Exogenous Thiamin Application Enhances Betalain Production, Bioactive Compound Accumulation and Physiology Performance in Celosia argentea L. var. cristata

(1) Background: Natural pigments and bioactive compounds have attracted increasing interest as sustainable alternatives to synthetic colorants in food and industrial applications. This study evaluated the effects of foliar-applied thiamin (vitamin B1) on the physiological, growth, and biochemical responses of Celosia argentea L. var. cristata. (2) Methods: Plants are exposed to different concentrations of thiamin (0, 50, 100, 150, and 200 mg L−1) and cultivated under protected conditions, in a semi-hydroponic system. (3) Results: Increases of 17.53% in CO2 assimilation rate, 34.0% in carboxylation efficiency, 15.06% in flower fresh weight, and 14.63% in flower dry weight were observed under lower concentrations (50–100 mg L−1). In contrast, higher concentrations (150–200 mg L−1) stimulated the accumulation of flavonoid content, increased by 9.93%. At 200 mg L−1, betacyanin, betaxanthin, and total betalain contents increased by 22.04%, 25.97%, and 23.55%, respectively. (4) Conclusions: Thiamin application induces concentration-dependent responses in Celosia argentea L. Concentrations up to 100 mg L−1 enhance flower production and biomass accumulation, while higher doses stimulate the accumulation of bioactive compounds, antioxidant metabolites, and betalains in flowers. These effects highlight thiamin’s potential to improve ornamental quality and support natural colorant production for food, pharmaceutical, cosmetic, and textile applications.