Effects of Gamma Stress and Carbon Dioxide on Eight Bioactive Flavonoids and Photosynthetic Efficiency in Centella asiatica

Abstract

Increased atmospheric CO2 and gamma irradiation have a significant impact on the plant photosynthetic apparatus and organic compound production. In this study, we evaluated the effect of elevated CO2 on the photosynthetic efficiency and production of defensive secondary metabolites (flavonoids) induced by gamma irradiation as a physical elicitor in Centella asiatica. Irradiated and non-irradiated 10-week-old plants of C. asiatica were exposed to 400 and 800 μmol mol−1 of atmospheric CO2 in growth chambers for 2 h every day until six weeks. A CO2-enriched atmosphere initially improved the photosynthetic efficiency and ameliorated the detrimental impact of gamma irradiation on the photosynthetic apparatus, increasing carbon allocation into the flavonoid pathway. Elevated CO2 combined with gamma irradiation resulted in the highest concentration of flavonoids in C. asiatica tissues compared with the other treatments. There was an enhancement in rutin (2.49 fold), naringin (2.15 fold), fisetin (4.07 fold), and morin (4.62 fold) with rising CO2 concentrations from 400 to 800 μmol mol−1 in the irradiated plants. With increasing CO2 concentration, the compensation point and the respiration declined, whereas the apparent quantum yield and the maximum net photosynthesis (Amax) rate increased. The efficiency of photosystem II (PSII) was improved in the irradiated plants grown under high concentrations of CO2. The total carbohydrate concentration reached the maximum value at the highest level of CO2, followed by gamma irradiation combined with the highest level of CO2. Irradiated plants of C. asiatica grown under elevated CO2 could be superior to non-irradiated plants due to increased carbon availability both for the flavonoid biosynthesis and for the photosynthetic pathway.