Physiological, Morphological, and Transcriptomic Basis of Biostimulant-Induced Abiotic Stress Resilience in Horticultural Crops: A Review

Climate change is increasing the frequency and severity of abiotic stresses, including salinity, drought, heat, and heavy metal toxicity, which strongly threaten productivity, quality, and market value in horticultural crops. Although plant biostimulants have been widely reviewed in agronomic crops, their role in horticultural systems remains less comprehensively synthesized, particularly across different crop groups, stress types, application methods, and molecular response mechanisms. This review addresses this gap by systematically compiling current evidence on the use of biostimulants to improve abiotic stress resilience in horticultural crops, with particular emphasis on morphological, physiological, biochemical, and recently emerging molecular responses, especially transcriptomic evidence with supporting metabolomic information where available. Publications were retrieved from the Web of Science Core Collection using two searches covering 2016–2025 for morphological/physiological responses and 2021–2025 for molecular/genetic responses. Of 780 records initially identified, 134 studies met the inclusion criteria. Across these studies, the most frequently evaluated biostimulants were seaweed extracts, humic and fulvic substances, protein hydrolysates, and microbial inoculants, particularly PGPR and AMF. Biostimulant application consistently improved stress tolerance by enhancing antioxidant capacity, osmotic adjustment, nutrient use efficiency, cell wall strengthening, and hormonal regulation. Foliar applications were frequently used for rapid mitigation of drought- and heat-induced canopy-level physiological responses, whereas soil/root-zone application was more common for salinity and heavy metal stress. Emerging molecular evidence, dominated by transcriptomic studies and supported by limited metabolomic data, indicates that biostimulants may induce molecular priming through stress-responsive gene networks and associated metabolic adjustments. Overall, biostimulants show strong potential to improve abiotic stress resilience in horticultural crops, but broader adoption is constrained by variable efficacy, limited mechanistic validation, and inconsistent regulatory frameworks. Future research should prioritize multi-environment validation and functional genetics to support more reliable and targeted biostimulant use.