Exogenous GA3 delays walnut fruit maturation and alters kernel fatty acid composition by modulating genes and hormones during development.

Regulation of walnut fruit maturation is crucial for delaying harvest and maintaining kernel quality. In this study, 'Qingxiang' walnut (Juglans regia L.) fruit were treated with 300 mg L-1 gibberellic acid (GA3) during 20-100 days after flowering (DAF) to assess its effects on fruit development, kernel nutrient composition and endogenous hormonal dynamics. GA3 treatment decreased walnut husk cracking and peeling incidences at 154 DAF by 46 and 59 percentage points, respectively, and delayed fruit abscission by 7 days compared to the control. In the husk, GA3 treatment postponed the accumulation peaks of acid-soluble pectin, suppressed pectin methylesterase and β-galactosidase activities, and downregulated JrPME-like2 and JrPMEI4 expression levels. In kernels, GA3 treatment delayed the protein accumulation pattern by approximately 12 days and reduced the relative content of oleic acid (OA) while promoting linoleic acid (LA) accumulation in kernels, consistent with altered expression peaks of JrFAD2 and JrFAD3. Significantly, in the GA3-treated group, OA and LA contents were highly correlated with the expression levels of JrGAI-like1, LEC2, JrSAD6, JrFAD2, and JrFAD3. GA3 treatment also elevated the peak levels of ZT, tZ-type CTK, IAA, and JA in walnut kernels, delayed the ZT peak by approximately 20 days, and increased endogenous GA3 content approximately 15-fold at 76 DAF relative to the control, whereas ABA and SA contents remained unaffected. Potential hormonal crosstalk among GA3, ABA, IAA, and tZ-type CTK suggests a complex regulatory network modulating nutrient accumulation in walnut kernels. Overall, GA3 delays walnut maturation by coordinating enzyme activities, hormonal balance, and gene expression, providing an effective strategy to extend harvest windows and enhance walnut quality.