ABSTRACT
The water relations and the stress tolerance of tissues are significantly improved with elevated abscisic acid (ABA) concentrations. ABA causes stomatal closure and stimulates uptake of water into roots. Davies and his group have shown that ABA as a root-to-shoot stress signal also regulates growth of the leaves. Soils of extreme habitats are not only very dry, they also often have high concentrations of different salts and can be very alkaline. Salinity (NaCl) stress caused a three to sixfold accumulation of ABA in tobacco leaves and maize roots for which the principle of cross-adaptation also seems to apply. In saline, alkaline soils, phosphate is scarcely available for plants. They suffer from severe phosphate deficiency as long as their roots are unable to acidify the rhizosphere. When root systems of a wide range of plants are salt-stressed, rate of ABA synthesis and conjugation is increased, and both compounds are transported in the xylem to the guard cells and the shoot meristems.
