Different light spectra elicited distinctive growth, metabolomics profiles and antioxidant capacity in Azolla filiculoides.

Abstract

Azolla are floating ferns suitable for low-input farming systems. Among others, Azolla filiculoides is a fast-growing fern whose rapid acclimation makes it a useful system to investigate light-dependent modulation of growth and specialized metabolism. In this study, A. filiculoides plants were exposed for 5 d to monochromatic blue or red light, two blue:red mixtures, and solar-like spectra at two irradiance levels. Thereafter, plants were characterized by integrating gas exchange, pigment profiling, oxidative markers, antioxidant assays and high-resolution mass spectrometry untargeted metabolomics. The relative abundance of Trichormus azollae, the A. filiculoides nitrogen-fixing symbiotic cyanobacterium, was also monitored. Red light maximized net CO2 assimilation, radiation-use efficiency and biomass accumulation, whereas blue light reduced the presence of the symbiotic cyanobacterium and Azolla plant biomass, while triggering a marked oxidative burst. At the metabolomics level, blue light preferentially elicited phenolic acids and other low-molecular-weight phenolics, and these metabolic changes correlated with an enhanced antioxidant response. By contrast, supplementation of blue light with a limited red fraction (4:1 blue:red) promoted the accumulation of anthocyanins, flavanones, flavonols, flavanols, isoflavonoids, phlobaphenes and 3-deoxyanthocyanidins while lowering hydrogen peroxide and membrane lipid peroxidation, compared to blue light alone. A high-irradiance solar-like treatment produced a partly overlapping metabolic output but required a substan\tially greater photon input. Overall, light spectral composition emerged as a primary determinant of the trade-off between growth and specialized metabolism in A. filiculoides. The results indicate that a blue-dominant spectrum containing a small red component is sufficient to promote flavonoid-rich photoacclimation, providing a physiological basis for light treatments aimed at obtaining phenylpropanoid - enriched Azolla holobiont.

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