Inducing drought tolerance in greenhouse grown Juglans regia by imposing controlled salt stress: The role of osmotic adjustment

Micropropagated walnut plantlets (Juglans regia) are usually grown in the greenhouse to obtain a suitable size prior to transplantation. Meanwhile, maintaining the plantlets under optimal growing conditions for long durations in the greenhouse makes them more susceptible to harsh environmental conditions such as drought. This study was aimed at inducing drought tolerance in one-year-old micropropagated walnut 'Chandler' plantlets by preconditioning the plantlets by exposing to controlled salt stress (100 mM Na2SO4, 100 mM CaCl2, 100 mM KNO3, 50 mM Na2SO4 + 50 mM CaCl2, 50 mM Na2SO4 + 50 mM KNO3, 50 mM CaCl2 + 50 mM KNO3, and 30 mM of each salt solution in half strength Hoagland's nutrient solution). Water relations, osmotic adjustment and the gas-exchange of the plants' were measured after the preconditioning phase to understand the plants' defense responses. Then, the irrigation of plants was stopped for 30 days and the above mentioned parameters were examined again to determine drought tolerance among the preconditioned plants. The preconditioning treatments induced osmotic adjustment in the leaves by imposing osmotic stress. With osmotic adjustment, the transpiration rate decreased and the water use efficiency (WUE) was enhanced. Furthermore, the preconditioned plants exhibited a slower rate of water loss from their detached leaves. After the drought period, the leaves of preconditioned plants showed a greater leaf area, water content, WUE, membrane thermostability index and chlorophyll concentration, compared to the control plants. These observations were mainly due to a stronger osmotic adjustment. Proline was the major osmolyte involved in the active osmotic adjustment in the leaves. The Na2SO4 proved to be the most successful treatment which caused the accumulation of proline and the induction of drought tolerance in J. regia.