Karrikinolide1 (KAR₁), a Bioactive Compound from Smoke, Improves the Germination of Morphologically Dormant <i>Apium graveolens</i> L. Seeds by Reducing Indole-3-Acetic Acid (IAA) Levels

Smoke-water (SW) and Karrikinolide1 (KAR₁) release dormancy and improve seed germination in many plant species. Therefore, we tested SW (1:2500 <i>v</i>/<i>v</i>) and KAR₁ (10⁻⁷ M) to break the morphological dormancy of celery cultivar (<i>Apium graveolens</i> L.). In the first trial, seeds were subjected to a 21-day incubation period at 20 °C with SW and KAR₁ applied as single treatments. KAR₁ showed significantly improved germination (30.7%) as compared to SW (17.2%) and a water control (14.7%). In seed soaking experiments, SW, KAR₁, and gibberellic acid (GA₃) treatments showed higher germination percentages than the water control after 3 and 6 h of soaking. However, prolonged soaking (12 h) reduced germination percentages for all treatments, indicating a detrimental effect. Analysis of KAR₁ content dynamics in 7-day- and 21-day-old celery seeds indicated its prolonged effects on germination and dormancy alleviation. Phytohormones, including auxins in 7-day-old and cytokinins in 7-day- and 21-day-old celery seedlings, along with their precursors and metabolites, were analyzed using ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) after treatment with KAR₁ and SW. The analysis of auxin levels in 7-day-old seeds revealed a negative correlation between seed germination and auxin (indole-3-acetic acid, IAA) content. Notably, it was found that KAR₁-treated seeds significantly reduced IAA levels in all treatments. SW and KAR₁ did not significantly affect cytokinin levels during celery germination except for N6-Isopentenyladenine. Hence, further research is needed to understand their precise role in celery seed germination. This work will improve our understanding of the role of bioactive compounds from plant-derived smoke and how they regulate hormonal responses and improve germination efficiency in celery.