EFFECT OF SOIL COMPACTION ON BARLEY (HORDEUM-VULGARE L) GROWTH .1. POSSIBLE ROLE FOR ABA AS A ROOT-SOURCED CHEMICAL SIGNAL

Wild-type (Steptoe) and abscisic acid (ABA)-deficient mutant (Az34) ge notypes of barley were grown in compacted soil to examine the potentia l role of ABA as a root-to-shoot signal. Root and shoot growth and lea f conductance were all reduced when plants were grown in compacted soi l with a bulk density of 1.7 g cm(-3) relative to uncompacted control plants (1.1 g cm(-3)), These effects occurred in the absence of detect able changes in leaf water status or foliar abscisic acid (ABA) conten t, Analysis of Steptoe and Az34 xylem sap showed that the ABA concentr ation was greatly increased at 6 d after emergence (6 DAE) when seedli ngs were grown in compacted soil (1.7 g cm(-3)); however, ABA concentr ations were never as high in the mutant as in the wild type, The incre ase in xylem sap ABA concentration observed at high bulk density was c losely correlated with reductions in leaf conductance, but not leaf ar ea, These increases were transitory, and xylem sap ABA concentrations subsequently decreased towards the control level by 18 DAE in both gen otypes. The ABA-deficient mutant, Az34, produced a much lower leaf are a than Steptoe at a bulk density of 1.6 g cm(-3). Examination of epide rmal characteristics indicates that this effect resulted mainly from r eductions in cell expansion rather than cell division, suggesting that the higher ABA concentrations detected in xylem sap from the wild-typ e Steptoe may have exerted a positive role in maintaining leaf expansi on in this treatment, The possible involvement of ABA as a root-to-sho ot signal mediating the effects of compaction stress is discussed.