Cultivar differences in heat tolerance of Oncidium orchids: physiological mechanisms and implications for breeding strategies

Oncidium orchids are a core economic genus in the global tropical ornamental flower industry, while sustained high temperature has become the key bottleneck restricting its commercial production. The coordinated physiological mechanism of heat adaptation, unified screening temperature threshold, and practical physiological biomarkers for heat tolerance evaluation in Oncidium remain insufficiently characterized.

This study systematically evaluated the heat tolerance of 36 Oncidium cultivars through natural field heatwave screening, controlled environment validation, and physiological mechanism dissection under graded temperature stress.

We screened 8 highly heat-tolerant cultivars from 36 tested materials, and identified 40.00°C as the optimal discriminative temperature for heat tolerance screening. Relative electrolyte conductivity (REC), proline content, and peroxidase (POD) activity were confirmed as the core physiological biomarkers (coefficient of variation > 50.00%). Principal component analysis revealed that membrane stability and osmotic adjustment were the primary determinants of Oncidium heat adaptation, explaining 80.95% of the total variance. Notably, our data suggested that excessive proline accumulation in heat-sensitive cultivars was a passive compensatory response after cell damage, rather than an active protective adaptation, which differs from the conventional view that higher proline accumulation indicates stronger stress tolerance.

These findings characterize physiological responses associated with heat tolerance in Oncidium and provide practical reference markers and a physiological basis for future genetic and molecular studies in orchids.