Effect of deficit irrigation on physiological, morphological and fruit quality traits of six walnut tree cultivars in the inland area of Central Asia

Persian walnut, a drought-sensitive tree, exhibits significant genetic variation in functional traits in response to drought—a domain that remains largely unexplored. This study examines the impact of two distinct irrigation regimes—conventional irrigation (CI) and deficit irrigation (DI)—on physiological, morphological, and fruit quality traits across six walnut tree cultivars in a scion orchard during the summer drought period. Our findings revealed significant effects of irrigation treatments on soil water content, with notable drought stress observed in Win 185, Xinjufeng, and Zha71 under DI. Win 185 and Xinfeng, subjected to DI, exhibited diminished photosynthetic rates (A) and stomatal conductance (gs), whereas Win 185, Xinjufeng, Xinxin 2, and Zha71 displayed heightened instantaneous water use efficiency (WUEi) under DI. The maximum photochemical efficiency of photosystem II (Fv/Fm) and chlorophyll index were also affected in Win 185, Xinfeng, Xinjufeng, Xinxin 2 and Zha71 subjected to the DI treatment. Identified as primary drought response strategies, stomatal regulation, osmotic adjustment, and morphological adaptations varied uniquely among cultivars, potentially mitigating the adverse effects of drought on fruit quality. Notably, DI induced minor changes in fruit quality for Win 185, Xinlu, and Zha71, resulting in varying reductions in fruit diameter and weight. This suggests the possibility of achieving reduced water consumption while preserving fruit quality in specific cultivars. Phenotypic plasticity was evident across all traits; however, its response to drought exhibited cultivar-specific variations. A nuanced understanding of phenotypic plasticity's role in fruit quality is essential for optimizing deficit irrigation practices across diverse walnut cultivars.