Early Physiological and Transcriptomic Responses of Container‐Grown Blueberry <i>(Vaccinium corymbosum</i> L. ‘Sierra’ <i>)</i> to Partial Substrate Replacement With Biochar

ABSTRACT Biochar has been widely investigated for its potential to improve substrate properties and plant performance, offering a potential partial replacement for peat in soilless cultivation amid declining peat availability. However, information on its short‐term physiological and molecular effects during the early establishment of container‐grown blueberry plants remains limited. This study evaluated the short‐term effects of biochar incorporation (0% (BC0), 5% (BC5), and 10% (BC10)), as a partial replacement of a peat‐based substrate, on growth, photosynthetic traits, and gene expression in potted highbush blueberry ( Vaccinium corymbosum L.), cv. “Sierra”, at the establishment stage (25–40 cm in height). A non‐fertilized commercial biochar was incorporated into the substrate in April 2024, and plant responses were evaluated in September 2024. Plant height was not affected by BC5, while BC10 reduced it by 10%. Leaf chlorophyll content (SPAD) decreased following both biochar treatments, while maximum PSII efficiency (Fv/Fm) remained unaffected. Gas‐exchange analyses indicated that BC10 plants exhibited altered light‐response traits, including lower dark respiration and a reduced light‐compensation point, although no significant changes were observed in V cmax or J max . Gene expression analysis showed consistent upregulation of growth‐related genes (SAUR49, STP, TCH4, DRM1) in biochar‐treated plants, whereas defence‐related genes (AOS, CHIB, JAS5, NPR1) displayed heterogeneous responses. Overall, biochar application modulated physiological and transcriptional processes in young blueberry plants, highlighting early responses occurring during establishment rather than direct improvements in growth. Within the tested range, results suggest that partial substitution of the substrate with biochar (5–10% v/v) does not enhance short‐term growth and may induce dose‐dependent effects. These findings should be interpreted as preliminary and hypothesis‐generating, supporting the need for further studies integrating substrate chemistry, plant physiology, and long‐term performance across multiple growing seasons under northern European climate conditions.