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

Novel techniques were devised to explore the mechanisms mediating the adver se 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 con taining moderately (1.4 g cm(-3)) and severely compacted (1.7 g cm(-3)) soi l, 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 an d 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 se verely 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) in to 1.4 g cm(-3) soil. Conductance was again reduced by a second 1.7 g cm(-3 ) horizon, These responses were inversely correlated,vith xylem sap ABA con centration. No equivalent stomatal responses occurred in Az34 (ABA deficien t 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 unaffe cted relative to 1.4 g cm(-3) control plants in Steptoe, but was significan tly reduced in Az34, Excision of the roots in compacted soil restored growt h to the 1.4 g cm(-3) control level in Az34, Stomatal conductance was reduc ed 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 1.7 g cm(-3) control plants but subsequently returned to the control level; no recovery occurred if the roo ts in compacted soil were left intact. Xylem sap ABA concentration in the s plit-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 dem onstrate the involvement of a root-sourced signal in mediating responses to compacted soil; the role of ABA in providing this signal and future applic ations of the compaction procedures reported here are discussed.