Integrated transcriptomic and functional trait responses reveal ecological strategy-associated biomass accumulation and allocation under drought in mulberry seedlings.

Drought alters both plant growth and biomass accumulation and allocation, but how molecular responses and functional traits jointly reflect these changes in woody plants remains insufficiently understood. We investigated whether plant ecological strategies, quantified using the competitive-stress tolerant-ruderal (CSR) framework, could provide an integrative description of drought-associated biomass responses in mulberry (Morus atropurpurea) seedlings. In this study, mulberry seedlings were maintained under well-watered, mild-drought, or moderate-drought conditions for 30 days, after which biomass accumulation and allocation, leaf and root functional traits, physiological and biochemical responses, and leaf transcriptomes were assessed. Transcriptomic and functional trait analyses revealed coordinated tradeoffs between growth-related processes and stress-defense responses. Total biomass was positively associated with photosynthetic performance, leaf and root growth traits, and genes related to light harvesting and carbon metabolism, whereas root-to-aboveground biomass ratio was positively associated with osmotic adjustment, antioxidant defense, ABA-related signaling pathways, and conservative root traits. The CSR scores revealed a corresponding shift from a predominantly competitive profile under well-watered conditions toward greater stress tolerance under moderate drought. Competitive scores covaried with resource-acquisitive traits and total biomass, whereas stress-tolerant scores covaried with cellular defense and increased relative belowground allocation. This result indicated that the shift in mulberry ecological strategies under drought stress represents an integrative adaptive syndrome that aligns with coordinated changes in biomass accumulation and allocation. The enhancement of competitive strategy is associated with greater aboveground growth and biomass accumulation, while the enhancement of stress-tolerant strategy corresponds to reduced overall growth and increased underground biomass allocation-patterns that collectively shape the balance between drought resistance and growth productivity in mulberry.