Glycine Betaine-Induced Responses Under Saline Stress in Passiflora edulis f. flavicarpa

Salinity constrains tropical fruit production, yet the physiological and metabolic responses of Passiflora edulis Sims f. flavicarpa to salt stress and glycine betaine (GB) remain insufficiently characterized. Passion fruit seedlings were evaluated in a completely randomized 2 × 3 factorial design with two GB treatments (0 and 100 mM), three salinity levels (0, 1.6, and 3.2 dS m−1), and five biological replicates per combination. Two-way ANOVA followed by Tukey’s test at 5% was used where applicable. Leaf water potential, SPAD, detached-leaf water loss, and GC–MS-based metabolite profiles were assessed. At 3.2 dS m−1, plants without GB exhibited the most negative leaf water potential, whereas GB maintained a significantly less negative value. SPAD remained relatively stable, while detached-leaf water-loss patterns varied among treatments. Metabolomic profiling revealed changes in sugars, amino acids, and organic acids. Under the higher salinity treatment, GB was associated with higher levels of glucose, sucrose, glycine, proline, citric acid, malic acid, and succinic acid, together with a distinct PCA profile. These findings indicate that GB partially alleviated effects of the higher salinity treatment by improving plant water status and modifying primary metabolism.