Biosynthesis of rubber and inulin depending on the spectral composition of light and activity of the photosynthetic apparatus during aeroponic cultivation of Taraxacum kok-saghys E. Rodin

Due to the intensive development of industry and new technologies, the demand for natural rubber is increasing. The synthetic rubber cannot replace this biopolymer due to its unique consumer and operational characteristics. Along with the traditional source of natural rubber production from the latex of Brazilian Hevea Hevea brasiliensis (Willd. ex A.Juss.) Müll. Arg., work is underway to obtain it from kok-saghyz plants Taraxacum kok-saghys E. Rodin which can be grown both in natural and controlled conditions. The determination of the most favorable light conditions, taking into account the physiological state of plants, is important to obtain a high yield of target products. In this study, we have shown for the first time an increase in the rate of rubber biosynthesis when irradiating kok-saghyz plants with light with a greater proportion of blue spectrum. The paper also describes the changes in light and dark reactions of the photosynthetic apparatus, and in sucrose and glucose accumulation in plants when changing the light regime for several hours. The aim of the work was to study the influence of light conditions on the physiological and biochemical processes and biosynthesis of rubber and inulin in kok-saghyz plants grown under controlled phytotron conditions. Kok-saghyz seeds (a collection form 391 from the VIR collection of the Vavilov All-Russian Institute of Plant Genetic Resources, St. Petersburg) were germinated under sterile conditions. From days 19-20, when 3-4 true leaves appeared, the plants were grown in the aeroponic phytotron with full-spectrum light-emitting diodes (LED) of photosynthetic active radiation PAR400-700 nm of 400±28 µmol photons·m-2·s-1. In the first chamber of the phytotron, there was a 255,6 µmol photons·m-2·s-1 exposure for PAR400-700 nm blue spectrum (BS, λmax = 460 nm) and 75,6 6 µmol photons·m-2·s-1 for PAR600-700 nm red spectrum (RS, λmax = 660 nm), with the RS/BS ratio of 0.30. In the second chamber, the RS irradiation intensity (PAR600-700 nm) was 259.6 µmol photons·m-2·s-1, the BS irradiation intensity (PAR400-500 nm) was 71.8 µmol photons·m-2·s-1, with the RS/BS ratio of 3.6. The revealed parameters of growth, photosynthetic activity, and accumulation of glucose and sucrose in leaves and rubber and inulin in roots under various spectral modes during long-term growth and with changing the irradiation mode draw us to the following conclusions. When growing plants for 28 days in phytotron chambers with an increased proportion of RS in the spectrum, the content of rubber increased 3-fold, of inulin 4.1-fold compared to the initial values. With an increase in the BS proportion in PAR, the levels of rubber and inulin rose 5.4 times and 4.6 times, respectively. Ultimately, when irradiated with light with a higher proportion of BL, plants accumulated 1.75 times more rubber. The change in the irradiation spectrum from BS to RS led to a short-term increase in the concentration of glucose and sucrose in the leaves compared to the initial values. This dependence persisted for 2 hours, after which the sucrose content did not change, but there was a decrease in glucose content. When the irradiation mode changed from BS to RS, the activity of the photosynthetic apparatus decreased, i.e., the rate of photosynthesis from 26.7 to 15.2 µmol CO2·m-2·s-1 at light saturation, the rate of dark respiration from 2.80 to 2.38 µmol CO2·m-2·s-1, and the quantum yield of photosynthesis from 0.066 to 0.055. Switching from RS to BS led to opposite results. It follows from the obtained data that the change in the concentration of soluble carbohydrates in plants is associated with a change in the spectral composition of irradiation and, as a consequence, with a change in the activity of the photosynthetic apparatus. When the irradiation changed from RS to BS, there was an increase in the rate of photosynthesis and activity of photosystem II, but a decrease in the accumulation of glucose and sucrose during the first 2 hours with a return to the initial values after 3 hours.