Foliar spraying of biogenic CuO nanoparticles protects the defence system and photosynthetic pigments of lettuce (Lactuca sativa)

Engineered nanoparticles (NPs) can act as biostimulants (fertilizers) or cause toxicity in plants (phytotoxicity) depending on several factors. When toxicity is produced, it can lead to oxidative stress, disturbing the plant's oxidative defence system. Biogenic NPs, obtained by green synthesis, are an interesting alternative which obviates toxicity and oxidative stress-related effects. The object of this study was to evaluate how biogenic CuO NPs synthesized with green tea (20 mg per plant) influence the defence system and the photosynthetic pigment content of Lactuca sativa in pot experiments under greenhouse conditions. Moreover, the exposure routes (foliar spraying and soil irrigation) and the forms of Cu applied (nanoscale and ionic) were compared. The defence system of L. sativa was accessed by antioxidant enzymatic activity [ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD)]; secondary metabolites or antioxidant non-enzymatic activity (total flavonoids and phenolic compounds); photosynthetic pigments [chlorophyll a and b and carotenoids]; and nitric oxide levels [R-S-nitrosothiols (RSNOs) and nitrite (NO2−)]. Foliar spraying of CuO NPs reduced APX, but there were no variations in CAT, POD, or SOD activities or photosynthetic pigment contents. However, levels of RSNOs and secondary metabolites increased. In contrast, treatments with CuSO4 raised the APX level and reduced levels of RSNOs, chlorophyll b and carotenoids. Soil irrigation with CuO NPs did not increase enzymatic activity, however it reduced the chlorophyll a and b contents. The use of biogenic CuO NPs via foliar spray showed promising results in protecting lettuce from oxidative stress.