Timing Matters: Post-Stress Biostimulant Application Promotes Growth Recovery and Physiological Rebalancing in Drought-Stressed Durum Wheat (Triticum durum Desf.).

As climate change intensifies drought across the Mediterranean basin, safeguarding durum wheat, a cornerstone of regional agriculture, is a global priority. This study evaluated the efficacy of a novel biostimulant, containing glycine betaine, vaterite, and Pseudomonas protegens, with a specific focus on a critical factor: the application timing. To distinguish preventive (pre-stress) and restorative (post-stress) strategies, the treatment was applied either before or after drought induction in drought-sensitive durum wheat seedlings. Drought severely inhibited growth, increased lipid peroxidation, and altered nutrient profiles. The effect of the biostimulant was highly dependent on application timing. Pre-stress application mainly induced tissue-specific metabolic and oxidative responses. In contrast, post-stress treatment promoted recovery of shoot biomass to values comparable to well-watered controls and was associated with increased stomatal density and leaf greenness relative to untreated drought-stressed plants. Post-stress treatment also produced a root morphological profile closer to that of control plants and a shoot ionomic pattern more similar to well-watered plants. Selected stress-responsive genes showed lower transcript levels in biostimulant-treated than in untreated drought-stressed plants, indicating treatment-associated changes in the expression of specific stress-related pathways during recovery. These findings highlight post-stress biostimulant application as a promising timing-dependent strategy to support the recovery of drought-sensitive durum wheat seedlings.