Biostimulating activity of biomass extracts and supernatants from a culture of Arthrospira platensis enriched with L-tryptophan

The present study was conducted with the aim of evaluating the effects, over 6 days, of different intensities of continuous light (20, 40, and 60 µmol photons m^-2 s^-1) on the growth of Arthrospira platensis and its impact on the production of phycocyanin, carotenoids, and intracellular/extracellular auxins, using a medium supplemented with 0.5 g L^-1 of L-tryptophan. Additionally, the study aimed to assess the effect of treatments on the biostimulant activity of the supernatant (T₀: untreated, T₁: treated) compared to that of biomass extracts (T₂). T₀, T₁, and T₂ were adjusted to a concentration of 0.1 mg L^-1 of indole-3-acetic acid (IAA) and a pH of 7.4. The results revealed that light intensities of 40 and 60 µmol photons m^-2 s^-1 produced the highest levels of extracellular IAA on day 2 (20.1 and 33.01 mg L^-1, respectively), while an intensity of 20 µmol photons m^-2 s^-1 reached peak IAA production on day 3 (25.74 mg L^-1), followed by a subsequent decrease. Phycocyanin concentrations markedly decreased after maximal IAA production under all light conditions. Tukey's analysis indicated that treatments T₁ and T₂ significantly increased the number and length of secondary roots in mung beans by 157% and 350%, respectively, with no significant differences between them, while the untreated supernatant (T₀) exhibited a minor effect. The study concludes that under all assessed light conditions, concentrations of phycocyanin and carotenoids drastically decreased after peak IAA production, gradually recovering in the following days. Additionally, the treated supernatant and biomass extract significantly enhanced root growth in mung beans, highlighting the importance of the treatment method for biostimulation and suggesting potential for improved storage and transportation. Future research should focus on refining supernatant treatments to optimize biostimulant activity and facilitate commercialization.