Variation in seedling growth, root exudates and rhizosphere microbial communities of two temperate tree species under fluctuating light and extreme drought

Background and aims: Root exudates play key roles in supporting plant growth and their response to environmental changes. However, it is unclear whether changes of root exudates induced by aboveground (fluctuating light) and belowground (water) stress drive the recruitment of specific rhizosphere microbes. Although two alternative hypotheses (trade-off vs. facilitation) offer explanations, how fluctuating light and extreme drought drive plant growth is still unknown. Methods: A controlled experiment was conducted with three water regimes (well-watered, mild, and extreme drought) and four light treatments (high-steady, high-fluctuating, low-steady, and low-fluctuating) with seedlings of two temperate forest tree species, drought-sensitive Tilia amurensis and drought-tolerant Quercus mongolica. We analyzed plant growth and links between root exudates and rhizosphere microbial communities. Results: Water and light treatment had no interactive effects and neither hypothesis was supported. Extreme drought and high light individually significantly reduced plant growth of both tree species, while high fluctuating light promoted plant growth compared to high steady light. Chemical compositions in root exudates under extreme drought differed significantly between high-steady and high-fluctuating light. Organic acids and carbohydrates were closely correlated with rhizosphere microbial communities, under extreme drought, they recruited specific microbes at the phylum level (i.e. Actinobacteriota) in response to varying light conditions. Besides, variations in microbial communities and root exudates of Q. mongolica were less significant than those of T. amurensis in response to drought. Conclusion: Root exudates were closely associated with rhizosphere microbial communities, which provides new insights into the potential to improve plant growth and resistance under dual abiotic stresses.