Differences in Phenylpropanoid and Reactive Oxygen Species Genes Drive Plastic and Adaptive Responses to Drought Stress Between Douglas‐Fir Varieties and Hybrids

Warming climates bring significant challenges to the survival and growth of conifer species, yet the genomic basis of drought resistance is poorly understood. In this study, we used a common garden experiment to combine genomic, physiological and transcriptomic analysis to dissect plastic and adaptive responses to drought between Douglas‐fir varieties and intervarietal hybrids. Coastal and interior Douglas‐fir varieties naturally hybridize in British Columbia and the Washington Cascades. We identified a total of 735 SNPs associated with drought‐related phenotypic traits or environmental variables. A total of 2669 genes with full functional annotations were identified by differential expression analysis. Pathways identified by enrichment analysis and a co‐expression network were phenylpropanoid biosynthetic process, response to osmotic stress, and terpenoid, diterpenoid, flavonoid, and isoprenoid. Significant differences in drought tolerance were found among varieties, with intervarietal hybrids having the highest water‐use efficiency and photosynthetic capacity. Under moderate and severe drought conditions, hybrids showed upregulation of genes related to response to reactive oxygen species, protein maturation and folding compared to pure varieties. This study advances our understanding of the molecular basis of drought resistance in long‐lived conifer species and can contribute to management and breeding efforts.