The abscisic acid (ABA)-deficient mutant of barley, Az34, exhibited a much reduced rate of leaf expansion at a bulk density of 1.6 g cm−3 as compared to the isogenic wild-type variety, Steptoe. Az34 had a consistently lower xylem sap ABA concentration at 7 d and 14 d after emergence (DAE). The xylem sap data suggest that ABA present at Steptoe concentrations may have a direct role in maintaining leaf expansion at the sub-critical bulk density (1.6 g cm−3 To test this hypothesis, addition of synthetic ABA either to the rooting environment (100 nM) or directly to the xylem sap (5 pg μl−1 to reproduce the xylem sap ABA concentrations found in Steptoe, increased leaf 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 role for ABA as a root-to-shoot signal which assists in maintaining leaf growth in plants experiencing subcritical levels of compaction stress.