Modulations of physiological responses and possible involvement of defense-related secondary metabolites in acclimation of Artemisia annua L. against short-term UV-B radiation

UV - B radiation exposure for upto 3 h did not cause direct damage to physiology, but adjusted secondary metabolism and metabolites accumulation as an effective acclimation mechanism to mitigate the adverse effects of radiation. Artemisia annua L. plants were irradiated with UV-B radiation (280-315 nm; 2.8 Wm(-2)) for different short-term (1, 2, 3 and 4 h) durations. UV-B irradiation of 3 h reduced the photosynthetic rate, stomatal conductance and transpiration rate. However, F (v)/F (m), a sensitive indicator of photosynthetic inhibition, remained stable (0.78) upto 3 h, thereafter it declined sharply (0.72). Interestingly, transcript level of LHCB1, PSBA and PSBO genes related to photosystem II (PSII) were induced under UV-B exposure. In addition, genes coding for Rubisco small (RBCS1B) and large (RBCL) subunits were also upregulated upto 3 h. To mitigate the adverse effects of UV-B radiation, plants tremendously induced defense-related secondary metabolites such as antioxidative phenolics, UV-B absorbing flavonoids, anthocyanins and protective terpenes. The GC-MS analysis of essential oils revealed relatively higher production of monoterpenes over sesquiterpenes as well as 1.2-folds higher total oil yield under UV-B radiation. Owing to its diverse biological activities, the altered quantity and quality of essential oil of A. annua may contribute towards improving its therapeutic properties. The results suggest that UV-B irradiation upto 3 h reduced photosynthesis, probably due to stomatal limitations rather than any direct injury to photosynthetic apparatus as evident from stable F (v)/F (m) value, upregulated genes and greater accumulation of their corresponding proteins which gauge PSII health, elevated UV-B absorbing compounds and other protective metabolites. Correlation analysis indicates a significant positive correlation of photosynthetic rate with stomatal conductance while a negative correlation with anthocyanin and monoterpene contents under UV-B radiation. The present study provides first hand information regarding photosynthesis, related physiological parameters and essential oil profiling in response to UV-B radiation in A. annua.