Root Hydraulic and Metabolic Regulation Drives Drought Tolerance in Napier Grass

Napier grass (Cenchrus purpureus syn. Pennisetum purpureum), a perennial C₄ forage and bioenergy crop, exhibits strong drought resilience, yet the integrative mechanisms underlying this tolerance remain incompletely understood. This study examined physiological, hydraulic, and metabolic responses of four Napier grass cultivars under PEG-induced osmotic stress and progressive soil water deficit. Drought significantly increased the root-to-shoot ratio, indicating preferential biomass allocation to roots, which supported maintenance of shoot growth and tissue water status. All cultivars showed an approximate twofold increase in water-use efficiency (WUE) under water deficit, with cv2 and cv7 displaying superior performance. Upregulation of aquaporin genes (PIP2;2 and PIP2;3) suggested active hydraulic regulation that sustained carbon assimilation under reduced transpiration. Metabolic profiling revealed pronounced root-centered osmotic adjustment, including accumulation of galactinol, myo-inositol, raffinose family oligosaccharides, proline, and several amino acids. Enhanced expression of the galactinol synthase gene confirmed activation of raffinose biosynthesis pathways. Genotypic variation highlighted cv2 as particularly drought resilient. Rapid post-stress regrowth further underscored the importance of perennial root persistence. In conclusion, drought tolerance in Napier grass arises from coordinated hydraulic resilience, osmotic adjustment, and C₄ photosynthetic efficiency, supporting its suitability for forage and bioenergy production in water-limited environments.