Exogenous zeaxanthin enhances cold tolerance in Phalaenopsis-type Dendrobium seedlings through physiological regulation and transcriptomic reprogramming

Phalaenopsis-type Dendrobium , a significant tropical orchid, is highly sensitive to low temperatures, limiting its cultivation in cooler climates. This study investigated the physiological and transcriptomic responses of Dendrobium seedlings to exogenous zeaxanthin to develop cold-tolerance strategies. Among the five non-brassinosteroid candidate compounds analyzed in the present study, zeaxanthin (optimal concentration: 50 mg L-1 ) provided the most consistent protection. Its application significantly mitigated cold-induced damage through several mechanisms: Cold tolerance: it reduced leaf chlorosis and lowered the semi-lethal temperature (LT50) from 6.67 °C in the control to 3.02 °C at 50 mg L-1 zeaxanthin. Membrane and osmotic stability: malondialdehyde (MDA) levels dropped by 64.2%, while soluble sugars (+37.8%), soluble proteins (+88.7%), and proline (+118.9%) increased. Photosynthetic integrity: zeaxanthin stabilized chlorophyll levels and the photosynthetic apparatus during stress and recovery. Transcriptional reprogramming: RNA-seq and WGCNA revealed zeaxanthin-responsive genes enriched in hormone signaling, MAPK cascades, photosynthesis, and stress-related metabolism. The transcriptomic patterns were consistent with a putative resource reallocation between photosynthesis-related and defense-related programs; because carbon assimilation, electron transport rate, and non-photochemical quenching were not directly measured, this interpretation remains inferential. Efficacy varied with cultivar and seedling age, with ‘Reke Mini’ and younger seedlings showing higher responsiveness. Overall, exogenous zeaxanthin enhances cold resilience in Dendrobium through coordinated membrane stabilization, osmotic adjustment, and transcriptional regulation, providing a foundation for stage-specific cultivation strategies.