Dual environmental thresholds and indications of reversible growth-mode regulation in Adenium dhofarense under Dhofar’s hyper-seasonal climate

抄録

Adenium dhofarense is endemic to the Dhofar region of southern Oman, where it experiences an extreme hyper-seasonal climate: a prolonged cool, clouded monsoon (khareef) followed by a brief post-monsoon interval of intense heat and high irradiance. We monitored 20 seedlings over 216 days under outdoor conditions in Japan, documenting caudex growth across varying temperature and light regimes. Under conditions adequate for congeners, A. dhofarense exhibited severe growth suppression. Rapid growth commenced only when high temperature (≥35°C) and high daily light integral (≥23 mol m⁻² day⁻¹) co-occurred, producing 3.5-fold growth acceleration. These thresholds closely match Dhofar's post-monsoon environmental conditions (40–45°C, estimated 25–31 mol m⁻² day⁻¹). The abrupt, reversible transition between slow-growth and rapid-growth states—triggered strictly by dual-threshold attainment—suggests a potential shift between distinct metabolic modes, possibly involving facultative engagement of C₃-like photosynthesis from a CAM-like default state. This interpretation, while requiring direct physiological validation, is consistent with adaptation to Dhofar's narrow annual growth window. The strict environmental dependency renders A. dhofarense vulnerable to climate-driven alterations in monsoon timing or intensity, with direct implications for conservation of this endemic species in one of Arabia's most climatically unique ecosystems. Keywords: Adenium dhofarense; photothermal threshold; Dhofar monsoon; hyper-seasonal adaptation; facultative CAM; climate vulnerability; desert plant conservation