UCLA

Drought is a major stress for plants, and creates strong selection pressure for adaptive responses. Many drought responses will be conserved, species-wide responses, but when populations are distributed across heterogeneous environments, local selection pressures may shape differences in their response to drought. This study tests whether populations of valley oak (Quercus lobata), a widely-distributed California endemic oak, are locally adapted in their response to water stress. Using groups of seedlings sampled from dissimilar climates and exposed to soil-drying or high water treatments, we measured ecophysiological traits and gene expression (RNA-seq) data. Valley oak seedlings under water stress had a lower leaf water potential and turgor loss point, but populations were not significantly different from each other, indicating a generalized species-wide response. However, most genes that were differentially expressed between treatments responded in only one seedling population, indicating that populations generally have different responses to water stress. Additionally, gene modules (groups of genes with similar expression patterns, identified using weighted gene co-expression networks) often responded differently to water stress treatment among populations, potentially identifying differences in drought response that occur through differential regulation of gene networks. This study provides evidence that valley oak populations are locally adapted to respond to water stress. As drought is projected to increase in California due to climate change, this may be useful for predicting the response of different populations and devising management strategies.