| Abstract: | The abscisic acid (ABA)-deficient mutant of barley, Az34, exhibiteda much reduced rate of leaf expansion at a bulk density of 1.6g cm–3 as compared to the isogenic wild-type variety,Steptoe. Az34 had a consistently lower xylem sap ABA concentrationat 7 d and 14 d after emergence (DAE). The xylem sap data suggestthat ABA present at Steptoe concentrations may have a directrole in maintaining leaf expansion at the sub-critical bulkdensity (1.6 g cm–3 To test this hypothesis, additionof synthetic ABA either to the rooting environment (100 nM)or directly to the xylem sap (5 pg µl–1 to reproducethe xylem sap ABA concentrations found in Steptoe, increasedleaf expansion in Az34 to the wild-type level. Furthermore,feeding Steptoe xylem sap to Az34 produced similar effects.These experiments provide direct evidence of a positive rolefor ABA as a root-to-shoot signal which assists in maintainingleaf growth in plants experiencing subcritical levels of compactionstress. Key words: ABA-deficient mutant, leaf expansion, xylem sap, ABA |
