Novel approaches for examining the effects of differential soil compaction on xylem sap abscisic acid concentration, stomatal conductance and growth in barley (Hordeum vulgare L.)

Novel techniques were devised to explore the mechanisms mediating the adverse effects of compacted soil on plants. These included growing plants in: (I) profiles containing horizons differing in their degree of compaction and; (ii) split-pots in which the roots were divided between compartments containing moderately (1.4 g cm(-3)) and severely compacted (1.7 g cm(-3)) soil. Wild-type and ABA-deficient genotypes of barley were used to examine the role of abscisic acid (ABA) as a root-to-shoot signal. Shoot dry weight and leaf area were reduced and root : shoot ratio was increased relative to 1.4 g cm(-3) control plants whenever plants of both genotypes encountered severely compacted horizons. In bartey cultivar Steptoe, stomatal conductance decreased within 4 d of the first roots encountering 1.7 g cm(-3) soil and increased over a similar period when roots penetrated from 1.7 g cm(-3) into 1.4 g cm(-3) soil. Conductance was again reduced by a second 1.7 g cm(-3) horizon. These responses were inversely correlated with xylem sap ABA concentration. No equivalent stomatal responses occurred in Az34 (ABA deficient genotype), in which the changes in xylem sap ABA were much smaller. When plants were grown in 1.7:1.4 g cm(-3) split-pots, shoot growth was unaffected relative to 1.4 g cm(-3) control plants in Steptoe, but was significantly reduced in Az34. Excision of the roots in compacted soil restored growth to the 1.4 g cm(-3) control level in Az34. Stomatal conductance was reduced in the split-pot treatment of Steptoe, but returned to the 1.4 g cm(-3) control level when the roots in compacted soil were excised. Xylem sap ABA concentration was initially higher than in 1.4 g cm(-3) control plants but subsequently returned to the control level; no recovery occurred if the roots in compacted soil were left intact. Xylem sap ABA concentration in the split-pot treatment of Az34 was initially similar to plants grown in uniform 1.7 g cm(-3) soil, but returned to the 1.4 g cm(-3) control level when the roots in the compacted compartment were excised. These results clearly demonstrate the involvement of a root-sourced signal in mediating responses to compacted soil; the role of ABA in providing this signal and future applications of the compaction procedures reported here are discussed.