Plant Biostimulants Under Abiotic Stress: Functional Convergence from Redox Regulation and Photosynthetic Protection to Crop Productivity, with Glycine Betaine as a Mechanistic Case Study

Plant biostimulants differ in composition, and agronomic value depends on whether they modify physiological processes that limit crop performance. This review examines functional convergence under abiotic stress, linking root and rhizosphere responses with yield formation. Foundational and regulatory sources, mechanistic studies, controlled applications, and greenhouse and field trials were evaluated for the inferences they support. Humic substances, protein hydrolysates, amino acid products, seaweed extracts, chitosan, beneficial elements, and microbial inoculants act through different pathways. Reported responses include nutrient acquisition, osmotic adjustment, ion homeostasis, membrane and protein stability, ROS homeostasis, stomatal conductance, PSII function, and net CO2 assimilation. Glycine betaine (GB) is used as a mechanistic case study, not a universal model. Mechanistic evidence links compatible solute function with D1 turnover, PSII repair, Rubisco and Rubisco activase stability, thylakoid energy coupling, metabolic energy balance, and redox control. These responses may matter when they preserve source activity, reproductive organ establishment, or sink filling during yield formation. Field responses depend on genotype, stress intensity, soil and nutrient status, formulation, dose, application method, and phenological stage. Biostimulant assessment should consider diagnosed constraints and observed effects on yield, quality, nitrogen use efficiency, water use efficiency, mineral nutrition, and overall crop productivity.