EFFECT OF SOIL COMPACTION ON BARLEY (HORDEUM-VULGARE L) GROWTH .2. ARE INCREASED XYLEM SAP ABA CONCENTRATIONS INVOLVED IN MAINTAINING LEAF EXPANSION IN COMPACTED SOILS

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 con sistently lower xylem sap ABA concentration at 7 d and 14 d after emer gence (DAE), The xylem sap data suggest that ABA present at Steptoe co ncentrations may have a direct role in maintaining leaf expansion at t he sub-critical bulk density (1.6 g cm(-3)), To test this hypothesis, addition of synthetic ABA either to the rooting environment (100 nM) o r directly to the xylem sap (5 pg mu l(-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 Az3 4 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 c ompaction stress.