Plant Growth Regulators for Abiotic Stress Alleviation in Spice Crops: A Critical Appraisal of Mechanisms, Evidence Quality and Translational Gaps

Abstract

Spice crops occupy a minor share of global cropland yet underpin export earnings, smallholder livelihoods and food quality across the tropics and subtropics. Their production systems are unusually exposed to water deficit, soil salinity and temperature extremes, and their value depends on secondary metabolites whose accumulation is itself stress sensitive. Exogenous plant growth regulators have been promoted as a rapid, low-capital means of buffering these constraints, but the evidence supporting that promise has not been appraised critically for spice crops as a group. This review synthesises experimental and mechanistic literature on regulator application under abiotic stress in turmeric, ginger, black pepper, cardamom, chilli, coriander, cumin, fennel, fenugreek, ajowan and saffron, retrieved through structured searching of scholarly databases and citation tracking. Evidence is organised by regulator class and by crop group, and is evaluated against experimental scale, dose design, outcome selection and genotype sampling. Salicylic acid dominates the literature and produces consistent redox and photosynthetic benefits at low concentrations, yet its effects on marketable yield remain sparsely documented. Melatonin and brassinosteroids show rapidly accumulating support in solanaceous and leguminous spices, while polyamines, jasmonates and abscisic acid have been examined in only a few species. Rhizomatous and perennial spices, particularly black pepper and cardamom, remain almost untested despite well-characterised stress sensitivity. Three weaknesses recur: reliance on short-term pot and in vitro systems, narrow dose ranges that obscure concentration-dependent reversals, and outcome sets dominated by biochemical surrogates rather than yield and quality. A further unresolved tension concerns metabolite concentration and metabolite yield, which stress and regulator treatments can move in opposite directions. Confidence in current conclusions is therefore moderate for physiological protection, low for field-scale productivity, and very low for economic returns. Multi-season, multi-genotype field experiments reporting dose–response relationships and harvestable quality are the priority.