Environmental and transcriptomic determinants of drought response in critically endangered Siamese rosewood

Rosewoods account for up to 40% of the global illegal wildlife trade, with Dalbergia cochinchinensis (Siamese rosewood) being the most heavily exploited species in Southeast Asia. Its survival is further threatened by intensifying drought linked to climate change and hydrological alteration. Here we combine greenhouse drought experiments across six provenances with full-length cDNA-seq to uncover how water-relations and carbon-use strategies vary within this species. Multivariate trait analysis resolves a two-dimensional isohydry space, in which a water-flux stringency axis (gs-E) is largely orthogonal to a carbon-economics axis (A-WUEi). Provenances differed strikingly, where two (KKH and DN) showed a rare E{downarrow}A{uparrow} response, achieving high WUEi and maintaining growth under drought. Contrary to expectation, precipitation of the wettest month, not the driest, predicted isohydry, indicating that wet-season conditions set a developmental and hydrological floor for later drought responses. We identified 76 drought-responsive genes and two genes associated with isohydry axes, SEOR1 and a poorly characterised Notch-like protein AT4G14746. We also detected provenance-specific isoform switches, where drought favoured a loss-of-function PRX52 isoform lacking its signal peptide in the anisohydric provenance THB, and gain-of-function isoforms of ANN3 and LTPG5 in NP. These results reveal previously hidden diversity in drought strategies, identify mechanism-related markers for screening, and provide a simple climatic lever for climate-adjusted provenancing. We reveal post-transcriptional regulation as a novel candidate substrate for local adaptation in a threatened tropical tree, directly linking ecophysiology, climate, and genomics for conservation and restoration.