Resistance to root growth and changes in the concentrations of ABA within the root and xylem sap during root-restriction stress

The possibility that increased soil resistance to root growth may mediate the dwarfing response associated with root-restriction stress (RRS), via an abscisic acid (ABA) transduction mechanism, was investigated by characterizing the responses of tomato plants (Lycopersicon esculentum Mill cv. Red Dwarf) and changes within the soil environment at three rooting volumes (RV) (200, 400 and 800 cm³). Plant dry weight, leaf area and stomatal conductance decreased with RRS, although leaf water potential was unaffected by RRS. The concentration of ABA within the root system (ABA]rt) and xylem sap (ABA]xy) increased with RRS. Increased bulk density caused soil resistance to root growth to increase with increasing RRS. Changes in the soil environment, other than bulk density, which may have induced this variation in concentrations of ABA, were either eliminated or shown not to limit plant growth. The proportional relationships between RRS and soil resistance, ABA]rt and ABA]xy, and the inverse relationship between RRS and plant growth, are possibly indicative of the restricted root system experiencing increased resistance to root growth, with the subsequent initiation of a cascade of growth inhibiting responses.