Mechanistic insights on physiological, biochemical, and metabolite profiling of pearl millet cultivars focusing bioactive compounds under elevated UV-B radiation.

Ultraviolet-B (UV-B) radiation is a potent stressor showing functional duality in plants. The escalating impact of climate change and UV-B radiation trigger a series of stress acclimation responses in plants, which are initiated through UVR8 signaling and result in accumulation of secondary metabolites. The impact of UV-B on major cereal crops has been explored, but its impact on pearl millet has not been investigated, specifically secondary metabolites linked with beneficial bioactive compounds. Four tropical cultivars of Pearl millet (Pennisetum glaucum L.), were grown under elevated UV-B radiation (eUV-B; ambient+7.2 kJ m day) in natural field conditions to assess changes in physiology, leaf alterations, oxidative stress, antioxidant profile and secondary metabolites. Chlorophyll fluorescence measurements showed decline in F/F ratio at PDS (panicle development stage) in MPMH-21, and MPMH-17, which leads to higher ROS as evidenced through spectrophotometric observations and histochemical localization. Protection from excessive O radical and HO was manifested by stimulation of antioxidative defense and UV-B induced metabolites (phenolics, flavonoids, anthocyanin). Alterations in stomatal aperture and wax deposition was also observed through SEM. Both MPMH-21 and MPMH-17 cultivar showed less biomass partitioning to grains, while HHB-272 and HHB-67 resulted in increased grain yield up to 28% and 23.2%, respectively. GC-MS analysis revealed an increase in phytosterol and triterpenoids like β-sitosterol, stigmasterol, and squalene, under eUV-B. This suggests that plants adapt to UV-B stress by shifting the primary metabolites towards the production of secondary metabolites. Further, eUV-B promotes synthesis of few compounds, such as neophytadiene, and Phenol, 2,4-bis(1,1-dimethylethyl) (2,4-DTBP), which possesses anti-cancerous activities. The findings indicated differential responses of the test plant might be due to mechanistic variations in the antioxidants and metabolites during the panicle development stage and grain filling stage, which protected these plants from oxidative stress induced via eUV-B.