Exogenous Application of Plant Growth Regulators Enhances Short-Term Cold Stress Tolerance in African Marigold Under Field Conditions

Cold stress is a major environmental constraint limiting the growth, physiological performance, and productivity of African marigold (Tagetes erecta L.) under open-field conditions. This study evaluated the comparative effectiveness of salicylic acid (SA), silicon (Si), and methyl jasmonate (MeJA) in alleviating cold-induced damage and enhancing stress tolerance. Field experiments were conducted under naturally occurring cold stress using foliar applications of SA (0, 0.1, 0.5, and 1 mM), Si (0, 1, 5, and 10 mM), and MeJA (0, 10, and 50 µM) in a complete randomized block design with three replications. Moderate concentrations of all three regulators significantly (p < 0.05) improved plant growth and physiological stability relative to untreated controls. Salicylic acid at 0.5 mM produced the most consistent protective response, increasing biomass accumulation, chlorophyll content, and relative water content while reducing membrane damage, as indicated by a 42.3% decrease in leaf electrolyte leakage at 2 °C. Silicon at 10 mM enhanced membrane integrity, plant water status, and vegetative growth under low-temperature conditions, while methyl jasmonate at 10 µM mitigated cold-induced membrane damage and improved physiological tolerance, whereas higher concentrations (50 µM) were less effective. At their optimal doses, SA, Si, and MeJA increased plant dry mass by 39.7%, 30.1%, and 38.5%, respectively. Correlation analysis confirmed these results, revealing strong positive relationships among growth, chlorophyll, and relative water content. Conversely, electrolyte leakage was negatively correlated with biomass and water status, identifying membrane stability as a key determinant of cold tolerance. Overall, 0.5 mM SA, 5–10 mM Si, and 10 μM MeJA improved growth and key physiological responses in African marigold under cold stress under field conditions. Future studies should integrate mechanistic and economic analyses to refine growth-regulator-based cold-stress management strategies.