Flash drought as possible contributor to seedling dieback in the endangered conifer Abies koreana.

Tree species grown at high altitudes experience significantly greater stress than those at lower altitudes. A notable example is Abies koreana, a conifer recently classified as endangered due to a decline in normal seedling distribution within Korean natural forests. While several hypotheses have been proposed to explain this phenomenon, the underlying causes remain unclear. Recent studies highlight that Korean forest tree species are increasingly vulnerable to flash drought (FD) events. However, it is still unknown whether this intense FD event affects the growth and distribution of high-altitude grown and endangered species like Abies koreana. To address this gap, we investigated the effects of FD on root carbon allocation, volatile biosynthesis, fatty acid modulation, and genome-wide modifications. Exposure to FD in three-year-old A. koreana seedlings primarily disrupted leaf chlorophyll biosynthesis, likely due to the depletion of root water and non-structural carbohydrates (NSC) transport to above-ground parts. Additionally, FD caused severe morphological changes, including reductions in root collar diameter along with root cortical senescence. These alterations are linked to transcriptomic variations, particularly mRNA decay and the repression of genes coding for ribosomal proteins. Seedlings exposed to FD also exhibited increased levels of abscisic acid (ABA) and poly-unsaturated fatty acids. The observed patterns and molecular mechanisms in FD-treated seedlings differed significantly from those observed for control and mild drought (MD) treatments. These findings suggest that FD conditions trigger rapid carbon reserve depletion and gene repression associated with root structural integrity, potentially leading to seedling mortality in Abies koreana seedlings.