Delivery rates of abscisic acid in xylem sap of Ricinus communis L plants subjected to part-drying of the soil

Abscisic acid (ABA) moving from roots to shoots in the transpiration stream is a potential hormonal message integrating perception of a root stress with adaptive changes in the shoot. A twin root system was used to study ways of estimating effects of droughting the upper roots of Ricinus communis L. on abscisic acid (ABA) transport to the shoot in the transpiration stream. Droughted plants transpired more slowly than controls. Droughting also increased concentrations of ABA up to 11-fold in sap induced to flow from the roots of freshly decapitated plants at rates of whole plant transpiration. However, because of dilution effects arising from the different sap flows in control and droughted plants, these changes in ABA concentration in the xylem sap did not accurately reflect amounts of ABA transported. To overcome this problem, delivery rates were calculated by multiplying concentration with sap flow rate to generate ABA delivery in terms of mu mol s(-1) per plant. Droughting for 24 h or more increased ABA delivery from roots to shoots by 5-fold. Since droughting can alter the relative sizes of the roots and shoots and also the root:shoot ratio these delivery rates were refined in several ways to reflect both the amount of root generating the ABA message and the size of the recipient shoot system.