Concurrent effects of elevated carbon dioxide and temperatures on the polyphenolics profile, in-vitro selected antioxidant and antimicrobial activities in Carpobrotus edulis (L.) leaves

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Anthropogenic activities have led to the accumulation of carbon dioxide in the atmosphere and consequently the elevation of temperature. Carbon dioxide and temperature influence the physiological and biochemical activities in plants and thus, are important for plant survival, growth, and development. The sessile nature of plants prevents them from physically avoiding environmental factors such as high CO2 and temperatures, as a result they have developed a physiological response mechanism for protection. This mechanism involves the production of secondary metabolites, which in return have human-health benefits. How then is the production of secondary metabolites impacted by rising atmospheric CO2 concentrations and temperatures? Moreover, is the pharmaceutical efficacy of medicinal plants impacted as the atmospheric CO2 concentrations and temperature conditions rise due to climate change? This study therefore, aimed to investigate the possible modifications in the composition of polyphenolic compounds, antioxidant and antibacterial activities in Carpobrotus edulis leaves under controlled concurrent elevated atmospheric CO2 and temperatures. A total of 36 C. edulis potted plant samples, constituting 12 pots, divided into 3 pots per treatments were exposed to combined 600 ppm and 35/30°C (day/night), 600 ppm and 45/35°C (day/night), 800 ppm and 35/30°C (day/night), 800 ppm and 45/35 °C (day/night), respectively. The control samples were kept at ambient conditions of combined 400 ppm and 28/25°C (day/night). The plant samples were exposed to these conditions for up to 192 hours, and leave samples were harvested episodically every 48 hours (48, 96, 144 and 192 hours) during the exposure period. All harvested leave samples were air-dried under 40°C and crude extracts were obtained using methanol. Preliminary phytochemical screening was performed to test the presence of tannins, phenolics, flavonoids, steroids, terpenoids, 6 glycosides, and saponins. The LC-MS/MS method was used to profile the polyphenolic compounds and 2,2-diphenyl-1- picrylhydrazyl (DPPH) used to measure the antioxidant activity of the plant. The antibacterial activity of C. edulis was determined by the use of two popular bacterial strains, Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive), using the Agar well diffusion method. The preliminary phytochemical screening results showed the consistent presence of tannins, phenolics, flavonoids, steroids, terpenoids, glycosides, and saponins throughout the treatments as compared with the control, however flavonoids were not detected in the samples under combined 800 ppm and 45/35°C, 600 ppm and 35/30°C as well as 800 ppm and 35/30°C. This indicated that combined elevated CO2 and temperatures could have impacted on the production of flavonoids. The LC-MS/MS results showed the presence of 24 polyphenolic compounds in Carpobrotus edulis leaves and of those only 14 (58.83%) were present in C. edulis leaves under ambient/control conditions. Furthermore, the concurrent increment of CO2 concentrations and temperatures prompted the appearance of 10 (41.67%) more compounds. A total of 17 (70.83%) polyphenolic compounds were identified under the 600 ppm and 45/35°C treatment. The presence of these 17 compounds seemed to be influenced by the duration of exposure to these conditions. Polyphenolic compounds profiling showed the disappearance of compounds and appearance of new compounds. The disappearance of some compounds was mainly observed under extreme conditions. Antioxidant activity decreased with increasing combined CO2 concentration and temperature exposure. Antimicrobial activity showed some inhibition of S. aureus and E. coli, and the inhibition activity remained constant in all the climatic conditions. This response maybe attributed to the appearance of new polyphenolic compounds. These results suggested that C. edulis is a strong antioxidant and antimicrobial agent, owing it to the polyphenolic compounds composition. However, these properties could be negatively impacted by elevated CO2 and temperatures, thus influencing the efficacy of C. edulis. Future studies could investigate the influences of elevated CO2 and temperatures independently to assess which factor plays the biggest role in the pharmaceutical efficacy of C. edulis
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science in Animal Plant and Environmental Sciences to the Faculty of Science, University of the Witwatersrand, Johannesburg, 2022
Antibacterial activity, Carpobrotus edulis, Antioxidant activities