Water relations, stomatal conductance, and abscisic acid content of container-grown apple (Malus domestica Borkh.) plants in response to sorbitol-induced osmotic stress

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 leafwater potential ({dollar}Psisb{lcub}rm w{rcub}{dollar}), osmotic potential {dollar}rm(Psisb{lcub}s{rcub}),{dollar} turgor potential {dollar}(Psisb{lcub}rm p{rcub}),{dollar} stomatal conductance {dollar}rm(gsb2),{dollar} relative water content (RWC), and abscisic acid (ABA) content of the leaf and xylem sap were determined. Sorbitol ({dollar}-{dollar}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 leafwater status {dollar}rm(Psisb{lcub}w{rcub}, Psisb{lcub}p{rcub},{dollar} and RWC). The SIOS-induced earlier reduction in {dollar}rm gsb{lcub}s{rcub}{dollar} 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, {dollar}rm gsb{lcub}s{rcub}{dollar} 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 {dollar}rmPsisb{lcub}p{rcub};{dollar} baseline {dollar}rmPsisb{lcub}p{rcub}{dollar} was much higher in the outdoor-adapted plants than in the greenhouse-adapted plants. In the greenhouse-adapted plants, {dollar}rmPsisb{lcub}p{rcub}{dollar} decreased by 61% (from 1.15 to 0.45 MPa) within two hrs after SIOS treatment, whereas {dollar}rmPsisb{lcub}p{rcub}{dollar} 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 g{dollar}sb{lcub}rm s{rcub}{dollar} and leaf {dollar}rmPsisb{lcub}w{rcub}{dollar} (thus {dollar}rmPsisb{lcub}p{rcub}{dollar} 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 {dollar}rm gsb{lcub}s{rcub}{dollar} data suggested that the immediate reduction in {dollar}rm gsb{lcub}s{rcub}{dollar} by SIOS treatment may have resulted from hydraulic effects on {dollar}rm gsb{lcub}s{rcub},{dollar} rather than chemical effects (e.g., root ABA signal), at least for the outdoor-adapted plants.