Luminosidade no cultivo in vitro de Mentha pulegium L. (Lamiaceae): impactos no crescimento e produção de constituintes químicos voláteis e fenólicos

Abstract

Mentha pulegium L. (Lamiaceae) is a species with a complex phytochemistry, including phenolic and volatile compounds responsible for acaricidal, insecticidal, antiviral, and antimicrobial activities. The intensity and spectral quality of light can cause modifications in the totipotency of plant cells, leading to structural and chemical responses. The aim was to evaluate the impacts of light intensity and quality on the growth and production of photosynthetic pigments, total phenolic and flavones/flavonols compounds, and volatile constituents of in vitro cultivated M. pulegium. Apical segments were excised from mother plants cultivated in beds at the Medicinal Plants Garden of UFLA, washed in running water for 30 min, followed by immersion in a 50% (v/v) commercial sodium hypochlorite solution for 15 min with constant agitation. These segments were established and multiplied on basic MS medium (Murashige and Skoog, 1962) until obtaining the necessary propagules for the experiments. Nodal segments (1 cm) were excised from pre-established plantlets and inoculated into tubes containing 15 mL of MS medium, supplemented with 30 g L-1 of sucrose and 5.5 g L-1 of agar. Two independent experiments were conducted. The first experiment consisted of 5 treatments with different light intensities (26, 51, 69, 94, and 130 μmol m-2 s -1 ), and the second one with 6 treatments of light spectral quality: blue, red, 30% blue:70% red, 70% blue:30% red, 50% blue:50% red, and white. After 30 days, growth analyses, photosynthetic pigment quantification, total phenolic and flavones/flavonols compounds content, and chemical analysis of the volatile fraction were performed. Intensity levels of 94 and 130 μmol m-2 s -1 resulted in higher dry matter accumulation, but did not affect the content of photosynthetic pigments and pulegone. However, principal component analysis showed a tendency for higher pulegone accumulation in seedlings grown under low light intensities of 26 and 51 μmol m-2 s -1 . Seedlings grown under 130 μmol m-2 s -1 accumulated the highest content of phenolic compounds, and those cultivated under an intensity of 94 μmol m-2 s -1 produced more flavones/flavonols. In the light spectral quality experiment, blue light and the combination of 50% blue:50% red showed the greatest biometric responses. Monochromatic blue light also provided greater in vitro pulegone production (80.64%), while monochromatic red light exhibited a decrease of about 10% in content compared to blue (72.95%). Light spectral quality did not affect the accumulation of flavones/flavonols, but the blue component increased the content of phenolic compounds in in vitro M. pulegium. The highest accumulations of photosynthetic pigments were recorded in white light and in the combination of 30% blue and 70% red. Therefore, in vitro cultivation of Mentha pulegium should be conducted under light intensities in the range of 94 to 130 μmol m-2 s -1 or under total or partial presence of blue LED.