Anatomical and physiological traits to identify low-vigor Persian walnut accessions as candidate dwarfing rootstocks.

Size-controlling planting materials are essential for improving orchard sustainability, enabling high-density systems, and reducing management costs in nut tree production. However, reliable sources of growth-restricting planting materials remain scarce in walnut improvement programs. This study quantified differences in growth performance among diverse Juglans regia germplasm using an integrative evaluation of morphological, physiological, and anatomical characteristics. Fifteen accessions with varying growth vigor collected from eight regions of Iran were propagated as half-sib families and grown under uniform field conditions. Growth attributes were recorded over two consecutive seasons, after which contrasting groups were subjected to detailed wood structure and water-relation analyses. Based on morphological evaluations, four of the fifteen accessions-representing high, low, and intermediate growth vigor-were selected for anatomical and physiological analyses to explore their differential characteristics. The less vigorous accessions, 'Qazvin 1' and 'Qazvin 2', showed significantly smaller vessel diameters, lower hydraulic conductance, reduced water potential (WP), decreased relative water content (RWC), and shorter stomatal length compared with the high-vigor genotypes, 'Urmia' and 'Damavand'. Moreover, 'Qazvin 1' and 'Qazvin 2' showed a greater frequency of small-diameter vessels. Collectively, these features are consistent with mechanisms associated with compact tree architecture and improved resource-use efficiency. The results highlight two accessions ('Qazvin 1' and 'Qazvin 2') as promising candidates for breeding and propagation programs aimed at developing size-controlling planting materials for modern walnut orchards.