| Summary: | Walnut (Juglans regia L.) is a drought and heat-sensitive plant. These sensitivities are intensified with young plants which are grown in greenhouse nurseries. We subjected one-year-old greenhouse-grown walnut 'Chandler' trees to different pretreatments [long-term water stress, controlled salinity stress, and foliar application of β-aminobutyric acid (BABA)] for acclimation to unfavorable conditions after transplantation. Then, the plants were transplanted into hot and dry conditions to evaluate efficiency of different preconditioning treatments in acclimation of the plants to deleterious environmental conditions. Investigating the plants' responses to the pretreatments for understanding activated adaptation mechanisms indicated that the pretreatments have induced osmotic adjustment (OA) in leaves. In this regard, the salinity pretreatment was the most effective treatment. For OA, proline accumulation was more effective than soluble sugars. However, glycine betaine was not involved in OA. The pretreatments sensitized stomatal responses to drought, according to limitation of water loss from detached leaves. Ninety days after transplantation, the preconditioned plants had higher biomass than the control plants. The major reasons were the prevention of leaf abscission and chlorophyll degradation. Moreover, improved water conservation ability in the preconditioned plants enhanced leaf expansion, photosynthesis rate and biomass accumulation in plants' organs. Restriction of stomatal conductance for water conservation in the control plants resulted in photosynthesis inhibition, heat accumulation, and vigor loss. The efficiency of BABA and salinity stress in acclimation of walnut to deleterious environmental conditions was higher than water stress. However, BABA foliar application as the safest and least complicated pretreatment was suggested in a drought/heat acclimation strategy in walnut.
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