Physiological and transcriptomic responses of Mallotus paniculatus to sustained drought stress: Implications for tropical coastal shelter forest restoration

Drought constrains seedling establishment and ecological restoration in tropical coastal forests, where native pioneer species are needed to diversify vulnerable shelterbelt plantations. Mallotus paniculatus is a common pioneer tree with broad environmental adaptability, but its drought-response mechanisms remain poorly understood. We integrated physiological measurements, full-length and short-read transcriptomics, weighted gene co-expression network analysis (WGCNA), and qRT-PCR to characterize the responses of M. paniculatus seedlings to sustained drought stress. Following sustained water withholding, stomatal conductance, carbon assimilation, apparent photorespiration, and the maximum quantum efficiency of PSII photochemistry (Fv/Fm) decreased, whereas water-use efficiency increased. Increased MDA and H₂O₂ levels indicated oxidative injury, while enhanced POD and APX activities and osmolyte accumulation reflected antioxidant and osmotic adjustments. An exploratory WGCNA prioritized two candidate modules with contrasting expression patterns. Putative hub genes within these modules were associated with cellular homeostasis, redox and osmotic metabolism, regulation, photosynthesis, central metabolism, and photorespiration. qRT-PCR analysis of six representative candidate genes supported their drought-responsive transcript abundance patterns. Overall, sustained drought elicited a response characterized by water conservation and cellular protection alongside reduced carbon assimilation and apparent photorespiratory metabolism. These findings provide preliminary physiological and transcriptomic evidence for further evaluating M. paniculatus as a candidate native tree for tropical coastal shelter forest restoration.