Phytotoxicity of Quillaja brasiliensis leaf saponins on Lactuca sativa and Echinochloa crus-galli and their potential as a bioherbicide

Abstract

The challenge of food security to meet the needs of an increasing human population demands the adoption of new approaches leading to sustainable and sufficient crop production. A high percentage of crop products are lost every year due to agricultural pests. Weeds are the most damaging class of pests and threaten the integrity of agricultural and natural environments due to their invasive and competing potential. Pesticide use in agriculture has increased in recent years and although it increases agricultural production, it may cause undesired negative environmental impacts. Bioherbicides, which can be used as weed management tools, consist of substances based on natural compounds already present in the environment being biodegradable and having low residual effects. The use of plant species able to produce and release phytotoxic compounds may represent effective bioherbicide sources. Quillaja brasiliensis produces secondary metabolites called saponins that could be evaluated for phytotoxic activity and potentially become a natural herbicide. Therefore, this study was conducted to examine the phytotoxic activity of Q. brasiliensis saponins aqueous extract (AE) and saponin fraction (QB) on morpho-physiological parameters of Lactuca sativa (lettuce) and Echinochloa crus-galli (barnyardgrass), in pre and post-emergence bioassays. In the pre-emergence bioassays, germination rate and speed of germination were determined. The seedlings of the same species were evaluated regarding effects on initial growth by measuring seedling root and shoot length, dry mass and chlorophyll content. Both aqueous extract and saponin fraction had high inhibitory impact on germination of lettuce and barnyardgrass. Osmotic potential analyses revealed that this parameter was not important in the observed responses. Saponin fraction at 1% and 2% (w/v) concentration significantly decreased shoot length of lettuce seedlings by more than 10-fold. Results also showed a phytotoxic effect on post-emergence growth of lettuce, especially at the highest concentration tested of AE (10% w/v). These results show that both saponin enriched fraction and aqueous extracts of Q. brasiliensis are phytotoxic. Further studies should aim at detailing their phytotoxic mechanism on plants aiming at their possible use as bioherbicides.