Irradiance-based treatments of Quillaja brasiliensis leaves (A. St.-Hil. & Tul.) Mart. as means to improve immunoadjuvant saponin yield

Quillaja brasiliensis (soap tree) is native of southern Brazil. A purified fraction of saponins from leaves of Q. brasiliensis, QB-90, had significant immunoadjuvant activity in several experimental vaccines. The accumulation of QB-90 in cultured leaf disks and seedlings was previously shown to be responsive to different stresses, including UV-C and higher white light irradiance. An investigation on the regulation of QB-90 accumulation by irradiance treatments in young plants and in harvested leaves of trees of different ages was carried out. Young plants exposed to acute UV-C for 2-4 d or to red light enrichment for 30 d accumulated significantly more QB-90 in leaves than control plants. Red light treatment of plants did not increase carbohydrate content, indicating that its effects on saponin yield were not due to photosynthesis changes. The influences of irradiance treatments on QB-90 content of leaves from juvenile and adult trees, as well as the impact of temperature on saponin content of adult leaves, were investigated during post-harvest periods. QB-90 yield of adult leaves was not affected by various temperature treatments. A significant increase in the concentration of QB-90 both in juvenile and adult leaves was observed after exposure to UV-C radiation for 15 min. Exposing leaves to visible irradiance enriched in red color was effective in increasing QB-90 yield after 48 h of treatment in adult leaves and after 12 h in juvenile leaves. QB-90 content in detached leaves was relatively stable over a 10 d dark period, indicating that results of exposure to UV-C and red irradiance were a direct consequence of metabolic elicitation of bioactive saponins in leaves. Out of the best results of each light treatment on detached juvenile leaves, UV-C proved to be the most efficient to increase QB-90 content. Irradiance-based treatments can be a valuable tool to improve yields of immunoadjuvant saponins in soap tree.