The role of solute accumulation, osmotic adjustment and changes in cell wall elasticity in drought tolerance in Ziziphus mauritiana (Lamk.)

Abstract

Ber (Ziziphus mauritiana Lamk.) is a major fruit tree crop of the north-west Indian arid zone. In a study of the physiological basis of drought tolerance in this species, two glasshouse experiments were conducted in which trees were droughted during single stresscycles. In the first experiment, during a 13 d drying cycle, pre-dawn leaf water (Ψ(leaf)) and osmotic (Ψ(π)) potentials in droughted trees declined from -0.5 and -1.4 MPa to -1.7 and -2.2 MPa, respectively, for a decrease in relative water content (Θ) of 14%. During drought stress, changes in sugar metabolism were associated with significant increases in concentrations of hexose sugars (3.8-fold), cyclitol (scyllo-inositol; 1.5-fold), and proline (35-fold; expressed per unit dry weight), suggesting that altered solute partitioning may be an important factor in drought tolerance of Ziziphus. On rewatering, pre-dawn Ψ(leaf) and Θ recovered fully, but Ψ(π) remained depressed by 0.4 MPa relative to control values, indicating that solute concentration per unit water content had changed during the drought cycle. Evidence for osmotic adjustment was provided from a second study during which a gradual drought was imposed. Pressure-volume analysis revealed a 0.7 MPa reduction in osmotic potential at full turgor, with Ψ(leaf) at turgor loss depressed by ~1 MPa in drought-stressed leaves. Coupled with osmotic adjustment, during gradual drought, was a 65% increase in bulk tissue elastic modulus (wall rigidity) which resulted in turgor loss at the same Θ in both stressed and unstressed leaves. The possible ecological significance of maintenance of turgor potential and cell volume at low water potentials for drought tolerance in Ziziphus is discussed.