conclude that, after the PEG treatment to the roots, the initial increase of the ABA content of sap, and its attenuation with time, may be ascribed to synthesis in the roots whereas, thereafter, ABA derived from the leaves makes a major contribution to the ABA found in the xylem sap.