PHLOEM TRANSPORT OF ABSCISIC-ACID IN RICINUS-COMMUNIS L SEEDLINGS

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2-0.5 mmol m(-3) abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceed ed that in root pressure exudate under a wide range of water supply. E xudation of sieve tube sap from the cut hypocotyl caused water loss, a nd this induced 'water shortage' in the cotyledons which resulted in t he ABA concentration in the cotyledons increasing by 3-fold and that i n the sieve tube sap increasing by up to 50-fold within 7 h. The wound ed surface of the cut hypocotyl was not responsible for the ABA increa se. Incubation of the cotyledons of endosperm-free seedlings in variou s ABA concentrations (up to 100 mmol m(-3)) increased the ABA concentr ation in sieve tube sap. The concomitant increase in ABA, both in coty ledons and in sieve tube sap, had no effect on the phloem loading of s ucrose, K+ and Mg2+ within the experimental period, i.e. up to 10 h. I t can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem -mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA c oncentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.