Melatonin mitigates yield losses in off-season common bean (Phaseolus vulgaris L.) under combined heat and drought stress conditions

ABSTRACT Melatonin has emerged as a promising bio-stimulant for enhancing stress tolerance, yet its effectiveness across common bean genotypes and multiple stress factors remain insufficiently explored. This study evaluated the influence of foliar-applied melatonin on yield components, antioxidant responses, and physiological traits in diverse common bean genotypes exposed to heat stress (HS) and combined drought and heat stress (CDHS) conditions. Melatonin application enhanced yield by 8.7% under HS and 22.4% under CDHS compared with the water-spray control ( p < 0.01), with maximum improvements observed in genotype NIAM-236-20 under HS (15.4%) and in BIO-98 (36.4%) and IPR-11-22 (32.9%) under CDHS. Improvements in pods per plant, seeds per pod, and 100-seed weight with melatonin spray were more pronounced under CDHS (14.5–15.6%) than under HS (6.7–9.3%). Melatonin application further boosted catalase, peroxidase, superoxide dismutase (SOD), ferric reducing antioxidant power (FRAP), and DPPH radical scavenging activities under both stress regimes, while significantly lowering malondialdehyde (MDA) levels. Melatonin-induced improvements in chlorophyll and carotenoid content, photosystem II efficiency (Fv/Fm), and proline accumulation were more pronounced under CDHS. Correlation analysis demonstrated that SOD and DPPH activities were the strongest positive predictors of yield under control and melatonin-treated plants, respectively, whereas MDA showed the strongest negative association. PCA and stepwise regression further highlighted the influence of antioxidant traits, especially FRAP, to grain yield variability. These findings demonstrate the potential of melatonin as an exogenous regulator for enhancing stress resilience in common bean, while genotype-specific responses suggest opportunities for targeted screening and melatonin-integrated management strategies in climate-vulnerable conditions.