Melatonin's dynamic instability in Portulaca: Decoding its probable link with non-directional responses and abiotic factors

• Root-specific melatonin peaks show clear diel and seasonal rhythms. • Temperature governs melatonin dynamics more than light. • Prolonged stress limits melatonin’s protective nastic functions. • Melatonin is structurally conserved but synthesized via plant-specific enzymes. • Portulaca emerges as a model for thermoresponsive chronohormonal studies. Melatonin, a plant-derived indoleamine structurally analogous to auxin, is proposed to govern circadian rhythms, abiotic stress responses, and turgor-driven nastic movements. This study dissects its spatiotemporal dynamics, thermal sensitivity, and tissue redistribution in CAM-adapted Portulaca , positioning melatonin as a thermoresponsive chronohormone orchestrating environmental and physiological synchronization. Portulaca oleracea and P. grandiflora were grown under controlled, pest-free conditions and observed for nastic movements and endogenous melatonin content across tissues, zeitgeber times, and four seasons. Conditioning studies employed specific light/dark regimes and measured melatonin by RP-HPLC, LC-MS/MS, and NMR. Melatonin degradation under thermal and photic stress was assessed. All quantifications were performed in triplicate, with statistical analysis by one-way ANOVA and heat map visualization for group comparisons. Portulaca exhibits a pronounced root-centric enrichment and rhythmic, temperature-driven redistribution of melatonin, with peak concentrations aligning with pre-summer thermal maxima. The consistently low melatonin content in aerial organs, especially flowers, directly challenges existing paradigms of reproductive enrichment. Thermal stress markedly compromises melatonin stability, confirming its rapid turnover and highlighting its role as a dynamic, but conditional, stress buffer. Notably, elevated melatonin alone does not fully sustain rhythmic nastic movements under persistent stress. Structural analyses affirm evolutionary conservation, while observed genotypic and tissue-specific differences underscore the sophistication of melatonin regulation in this CAM-adapted system. Melatonin in Portulaca acts as a dynamic, temperature-driven signal, coordinating tissue-specific adaptation and stress resilience, with root-centric enrichment and rapid aerial turnover, establishing Portulaca as a model for chronohormonal research.