HARNESSING SEAWEED-BASED BIOSTIMULANTS FOR ENHANCING ABIOTIC STRESS TOLERANCE IN FIELD CROPS: MECHANISMS, PROSPECTS AND FUTURE DIRECTIONS

Abstract

Field crops like cereals, pulses, oilseeds and fibre and sugar crops supply the bulk of global food, feed and fibre; however, a large proportion of the area under these crops is rainfed or only marginally irrigated. Under such conditions, drought, heat, salinity and the physical constraints of heavy-textured soils depress productivity and quality. Reported penalties are substantial and crop-specific: grain yield losses approaching 46 % in soybean at 40 % field capacity, marked declines in shoot biomass, pigments and starch in wheat under terminal water deficit and yield reductions of 10–30 % in cotton under field drought. Against this background, seaweed-derived biostimulants have gained attention as low-toxicity, multi-functional inputs capable of raising abiotic stress tolerance without the regulatory and cost barriers associated with genetic or high-input approaches. This review synthesises current evidence on macroalgal biostimulants across the major field crop groups. It first characterises the chemical basis of activity alginates, fucoidans, laminarins, carrageenans and ulvans, together with phlorotannins, betaines, hormone analogues, free amino acids and chelated nutrients and relates compositional variability to taxonomy, phenology, harvest origin and extraction route. It then integrates the mechanistic literature into six converging domains: antioxidant priming and reactive oxygen species homeostasis; phytohormone modulation, particularly abscisic acid sensitisation and cytokinin-like activity; osmolyte accumulation and cellular hydration; photosynthetic apparatus protection; activation of stress-responsive transcription factor networks; and membrane stability with ion homeostasis. Crop-wise agronomic evidence is compiled for wheat, rice, maize, sorghum, soybean, faba bean, chickpea, green gram, canola, sugarcane, cotton and forage grasses, alongside soil-mediated benefits in root architecture, aggregate stability, water retention and rhizosphere function. Meta-analytical estimates converge on pooled yield gains of roughly 10–20 % under stress, but efficacy is strongly conditioned by application strategy, formulation and extraction method. Persistent constraints product heterogeneity, absence of standardised potency assays, dominance of single-season single-site trials, hormetic dose responses and thin economic evidence for smallholders are examined and a conceptual framework with prioritised research actions is proposed for integrating seaweed biostimulants into climate-resilient field crop production.