Influence of soil compaction on xylem sap composition in barley (Hordeum vulgare L.)

Barley (Hordeum vulgare L. cv. Steptoe) was grown at various soil bulk densities under non-limiting water and nutrient conditions to assess the role of changes in xylem sap pH and the concentrations of nitrate, calcium, potassium, magnesium and sodium as potential root-to-shoot signal(s) mediating plant responses to soil compaction. Significant reductions in leaf conductance, growth and changes in xylem sap composition were observed at a bulk density of 1.7g cm(-3) (severe compaction) when compared with the uncompacted 1.1g cm(-3) control treatment (low compaction). Xylem sap calcium concentration was increased, while stomatal conductance was reduced in severely compacted soil at 6, 12 and 18 days after emergence (DAE). The concentration of magnesium in the xylem sap was also increased in the 1.7g cm(-3) treatment at 6 and 12 DAE, while that of potassium was up to S-fold lower than in the 1.1g cm(3) treatment. The most notable effect of severe compaction on the ionic composition of the xylem sap was a substantial decrease in nitrate concentration relative to uncompacted control plants at 6 and 12 DAE, which was associated with the observed reductions in leaf area. pH and sodium levels were both unaffected by the compaction treatments examined. The role of changes in the ionic concentration of xylem sap as a root-to-shoot signal during compaction stress is evaluated.