Abstract:
Sorbitol-induced osmotic stress (SIOS) to the root was found to be effective in
reducing transpirational water loss in plants during water stress. Mechanisms of SIOS in
stomatal control were investigated in containerized apples (Malus domestica Borkh.
'Gala'/M26) when leaf water potential (Ψw), osmotic potential (Ψs), turgor potential (Ψp),
stomatal conductance (gs), relative water content (RWC), and abscisic acid (ABA)
content of the leaf and xylem sap were determined. Sorbitol (-1.2 MPa) was applied as a
soil drench. For ABA determination, crude leaf extract and xylem sap were used for the
radioimmunoassay method.
Stomatal conductance decreased rapidly within one hr after SIOS treatment,
which was closely associated with a decline in leaf water status Ψw, Ψp, and RWC). The
SlOS-induced earlier reduction in gs caused plants to resist greater desiccation stress than
the untreated controls.
Leaf ABA production was stimulated by SIOS treatment, but the effect differed
between plants adapted in greenhouse and outdoor conditions. In greenhouse-adapted
plants, SIOS caused leaf ABA to increase significantly two hrs after treatment, but had no
effect in outdoor-adapted plants. However, gs was similarly reduced in both conditions
by about 55%. This difference in leaf ABA production between plants adapted in both
conditions could be associated with the concurrent differential decrease in Ψp; baseline
Ψp was much higher in the outdoor-adapted plants than in the greenhouse-adapted plants.
In the greenhouse-adapted plants, Ψp decreased by 61% (from 1.15 to 0.45 MPa) within
two hrs after SIOS treatment, whereas Ψp decreased by 27% (from 2.15 to 1.58 MPa) in
the outdoor-adapted plants.
Xylem sap ABA, measured two hrs after SIOS treatment from the outdoor-adapted
plants, was not affected, whereas gs and leaf Ψw (thus Ψp) significantly
decreased. Significant increase in ABA content in the xylem sap and leaf, however,
occurred 1 and 3 days after treatment, respectively. ABA level and gs data suggested that
the immediate reduction in gs by SIOS treatment may have resulted from hydraulic
effects on gs, rather than chemical effects (e.g., root ABA signal), at least for the outdoor-adapted
plants.
