Abstract
High temperature is a major environmental stress that severely limits passionfruit (Passiflora edulis) productivity by impairing floral initiation. However, the physiological and molecular mechanisms underlying this process remain poorly understood. In this study, we investigated the effects of varying durations and intensities of heat stress on flower bud differentiation in passionfruit. Our results showed that prolonged exposure to temperatures above 35 °C significantly delayed or completely inhibited bud formation, accompanied by altered carbohydrate and nitrogen metabolism, accumulation of osmolytes (soluble protein and proline), and dynamic changes in antioxidant enzyme activities (SOD, POD, CAT). Notably, short-term heat stress induced a transient increase in salicylic acid (SA) levels and upregulation of SA biosynthesis genes (PeEDS1.2, PeICS1) and WRKY transcription factors (PeWRKY11/15), which were associated with sustained floral initiation. In contrast, prolonged stress suppressed SA accumulation and signaling, leading to bud abortion. Comparative transcriptomic analysis further revealed enrichment of pathways related to secondary metabolite biosynthesis, plant hormone signal transduction, and phenylpropanoid biosynthesis under heat stress. These findings highlight the critical role of SA in balancing heat tolerance and reproductive development and provide candidate gene resources for the molecular breeding of heat-resistant passionfruit varieties. This study offers new insights into the thermotolerance mechanisms of fruit crops under sustained high-temperature stress.